2. Introduction
About NKIF
Norsk Kinesisk Ingeniørforening (NKIF) is a non-profit, professional
association dedicated to providing professional networking
opportunities and promoting technology application. It is officially
founded and registered in Oslo in 2014 and is open to all professions
in Oil & Gas, Maritime and other relevant industries. The NKIF
members include engineers, professors, research scientists,
university postgraduate and undergraduate students etc. from both
China and Norway.
NKIF is organized by a Board with board members elected every
second year by all NKIF individual and corporate members. The
board members are unpaid volunteers with supports from all the
members. The operation of NKIF will be open and transparent.
NKIF is committed to:
Promoting the professional network and collaboration both
within NKIF and with other associations
Encouraging experience and knowledge sharing
Supporting professional development
Strengthening cooperation between industries and academia
world widely
Being the bridge between the industries in China and Norway
I
3. NKIF provides:
Technical seminar and lectures
Career development forum
Continuously updated latest industry events
Publication of NKIF newsletter
NKIF journal with technical and overview articles for relevant
engineering disciplines
Posting of job opportunities from NKIF corporate members
Benefits as a NKIF Member:
Free to all NKIF organized events, e.g. technical
seminars/workshops
Free subscription to NKIF newsletters and journals
Informed with job opportunities in both Norway and China
Expanded professional network towards companies and
engineers
II
4. Disclaimer
The materials in all the articles have been prepared by the
corresponding authors with the purpose to share general
information among the NKIF members. If you own rights to
any of the materials and do not want them to appear in the
NKIF eJournal, please contact the author or NKIF and they
will be promptly removed.
The views and opinions expressed in the articles are those of
the authors and are not necessarily reflective of NKIF.
Any form of redistribution of the materials in the articles in
NKIF eJournal is not allowed without permission from the
authors and NKIF.
NKIF eJournal Chief Editor
Haifeng Wu
2015.04.30
III
5. Contents
Introduction ………………………………………………… I
Disclaimer…………………………………………………. III
Marine Structures – From Conventional Ships and Offshore
Oil & Gas Platforms to Recent and Future
Developments………………………………………………. 1
An introduction of Sesam package with its application to
offshore structure design………….………………………..17
Risk based inspection analysis of structures.……………… 23
Arctic Offshore Operation: Challenges and Solutions…..... 37
How well can we predict the loads from ice……………….50
About the authors ………………………………………… 61
IV
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Marine Structures – From Conventional Ships and Offshore Oil & Gas Platforms to
Recent and Future Developments
Zhen Gao
Centre for Ships and Ocean Structures, Centre for Autonomous Marine Operations and
Systems and Department of Marine Technology, Norwegian University of Science and
Technology
Introduction
We live on the Earth with our major activities being carried out onshore. Although the oceans
are not suitable for human beings to live in directly, they cover more than 70% of the Earth’s
surface and do provide us the opportunities for sea transportation, exploitation of oil and gas,
production of seafood, utilization of offshore renewable energy, and infrastructure for
recreations. These opportunities are realized through man-made marine structures.
In this article, a brief introduction will be given to the historical development of marine
structures with focus on ships for sea transportation and offshore platforms for exploitation of
subsea oil & gas resources. The focus here are offshore platforms. Recent developments of
offshore renewable energy devices will be discussed, in view of the design challenges and the
needs for accurate numerical models for load and response analysis. The article also provides
an outlook on the concepts of future marine structures with unprecedented designs such as
floating bridges. Marine structures are designed on one hand to fulfil a certain function, and
on the other hand to ensure safety during the life-time operation. Design aspects concerning
safety for ships and offshore platforms will be discussed in detail. The difference between the
traditional ship design method using reference (or mother) ships and the first-principle design
approach for offshore platforms will be emphasized. Design analysis procedures considering
ultimate and fatigue limit states will be explained and in addition, the probabilistic design
approach as well as the principle of accidental limit state design will also be introduced.
Category of marine structures
Ships
Ships have a long history for transportation of materials, goods and passengers and now
become an important component of the world trade. The non-uniform distribution of natural
resources (such as coal, oil and gas, minerals, etc.) around the globe and the uneven use of
these resources in different countries call for an increasing world trade via sea transportation.
Internationalization of the world market and specialization of the manufacturing and
fabrication work encourages such an interconnected world trade network for transportation of
various goods from where they are produced to where they are consumed. Modern ships are
purposely designed and built to carry different types of raw materials and goods in order to
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improve the efficiency and reduce the cost of sea transportation. These include bulk carriers,
oil tankers, LNG tankers, container ships, passenger ships, supply vessels for the offshore oil
& gas industry, war ships, etc. For ships, low resistance in order to limit power consumption
is an important requirement and the overall hull shape is commonly determined by transport
economics. Along with it, there is a significant development of international ports with highly
efficient loading and offloading systems and complex and effective logistics.
Bulk carriers are the most frequently used ships nowadays, making up 40% of the
international fleet and carrying 66% of the world trade. Oil tankers are becoming bigger and
bigger, transporting crude oil from the oil production countries (for example in the Middle
East) to the giant oil consumers (such as US, Japan, China, etc.). In LNG tankers, the gas is
liquefied at low temperature of -163°C and it is challenging to design a proper containment
which carries the fluid loading and yet provides an effective thermal insulation.
In the modern world, the majority of various goods are produced in a few developing
countries (such as China and India) where the labour cost is relatively low and they are
standardized for easy transportation by container ships to the developed countries in the North
America and Europe. Container ships are developed along with the need for distribution of all
kinds of goods to a vast number of end users in the form of standardized containers, which
can also be easily transported by trucks and trains onshore.
Comfortability and functionality with a number of choices of entertainments are the first
important features of a large cruise vessel. The recent trend of an increase in ship size and
cabin capacity demonstrates this. Safety is another crucial factor to consider for such ships
since they normally have a huge number of passengers on board. It is also important to
operate and manoeuvre safely in coastal waters.
Another category of ships are related to supply vessels or purpose-built offshore vessels for
supporting activities for the offshore oil & gas industry, such as transport of equipment and
personnel, vessels for installation of infrastructures (like subsea templates, pipelines and
power cables). These vessels normally have a ship shape, but an important concern is their
dynamic behaviour in waves during the operation at sea.
Oil & gas platforms
Different types of platforms [1] are envisaged (as shown in Figure 1) in the offshore oil & gas
industry at various geographical locations, including the North Sea, the Gulf of Mexico,
Brazil, the West Africa, the Persian Gulf, the Caspian Sea, Asia, etc. These platforms are
either bottom-fixed typically with gravity base or jacket foundations for small or moderate
water depths (up to 200-300m), or floating in deep waters with different hull shapes and
mooring systems, such as semi-submersibles, spars or ship-shaped Floating Production,
Storage and Offloading (FPSOs) with catenary mooring systems or Tension Leg Platforms
(TLPs) with tendons. Floating platforms are categorized based on the way they achieve the
static stability. A spar platform has a low Centre of Gravity (CoG) with heavy ballast at the
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bottom, while a ship-type structure has a very large water-plane area and a semi-submersible
has well separated surface-piercing columns, providing sufficient restoring stiffness in pitch
and roll. TLPs reply on the design with excessive buoyancy of the floater (much larger than
the gravity), leading to a high pre-tension and stiffness in tendons. In recent years, similar
floating structures are proposed for supporting offshore wind turbines.
Figure 1 Offshore oil & gas platforms (1, 2) conventional bottom-fixed platforms; 3)
compliant tower; 4, 5) vertically moored tension leg and mini-tension leg platform; 6) spar; 7,
8) semi-submersibles; 9) floating production, storage, and offloading facility; 10) sub-sea
completion and tie-back to host facility) [1]
Based on the function of an offshore platform, it can be categorized as drilling platform or
production platform. The first type of platforms are required for exploratory drilling to
identify hydrocarbons in the subsea reservoir and therefore need large payload capacity and
deck area for drilling equipment with limited motions and good mobility. Production
platforms carry chemical plants which consist of separators, pumps, etc. and normally are
permanently moored for the production period corresponding to the platform service life.
The vast majority of offshore structures used today are bottom-fixed platforms. As compared
to floating platforms, bottom-fixed ones exhibit apparent advantages of having no rigid-body
motions in particular in heave, which are one of the major concerns for drilling platforms.
However, in deep or ultra-deep waters, floating platforms are inevitably deployed since
bottom-fixed structures for such water depths are too expensive.
Offshore renewable energy devices
Utilization of offshore renewable energy for electricity generation has a relatively short
history. During the oil crisis in late 70s, there were intensive pioneering research activities on
developing the technologies to utilize offshore renewable energy, in particular wave energy.
However, it did not result into a commercial development of wave energy technology. Since
90s, there is a significant development on offshore wind technology due to the success of the
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onshore wind industry. Recently, there is an increasing interest in offshore renewable energy,
including offshore wind, wave and marine current (tidal and ocean current) energy. Nowadays,
offshore wind technology is by far the most developed technology, while both the wave and
marine current energy have not been developed into a fully commercial stage. The discussion
here will focus on offshore wind turbines and wave energy converters.
According to the types of foundations, offshore wind turbines may have bottom-fixed support
structures (such as monopile, gravity base, tripod or jacket) or floating support structures
(such as TLP, semi-submersible or spar), as shown in Figure 2. All of these structures support
a three-blade horizontal axis wind turbine (with variable speed and pitch control), which is
more or less standardized based on the development of the onshore wind industry. Vertical
axis wind turbine (which has a less power absorption coefficient) has not been widely used
onshore, but recently received a particular attention for floating concepts due to its advantages
of low CoG and independence of wind direction.
Figure 2 Bottom-fixed and floating wind turbine concepts [2]
As mentioned above, these concepts are ‘borrowed’ from the offshore oil & gas industry. The
experiences from both the onshore wind industry and the oil & gas industry have led to a
rapid development of offshore wind technology in particular floating wind technology in
recent years. Figure 3 shows three prototypes of floating wind turbines (one on spar and the
other two on semi-submersible floaters). However, it should be noted that most of the wind
turbines installed in the commercial offshore wind farms today are bottom-fixed monopile
and jacket wind turbines. The choice of foundations are mainly determined by the
consideration of cost. Floating wind turbines are not economically feasible for small water
depths (say less than 50-100m). In some parts of the world (such as Japan, Scotland, east
coast of US, South China Sea), the large water depth calls for floating wind turbine concepts.
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Figure 3 Prototypes of floating wind turbines (left: Hywind [3]; middle: WindFloat [4];
right: Fukushima semi-submersible [5])
Although the wave power density is larger than that of the wind power, it is much more
difficult to convert wave power into electricity in particular at a commercial scale. In contrast
to wind and tidal energy, wave energy converters span a wide range of different concepts with
over a hundred different designs being proposed over the years, many of which are under the
active development. This might be one of the reasons that the wave energy technology has not
been commercialized since the research efforts have not been concentrated on one particular
technology.
According to the working principle, these devices can be classified into three main categories
[6], namely oscillating water column, oscillating bodies and overtopping, as shown in Figure
4. Many concepts have been developed into prototypes, such as Pelamis, WaveBob, Pico and
WaveDragon, as shown in Figures 5 and 6.
As we can see, compared to floating offshore oil & gas platforms, wave energy converters
may have a very different shape of floaters, which is a direct result of functionality
requirement of wave power absorption. In addition, the concept of oscillating bodies
maximizes the motions by resonance in waves and therefore the wave power absorption. On
the other hand, the structural responses will also be larger due to the resonant motions. This is
contradictory to the design principle to minimize the motions for offshore floating oil & gas
platforms. As a result, it will be more challenging to ensure the structural integrity for a wave
energy converter, although most of the research today still focuses on power maximization.
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Figure 6 Pico (left) [9] and WaveDragon (right) [10]
Floating bridges
Recently, the Norwegian Public Roads Administration has initiated a study on the potential to
replace ferries with fjord crossing concepts (bridges or tunnels) along the E39 route between
Kristiansand and Trondheim. The Sognefjord, which is about 4km wide and up to 1300m
deep, is the pilot site among the seven fjords for developing such concepts. Floating
suspension bridge concept (as shown in Figure 7) and submerged floating tunnel concept (as
shown in Figure 8) were proposed by different research institutes and industry companies.
The fjord width of 4km does not allow a suspension bridge with a single span. Therefore, the
design in Figure 7 considers two towers, sitting on floaters (rather than on the sea bed) in the
fjord with a depth of 1300m. The floaters are then moored to the sea bed by mooring lines.
Two additional bottom-fixed towers are placed close to the shore. The long span of the bridge
and the floating support structures present unique challenges for design in particular under the
simultaneous wave and wind loads.
The submerged floating tunnel concept in Figure 8 consists of two tunnels submerged in the
water and interconnected by cross tubes, and many surface floaters to support the submerged
tunnels and to provide vertical stiffness to ensure the rigidity of the complete system. The
non-homogeneous wave and current loads on the tunnels and the floaters might excite both
vertical and horizontal eigenmodes of the structure and are particularly difficult to model.
So far, these are just concepts that could be used for fjord crossings, but they represent a very
different marine infrastructure as compared to an offshore oil & gas platform. More research
and engineering efforts are required to build, install and operate such floating bridges.
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Traditionally, conventional ships are designed based on empiricism, using reference ships (or
mother ships) and prescriptive ‘rule-book’ approaches. Such approaches were developed
gradually in the long history of ship technology and have been very useful and efficient to
extrapolate existing ship designs in small steps to those with larger dimensions, during the
years when direct calculation of loads/load effects and structural strength were not feasible.
However, new hydrodynamic or structural phenomena experienced by large ships or new
types of ships call for a different and a more rational approach for design by first principles
using analysis. The development of fundamental theory in hydrodynamics and structural
mechanics and dynamics, numerical analysis methods as well as computer science and
technology in recent decades enable the development and the application of first-principle
design approaches. Moreover, such approaches were practiced along the development of
offshore platforms for the oil & gas industry for which there were no experiences at all in its
early days.
Design based on first principles
Design by first principles requires explicit criteria for serviceability and safety. The most
important safety requirements for ships and floating platforms refer to avoidance of capsizing
or sinking and structural failure, which otherwise will occur and lead to catastrophic
consequences with fatalities, pollution or loss of property.
Static stability of a floating system is achieved by sufficient restoring stiffness against heeling
or tilting under mean external environmental (wind, wave and current) loads. This can be
realized by a proper design of centers of gravity and buoyancy, water-plane area of the floater
or mooring system. Typically, both intact and damage stability criteria need to be satisfied for
offshore oil & gas platforms. For floating wind turbines, the mean thrust force acting on the
wind turbine rotor will induce a significant overturning moment and it also varies as function
of mean wind speed with a maximum occurring at the rated wind speed. The design of the
floater needs take due consideration of this unique feature. However, the damage stability
criteria might not be necessary for floating wind turbines since the consequences of such
failure will normally only be loss of property. Stability check is not only applicable to floating
systems during normal operations, but also during temporary phases of transport and
installation. For example, a tension-leg platform is normally freely floating, possibly
supported with extra buoyancy during transport, while it has excessive buoyancy and a pre-
tensioned mooring system for normal operations. For bottom-fixed structures, like monopiles
and jackets, the overall stability is replaced by a strength criterion of the foundations (piles or
buckets), involving soil-structure interaction.
Structural safety is ensured in terms of load effects and strength depending upon relevant
failure modes. For marine structures, limit state criteria include ultimate limit state (ULS),
fatigue limit state (FLS) as well as accidental limit state (ALS). The ULS design ensures that
the extreme structural response (in a wider sense the extreme load effect) is smaller than the
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ultimate strength of the component or the system. The failure modes considered are buckling
and yielding. In design codes, a load factor resistance design (LFRD) format is typically used
with both a load factor and a material factor to take into account the uncertainties in the
estimations of load effect and structural strength, respectively. A different set of the two
safety factors represents a different safety level, for example a different annual failure
probability. In a FLS design check, the life-time fatigue damage should be smaller than the
allowable fatigue damage, which are determined considering the consequences of such fatigue
failure and the access for inspection and repair of the potential fatigue cracks. Most of the
codes today still use the SN-curve approach for fatigue design. The fracture mechanics
approach is applied in connection with crack inspection planning, but it still has a big
uncertainty in modeling of crack initiation and propagation in real conditions. ALS criteria
deal with the design concerns for marine structures under abnormal loads, such as ship
collision, fire and explosion, loss of one mooring line, etc.
An important step in design of marine structures is to predict the structural responses under
the external environmental loads. Certainly, wave loads are of primary concern. Floating
structures are highly dynamical systems and need to be designed with a good dynamic
performance in waves. That means the rigid-body motions should be minimized and in
particular, a floating system should avoid resonant motions due to the first-order wave loads.
Otherwise, excessive motions and the associated inertial loads will lead to too large structural
responses and expensive designs. Therefore, the natural periods of rigid-body motions should
be designed outside the period range of main wave conditions, typically 5-25s. Two different
solutions are envisaged, one with semi-submersibles or spars and soft mooring systems to
have natural periods larger than 25s, and the other with TLPs and tendons to have natural
periods of the vertical motion modes (heave, pitch and roll) less than 5s. However, second-
order (or even higher-order) wave loads will excite these resonant motions, but the magnitude
of the induced responses are much lower. As mentioned above, some wave energy concepts
utilize the wave-induced resonant motions to maximize the power absorption and accordingly
become expensive due to the large structural responses. A tradeoff between the power and the
cost needs to be found for such systems.
Motion characteristics are not explicit safety criteria for design of marine structures.
Eventually, one needs to estimate the structural responses (at a stress level) in order to do a
design check. This requires analysis methods to predict hydrodynamic loads, to perform
motion response analysis and to do structural response calculation. Such design analysis is
normally performed using numerical methods and numerical models for load prediction and
response analysis. More and more, time-domain simulations are applied in which nonlinear
external loads can be modeled and the coupling between the responses induced by different
sources of loading can be included. Figure 9 shows the complexity of external loadings that a
TLP floating wind turbine might experience. In particular, both wind and wave loads might be
nonlinear and coupled to the induced motion and structural responses, and in addition the
wind turbine automatic control is typically applied in the time domain. The floating structure
and certainly the wind turbine rotor exhibit geometrical nonlinearities with large rigid-body
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motions or deformations. These call for a nonlinear time-domain formulation of the dynamic
problem.
Figure 9 External loads on a TLP floating wind turbine [12]
From the structural response point of view, besides the quasi-static wind- and wave-induced
responses, responses of floating structures are typically governed by resonant rigid-body
motions and/or structural vibrations. Under such conditions, the damping from various
sources or mechanisms is crucial since the inertia loads cancel the restoring forces at the
resonance, and the damping forces are only the parameter that determines the response level
under the given excitations. An accurate estimation of the damping effect (for example soil
damping or structural damping) is difficult and requires further research efforts. Damping
cannot be measured directly and this adds another difficulty in the experimental study on
damping. On the other hand, the aerodynamic, hydrodynamic, structural or soil damping
effect strongly depends on the motion or vibration modes of the structure. This is a principle
that has been used in some of the active damping devices which are typically placed at the
position with a maximum displacement of a certain mode.
Structural analysis for design checks
Structural design of floating platforms can be broken down in two fundamental levels: local
strength and global strength, considering local loading and global loading, respectively. For
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example, in a semi-submersible floating platform with braces, the dimension of the columns
or pontoons are determined by a local strength check, while the size of connecting braces are
decided from a global analysis, in which the global loads on the columns or pontoons are
balanced by the cross-sectional forces/moments in the braces.
Global strength check uses a stress-based, rational analysis to examine the entire structure as a
space frame for example for a semi-submersible with braces or, in the case of a spar, as a
single slender beam. Structural response analysis is based on the force and moment
equilibriums of the floater considering the distributed gravity/buoyancy loading, the external
loading from wind, current and waves, as well as the inertial forces due to platform motions
and the reaction forces from mooring lines and tendons. Local structural design check is
mostly based on empirical, classification rules (similar to those for ship structures) and
gravity/buoyancy loading. Loading on the floating structures is generally expressed as an
equivalent hydrostatic head.
Typically, the governing load cases for offshore platforms are related to the normal
operational cases, but in some cases, the loading in the transient phases (such as transportation
or upending of a spar) might be governing. Floating oil & gas platforms are wave-load
dominated, and the responses normally increase with the severity of wave conditions.
Therefore, the ultimate loads and load effects are related to the extreme design wave
conditions. A contour line (or surface) method with a certain correction factor might be used
to predict the long-term extreme responses. However, for offshore wind turbines dominated
by wind loads, the rotor is parked during the extreme wind conditions to reduce the
aerodynamic loads and the governing wind loads might be associated with a lower wind speed
around the rated value. Similar considerations are made for wave energy converters. It is then
important to notice that the design loads for these structures should be determined taking due
considerations of operational limits and survivability adjustments.
In recent decades, finite element (FE) and multi-body dynamics methods have been widely
used in static and dynamic analyses for design of marine structures. Such analysis includes
both analysis of structural responses under external environmental loads and analysis to
determine the ultimate strength of structures. As mentioned, both global and local analyses
can be performed using FE methods, see Figure 10. For FLS design checks, FE methods with
refined meshes (in the order of thickness by thickness for shell meshes) are also used for
determining the stress concentration factors (SCFs) via a linear structural analysis. Nonlinear
finite element analysis is normally performed for ultimate strength (for example buckling
strength) analysis of marine structures’ components or systems. For designs considering ALS
load conditions involving collision, fire and explosion, time-domain nonlinear finite element
analysis has to be applied.
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Figure 10 FE models of marine structures (from left: a global semi-submersible model; a
global catamaran model; a refined column-brace model) [13]
To achieve safety, it is crucial to avoid errors in design, fabrication and operation. The design
phase is the most important phase from a life cycle perspective, since most of the important
decisions are made during this phase, regarding fabrication method, serviceability during
operation and safety during operation. Offshore oil & gas platforms are normally one of its
kind and prototype testing of such system is not practical. Due to the complexity, numerical
analysis using validated tools is crucial for design assessment. Numerical methods and codes
have been developed and validated against lab and field measurements and used for design
checks. For offshore renewable energy devices, one has to take into account the advantage of
mass production or mass installation in order to reduce the capital cost and therefore the cost
of energy.
Ships and offshore platforms are traditionally and probably will be steel structures in the
future. In particular, high tensile steel (HTS) has been widely used now and led to a reduction
of the required structural dimensions. However, from a material strength point of view, the
fatigue property of such steel has not been improved and the fatigue problem becomes more
and more important for design assessment. A better understanding of the development of
fatigue cracks into fracture is of concern. This is an issue especially relevant in view of
conversions for other use, and extended service life of existing marine structures. On the other
hand, recent development in the welding technology has significantly improved the welding
quality and therefore the fatigue strength of marine structures. Materials technology has
enabled the development of innovative marine structures. For example, aluminium, titanium
and fibre-reinforced plastics have been used in high-speed/passenger vessels for which light
weight and high strength are important concerns.
Probabilistic design of marine structures
Marine structures are subjected to environmental loads from wind, waves and current, which
are of stochastic nature. The fabrication process, although highly automated today, introduces
a variation in the strength property of fabricated structures. Numerical methods or models
used to determine the loads/load effects as well as the strength of marine structures are not
perfect. Therefore, design of marine structures needs to consider these uncertainties and the
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design codes should reflect them in the specified safety factors in the corresponding design
format.
In connection with a ULS design, a more relevant question is what will be the life-time
extreme response, rather than when the extreme response will occur. On the other hand, based
on the technology today, we are not able to predict the exact time series of environmental
conditions and therefore structural responses in the order of the life-time (20-50 years) of
marine structures. Statistical assessment is therefore needed. In general, this requires a
probabilistic rather than a deterministic assessment of load effects and structural strength.
The overall aim of structural design should be to reach an agreed acceptable safety level (for
example a target annual failure probability) by appropriate probabilistic definitions of
loads/load effects, and strength (or resistance) as well as safety factors. Such criteria should
be verified by reliability and risk approaches. Typically, a target annual failure probability of
10^-4 or 10^-5 depending on the consequences of the failure is considered for ULS and ALS
design of marine structures and 10^-3 or 10^-5 for FLS design. A higher safety factor would
imply a lower annual failure probability. In other words, the safety factors should be
calibrated by structural reliability analysis to reflect a target safety level. A higher safety
factor also means a more conservative and therefore costly design. Safety factors should be
specified differently for oil & gas platforms with failures leading to severe consequences
(such as fatalities, pollution and/or loss of property) and for offshore renewable energy
devices with loss of property as the major consequence. For offshore renewable energy
devices, cost reduction is the most important consideration for commercial development and
this requires more accurate numerical methods and models in order to reduce the uncertainties
associated with the prediction of load effects and to achieve a cost-effective design.
Design based on a design format with the above-mentioned safety factors is called a semi-
probabilistic design approach, and it is widely used now in the design of offshore platforms. A
complete probabilistic design requires an explicit assessment of the uncertainties in the
modeling of environmental conditions, external loads, motion and structural responses, as
well as structural strength and a direct calculation of the failure probability (typically
represented as annual failure probability) of a limit state function. Such limit state function is
based on a load effect-resistance formulation and corresponds to a certain failure mode (for
example due to ultimate load or fatigue load).
The theory of structural reliability has been well developed and it has been also used for
design of civil structures, such as buildings, bridges, etc. The most important work for
different applications are related to the uncertainty modeling and quantification. This is an
area requires further research efforts. Normally, the uncertainties associated with the load
effect prediction are much larger than those in the strength. In particular, the uncertainties
related to the environmental conditions require a collection of relevant wind, wave and
current data for a long-term period, either based on field measurements or hindcast numerical
models. To obtain an explicit safety measure for structures, the model uncertainty of the
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relevant calculation method should be determined. The possible statistical error due to
limiting sampling size in time domain analyses should also be assessed.
Concluding remarks
Marine structures have been developed for the need of mankind for sea transportation,
exploitation of oil and gas, utilization of offshore renewable energy and will be further
developed in view of other use of the ocean space, such as production of seafood and
infrastructure for recreations. Along with these opportunities that the oceans provide to us,
there are still many technological challenges that we need to overcome for the development of
future marine structures.
Ships have a long history of development and design of ships have been mainly rule-based.
Offshore oil & gas platforms are normally designed based on first principles through direction
analysis which is enabled by the fast development of the computer science and technology, as
well as the numerical methods and codes. The rapid development of offshore renewable
energy devices in recent years benefits from such design principles and approaches. It can be
foreseen that a rational design approach for future marine structures should be based on [13]:
- Goal-setting; not prescriptive
- Probabilistic; not deterministic
- First principles; not purely experimental
- Integrated total; not separately
- Balance of safety elements; not hardware.
References
[1] Office of Ocean Exploration and Research (2008). Types of Offshore Oil and Gas
Structures. NOAA Ocean Explorer: Expedition to the Deep Slope. National Oceanic and
Atmospheric Administration.
[2] De Vries, W.E., van der Tempel, J., Carstens, H., Argyriadis, K., Passon, P., Camp, T. &
Cutts, R. (2010). Assessment of Bottom-mounted Support Structure Types with Conventional
Design Stiffness and Installation Techniques for Typical Deep Water Sites. Deliverable
D4.2.1 (WP4: Offshore Foundations and Support Structures), Project UpWind EU.
[3] Statoil (2015).
http://www.statoil.com/en/TechnologyInnovation/NewEnergy/RenewablePowerProduction/O
ffshore/Hywind/Pages/HywindPuttingWindPowerToTheTest.aspx?redirectShortUrl=http%3a
%2f%2fwww.statoil.com%2fhywind
[4] Principle Power (2015). http://www.principlepowerinc.com/products/windfloat.html
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[5] FOWC (2013). Fukushima Floating Offshore Wind Farm Demonstration Project
(Fukushima FORWARD) – Construction of Phase I. Fukushima Offshore Wind Consortium.
[6] Falcão, A. F. O. (2010). Wave Energy Utilization: A Review of the Technologies.
Renewable and Sustainable Energy Reviews, 14 (3): 899-918.
[7] Pelamis (2015). https://www.youtube.com/user/PelamisWavePower
[8] WaveBob (2015). https://www.youtube.com/watch?v=0hGoDXCyr54
[9] Pico (2015). http://www.pico-owc.net/
[10] WaveDragon (2015). http://www.wavedragon.net/
[11] Ferjefri E39 Project (2015).
http://www.vegvesen.no/Vegprosjekter/ferjefriE39/English/Fjordcrossings
[12] Butterfield, S., Musial, W., Jonkman, J. & Sclavounos, P. (2005). Engineering
Challenges for Floating Offshore Wind Turbines. In: Proceedings of the 2005 Copenhagen
Offshore Wind Conference, October 26-28, Copenhagen, Denmark.
[13] Moan, T. (2003). Marine Structures for the Future. Presentation for the Inaugural Keppel
Lecture held at the National University of Singapore on July 18, 2003.
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An introduction of Sesam package with its application to offshore
structure design
Zhiyuan Pan
DNV GL
1. History
Sesam (Super Element Structural Analysis Modules) has a history that began with the
discovery of oil in the North Sea and with the first advances in computer technology in the
1960s. Created as a joint research project between Det Norske Veritas and the Norwegian
University of Science and Technology in Trondheim, it was the first software for structural
analysis of maritime and offshore structures based on the revolutionary finite element (FE)
methodology previously used in the aerospace industry. Since 1969, Sesam has been owned
and developed by DNV (DNV GL now). Over the last 45 years, the FE methodology in its
first version has been maintained and developed, serving as the central role in Sesam program
family.
Fig. 1. The 472 meter-height GBS platform Troll A with it Sesam FE model
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In 1980’s, there were extensive activities in development of the Sesam package. The graphic
user interface for displaying and modelling was brought in. Moreover, the package as a whole
became versatile, such that it can be applied to all kinds of ships and offshore structures (GBS,
Jacket, FPSO and Semi-submersible) with more modules introduced. Among them, the wave
load analysis programs are crucial with the offshore structure going into deeper water.
During 1990’s, the development of new generation of Sesam was initiated. “Concept
modelling” was introduced in FE modelling tool GeniE. The users do not need to make very
single element as in the old days. With the concept model and properties assigned, the FE
model can be generated by a single click of keyboard. With the same concept model retained
for different purpose (global or local structure analysis, hydrodynamic analysis), more effort
can be paid to the analysis and design work, as the most engineer would like to experience.
The leading position in the offshore structure design market had been secured by the
integrated programs GeniE, HydroD and DeepC, where an increased focus was paid on
advanced 3D visualization, user-friendliness, the interaction with the background solvers are
controlled in a straightforward manner.
Today, Sesam is owned and marketed by DNV GL Software. There are hundreds of world-
wide users including the major shipyards, oil and marine & offshore design companies. With
the income re-invested into development, more cutting-edge and market required features are
in their way into the Sesam package.
2. SESAM as a family
Fig. 2. Family picture of Sesam
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SESAM package includes dozens of modules, which can be classified roughly into four
groups: 1) preprocessing which make or refine analysis models, 2) motions and/or
environmental loads solvers for large volume and/or frame structures, 3) linear/nonlinear
structural analysis solvers, 4) post-processing tools, see Fig.2. Some of them have their own
user interface, whereas others are pure background solvers. These modules are communicating
with each other through the unique data storage format called “Sesam interface file”. Each
module can make its single step in the whole analysis loop, without knowing how its input
files are prepared or how its output files will be handled, as long as they are following the
same format. Such a common data format makes the Sesam package easy to be extended or
ported. With some moderate effort in adopting its I/O format, a new program can easily
position itself in the family. Among these Sesam modules, there are some 3rd
part tools, such
as Patran-pre and Xtract. Moreover, the cooperation with Marintek had brought in Mimosa,
Simo, Riflex for analysis with mooring lineas and Usfos for pushing over analysis for Jackets.
Over years, new modules enter into Sesam family, while some out-of-date modules are
replaced or quietly passing away. As the outcome of the collaboration with universities, the
core part of the FE analysis methodology in Sesam first version can still be found in the FE
solver, Sestra. The main hydrodynamic analysis program Wadam and Wasim are originated
from the academic research activities in Massachusetts Institute of Technology (MIT). Such
cooperation with universities and other 3rd
parties are still ongoing and it keeps Sesam as a
whole package alive and in the front of the market.
Using Sesam for structural design will in practical involve many modules. Managing input
and output files for all analysis steps could be challenging without any help. Integrated
program environment are provided which can facilitate users in their interacting with the
functioning modules. Today, the end users always relate Sesam to these integrated programs
GeniE, HydroD and DeepC with their application in FE modelling and structural analysis,
hydrodynamics and stability analysis, as well as mooring and coupled analysis of deep water
systems. In case where the analysis covers many territories, it is possible to manage the whole
analysis workflow by using Sesam manager. With all relevant controlling parameters to each
involved module are scripted and the entire process is kept in a clean way in Sesam Manager,
it is possible to have the whole design loop automated or re-established after modification in
one or several steps.
3. Application in offshore structure analysis
Different from ship hull structure design for oil tanker or bulk carriers, where the rule based
loading conditions are given in prior, offshore floaters vary from one to the other, so that
direct wave load analysis is always required. The accurate wave load & global response
computation and load transfer is crucial to the structural design.
To start with the hydrodynamic analysis, the equilibrium position should be found where the
buoyancy force is balanced with the gravity force plus the static pretention forces from the
mooring system. Two types of hydrodynamics analysis can be offered from SESAM: the
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linear frequency domain analysis which can be used for short term or long term statistics
analysis (to find extremes, or for a stochastic fatigue check), or the time domain analysis
where current or forward speed can be accounted together with nonlinear effects. In the
frequency domain analysis, the dynamic forces shall be computed for a group of regular wave
conditions with a combination of wave periods and directions which cover the major
environmental and operational conditions of the offshore unit in question. The nonlinear time
domain analysis is normally used to evaluate the most critical states where the extremes (for
motions, sectional loads or local pressure) are found. The deviations from the linear analysis
shall be illustrated. The following dynamic loads shall be considered: inertial forces, wave
pressure acting on wet-hull, line loads on Morison beams and point loads from the mooring
systems. All these loads should be in balance in the quasi-static condition for FE analysis.
Fig. 3. Computational structual analysis workflow chart
In general, 3 types of material failure of the hull structure, yielding, buckling and fatigue, shall
be considered covering the whole life cycle of the offshore structure in question. The yielding
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and buckling can be found in the Ultimate Limit State (ULS), where a group of design wave
loading condition shall be applied. In each of these conditions, a design wave is determined by
the long term statistics of a design criterion, such as sectional loads, accelerations and local
pressures. The Fatigue Limit State (FLS) assessment is based on complex stress transfer
functions established through direct wave load calculations combined with subsequent stress
response analyses. It is recommended to carry out a fatigue screening analysis on the global
structure model and a group of local models with refined mesh. It should be noted that the
same hydrodynamic analysis can be applied for both global structure and local models with
automatic load transfer. Moreover, the local fine mesh model does not need to be redefined in
global model, but can be modelled separately. A mapping process can be utilized to find the
deformation of the global model and apply as the boundary constrains to the local model. The
workflow of the ULS and FLS check on an offshore structure is illustrated in Fig. 3, where
each analysis step involves at least one Sesam module. Such a practice can be found in many
DNV codes or guidelines for offshore structure strength assessment.
Tab. 1 shows the modules used in the workflow in Fig. 3 with handling of the input and/or
output files. The modules found their position also in Fig. 2. Here, the super element number 1
corresponds to the global structure model, and 10 for the local structure model. The
numbering used here is just an instance, and could be different from case to case. All types of
input or output files involved, which are following the Sesam interface file format, are
illustrated with the table.
Sesam
Module
Application Input Output
GeniE FE Modelling T1.FEM
T10.FEM
HydroD/
Wadam
Global motion, ULS Loads, FLS
Loads to global model
T2.FEM T1.FEM G1.SIF,L1.FEM,S1.FE
M
FLS Loads to local model T2.FEM T10.FEM L10.FEM,S10.FEM
Sestra FE analysis of global model
T1.FEM, L1.FEM,
S1.FEM,
R1.SIN
FE analysis of local model
T10.FEM, L10.FEM,
S10.FEM
R10.SIN
Cutres Sectional loads verification R1.SIN
Submod Setup boundary condition for sub-
model structure from the global
analysis result
R1.SIN, T10.FEM T10.FEM
Stofat Stochastic fatigue &
long term stress level
R1.FEM, R10.FEM
Postresp Short term/long term statistics G1.SIF
Xtract General graphical post-processing T#.FEM, G1.SIF,
L#.FEM, R#.SIN,
Tab. 1. Sesam modules usage with input & output files
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T1.FEM: Global structure model
T2.FEM: Panel model
T10.FEM : Sub structure model
G1.SIF: Contains the global response analysis results
L#.FEM: Contains the hydrodynamic and inertial loading
R#.SIN: FE analysis result with stresses and deformation
4. Future
In recent years, to pave the way for the future, major efforts from DNV GL Software have
been paid, among which a couple of these activities can be highlighted as follows. First, the
infrastructure of the source codes is rebuilt to make Sesam more extensible and portable. To
name a few advantages, we could easily to port the application to new platform like Android,
or could for instance have the possibility to get the analysis job done on web-based application,
and have the model or the result stored in cloud, instead of offering Sesam as a product which
is normally to be installed with a CD into a desktop. Moreover, the size of the computational
model increases with the rapid development of the PC hardware. Transporting data between
modules using the interface file in the old way for large models could be very time-consuming.
Investigation has been carried out to make each SESAM modules as in-core services having
the common access to the computational data, so that writing and reading data files will be not
necessary. For example, meshing and re-meshing could be done with the storage of the model
file in memory, and the FE solver could be called directly with the access to the mesh data.
Sesam as a package has been undergone three major campaigns of development and moving
from different generations of computational platforms. Today, it is regarded as the most
comprehensive software solution for offshore structure design. It will continue to keep track
on the development of the best engineering practices of DNV GL in maritime and offshore
industries, and offers the best part of them to our world-wide users.
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RISK BASED INSPECTION ANALYSIS OF OFFSHORE STRUCTURES
WenBin Dong
DNV GL
ABSTRACT
Offshore structures are subjected to environmental loads due to waves, current and wind, as
well as the effect of corrosion from salt water, and in some cases floating ice. In order to
maintain safety of offshore structures in service life with respect to fatigue, wear and other
deterioration phenomena especially, inspection, monitoring and repair are important measures. In
this paper the motivation of Risk Based Inspection planning is described. Operational
experiences with respect to degradation of various types of offshore structures are summarized.
The basic methodology and useful guidelines are introduced.
1. INTRODUCTION
Oil and gas are the dominant sources of energy in the world. Twenty percent of these
hydrocarbons are recovered from offshore. Various kinds of platforms are designed and used to
support exploratory drilling equipment, and the chemical (production) plants required to process
the hydrocarbons, see Figure 1. Safety is a significant challenge for offshore structures due to the
harsh ocean environment and the fire and explosion risk associated with hydrocarbons. Fatigue is
an important consideration for structures in areas with more or less continuous storm loading,
such as offshore structures in the North Sea and ships in worldwide operation, and especially for
dynamically sensitive structures and welded joints with high stress concentration [1]. The first
rules for offshore structures appeared around 1970 and included fatigue requirements, which
were later refined, especially after the fatigue-induced total losses of the jack-up Ranger I and
semisubmersible Alexander L. Kielland in 1979 and 1980, respectively [1]. Corrosion is another
important strength degradation phenomenon widely existed in the offshore structures due to the
effect of harsh environment, which is normally treated in the design by providing a corrosion
protection (e.g. coating protection) and a thickness allowance. In order to maintain safety of
offshore structures in service life with respect to fatigue, corrosion and other deterioration
phenomena especially, Inspection, Monitoring and Maintenance and Repair (IMMR) are
important measures. IMMR are increasingly focused on fatigue and other degradation
phenomena in the last 20 to 25 years. In order to optimize IMMR plan, risk based inspection
planning (RBI) for offshore structures is developed as a systematic, qualitative and quantitative
approach which combines theoretical models, test results and in-service experiences, e.g. the RBI
methodology developed by DNV GL. The method is specially developed for offshore structures
such as:
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● Jacket ● TLP
● FPSO ● Deep Draft Floaters
● Jack-up ● Semi-submersible
● Concrete GBS ● Subsea template
In this paper the motivation of RBI analysis for different offshore structures is presented.
Operational experiences with respect to fatigue degradation are summarized. The basic work
principle of the approach and useful guidelines are introduced.
Figure 1 Selected offshore platforms
Jacket
(http://www.scivita.com/)
Jack-up
(http://www.saff-rosemond.com/)
Concrete GBS
(http://www.arcmachines.com/)
Semi-submersible
(http://www.basstech.se/)
TLP
(http://www.marinetechnology.mobi/)
FPSO
(http://www.offshore-technology.com/)
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2. CHARACTERISTIC FEATURES OF OFFSHORE STRUCTURES
Various types of platforms are applied in the offshore oil and gas industry, as presented in Figure
1. Their main function is to provide support of facilities for drilling operation or the oil and gas
production. While drilling units have to be designed to be mobile and appear as ships, semi-
submersibles and other shapes, production platforms will be located permanently on a site and
involve jackets, guyed tower, tension-leg platforms (TLPs), semi-submersibles and other types
[1]. While jackets consist of relatively slender tubular members, ships are composed of stiffened
panels. Semi-submersibles may consist of stiffened flat or curved panels and some slender
tubular braces.
2.1 LIMIT STATES
Design criteria for offshore structures are based on limit state formulations and semi-
probabilistic design principles, see e.g. ISO 19900[2] and NORSOK N-001[3]. The relevant limit
states are summarized in Table 1[1].
Table 1 Safety criteria.
Limited states Description Remarks
Ultimate (ULS) Overall “rigid body” stability
Ultimate strength of structure,
mooring or possible
foundation
Different types of criteria
apply
Component design check
Fatigue (FLS) Failure of (welded) joints Component design check
depending on residual system
strength after fatigue failure
Accidental collapse (ALS) Ultimate capacity1
of damaged
structure (due to fabrication
defects or accidental loads) or
operational error
System design check
1
Capacity to resist “rigid body” instability or total structural failure.
2.1.1 ULTIMATE LIMIT STATE
ULS criteria for overall stability of bottom-fixed structures are based on overturning forces due
to wave, current and wind and stabilizing forces due to permanent and variable payloads.
Stability of floating structures is analyzed in terms of overturning moment by wind only, and
uprighting moment due to the hydrostatic pressure on the inclined body [4].
Load effects (member and joint forces) due to permanent and variable deck loads as well as wave
and current loads, are usually used to check the ultimate structural strength of marine structures,
which are obtained by a linear global analysis. Stiffened flat panels and cylindrical shells are
commonly used in offshore structures. Ultimate strength formulations for such components are
traditionally obtained from strength of material formulations and substantiated by extensive test
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results. However, direct ultimate strength analysis, using finite element methods and accounting
for nonlinear geometric and material effects are being used together for design. Usually
acceptable safety is achieved by designing individual platform components based on
characteristic values of load effects and resistances, and appropriate partial safety factors.
2.1.2 FATIGUE LIMIT STATE
Fatigue criteria have been originally considered for fixed offshore platforms sincere the early
1970s. Fatigue also became an important consideration for mobile units around 1980 due to the
severe accidents since then, if not before. In the ship industry explicit fatigue assessment became
a common part of ship design since the early 1990s, when the major classification societies
introduced explicit fatigue design and assessment procedure in their rules.
Fatigue-induced catastrophic accidents for semi-submersible and jack-up platforms occurred
around 1975-1980. The fatigue cracking of the on tankers had some impact on the concern about
fatigue in design around 1990. The most
Severe accidents induced by fatigue were usually caused by gross errors, e.g. complete absence
of fatigue design check, bad design detailing, gross fabrication defects, non-redundant structure,
as well as lack of or deficient inspection.
Fatigue is an important consideration for structures in areas with more or less continuous storm
loading (such as the North Sea) and especially for dynamically sensitive structures [1]. Fatigue
strength is usually described by SN curves that have been obtained by laboratory experiments.
Fracture mechanics methods have been applied to assess the different stages of crack growth,
including calculation of residual fatigue life beyond through thickness crack, which is normally
defined as fatigue failure. The detailed information about crack propagation is also required to
plan inspections and repair. The basic design formula based on SN-curves and Miner-Palmgren’s
hypothesis could be written as:
𝐷 = ∑
𝑛 𝑖
𝑁 𝑖
𝑖 ≤ ∆ (1)
Where ni and Ni are the number of loading cycles and number of cycles to failure, respectively.
The calculation of the fatigue loading involves estimating stress ranges in various sea states in
the long-term period, see e.g. API [5], NORSOK N-003 [6].
It is assumed that the stress range only characterizes the fatigue strength and using the SN data
according to N=KS-m
and the Weibull distribution (with scale parameter A and shape parameter
B) for the stress range, the long-term cumulative damage may be written as
𝐷 =
𝑁 𝑇
𝐾
[
𝑠0
𝑙𝑛𝑁0
1
𝐵
] 𝑚
𝛤(𝑚 𝐵⁄ + 1) =
𝑁 𝑇
𝐾
𝑆̅ 𝑚
(2)
Where NT is the total number of cycles in the long-term period considered, s0 is the wave
induced stress response with an exceedance probability of 1/N0, m is the inverse slope of the SN
curve, Γ() is the Gamma function. The scale parameter A in the Weibull distribution is
A=s0/(lnN0)1/B
. 𝑆̅ is an equivalent constant stress range that represents the random loading. K is
the material parameter in the SN curve.
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Equation (2) could be used as a basis for an early screening of fatigue proneness, by using a
simple (conservative) estimate of the extreme response, s0 and by assuming the shape parameter,
B of the Weibull distribution based on experience.
Fatigue design criteria for offshore structures in Norway are dependent upon inspectability and
consequences of failure since 1984, as given in Table 2. Therefore, the acceptable fatigue
damage depends upon whether there is inspection or not. The acceptance criterion in Table 2 is
based on two consequence classes. The treatment of both the consequence and inspection issue,
however, could be improved, e.g. by taking the fatigue design factor, FDF (Table 2) as a function
of a more precise measure of residual strength and an explicit measure of the effect of inspection
including the quality of the inspection [1].
Table 2 Fatigue design factor (FDF) to multiply the planned service life to obtain required
the design fatigue life [3]
Classification of structural
components based on damage
consequence1
No access or in
the splash zone
Access for inspection and repair
Accessible (inspection according to
generic scheme is carried out)
Below splash zone Above splash zone
or internal
Substantial consequences 10 3 2
Without substantial consequences 3 2 1
1
The consequences are substantial if the accidental collapse limit state (ALS) criterion is not
satisfied in case of a failure of the relevant welded joint considered in the fatigue check.
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2.1.3 ACCIDENTAL COLLAPSE LIMIT STATE
Structural robustness checks are usually based on resistance against progressive failure after
removal of any one component, alternate paths and redundancy. The ALS check given in
NORSOK N-001[3] is a more explicit and quantitative survival check of a damaged structural
system. It is assumed that the damage is due to accidental loads such as fires, explosions, ship
impacts or fabrication defects corresponding to an annual exceedance probability of 10-4
and
should be specified by risk analysis [7], considering relevant risk reduction actions such as use of
sprinkler/inert gas system or fire walls for fires and fenders for collisions. Permanent
deformation, rupture of parts of the structure, nonlinear material and geometrical structural
behavior need to be accounted for to estimate damage.
The structure should be able to survive the various damage conditions – without global failure,
considering environmental loads with an annual exceedance probability of 10-2
. A conventional
ULS design check, based on a global linear structural analysis and component design checks
using truly ultimate strength formulations could be used. More accurate nonlinear analysis
methods could be also applied.
Figure 2 Fatigue cracks: (a) crack developing from location II in brace D-6 in ALK; (b) crack in tubular
joint; (c) 24m crack in tanker Castor
(a)
(b) (c)
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Figure 3 Development of cracks into ultimate consequences, and barriers to prevent such consequences
2.1.4 EFFECT OF CORROSION
Corrosion is an important strength degradation phenomenon widely existed in the offshore
structures due to the effect of harsh environment. Coating (paint or monel wrap), cathodic
protection and/or a plate thickness allowance are usually used to prevent or reduce the effect of
corrosion for design. Corrosion and its negative effects on ultimate strength and fatigue
resistance are to be considered during operation. The main types of corrosion patterns are general
corrosion, pitting corrosion, grooving corrosion and weld metal corrosion. Studies have shown
that the corrosion rate exhibits a very large scatter depending upon location in the structure. The
sea environment, e.g. in North Sea as compared to West Africa or Gulf of Mexico, is also a factor
of influence on corrosion rates. Once the corrosion protection system breaks down, free
corrosion effects start to take place on the surface of the structural component. The stress level is
increased and the strength is reduced due to the thickness reduction from corrosion. Increased
stress increases the fatigue crack growth rate. The effect of corrosion on the crack propagation
rate may be presented by the SN-curves or by introducing a correction factor Ccorr to the material
or crack growth parameter C used in the fracture mechanics models. As there is obviously a large
implied uncertainty, this factor should be modelled as random variable in reliability analysis.
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2.2 OPERATIONAL EXPERIENCES
The event sequence caused by a crack would depend upon the geometry of the structure. At the
design stage, scantlings and local geometry are determined to ensure a certain fatigue life, under
a limited stress level, plus crack initiation and growth rate as well as a high fracture resistance.
Several examples of cracks in platforms and ships are illustrated in Figure 2 [8]. The possible
sequences can occur and be controlled as illustrated in Figure 3 [8]. The residual resistance
against progressive crack propagation and ultimate collapse as well as inspection and repair
efforts have dominate effects on various sequences. All sequences in principle imply costs,
fatalities usually result only if there is total collapse of the whole structure or deck structure.
Through thickness cracks may imply loss of containment, hence leakage of oil or gas, with
undesirable consequences.
An overview of experiences with fatigue cracks in offshore structures operating in the North Sea
[1] and examples of fatigue crack experiences in ships [9] are briefly highlighted below.
Jackets. Proper fatigue design practice for North Sea jackets appeared around 1970–80.
Inspections have been carried out on the outside by divers or by remotely operated vehicles due
to lack of access inside the underwater jacket structure. Inspection/repair costs per joint are in
general much higher than those for ships and semisubmersibles.
A large amount of inspections has been devoted to North Sea fixed platforms since the last part
of 1970, throughout the 1980s. The fatigue failure of the Alexander L. Kielland contributed to
the attention to inspections. However, in the 1990s the limited amount of cracks detected
suggested that the prediction methods were conservative, and that the likelihood of fatigue cracks
was much less than initially anticipated. Studies found that the number of propagating cracks
predicted is typically 3 to 10 times too high, and it is most conservative for new structures [10].
On the other hand it should be noted that 2% to 3% of the fatigue cracks detected occurred in
joints which are not predicted to be susceptible to fatigue. This fact is mainly due to the
occurrence of gross fabrication defects. The average crack depth of the propagating cracks
detected was 4.8 mm, with a small percentage of through thickness cracks. Another lesson is the
big difference in relative crack occurrences in platforms installed before and after 1978.
Semisubmersibles. The most critical joints in semisubmersibles are tubular joints, which are
normally designed to transfer loads by means of membrane stresses, with much less bending than
in unstiffened tubular joints in jackets. Design requirements were initiated due to fatigue failures
that occurred in semisubmersibles in 1965–70. But the application of fatigue criteria varied, even
for platforms built in 1970–80. The total loss of the Alexander Kielland platform in 1980 was
initiated by a fatigue failure. Cracks were especially observed on Alexander Kielland and its
sister rigs at locations similar to the critical joint of Alexander Kielland. However, these cracks
were smaller than the one that caused failure of the brace, because of the absence of the
fabrication defect on these other locations. Although the total length of such defects maybe quite
long, repair is easy to complete by grinding. Extraordinary surveys carried out after this accident
on these North Sea platforms revealed many cracks, especially in brace-column connections
[11]. Even today the brace-column and column-pontoon connections for semis with long
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pontoons are still a challenge due to the complex geometry and high stress concentration (SC)
involved. For this reason cast pieces are applied at the high SC areas, while welds are located in
lower stress regions. The most fatigue-prone and critical areas of a semisubmersible are much
more limited in extent than in ships. This allows inspections to be focused. In addition,
inspections can be carried out from inside the structure. This has a significant effect on the
quality and costs of inspections.
Ships. Cracks have been known to be a common phenomenon in ships for decades. Cracks in the
main hull girder of a ship will grow continuously until global rupture of the hull. Fatigue failure
is normally defined as a crack through the plate thickness. The information about the crack
propagation from a through-thickness crack until fracture is necessary for the assessment of
critical crack size of ship hulls.
Fatigue was considered a serviceability requirement since periods with crack occurrences in the
1960s and 1970s. However, explicit fatigue requirements for hatch-corners in containerships and
LNG tanks were not introduced before 1991-92. The new fatigue design rules were introduced
due to the significant fatigue problems experienced for side longitudinals of 2- to 5-year-old
VLCC tankers in the Alaska-California trade [8]. A large amount of cracks occurred at the
intersections between side longitudinals and primary members in tankers, especially at bulkheads
and adjacent web frames. It has become evident that fatigue of the hull girder was a governing
strength criterion since 1985 with the first purpose-build ship for oil production in the North Sea.
However, it was realized too late to ensure that the vessel was built with adequate fatigue life.
The required safety level was then achieved by a more extensive inspection program, but with
the economic penalty of more inspections and crack repairs [12]. Similar experiences also exist
with shuttle tankers [13].
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3. RBI ANALYSIS FOR OFFSHORE STRUCTURES
Inspections are the basis for assessing the condition of the offshore platforms. The purpose of
inspection planning is to specify an inspection strategy that in a cost efficient way ensures that
legislative and operator requirements to safety are fulfilled and can be documented.
Risk based inspection (RBI) provides a consistent framework for decision making under
uncertainties. The main principle of this approach is that different inspection strategies are
compared in terms of the risk they imply. Risk is normally defined as the product between
likelihood and consequence of failure, which may be assessed for the safety of personnel as well
as for monetary costs or any other criterion of relevance for the installation. The RBI approach is
a condition based approach by which the inspection effort is fitted to the condition of the item
and prioritized in accordance with the importance of the individual items and the different
deterioration mechanisms. In practice, RBI analyses are usually performed for process systems
and structures separately. This paper is mainly focus on the RBI analyses of offshore structures,
and fatigue deterioration is highlighted.
3.1 RBI METHODOLOGY DESCRIPTION
Figure 4 shows the various activities undertaken in Risk Based Inspection [14]. The circle
illustrates the dynamic nature of the method. In practice RBI is usually divided into a so-called
risk screening process and a detailed assessment process. Figure 5 shows the typical tasks of
Risk Based Inspection planning for structures [14], which are briefly explained as follows:
Collection of Available Information The purpose of this task is to provide the required
information for subsequent assessment, and to document the basis for the study. The RBI
analysis is normally based on existing design documentation, reducing the need for additional
analyses.
Perform Portfolio Risk Ranking The purpose of this task is to perform a qualitative ranking of
a fleet of platforms to evaluate the need for further RBI analysis and prioritize the order of
platforms to be subjected to further work based on a qualitative, but consistent estimate of risk.
The likelihood and the consequences of failure due to fatigue, corrosion, scouring, and other
relevant deterioration mechanisms are evaluated, and the high risk structures are identified.
RBI Analysis including Cost Optimization The purpose of this task is to conduct RBI analysis
for individual structures. This can be the structures which have been specified as the most critical
ones in the Portfolio Risk Ranking, if performed. A suite of dedicated tools for structural RBI
assessments are available for the analysis, e.g. ORBIT Structure and SESAM programs from
DNV GL for maritime and offshore engineering analyses.
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Figure 4 Simplified illustration of the five tasks of the RBI process
Figure 5 Simplified illustration of the five tasks of the RBI process
Prepare Inspection Scheduling handbooks The purpose of this task is to collect and group the
proposed inspection plans obtained in Task 3 into suitable inspection intervals (campaign
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inspections). Deliverables from this task are handbooks, giving recommendation for inspection
scheduling.
Implementation into inspection management system The purpose of this task is to implement
the proposed inspection scheduling into the client’s inspection management system. The results
from the inspection planning should be prepared on such a form that electronic transfer of data
into the inspection management system is facilitated.
For the RBI analysis performed in Task 3, reliability methods have been identified efficient for
planning in-service inspection for fatigue cracks, accounting for both the detection accuracy and
the sizing accuracy for observed cracks. The time to first inspection and the inspection intervals
based on a specified required safety level can be assessed. Reliability methods could also be used
to optimize the design solutions. The most important issue for RBI analysis of structures is to
estimate the probability of a failure as function of time. Then the risk cost can be determined by
combining the probability of failure with the associated failure cost.
For fatigue failure of offshore structure, the failure criterion for fatigue limit state, based on the
fracture mechanics approach, may be stated by
𝑔(𝑋) = 𝑎 𝑐 (𝑋1) − 𝑎 𝑁 (𝑋2) (3)
where ac represents the critical crack size; aN represents the crack size after N cycles; N
represents
the cycle numbers; X1 and X2 represent a vector of stochastic parameters respectively (stress,
crack length, fatigue strength, etc.); X=[X1,X2].
The failure probability, e.g., the probability that the crack size exceeds a critical crack size within
the time period t (or N) is then
𝑃𝐹 = 𝑃(𝑔(𝑥) ≤ 0) (4)
First order reliability methods (FORM), second order reliability methods (SORM) and monte
carlo simulations could be used for the reliability calculations.
Figure 6 shows a typical example of the event tree for inspection planning. T0, T1, T2, T3 and T4
represents the inspection time. 1 represents crack has been found and repaired. 0 represents there
is no findings. More details could be found in [15].
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Figure 6 Inspection scheme
3.1 INSPECTION RELIABILITY
Non-destructive examinations (NDT) are commonly used to localize and size defects in
structures. The inspection reliability for the NDT method is defined as a function of a defect size,
through Probability of Detection (PoD) curves. PoD curves are available for the following
inspection methods:
Flooded Member Detection (FMD).
Eddy Current (EC).
Magnetic Particle Inspection (MPI).
Alternating Current Field Measurement (ACFM).
The probabilistic distribution functions of PoD for EC, MPI and ACFM could be presented as :
𝑃𝑜𝐷(𝑎) = 1 −
1
1+(
𝑎
𝑋0
)
𝑏 (5)
Where a = crack depth in mm
X0 = distribution parameter (= 50% median value for the PoD)
b= distribution parameter
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More details are given in [14].
4. CONCLUSIONS
RBI approach has been successfully applied to various offshore structures, e.g. jackets,
semisubmersibles, FPSO, jack-ups, as well as the pipelines and the mooring lines. It will be
playing a more and more important role for the safety of offshore structures in future.
In addition, with the development of offshore renewable energy utilization, safety is also
becoming a more and more important issue, especially for offshore wind energy. In offshore
wind industry, RBI approach has been applied to the support structures of offshore wind turbine
[16], and the mechanical components in the drive train of wind turbines [17]. The application is
still very limited up to now, and more research work is needed.
5. REFERENCE
[1] Moan,T., Reliability-based management of inspection, maintenance and repair of offshore structures. Structure
and Infrastructure Engineering, 2005, Vol.1, No. 1.
[2] ISO 19900, Petroleum and Natural Gas Industries – Offshore Structures –Part 1: General Requirements, 1994
(Int. Standardization Organization:London).
[3] NORSOK N-001, Structural Design, 1998 (Norwegian Technology Standards: Oslo).
[4] Clauss, G., Lehmann, E. and Østergaard, C., Offshore Structures, 1991, Vol. 1 (Springer Verlag: Berlin).
[5] API (1993/1997), Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms,
API RP2A-WSD July 1993 with Supplement 1 with Sect., 17.0, Assessment of Existing Platform, February 1997
(American Petroleum Institute: Dallas).
[6] NORSOK N-003, Actions and Action Effects, 1999 (Norwegian Technology Standards, Oslo).
[7] Vinnem, J.E., Offshore Risk Assessment, 1999 (Kluwer Academic Publishers:Dordrecht).
[8] Moan, T., Fatigue Reliability of Marine Structures, from the Alexander Kielland Accident to Life Cycle
Assessment. International Journal of Offshore and Polar Engineering, 2007, Vol. 17, No.1.
[9] Sucharski, D., Crude oil tanker hull structure fracturing: an operator’s perspective, in Ship Structure Committee,
in Proc. Symposium and workshop on the prevention of fracture in ship structure, Washington, D.C., 1997.
[10] Vårdal, O.T. and Moan, T., Predicted versus observed fatigue crack growth. Validation of probabilistic fracture
mechanics analysis of fatigue in North Sea jackets, in Proc. 16th OMAE Conference, Yokohama, Japan, 1997, paper
no. 1334.
[11] Potthurst, R., Coates, A.D. and Nataraja, R., Fatigue Correlation Study – Semi-submersible Platforms,
OTH88288, Report, 1989 (Department of Energy: U.K.).
[12] Bach-Gansmo, O., Carlsen, C.A. and Moan, T., Fatigue assessment of hull girder for ship type floating
production vessels, in Proc. Conf. on Mobile Offshore Units, 1987 (City University: London).
[13] Hansen, H.R., Nielssen, N.B., and Valsgård, S., Operational Experiences with Double Hull Tankers, Int Conf
Design & Oper of Double Hull Tankers, RINA, London, 2004.
[14] DNV ENERGY, RBI HANDBOOK FOR OFFSHORE STRUCTURES GENERAL, 2008 (Det Norsk Veritas:
Høvik).
[15] Dong W.B., Gao Z., Moan T., Fatigue reliability analysis of jacket-type offshore wind turbine considering
inspection and repair. In:Proceedings of EuropeanWind Energy Conference2010.Warsaw, Poland; 2010.
[16] Dong, W.B., Moan, T. & Gao, Z., Fatigue reliability analysis of the jacket support structure for offshore wind
turbine considering the effect of corrosion and inspection. Reliability Engineering & System Safety, 2012, Vol. 106.
[17] Dong, W.B., Xing, Y.H., Moan, T. & Gao, Z., Time do-main based gear contact fatigue analysis of a wind
turbine drivetrain under dynamic conditions. International Journal of Fatigue, 2013, Vol. 48.
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Arctic Offshore Operation: Challenges and Solutions
Biao Su
SINTEF
Introduction
There is a growing interest in hydrocarbon exploration and production in Arctic waters,
where one of the pronounced challenges is the presence of sea ice. The ice creates a number
of additional challenges compared to open water operations. Key factors are the physical
properties and dynamics of sea ice. In principle it is possible to distinguish between first-year
sea ice, multi-year sea ice and the presence of ice bergs. Ice bergs and multi-year sea ice may
show to impose actions that make structures to be costly to build and operate. There are also a
number of other factors that influence the design of Arctic structures in a conservative
direction, due to for instance lack of knowledge and operational experience, where an upper
bound conservative solution is chosen for the design (Bonnemaire et al., 2007).
Fixed structures may show to be attractive or the only possible solutions in shallow waters.
Such structures range from very shallow water artificial island concepts, to GBS (gravity-
based structure) solutions, loading towers and moored vessels possible for a range of water
depths. Moored, floating vessel concepts may show to be the most attractive solutions in an
Arctic environment. This applies to most operations, including drilling, production and
offloading of hydrocarbons. Several of the extreme ice events may be solved by effective ice
management, including ice intelligence, risk evaluation and icebreaker assistance.
Comprehensive use of ice management has shown to be a key factor when operating in ice
covered waters (Eik, 2010).
Arctic offshore challenges
Environmental issues are by most people considered to be more critical in the Arctic than in
other areas. Remoteness and climate factors will anyway make operations, for instance clean
up (i.e. after an oil spill) more difficult, and thereby the consequences of an accident higher.
By arguing that the risk is a product of the probability of failure and the consequences of an
accident, one may conclude that the probability of accidents in the Arctic should be reduced
as compared to other areas. This leads anyway to a focus on safety and reliability of Arctic
structures, which also imposes requirements for increased redundancy and backup solutions
for safe operations. This discussion is not taken any further here.
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Operational factors from the physical environment such as icing (see e.g. Figure 1), remote
location, and duration of daylight, temperature and wind, addressed as "winterization issues"
are not discussed here, nor the discussion of safety related to evacuation of personnel.
Figure 1 An example of the icing effect, SALM Offshore Sakhalin Island, December 16,
2004 (http://www.canatec.ca).
There are a number of other technical challenges that has to be addressed in a concept
evaluation for Arctic conditions. The key element in an evaluation is the capability to resist
and operate safely at the site specific physical environment, here focused on the operations in
ice.
The ice enviorment in the Arctic and Sub-Arctic seas ranges from areas with dynamic ice
conditions where ice is present occasionally to areas with ice cover every year and possible
more than 6 months of the year. In brief the ice conditions can be characterized by type of ice
(first-year, multi-year and icebergs), the cover in percentage, the drift characteristics and
intrusion of features like sea ice ridges, hummocks and stamukhas. Examples of the physical
environment and the ice cover are shown in Figures 2-3, and more details are found in Løset
et al. (2006).
The level of the ice actions on an offshore structure will depend on several main factors as
listed below:
The resistance in intact ice (level ice) is a function of the ice thickness, ice strength
properties and the shape and size of the structure. The mode of ice failure against the
structure has a significant effect on ice actions (see e.g. Figure 4).
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The ice drift and its characteristics will represent challenges if weather-vaning is
needed. Drift patterns may influence the action level in general, and sudden changes
in the ice drift may lead to high action situations and overloading of the system.
The ridges are normally thought to represent the ultimate loads when present. First-
year sea ice ridges consist of a consolidated upper layer, often considered as 2-3 times
level ice thickness, a sail, which is the observed part from the air, and piled ice blocks,
forming the keel (see e.g. Figure 5). The keels can extend from 20 to 30 m depending
on location. The total thickness for a multi-year ridge was reported to be 40 m
(Johnston et al., 2009). Ridges and their properties are also challenging to model in an
ice tank, due to thermodynamics or confinement scaling challenges.
In many cases, the iceberg impacts on offshore structures will give the design load in
accordance with the Abnormal Limit States (ALS). The need to avoid direct
interactions between offshore structures (including mooring lines, risers and
pipelines) and icebergs requires that icebergs must be reliably detected, so they can be
managed or avoided through disconnection. This is a significant challenge especially
when the icebergs are below 30 m wide and in the presence of sea ice (see e.g. Figure 6).
Figure 2 A hypothetical sea ice dynamics scenario (Wikipedia).
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Figure 3 Sea ice concentrations (amount of sea ice covering an area,
http://seaiceatlas.snap.uaf.edu).
Figure 4 Failure modes of sea ice, depending on parameters such as ice thickness, ice
velocity, ice temperature and the shape and size of the structure.
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Figure 5 Hypothetical interaction between two floes, resulting in a pressure ridge
(Wikipedia).
Figure 6 Small icebergs in sea ice.
Arctic offshore solutions
Offshore operations have been, and still are, successfully conducted in almost any kind of ice
regime. Spanning from ultra shallow waters of 3 m in the Caspian to depths more than 1000
m in the Arctic Basin, various types of drilling operations have been carried out. Oil
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production is safely carried out in the iceberg stream at Grand Banks and even the heavily
ridged multi-year ice in the Beaufort Sea has been handled in a safe way (Eik, 2010).
Based on experiences in the past, the main concepts of offshore structures and their feasibility
for Arctic waters are listed as below (Hannus and Bruun (2010)):
Bottom founded structure - concrete structure or steel caisson (see e.g. Figure 7)
Strengthes:
- Large topside weight
- Can resist large ice loads
- Can be designed to take iceberg
- Protection of riser systems and water intakes
- Can have drilling trough shaft
Weaknesses:
- Only applicable for shallow Arctic waters
Bottom founded structure - jacket platfrom
Strengthes:
- Straight forward fabrication of substructure
- Industry has long experience with jacket structures
- Have been applied in level ice (see e.g. Figure 8)
Weaknesses:
- Only applicable for shallow Arctic waters
- Requires offshore lift of topside
- Conductors open for ice interaction
- Small topside weight
- Vibration challenges in ice - self exitation of structure (see e.g. Yue et al., 2009)
- Cannot resist iceberg interaction
Ship-shaped structure
Strengthes:
- Well proven in Beaufort Sea (see e.g. Figure 9)
- Proven disconnection of risers and mooring
- Self-propelled after disconnection
- Straight forward deck integration
- Large deck carrying capacity
- Can be designed with dynamic positioning (DP) and icebreaking azimuth
propellers: ice milling and propeller wash
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Weaknesses:
- Limited capacity through swivel
- Subsurface ice transport (see e.g. Figure 10) can be a hazard for mooring lines,
risers, cathodic protection and water intakes
- Sudden changes in the ice drift may lead to high action situations and overloading
of the system (see e.g. Figure 11)
Semi-submersible platform
Strengthes:
- Proven concept (well established in harsh environment)
- Large topside capacity
- Can handle large number of risers
- Good motion characteristics in open water
Weaknesses:
- Unacceptable large ice loads: ice will crush towards and accumulate between the
vertical columns
- Mooring and riser disconnection systems not proven
- Flexible risers need special protection in ice zone
Shallow draft buoy
Strengthes:
- Well proven in Beaufort Sea (see e.g. Figure 12)
- Large deck carrying capacity
- Large hull volume for storage and marine systems
- Traditional and straight forward construction and fabrication methods
- May be transported to shore for dry docking and repair
- Short time for re-connection of mooring and riser systems
Weaknesses:
- Large diameter attracts large ice loads - heavy mooring system
- Motions in open water not proven.
- Not proven disconnection system with multiple risers and mooring connected in a
common riser buoy
- Subsurface ice transport due to shallow draught
Tension Leg Platform (TLP)
Strengthes:
- Proven concept (well established in harsh environment)
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- Large deck payload
- Can handle large number of rigid top-tensioned risers, dry trees
- Exellent motion characteristics in open water
- Single leg TLPs can be designed for limited ice loads
Weaknesses:
- Cannot be disconnected in case of ice conditions exceeding design criteria
SPAR
Strengthes:
- Proven concept (SPARS in the Gulf of Mexico)
- Acceptable ice loads
- Good motions in open water
- Can be disconnected and reconnected
- Only mooring and cathodic protection exposed to subsurface ice transport
Weaknesses:
- Limited deck capacities
- Long time for mooring re-connection after disconnection
- Disconnection/re-connection systems not proven
Figure 7 Steel Drilling Caisson (SDC) on location (http://www.canatec.ca).
Figure 8 Jacket platforms in JZ20-2 oil field, Bohai Sea (Yue et al., 2009).
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Figure 9 Beaufort Sea drilling operations from a moored drillship (http://www.canatec.ca).
Figure 10 Illustration of the subsurface ice transport which can be a hazard to for mooring
lines and risers (Bonnemaire et al., 2007)
Figure 11 Example of a 90° sudden change of drift direction observed during the IMD tests
(Spencer and Jones, 1995). The time series show the mooring load first in straight drift and
then during a 90 deg change of drift direction.
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Figure 12 Kulluk icebreaking drill barge and icebreaking supply vessel, Beaufort Sea
(http://www.canatec.ca).
Ice management
One of the most important lessons from the past is that the ice management system has been a
key factor when operating in ice covered waters (Eik, 2010). Without proper ice intelligence,
risk evaluation, ice breaker assistance (see e.g. Figure 13) and the possibility to escape the
drilling site, it would probably not have been possible to work in the strong multi-year ice in
Beaufort Sea. The associations with ice management may depend on the regions that are
under consideration: in Beaufort Sea ice management is typically about breaking and clearing
sea ice (see e.g. Figure 14), while ice management at Grand Banks typically concerns iceberg
deflection (see e.g. Figure 15). In some areas the presence of both sea ice and icebergs will be
expected, however, technology for handling icebergs frozen in the sea ice (see e.g. Figure 16)
is not proven.
The major conclusions regarding ice management are listed as below (by Eik (2010)):
Comprehensive use of ice management is explained as a key factor for the success in
Arctic offhsore operations.
Technology for iceberg handling in open water is considered as proven.
Technology for handling icebergs frozen in the sea ice is not considered proven.
Technology for breaking sea ice is proven for a wide range of severe conditions
including multi-year ice and ice ridges. However, it is expected that there may be ice
conditions more severe than the most powerful icebreakers can handle.
Use of azimuth propeller systems on icebreakers have been seen to contribute to
significant improvements in the icebreaking capability (the ability to clear ice around
a structure) and more important for offshore operations.
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Technology for detection and tracking of ice features will have to include a wide
range of tools. Use of unmanned aeroplanes, unmanned underwater vehicles and multi
beam sonar may be considered as possible future supplements to existing ice detection
tools.
It is recommended that evaluation of ice management capabilities is performed at an
early stage when planning new operations and in the evaluation of new drilling and
production concepts.
Future work regarding methodology for implementation of ice management
capabilities in concepts/operations is recommended.
Figure 13 Typical components of an ice management system (ISO/FDIS 19906).
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Figure 14 Illustration of two-stage ice management wherein two icebreakers reduce floe size
of the drifting ice to levels that exert manageable loads on the protected stationary vessel
(Hamilton et al., 2011).
Figure 15 Towing an iceberg from a collision course with an oil platform, Photo by Randy
Olson (http://www.amusingplanet.com).
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Figure 16 Iceberg frozen in sea ice.
References
Bonnemaire, B., Jensen, A., Gudmestad, O.T., Lundamo, T. and Løset, S., 2007. Challenges
related to station-keeping in ice. 9th
Annual INTSOK Conference, Houston, Texas, USA.
Eik, K. J., 2010. Ice management in Arctic offshore operations and field developments. Ph.D.
thesis, Norwegian University of Science and Technology, Trondheim, Norway.
Hamilton, J., Holub, C., Blunt, J., Mitchell, D. and Kokkinis, T., 2011. Ice management for
support of Arctic Floating operations. Proceedings of OTC Arctic Technology Conference,
Offshore Technology Conference, Houston, Texas, USA.
Hannus, H. and Bruun P.K., 2010. Conceptual design for Arctic waters [PowerPoint slides].
Lecture No. AT-327: Arctic Offshore Engineering, University Centre in Svalbard (UNIS),
Longyearbyen, Svalbard.
Johnston, M., Masterson, D. and Wright, B., 2009. Multi-year ice thickness: knowns and
unknowns. Proceedings of the 20th
International Conference on Port and Ocean Engineering
under Arctic Conditions (POAC), Luleå, Sweden.
Løset, S., Shkhinek K.N., Gudmestad O.T. and Høyland K.V., 2006: Actions from ice on
Arctic offshore and costal structures. Krasnodar, St. Petersburg, Russia, 2006, 271 p.
Spencer, D., and Jones S.J., 1995. Experimental Investigation into the Response of a Moored
Tanker to Changes in Ice Drift Angle. Institute of Marine Dynamics, Ottawa, Canada.
Yue, Q., Zhang, L., Zhang, W. and Kärnä, T., 2009. Mitigating ice-induced jacket platform
vibrations utilizing a TMD system. Cold Regions Science and Technology, Vol. 56, pp. 84-
89.
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How well can we predict the loads from ice
Fengwei Guo
DNV GL Oil & Gas
Abstract
The research activities contributing to ice load assessment are summarized. The industry
standards containing ice load equations are briefly reviewed, including the most updated ISO
19906 (Petroleum and Natural gas industries - Arctic Offshore Structures). In order to fill in
the large number of technical gaps and correct many inconsistency in ISO 19906, in 2009 a
Joint Industry Project called ICESTRUCT was launched by DNV (now DNV GL) and other
23 participants (operators, designers, academic institutes). ICESTRUCT was finished in 2012,
and the outcome is highly appreciated by the designers. Based on ICESTRUCT results, DNV
GL is developing a recommended practice to help designers facing challenges of ice loads
calculation. From the research point of view, the knowledge and resources needed to reduce
the uncertainty of ice load estimation are discussed.
1. Introduction
The attraction of Arctic resources (petroleum, minerals, fishery, etc.) becomes a popular topic
in the past years, both in public media and industry (Figure.1). In fact, the industry activity in
the Arctic region dates back to 1960’s – 1980’s, especially in Alaska and Canadian Beaufort
Sea. The industry interest in the Arctic has been always fluctuating with oil price and
profitability.
Figure.1 Petroleum in the Arctic
Apart from the oil & gas business, the research on high latitude areas has been always active
for many different reasons. For example, a bridge was built in 1997 in northeast of Canada
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(Figure.3), and the ice loading became a big challenge. In cold regions, river ice might
become a remarkable hazard, especially in the spring when ice cover breaks into large
amount of ice rubbles, which can damage the hydraulic facilities or lead to flooding
(Figure.4).
Figure.2 Two offshore structures in ice environment
Figure.3 Confederation bridge in Canada
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Figure.4 River ice break-up (Yellow river)
Dealing with these problems requires the knowledge on one subject: how does ice behave
under external loading? Researchers might be more interested in the fundamental questions
like:
Ice is a solid material, it looks quite similar to rock or concrete, how similar are they?
How strong is ice?
What are the differences between sea ice and fresh water ice?
On the other hand, the designers might ask the practical questions like:
I need to design a concrete platform in ice, how should I calculate the design ice load?
I need to design a container ship traveling across the Northern sea route, how should I
design the hull plating and stiffening?
I suppose the ice load depends on ice thickness, how can I estimate the ice thickness,
say, for 100 year return period?
If I need to consider ice berg, how can I estimate the frequency of ice berg impact?
In order to answer the fundamental questions, a lot of efforts have been made to improve the
understanding of mechanical behaviour of ice. Unfortunately, so far ice mechanics is a quite
immature field, because ice is an extremely complex material. Table.1 lists the most
important factors affecting the mechanical behaviour of ice.
Table.1 The governing factors to mechanical behaviour of ice
The crystal structure of ice
The temperature in ice
The porosity
The loading direction, if the ice is anisotropic
The loading rate
The boundary conditions
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Despite the difficulties and bottlenecks in ice mechanics, the industry has to take the
challenges making use of previous experience and knowledge. Based on limited information,
many ice load equations are developed and adopted in industry standards. In addition, some
private consultants also provide services on ice load assessment.
It is always interesting to compare the ice loads predicted by different methods. Figure.5 and
Figure.6 shows two surveys conducted in 1996 and 2006. In general, significant deviations
still exist among the predictions by different methods.
Figure.5 Ice load survey by Croasdale, 1996
(1.5 m thick ice on 100 m wide structure)
Figure.6 Ice load survey by Timco, 2006
(1.5 m thick ice on 100 m wide structure)
