1. ISOPE-2010- Lessons Learnt from Recent Deepwater Riser Projects
Jean-François Saint-Marcoux,
Acergy, London, UK
Marin Abelanet
Acergy, Singapore, Singapore
Stéphane Bombino
Acergy, Paris, France
ABSTRACT
Major Risers development Projects have been launched recently (Total
CLOV and Petrobras Pre-salt to name a few). The Business strategy of
major Oil and Gas companies with regards to Subsea Umbilicals
Flowlines and Risers is evolving towards a more segmented approach
requiring optimization of each individual element of the SURF
package. This in turn requires a thorough review of the capabilities of
the concept of each segment to a level that was not considered before.
Acergy has gathered experience in all types of flowlines (rigid,
flexibles, bundles) and risers (SCR’s, Flexibles, single and bundle
Hybrid riser Towers). In particular, Acergy has pioneered the use of
Hybrid Riser Towers and have completed in 2007 the largest bundle
HRT to-date.
The paper will focus on how Risers become industrialized products
with their components being systematically organized through a
technical hierarchy. This in turns allow a detailed FMECA and a
structured detailed engineering package. From the structured
engineering package, result robust interfaces for material sourcing and
Fabrication.
KEY WORDS: Risers, FMECA, Hybrid Riser Towers, Risers,
Ultra-deepwater, Industrial model
NOMENCLATURE
ANSI: American National Standard Institute
API: American Petroleum Institute
BT: Buoyancy Tank
CRA: Corrosion Resistant Alloys
FEED: Front End Engineering Design
FJC: Field Joint Coating
FMECA: Failure Modes, Effects and Criticality Analysis
FPSO: Floating Production Storage and Offloading [unit]
FPU: Floating Production Unit
GOM: Gulf of Mexico
HRT: Hybrid Riser Towers
IMR: Inspection, Maintenance, Repair
ISO: International Standard Organization
LRA: Lower riser assembly
NACE: National Association of Corrosion Engineers
OHTC: Overall Heat Transfer Coefficient
OREDA: Offshore Reliability Data
PIP: Pipe in Pipe
PLET: Pipeline End Termination
SCM: Supply Chain Management
SCR: Steel Catenary Risers
SLOR: Single Line offset Risers
SURF: Subsea Umbilicals Flowlines and Risers
URA: Upper Riser Assembly
INTRODUCTION
As ultra deepwater1
fields are being developed, it becomes more and
more clear that, with the notable exception of the Gulf of Mexico,
FPSO based field architecture with wet Xmas trees is becoming the
norm. Typical examples are Total Angola CLOV development and
Petrobras Brazil Tupi area fields.
Operators, to a large extent, handle themselves directly or through
engineering companies their overall field developments. These field
developments include: Flowlines, Risers, and Floating Production
Units. They expect cost effective solutions and, as they have done for
other areas of petroleum industry, they split their ultra-deepwater scope
of work into segments: Flowlines, Risers and Umbilicals. Therefore it
is for the contractors as they provide any of these segments to adapt
their offer accordingly. As has been well known for centuries,
industrialization is the only solution to achieve a step change in cost-
effectiveness. This paper specifically deals with ultra-deepwater risers
as one of these segments.
STAKES
Industrialization brings cost-effectiveness and reliability through
standardization and repeatable processes. For a basically tailor-made
equipment such as HRT, this implies adapting methods that are of
frequent use in serial manufacturing industries, for instance System
Engineering and Value Analysis concepts.
The main stake is to find relevant tools, among general industrial
engineering methodologies, that can apply to this specific problem, and
that meet operators needs.
The frame to this industrialization / standardization is given by the
following complementary objectives:
1) Structured feedback and lessons learnt through a standardized
description of studied equipment:
- Conceptual description, functional approach;
- Physical description, organized list of equipments and
technologies, maturity approach;
- Organizational description (packages and suppliers tiers
1
Taken as deeper than 1800m per ANSI/API 17A

2. models).
2) Assess failures:
- Record what went wrong or can go wrong
- Better identify where effort must apply to avoid critical
dysfunctions
- Complete functional knowledge of equipment through
dysfunctional approach
Dysfunctional
modes
Functional
modes
Unknown
Known
Dysfunctional
modes
Functional modes
Standards scope
Unknown
Known
Fig. 1 Failures assessment role in standardization
As described in Fig.1 failure phenomena and scenarios knowledge
improves general knowledge of behavior of equipment, hence allowing
better standardization of products and methods.
AN EXAMPLE OF METHODOLGY
John Milton remarked “Truth and understanding are not such wares as
to be monopolized and traded in by tickets and statutes and standards”.
Yet in spite of this early recognized difficulty, there has been a
continuous push to make available the firmer part of current
understanding into codes and standards and the ANSI-API 17/ISO
13628 series of recommended practices can be seen as the most recent
attempt to do so in the deepwater offshore industry. This effort is
further carried out internally by companies and contractors.
The Fig.2 delineates the simplified workflow process of new concepts
as they are being developed.
The deliverable of the first phase is a functional analysis whereby the
various requirements, from operational to construction (fabrication and
installation) are clearly identified. The major drivers are:
- Flow assurance requirements
- Met ocean conditions
- Site location
- Yard fabrication procedure
- Offshore construction
The second phase consists of the Front End Engineering Design. This
starts with the definition of a concept consistent with the functional
requirements. This is validated with:
- Pre-design analyses
- FMECA
The third and final phase is the delivery of the documents required for
Supply Chain Management (procurement services and their associated
management) and then for construction (fabrication and installation
procedures)
Of course, this straightforward scheme is in reality much more complex
because of overlaps between the phases, different owners of the phases,
not to mention changes along the duration of a particular project.
Fig. 2 Workflow of new riser concept development
To a certain extent it may be said that deepwater risers such as flexible
risers or SCR are single bore units and therefore do not require as much
standardization as HRT’s. By its inherent flexibility in design (selection
of the number of conduits HRT’s in a field and selection of the number
of HRT’s) it lends itself to fit for purpose solution for each field.
Therefore the remaining of the paper will concentrate on HRT’s.

3. The figures 3 and 4 below respectively describe
- a turret-moored FPSO in ultra deepwater with two HRT’s
(Fig.3)
- an HRT fabrication yard (Fig.4)
Fig. 3 Deepwater FPSO based development with HRT’s
Fig. 4 Deepwater HRT fabrication
HRT FUNCTIONAL ANALYSIS AND
STANDARDIZATION
Industrialization means that pragmatic choices have to be made, and
that those choices must be robust with intended projects and with
regards to potential changes. Functional analysis allows making those
choices judiciously.
Flow Assurance
Pressure containment at design temperature is usually the driver and
requires high strength steel. However because of the potential for
reservoir souring API 5L grade X65 is usually the highest grade used
because of its compliance with NACE MR-01-75. It is therefore
reasonable to standardize on API 5LX65, and to consider CRA as an
option in presence of high CO2. If CRA is justified it will require less
buoyancy and therefore will be even more cost effective compared to
other riser types.
Wet insulation cover a wide range of Overall Heat Transfer Coefficient
and long cool down time. Unlike other riser systems, HRT risers can be
coated with a large thickness of wet insulation without the limitation on
installation squeeze pressure of flexibles, or the limitation of w/D ratio
of dynamic risers. If an OHTC-value of less than 1.5 W/m2
.K is
required, Pipe-in-Pipe solutions are feasible. Therefore Wet insulation
is the selected standard with PIP as an option.
With regards to the number of risers per tower it can be seen that one
riser per tower (SLOR) may lead to layout difficulties whereas large
number of risers may lead to impractical sizes of buoyancy tank.
Standard sizes would be from 4 to 8 main risers2
.
Site location and Met ocean Conditions
The current ultra-deepwater field developments appear to target areas
with a maximum water depth of 3000 to 3500m. This is about the
maximum water depth encountered in the GOM, and in the
Mediterranean Sea. And therefore the concept should be acceptable to
these depths.
This is the case because buoyancy foam which provides buoyancy
along the bundle has been already qualified by drillers.
The in-place dynamics of an HRT is low and therefore a wide range of
met ocean conditions can be accommodated. Met ocean conditions
affect essentially:
- Towing (surface current and waves period and directionality)
- Depth of the buoyancy tank (depth at which the current is still
significant)
Fabrication and Installation
The basic principle is that under usual conditions onshore fabrication is
more cost effective. Additionally quality control techniques are more
easily implemented onshore or even inshore. Still onshore construction
can be adapted to the locally available constructions site:
- Complete onshore construction of the full length of the tower
- Construction segments with inshore jointing (patent pending)
2
This number refers to Production,, Water Injection, and Gas
Injection or export risers. Gas lift risers are smaller and not a
driver of the bundle architecture and therefore not included in
these figures.

4. The same standard design with minor differences can accommodate
onshore or inshore construction.
Reversible installation operations allow simpler de-commissioning or
re-location. This feature was used as early as in 1995 (Fisher and
Berner 1988; Fisher, Holley, and Brashier 1995).
TECHNICAL HIERARCHY
As HRT’s (both monobore –SLOR- and multibore) are becoming more
and more known a standardization of the HRT has appeared:
- Jumper
- Buoyancy Tank (BT)
- Upper Tower Assembly (URA)
- Bundle
- Lower Tower Assembly (LRA)
- Suction piles
- Spool
The HRT being considered a system, these elements are called “sub-
systems” as shown in Fig. 5
Sub-system
Bundle BUN
Riser base RBA
Buoyancy tank BTA
URA URA
LRA LRA
Jumpers/spools JPS
Others OTH
Fig.5 Identifiers of sub-systems of an HRT.
With the introduction of a technical hierarchy of system, sub-system,
unit, equipment, part, and component it becomes feasible to identify
completely all elements of the “System HRT”. As an example Fig. 6
shows the full decomposition of the production riser down to the
individual weld.
System
Sub-system
Unit
Equipment
Part
Component
Name
2 3 4 5 6 7
x Hybrid riser tower
x Bundle
x Production riser
x Rigid risers
x DJ
x Joint
x Weld
Fig. 6 Use of the Technical Hierarchy levels to describe the HRT down
to its lowest level
The Technical Hierarchy provides the frame to identify each
component of the system down to the individual fabrication operation.
It allows both a top-down and a bottom-up view. As such it allows
answering the following example questions:
- which part, unit or system can be affected by the failure of a
particular weld?
- Which welds would be affected by a change in service
conditions?
The Technical Hierarchy provides a mean of tracking each individual
element from engineering, to procurement, fabrication and installation.
In particular:
- Engineering documents can be referred to a particular level of
the technical hierarchy
- Interfaces are readily identified
- Procurement can be performed at the most cost effective level
with regards to cost and delivery
- Quality Control can be tracked to the same level
Besides the Technical Hierarchy allows proceeding with the FMECA.
HRT FAILURE MODES, EFFECTS AND CRITICALITY
ANALYSIS
The FMECA was conducted by Acergy on its preferred “standard”
architecture.
Principles
The FMECA covers all stages of the life of the HRT:
- Installation of the suction anchor
- Fabrication in the yard, and inshore as applicable
- Installation towing
- Installation upending
- Spools and jumper connections
- Pre-commissioning
- Service life
The following assumptions were made:
- Design is compliant with codes and industry standards
- Design is compliant with specific design analyses developed
by Acergy for similar systems (Total Angola Girassol, BP
Angola Greater Plutonio, ExxonMobil Angola Block 15 Gas
Gathering)
- Extreme and operating conditions as provided by operators
are not exceeded
- Causes originating in Fabrication that may affect service
conditions are included
- Detailed risk analyses will be performed for each specific
project
- Suitable maintenance is provided during service life
- Failures are mutually independent, but common mode
failures are included where necessary
- All parts are manufactured by qualified suppliers with an
approved quality control system
Fabrication
Fabrication risks are related to lifting and handling of parts and sections
of the bundles.

5. Connections can be monitored through onshore quality control and
hydrotest.
It is easier to consider the fabrication and the sheltered water assembly
as a single phase because the same tools are available there.
The Fig. 7 is a diagram of the fabrication process. To the left is the yard
fabrication of the sub-systems: buoyancy tank, upper riser assembly,
bundle, lower riser assembly and flexibles. To the right is the sheltered
water area where the bundle is assembled to the URA and the LRA.
Fig. 7 Location of Risks at Fabrication phase (low risks omitted for
clarity)
Towing Upending
Main risks are localised at interfaces between the various sub-systems
in particular at URA and LRA where moment and loads are
concentrated.
Unexpected human intervention should be minimized: contingency
plan should detail tasks to a sufficient level, in order to be able to
anticipate contingency risks assessment.
Normally the suction anchor will have been installed long before to
allow settling of the soil.
The Fig. 8 shows both the towing and upending configurations. Those
two operations re performed in a relatively short period of time,
typically a week, by a dedicated fleet of vessels and crew. During tow
care should be exercised to minimize fatigue. This is achieved by a
proper selection of the route and depth of tow. The upending operation
is conducted in a matter of hours.
Fig. 8 Risks at towing and upending phase (low risks omitted for
clarity)
Service
Risks can be grouped as per functions, as follows:
- Loss of containment which can be mitigated by pressure and
temperature monitoring
- Structural integrity: in particular tension provided by the
buoyancy tank;
This is the rationale behind the monitoring of the HRT. Monitoring can
also include the position of the top of the HRT for early assessment of
undesired trend.
The Fig. 9 shows the risks in service. The risks are essentially:
- loss of buoyancy of the buoyancy tank
- tether connection
- erosion in the goosenecks
- loss of permanent buoyancy of the bundle due to water creep
- integrity of the risers weld
- rotolatch connection
- integrity of the spools
All of these risks are well identified and mitigated through engineering
per design codes specialized material qualifications.
Such facilities have been in service for ten years and this contributes
greatly to the assessment of the risks.
Through the OREDA database it is possible to quantitatively assess the
risks of various subsea components and in particular connectors.

6. Fig. 9 Risks in service (low risks omitted for clarity)
Summary
Risk criticality is assessed from consequence severity and causes
frequency. See Tab.1.
It would be difficult to cascade FMECA into a detailed quantitative
RAM analysis, because it covers a huge variety of concepts, but a few
quantitative parameters could be analyzed:
- frequency provided by OREDA database
- number of failure causes reviewed
- relative proportion of high severity cases
Tab. 1 Table of frequency vs. severity
Frequency
Severity
1
Very
unlikely
2
Unlikely
3
Possible
4
Likely
5
Very
likely
5 Very serious M M H H H
4 Serious L M M H H
3 Moderate L L M M H
2 Slight L L L M M
1 Negligible L L L L M
The results of the FMECA are as follows:
- 2 High severity risks
- 194 Medium severity risks
- 437 Low severity risks
These results are summarized in Fig. 11
Total
0%
31%
69%
H
M
L
Fig. 10 distribution of risks for an HRT
It is interesting to note that the two high risks identified are the
connection at the top and bottom of the HRT, as resulting from the
OREDA (2002) database. These connections are of course inevitable,
and therefore it can be concluded that the proposed design of the HRT
does not provide any additional high risk.
PRACTICAL IMPLICATIONS
First immediate outcome of FMECA is a sorted list of critical failure
scenarios that are mitigated throughout detailed design phase. In other
words, it is a qualitative reliability and availability assessment.
Then, a check-list can be extracted from tables to improve technical
risk management in design:
Tab. 2 : Example of check list of failure modes coverage.
Standard or spec
Part Failure
mode
Cause /
phenomenon,
Ref § Comments
Joint Crack Overpressure ANSI/API…
Another classical outcome of a FMECA is IMR support dossier. It
justifies the preventative inspection and maintenance actions, and
corrective actions. It allows issuing a preliminary list of spare parts, to
be optimized by operating Company from own feedback and policy.
This justified, anticipated approach can simplify spare standardization.
CONCLUSIONS
The recent deepwater projects have fostered an independent review of
HRT concepts. This is opening the door to their industrialization.
It also can be seen that the level of risks of HRT is acceptable as not
significant risk is added by a carefully selected architecture.
Further standardization of engineering will allow shorter delivery and
improved costs.

7. ACKNOWLEDGMENTS
The authors acknowledge Acergy for allowing them to prepare this
paper. The contribution of Jean Pierre Branchut, Jean-Luc Legras,
Gregoire de Roux, Blaise Seguin and Johann Declerq to several aspects
of the subject of this paper is gratefully acknowledged.
Nevertheless this paper only reflects the opinion of its authors and does
not imply endorsement by the company to which acknowledgements
are made.
REFERENCES
ANSI/API RP 17 A (ISO 13628-1) Design and Operation of Subsea
Production Systems – General Requirements and Recommendations.
January 2006
ANSI/API RP 17 N Recommended Practice for Subsea Production System
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DNV RP-A203 Qualification Procedure for New Technology
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Green Canyon Block 29 Development”, OTC 5846, Houston, TX
Fisher, E., A., Holley, P., Brashier, S. (1995), “Development and
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