1. PIPING BASICS
Piping may be defined as a mode of transportation for
the liquids, gases, and fluidized solids from one place to
another. A pipe is basically a tubular structure which is
specified by its nominal bore diameter and its Schedule
number which generally gives the standards of its wall
thickness. The piping is laid and designed according to
some standards laid by some of the standard
organizations of the world. It forms the heart of
industries such as power plants, petrochemical projects
etc.
Hence it becomes very important for us to know the
designing of such structures. The sector where piping is
used as the backbone are :-
(i) Oil & Gas Industry
(ii) Refineries
(iii) Petro Chemicals
(iv) Chemical Plants
(v) Power Plants
(vi) Water Treatment Plants
(vii) Pharmaceuticals & Food Industry
(viii) Paper plants
In other few pages we shall gain the complete
knowledge on piping and its basics.
2. OFFSHORE FUNDAMENTALS
INTRODUCTION
Offshore industry is basically concerned with the
exploration, drilling and production of oil and gas from the sea bed
in shallow as well as deep water. Oil and gas are derived almost
entirely from decayed plants and bacteria. Energy from the
sun, which fuelled the plant growth, has been recycled into useful
energy in the form of hydrocarbon compounds - hydrogen and
carbon atoms linked together.
Offshore and gas originates from two sources. Gas from
beneath the southern North Sea and the Irish Sea formed from coals
which were derived from the lush, tropical rain forests that grew in
the Carboniferous Period, about 300 million years ago. Oil and most
gas under the central and northern North Sea and west of the Shet
land Islands formed from the remains of planktonic algae and
bacteria that flourished in tropical seas of the Jurassic and
Cretaceous Periods, about 140 to 130 million years ago (a significant
amount of the Kimmeridge Clay Format ion is Cretaceous in age) .
They accumulated in muds, which are now the prolific Kimmeridge
Clay source rock.
Crude oil is a complex mixture of hydrocarbons with
small amounts of other chemical compounds that contain
sulphur, nitrogen and oxygen. Traces of other elements, such as
sulphur and nitrogen, were also present in the decaying organic
material, giving rise to small quantities of other compounds in
crude oil. Hydrocarbon molecules come in a variety of shapes and
sizes, (straight -chain, branched chain or cyclic) , this is one of the
things that makes them so valuable because it allows them to be
used in so many different ways.
3. Oil and gas form as the result of a precise sequence of
environmental condit ions:
· The presence of organic material
· Organic remains being t rapped and preserved in sediment
· The material is buried deeply and then slowly "cooked" by
increased temperature and pressure.
Offshore platforms are used for exploration of Oil and Gas from under
Seabed and processing. The First Offshore platform was installed in
1947 off the coast of Louisiana in 6M depth of water. Today there are
over 7,000 Offshore platforms around the world in water depths up to
1,850M
Platform size depends on facilities to be installed on top side eg. Oil
rig, living quarters, Helipad etc.
Classification of water depths:
< 350 M- Shallow water
< 1500 M - Deep water
> 1500 M- Ultra deep water
US Mineral Management Service (MMS) classifies water depths
greater than 1,300 ft as deepwater, and greater than 5,000 ft as
ultra-deepwater.
4. OFFSHORE PROCESS
EXPLORATION
DISCOVERING THE UNDERGROUND STRUCTURE
Large-scale geological structures that might hold oil or gas
reservoirs are invariably located beneath non-productive
rocks, and in addition this is often below the sea. Geophysical
methods can penetrate them to produce a picture of the pat tern
of the hidden rocks. Relatively inexpensive gravity and
geomagnetic surveys can identify potentially oil-bearing
sedimentary basins, but costly seismic surveys are essential to
discover oil and gas bearing structures. Sedimentary rocks are
generally of low density and poorly magnetic, and are often
underlain by strongly magnetic, dense basement rocks. By
measuring ’anomalies’ or variations from the regional
average, a three-dimensional picture can be calculated. Modern
gravity surveys show a generalized picture of the sedimentary
basins. Recently, high resolution aero-magnetic surveys flown
by specially equipped aircraft at 70 - 100m altitude show fault t
races and near surface volcanic rocks.
Shooting seismic surveys
More detailed in format ion about the rock layers within such
an area can be obtained by deep echo sounding, or seismic
reflect ion surveys. In offshore areas these surveys are
undertaken by a ship (F52) towing both a submerged air or
water gun array, to produce short bursts of sound energy, and a
set of streamers of several kilometers length. Each streamer
contains a dense array of hydrophone groups that collect and
pass to recorders echoes of sound from reflecting layers.
5. The depths of the reflecting layers are calculated from the time taken
for the sound to reach the hydrophones via the reflector; this is
known as the two-way t ravel time (F50a & b) . The pulse of sound
from the guns radiates out as a hemispherical wave front , a port ion
of which will be reflected back towards the hydrophones from rock
interfaces (F50a) . The path of the minute port ion of the reflected
wave- front intercepted by a hydrophone group is called a ray path.
Hydrophone groups spaced along the streamer pick out ray paths
that can be related to specific points on the reflector surface (F50c) .
Graphs of the intensity of the recorded sound plot ted against the
two-way time are displayed as wiggle t races (F50b) . Seismic
recording at sea always uses the common depth point (CDP) method
(F50c & d) . A sequence of regularly spaced seismic shot s is made as
the survey vessel accurately navigates its course.
Processing
Processing recordings involves many stages of signal processing and
computer summing. Firstly, wiggle t races from a single CDP are
collected into groups. Displayed side by side in sequence they form a
CDP gather (F51a & b) . Reflections from any one reflector form a
hyperbolic curve on the gather because the sound takes longer to t
ravel to the more distant hydrophones. This effect is called normal
move out (NMO) . Correct ion is needed to bring the pulses to a
horizontal alignment , as if they all came from vertically below the
sound source (F51c) . The separate wiggle t races are added
together, or stacked (F51d) . Stacking causes t rue reflect ion pulses
to enhance one another, and hopefully, random noise will cancel out
. This process is repeated for all the CDPs on the survey line. The
stacked and corrected wiggle t races are displayed side by side to
give a seismic sect ion (F51e)
7. Interpretation
Seismic sect ions provide 2-dimensional views of underground
structure. By using special shooting techniques such as spaced air gun
arrays or towing the streamer slantwise, or by shooting very closely
spaced lines, it is possible to produce 3-dimensional (3D) seismic
images (F59) . These images comprise vertical sect ions and horizontal
sect ions ( ’time-slices’) .
DRILLING
DEALING WITH THE UNDERGROUND STRUCTURE
There are two basic types of drilling rigs - fixed plat form rigs and
mobile rigs. Fixed platform rigs are installed on large offshore plat
forms and remain in place for many years. Most of the large fields in
the North Sea such as Forties and Brent were developed using fixed
plat form rigs. Drilling fluid (also called "mud") , which is mainly
water-based, is pumped continuously down the drill string while
drilling. It lubricates the drilling tools, washes up rock cuttings and
most importantly, balances the pressure of fluids in the rock format
ions below to prevent blowouts.
In offshore drilling, the first step is to put down a wide-diameter
conductor pipe into the seabed to guide the drilling and contain the
drilling fluid. It is drilled into the seabed from semi-submersible
rigs, but on product ion plat forms a pile-driver may be used. As
drilling continues, completed sections of the well are cased with steel
pipe
cemented into place. A blowout preventer is attached to the top of the
casing. This is a stack of hydraulic rams which can close off the well
instantly if back pressure (a kick) develops from invading oil, gas or
water.
Drilling grinds up the rock into tea- leaf-sized cuttings which are
brought to the surface by the drilling mud. The drilling mud is passed
over a shale shaker which sieves out the cuttings .
9. In exploration drilling, the cuttings are taken for examination by a
geologist known as a mud logger who is constantly on the lookout
for oil and gas.
PRODUCTION
THE OFFSHORE CHALLENGE
Production facilities had to be designed to withstand wind gusts of
180 km/ hour and waves 30 met res high. Other problems included
the ever-present salt -water corrosion and fouling by marine
organisms. Dealing with the many underwater
construction and maintenance tasks falls to divers and remotely
operated vehicles. Giant floating cranes (F83) designed to lift ever
greater loads were commissioned and many other specialized craft
had to be developed to establish and service the offshore industry.
Huge helicopter fleet s were needed to ferry workers to and from
the plat forms and rigs.
Product ion Plat forms
Most oil and gas product ion plat forms in offshore Britain rest on
steel supports known as ’jackets’, a term derived from the Gulf of
Mexico. A small number of plat forms are fabricated from concrete.
The steel jacket , fabricated from welded pipe, is pinned to the sea
floor with steel piles. Above it are prefabricated units or modules
providing accommodation and housing various facilities including
gas turbine generating sets. Towering above the modules are the
drilling rig derrick ( two on some plat forms) , the flare stack in
some designs (also frequently cantilevered outwards) and service
cranes. Horizontal surfaces are taken up by store areas, drilling
pipe deck and the vital helicopter pad. Concrete gravity plat forms
are so- called because their great weight holds them firmly on the
seabed. They were first developed to provide storage capacity in
oilfields where tankers were used to transport oil, and to eliminate
the need for piling in hard sea beds.
10. The Brent D plat form (F87) , which weighs more than 200 000
tonnes, was designed to store over a million barrels of oil. But
steel plat forms, in which there have been design advances, are
now favored over concrete ones. Several plat forms may have to
be installed to exploit the larger fields, but where the capacity
of an existing plat form permits, subsea collecting systems
linked to it by pipelines have been developed using the most
modern technology. They will be increasingly used as smaller
fields are developed. For very deep waters, one solution was
the Hut ton Tension Leg Plat form: the buoyant plat
form, resembling a huge drilling rig, is tethered to the sea-bed
by jointed legs kept in tension by computer- cont rolled ballast
adjustments. Alternatively, a subsea collect ion system may be
linked via a product ion riser to a Floating, Production, Storage
and Offloading (FSPO) vessel (F88) ; either a purpose built ship
or a converted tanker or semisubmersible rig. The oil is
offloaded by a shut t le tanker.
Product ion Wells
To develop offshore fields as economically as
possible, numerous directional wells radiate out from a single
plat form to drain a large area of reservoir (F94) . For directional
drilling special weighted drill collars are used with a ’bent sub’
to deflect the drill bit at a certain angle in the required direct ion
(F93) . Wells which deviate at more than 65 degrees from the
vertical and reach out horizontally more than twice their
vertical depth are known as extended reach wells. More than
one horizontal sect ion can be drilled in one well as a
multilateral well (F96) . This technique is used to reduce drilling
costs and to maximize the number of wells that can be drilled
from small plat forms.
11. Platform Semi- T.L.P.
Land Jack- Submersible Drill
Rig up Ship
PRODUCTION
12. GETTING OIL AND GAS ASHORE
Most offshore oil and all offshore gas are brought to shore by
pipelines which operate in all weathers. Pipeline routes are
planned to be as short as possible. Slopes that could put stress
on unsupported pipe are avoided and seabed sediments are
mapped to identify unstable areas and to see if it will be
possible to bury the pipe. Pipeline construction begins
onshore, as lengths of pipe are waterproofed with bitumen and
coated with steel- reinforced concrete. This coating weighs
down the submarine pipeline even when it is filled with gas.
The prepared pipe- lengths are welded together offshore on a
lay barge (F101) . As the barge winches forward on its anchor
lines, the pipeline drops gently to the seabed, guided by a
’stinger’. The inside of pipelines need to be cleaned regularly
to remove wax deposits and water: to do this a collecting
device known as a pig is forced through the pipe. Where
tankers transport oil from small or isolated fields, various oil
storage systems may be used. These may range from
cylindrical cells contained in some of the massive concrete
structures, to seabed storage units such as that employed at the
Kittiwake field, or integral storage such as that contained in
the various Floating, Product ion, Storage and Offloading
vessels. In essence these FPSOs are floating storage tankers, as
well as product ion and processing installations. FPSOs
provide an important option for developing fields which may
be remote from existing infrastructure or where the
field recoverable reserves are uncertain, for example because
of difficult geological conditions.
13. OIL PLATFORMS
An oil platform or oil rig is a large structure used to house
workers and machinery needed to drill and/or extract oil and
natural gas through wells in the ocean bed. Depending on the
circumstances, the platform may be attached to the ocean
floor, consist of an artificial island, or be floating. Generally,
oil platforms are located on the continental shelf, though as
technology improves and crude oil prices increase, drilling
and production in deeper waters becomes both feasible and
profitable. A typical platform may have around thirty
wellheads located on the platform and directional drilling
allows reservoirs to be accessed at both different depths and
at remote positions up to 5 miles (8 kilometres) from the
platform. Many platforms also have remote wellheads
attached by umbilical connections, these may be single wells
or a manifold centre for multiple wells.
Offshore platforms can broadly be categorized
into two parts :-
Structures that extend to the sea bed
Jacketed or Fixed Steel platform
Concrete Gravity Structures
Compliant Tower
Structures that float near the water surface- Recent
Development
Tension Leg Platforms
SPAR
Ship shaped Vessels (FPSO)
14. STRUCTURES THAT EXTEND TO THE SEA BED
FIXED STEEL PLATFORMS
A Fixed Platform is a type of offshore platform used for the
production of oil or gas. These platforms are built on concrete
and/or steel legs anchored directly onto the seabed, supporting
a deck with space for drilling rigs, production facilities and crew
quarters. Such platforms are, by virtue of their
immobility, designed for very long term use (for instance the
Hibernia platform). Various types of structure are used, steel
jacket, concrete caisson, floating steel and even floating concrete.
Steel jackets are vertical sections made of tubular steel
members, and are usually piled into the seabed. Concrete
caisson structures, pioneered by the Condeep concept, often
have in-built oil storage in tanks below the sea surface and these
tanks were often used as a flotation capability, allowing them to
be built close to shore (Norwegian Fjords and Scottish Firths are
popular because they are sheltered and deep enough) and then
floated to their final position where they are sunk to the seabed.
Fixed platforms are economically feasible for installation in
water depths up to about 1,700 feet (520 m). Space framed
structure with tubular members supported on piled
foundations. Used for moderate water depths up to 400 M.
Jackets provides protective layer around the pipes. Typical
offshore structure will have a deck structure containing a Main
Deck, a Cellar Deck, and a Helideck. The deck structure is
supported by deck legs connected to the top of the piles. The
piles extend from above the Mean Low Water through the
seabed and into the soil. Underwater, the piles are contained
inside the legs of a “jacket” structure which serves as bracing for
the piles against lateral loads.
16. The jacket also serves as a template for the initial driving of
the piles. (The piles are driven through the inside of the
legs of the jacket structure).Natural period (usually 2.5
second) is kept below wave period (14 to 20 seconds) to
avoid amplification of wave loads. 95% of offshore
platforms around the world are Jacket supported.
CONCRETE GRAVITY BASE STRUCTURES
Whilst the vast majority of fixed offshore platforms employ
a tubular jacket to support the topside facilities, a number
of installations have been constructed using a base
manufactured from reinforced concrete. They are Fixed-
bottom structures made from concrete . They are heavy and
remain in place on the seabed without the need for piles.
They are widely used for moderate water depths up to 300
M. Part construction is made in a dry dock adjacent to the
sea. The structure is built from bottom up, like onshore
structure. At a certain point , dock is flooded and the
partially built structure floats. It is towed to deeper
sheltered water where remaining construction is
completed. After towing to field, base is filled wi1th
water to sink it on the seabed. Its main advantage is its less
maintenance. The first concrete structure to be installed in
the North Sea was constructed by the Norwegians in 1973
and used to develop the Ekofisk field. Since then those
people have installed a steady string of concrete structures
and it came as no surprise when their government elected
to develop the other fields with the same concrete structure
which stands in 350ft. of water and is currently the largest
offshore structure in Europe and the largest concrete
platform in the world.
18. COMPLIANT TOWER
A compliant tower (CT) is a fixed rig structure normally used
for the offshore production of oil or gas. The rig consist of
narrow, flexible (compliant) towers and a piled foundation
supporting a conventional deck for drilling and production
operations. Compliant towers are designed to sustain
significant lateral deflections and forces, and are typically used
in water depths ranging from 1,500 and 3,000 feet (450 and 900
m). With the use of flex elements such as flex legs or axial
tubes, resonance is reduced and wave forces are de-amplified.
This type of rig structure can be configured to adapt to existing
fabrication and installation equipment. Compared with
floating systems, such as Tension leg platforms and SPARs, the
production risers are conventional and are subjected to less
structural demands and flexing. This flexibility allows it to
operate in much deeper water, as it can 'absorb' much of the
pressure exerted on it by the wind and sea. Despite its
flexibility, the compliant tower system is strong enough to
withstand hurricane conditions. The first tower emerged in the
early 1980s with the installation of Exxon's Lena oil platform.
Narrow, flexible framed structures supported by piled
foundations. It has no oil storage capacity. Production is
through tensioned rigid risers and export by flexible or
catenary steel pipe. It undergos large lateral deflections (up to
10 ft) under wave loading. Used for moderate water depths up
to 600 M. Natural period (usually 30 second) is kept above
wave period (14 to 20 seconds) to avoid amplification of wave
loads.
19. STRUCTURES THAT FLOAT NEAR THE
WATER SURFACE
TENSION LEG PLATFORMS
A Tension-leg platform or Extended Tension Leg Platform
(ETLP) is a vertically moored floating structure normally used
for the offshore production of oil or gas and is particularly
suited for water depths greater than 300 meters (about 1000 ft).
Also proposed for wind turbines. The platform is permanently
moored by means of tethers or tendons grouped at each of the
structure's corners. A group of tethers is called a tension leg. A
feature of the design of the tethers is that they have relatively
high axial stiffness(low elasticity), such that virtually all vertical
motion of the platform is eliminated. This allows the platform
to have the production wellheads on deck (connected directly
to the subsea wells by rigid risers), instead of on the seafloor .
This makes for a cheaper well completion and gives better
control over the production from the oil or gas reservoir. The
first Tension Leg Platform was built for Conoco's Hutton field
in the North Sea in the early 1980s. The hull was built in the
dry-dock at Highland Fabricator's Nigg yard in the north of
Scotland, with the deck section built nearby at McDermott's
yard at Ardersier. The two parts were mated in the Moray Firth
in 1984. Tension Leg Platforms (TLPs) are floating facilities that
are tied down to the seabed by vertical steel tubes called
tethers. This characteristic makes the structure very rigid in the
vertical direction and very flexible in the horizontal plane. The
vertical rigidity helps to tie in wells for production, while, the
horizontal compliance makes the platform insensitive to the
primary effect of waves. It has large columns and Pontoons and
a fairly deep draught.
20. TLP has excess buoyancy which keeps tethers in tension.
Topside facilities , no. of risers etc. have to fixed at pre-design
stage. It is used for deep water up to 1200 M. It has no integral
storage. It is sensitive to topside load/draught variations as
tether tensions are affected.
SPAR
A SPAR, named for logs used as buoys in shipping and moored
in place vertically, is a type of floating oil platform typically
used in very deep waters. Spar production platforms have been
developed as an alternative to conventional platforms. A Spar
platform consists of a large-diameter, single vertical cylinder
supporting a deck. It contains a deep-draft floating
caisson, which is a hollow cylindrical structure similar to a very
large buoy. Its four major systems are hull, moorings, topsides
and risers. About 90% of the structure is underwater. The spar
design is now being used for drilling, production, or both. The
distinguishing feature of a spar is its deep-draft hull, which
produces very favorable motion characteristics compared to
other floating concepts. Water depth capability has been stated
by industry as ranging up to 10,000 ft. The first Spar platform in
the was installed in September of 1996. It follows the concept of
a large diameter single vertical cylinder supporting deck. These
are a very new and emerging concept: the first spar
platform, Neptune, was installed off the USA coast in 1997.Spar
platforms have taut catenary moorings and deep draught, hence
heave natural period is about 30 seconds.
22. FPSO {Floating Production Storage and Offloading}
A Floating Production, Storage and Offloading vessel
(FPSO; also called a "unit" and a "system") is a type of
floating tank system used by the offshore oil and gas
industry and designed to take all of the oil or gas produced
from a nearby platform (s), process it, and store it until the
oil or gas can be offloaded onto waiting tankers or sent
through a pipeline.
History
Oil has been produced from offshore locations since the
1950s. Originally, all oil platforms sat on the seabed, but as
exploration moved to deeper waters and more distant
locations in the 1970s, floating production systems came to
be used. The first oil FPSO was the Shell Castellon, built in
Spain in 1977. The first ever conversion of a LNG carrier
(Golar LNG owned Moss type LNG carrier) into an LNG
floating storage and regasification unit was carried out in
2007 by Keppel shipyard in Singapore. The last few years
concepts for LNG FPSOs has also been launched. An LNG
FPSO works under the same principles as an Oil FPSO,
but it only produces natural gas, condensate and/or LPG,
which is stored and offloaded.
23. Working principles
Oil produced from offshore production platforms can be
transported to the mainland either by pipeline or by tanker.
When a tanker solution is chosen, it is necessary to
accumulate oil in some form of tank such that an oil tanker is
not continuously occupied while sufficient oil is produced to
fill the tanker.
Often the solution is a decommissioned oil tanker which has
been stripped down and equipped with facilities to be
connected to a mooring buoy. Oil is accumulated in the FPSO
until there is sufficient amount to fill a transport tanker, at
which point the transport tanker connects to the stern of the
floating storage unit and offloads the oil. An FPSO has the
capability to carry out some form of oil separation process
obviating the need for such facilities to be located on an oil
platform. Partial separation may still be done on the oil
platform to increase the oil capacity of the pipeline(s) to the
FPSO.
Advantages
Floating Production, Storage and Offloading vessels are
particularly effective in remote or deepwater locations where
seabed pipelines are not cost effective. FPSOs eliminate the
need to lay expensive long-distance pipelines from the oil
well to an onshore terminal. They can also be used
economically in smaller oil fields which can be exhausted in a
few years and do not justify the expense of installing a fixed
oil platform. Once the field is depleted, the FPSO can be
moved to a new location.
25. Specific types
A Floating Storage and Offloading unit (FSO) is a floating
storage device, which is simplified FPSO without the
possibility for oil or gas processing. Most FSOs are old single
hull supertankers that have been converted. An example of
this is the Knock Navis, the world's largest ship, which has
been converted to an FSO to be used offshore Qatar.
A LNG floating storage and regasification unit (FSRU) is a
floating storage and regasification system, which receives
liquefied natural gas(LNG) from offloading LNG carriers, and
the onboard regasification system provides natural gas send-
out through flexible risers and pipeline to shore.
26. MODU’s { Mobile Offshore Drilling Units }
The basic work of the mobile units is to drill the well in
the sea bed and prepare the line for production. Offshore
drilling is divided into two parts i.e shallow water and
deep water. In shallow water, jack-up rigs, standing with
their feet on the seabed are used to drill the oil wells. In
deeper water, floating drilling units are used. There are
two basic types; Drill ships and Semi-submersible
drilling rigs. This is a very important process and is very
hard in nature as the environmental conditions are
always unfavorable for such a process to accomplish.
There are basically three type of drilling units that are
widely used over the world. They are :-
Semi-submersible drill rigs
Self elevated Jack up rigs
Drill ships
27. SEMI SUBMERSIBLE DRILLING RIG
A semi-submersible drilling rig is one in which:
Sea water is pumped into the hull of the vessel
causing the vessel to submerge to the desired
depth.
The submerged vessel maintains its position
over the well location by means of anchor chains.
A semi-submersible is not bottom-founded and
can work in much greater water depths than a
jack-up.
The maximum water depth is a function of the
length of the rig's riser, a bundle of utility tubes
through which drilling fluids and other material is
conducted, enclosed in an outer tube, suspended
to the seafloor.
DRILL SHIPS
Drill ships, a maritime vessel that has been fitted
with drilling apparatus. It is most often used for
exploratory drilling of new oil or gas wells in deep
water but can also be used for scientific drilling. It
is often built on a modified tanker hull and
outfitted with a dynamic positioning system to
maintain its position over the well.
30. ELEVATING JACK UP RIGS
INTRODUCTION
A Jack Up is an offshore structure composed of a hull, legs
and a lifting system that allows it to be towed to a site, lower
its legs into the seabed and elevate its hull to provide a
stable work deck capable of withstanding the environmental
loads. A typical modern drilling Jack Up is capable of working
in harsh environments (Wave Heights up to 80 ft, Wind
Speeds in excess of 100 knots) in water depths up to 500
feet. Because Jack Ups are supported by the seabed, they
are preloaded when they first arrive at a site to simulate the
maximum expected leg loads and ensure that, after they are
Jacked to full air gap and experience operating and
environmental loads, the supporting soil will provide a
reliable foundation. Jack Up Units have been a part of the
Offshore Oil Industry exploration fleet since the 1950’s. They
have been used for exploration drilling, tender assisted
drilling, production, accommodation, and work/maintenance
platforms. As with every innovative technology, Jack Up
Units have been used to their operational and design
limitations. These limitations include deck load carrying
limits when afloat, load carrying capabilities when
elevated, environmental limits, drilling limits, and soil
(foundation) limits.
31. The reasons for pushing these limits include the desire to explore
deeper waters, deeper reservoirs in harsher environments, and in
areas where soils and foundations
may be challenging or even unstable.
There are three main components of a Jack Up Unit:
the Hull, the Legs & Footings, and the Equipment. Each of the
component are described below :-
HULL
The Hull of a Jack Up Unit is a watertight structure that supports
or houses the equipment, systems, and personnel, thus enabling
the Jack Up Unit to perform its tasks. When the Jack Up Unit is
afloat, the hull provides buoyancy and supports the weight of the
legs and footings (spud cans), equipment, and variable load.
Different parameters of the hull affect different modes of
operation of the Unit. In general, the larger the length and breadth
of the hull, the more variable deck load and equipment the Unit
will be able to carry, especially in the Afloat mode (due to
increased deck space and increased buoyancy).
Also, larger hulls generally result in roomier machinery spaces
and more clear space on the main deck to store pipe, 3rd
Party Equipment, and provide for clear work areas. The larger
hull may have larger preload capacity that may permit increased
flexibility in preloading operations. Larger hulls generally have
the negative effects of attracting higher wind, wave and current
loads. Since Jack Ups with larger hulls weigh more, they will
require more elevating jacks of larger capacity to elevate and hold
the Unit.
32. The large weight also affects the natural period of the Jack Up
Unit in the elevated mode. The draft of the hull, or the distance
from the afloat waterline to the baseline of the hull, has a direct
effect on the amount of variable deck load that can be carried and
the stability when afloat. The draft of the hull has an opposing
relationship with the hull’s freeboard, or the distance from the
afloat waterline to the main deck of the hull. Every incremental
increase in the draft of a Jack Up decreases the freeboard by the
same increment.
LEGS AND FOOTINGS
The legs and footings of a Jack Up are steel structures that
support the hull when the Unit is in the Elevated mode and
provide stability to resist lateral loads. Footings are needed to
increase the soil bearing area thereby reducing required soil
strength. The legs and footings have certain characteristics which
affect how the Unit reacts in the Elevated and Afloat
Modes, while going on location and in non-design events. The
legs of a Jack Up Unit may extend over 500 ft above the surface of
the water when the Unit is being towed with the legs fully
retracted. Depending on size and length, the legs usually have
the most detrimental impact on the afloat stability of the Unit.
The heavy weight at a high center of gravity and the large wind
area of the legs combine to dramatically affect the Unit’s afloat
stability. For Units of the same hull configuration and draft, the
Unit with the larger legs will have less afloat stability. When in
the Elevated Mode, the legs of a Jack Up Unit are subjected to
wind, wave, and current loadings. In addition to the specifics of
the environment, the magnitude and proportion of these loads is
a function of the water depth, air gap (distance from the water
line to the hull baseline) and the distance the footings penetrate
into the seabed.
33. Generally, the larger the legs and footings, the more load
wind, wave, and current will exert on them. Legs of different
design and size exhibit different levels of lateral stiffness
(amount of load needed to produce a unit deflection). Jack Up
stiffness decreases with increases in water depth (or more
precisely, with the distance from the support footing to the
hull/leg connection). Furthermore, for deeper water
depths, flexural stiffness (chord area and spacing) overshadows
the effects of shear stiffness (brace). Leg stiffness is directly
related to Jack Up stiffness in the elevated mode, thereby
affecting the amount of hull sway and the natural period of the
Unit (which may result in a magnification of the
oscillatory wave loads).
EQUIPMENT
The equipment required to satisfy the mission of the Jack Up
Unit affects both the hull size and lightship weight of the Unit.
There are three main groups of equipment on a Jack Up
Unit, the Marine Equipment, Mission Equipment, and Elevating
Equipment. “Marine Equipment” refers to the equipment and
systems aboard a Jack Up Unit that are not related to the
Mission Equipment. Marine Equipment could be found on any
sea-going vessel, regardless of its form or function. Marine
Equipment may include items such as main diesel engines, fuel
oil piping, electrical power distribution switchboards,
lifeboats, radar, communication equipment, galley
equipment, etc. Marine Equipment, while not directly involved
with the Mission of the Jack up Unit, is necessary for the support
of the personnel and equipment necessary to carry out the
Mission. All Marine Equipment is classified as part of the Jack
Up Lightship Weight.
34. “Mission Equipment” refers to the equipment and systems
aboard a Jack Up Unit, which are necessary for the Jack Up to
complete its Mission. Mission Equipment varies by the mission
and by the Jack Up. Two Jack Up Units which are involved in
Exploration Drilling may not have the same Mission
Equipment. Examples of Mission Equipment may include
derricks, mud pumps, mud piping, drilling control
systems, production equipment, cranes, combustible gas
detection and alarms systems, etc. Mission Equipment is not
always classified as part of the Jack Up Lightship Weight.
Some items, such as cement units, are typically classified as
variable deck load as they may not always be located aboard
the Jack Up.
MODES OF OPERATION OF A JACK UP
Jack Up Units operate in three main modes: transit from one
location to another, elevated on its legs, and jacking up or
down between afloat and elevated modes. Each of these modes
has specific precautions and requirements to be followed to
ensure smooth operations. A brief discussion of these modes of
operations along with key issues associated with each follows.
TRANSIT FROM ONE LOCATION TO ANOTHER
The Transit Mode occurs when a Jack Up Unit is to be
transported from one location to another. Transit can occur
either afloat on the Jack Up Unit’s own hull (wet tow), or with
the Jack Up Unit as cargo on the deck of another vessel (dry
tow).
.
35. Main preparations for each Transit Mode address support of the
legs, support of the hull, watertight integrity of the unit, and
stowage of cargo and equipment to prevent shifting due to
motions. Though the Unit’s legs must be raised to ensure they
clear the seabed during tow, it is not required that the legs be fully
retracted. Allowing part of the legs to be lower than the hull
baseline not only reduces jacking time, but it also reduces leg
inertia loads due to tow motions and increases stability due to
decreased wind overturning. Lowering the legs a small distance
may also improve the hydrodynamic flow around the open leg
wells and reduce tow resistance. Whatever the position of the legs
during tow, their structure at the leg/hull interface must be
checked to ensure the legs can withstand the gravity and inertial
loads associated with the tow. Field Tow corresponds to the
condition where a Jack Up Unit is afloat on its own hull with its
legs raised, and is moved a relatively short distance to another
location. For a short move, the ability to predict the condition of
the weather and sea state is relatively good. Therefore, steps to
prepare the Unit for Field Tow are not as stringent as for a longer
tow. Most Classification Societies define a “Field Tow” as a Tow
that does not take longer than 12 hours, and must satisfy certain
requirements with regards to motion criteria. This motion
criterion, expressed as a roll/pitch magnitude at a certain
period, limits the inertial loads on the legs and leg support
mechanism. For certain moves lasting more than 12 hours, a Unit
may undertake an Extended Field Tow. An Extended Field Tow is
defined as a Tow where the Unit is always within a 12-hour Tow
of a safe haven, should weather deteriorate. In this condition, the
Jack Up Unit is afloat on its own hull with its legs raised, similar
to a Field Tow. The duration of an Extended Field Tow may be
many days. The motion criteria for an Extended Field Tow is the
same as for a Field Tow.
36. The main preparations for a Unit to undertake an Extended
Field Tow are the same as those for a Field Tow with the
additional criteria that the weather is to be carefully monitored
throughout the duration of the tow. A Wet Ocean Tow is
defined as an afloat move lasting more than 12-hours which
does not satisfy the requirements of an Extended Field Tow. In
this condition, the Jack Up Unit is afloat on its own hull with its
legs raised as with a Field Tow, but, for many Units, additional
precautions must be made. This is because the motion criteria
for a Wet Ocean Tow are more stringent than for a Field Tow.
The additional preparations may include installing additional
leg supports, shortening the leg by cutting or lowering, and
securing more equipment and cargo in and on the hull. A Dry
Ocean Tow is defined as the transportation of a Jack Up Unit on
the deck of another vessel. In this condition, the Jack Up Unit is
not afloat, but is secured as deck cargo. The motion criteria for
the Unit is dictated by the motions of the transportation vessel
with the Unit on board. Therefore, the precautions to be taken
with regard to support of the legs must be investigated on a
case-by-case basis. Generally, though, the legs are to be
retracted as far as possible into the hull so the Jack Up hull can
be kept as low as practicable to the deck of the transport vessel
and to reduce the amount of cribbing support. The other critical
precaution unique to Dry Ocean Tow is the support of the Jack
Up hull. The hull must be supported by cribbing on strong
points (bulkheads) within the hull and in many cases, portions
of the hull overhang the side of the transportation vessel. These
overhanging sections may be exposed to wave impact, putting
additional stress on the hull, and if the overhanging sections
include the legs, the resultant bending moment applied to the
hull (and amplified by vessel motions) can be significant.
Calculations should be made to ensure that the hull will not lift
off the cribbing with the expected tow motions.
38. ARRIVING ON LOCATION
Upon completion of the Transit Mode, the Jack Up Unit is said
to be in the Arriving On Location Mode. In this Mode, the
Unit is secured from Transit Mode and begins preparations to
Jack Up to the Elevated Mode. Preparations include
removing any wedges in the leg guides, energizing the jacking
system, and removing any leg securing mechanisms installed
for the Transit thereby transferring the weight of the legs to the
pinions.
SOFT PINNING THE LEGS
If an independent leg Jack Up Unit is going to be operated
next to a Fixed Structure, or in a difficult area with bottom
restrictions, the Jack Up Unit will often be temporarily
positioned just away from its final working location. This is
called “Soft Pinning” the legs or “Standing Off” location. This
procedure involves lowering one or more legs until the bottom
of the spud can(s) just touches the soil. The purpose of this is to
provide a “Stop” point in the Arriving On Location process.
Here, all preparations can be checked and made for the final
approach to the working location. This includes coordinating
with the assisting tugs, running anchor lines to be able to “winch
in” to final location, powering up of positioning thrusters on the
Unit (if fitted), checking the weather forecast for the period of
preloading and jacking up, etc.
39. FINAL GOING ON LOCATION
Whether a Unit stops at a Soft Pin location, or proceeds directly to
the final jacking up location, they will have some means of
positioning the Unit so that ballasting or preloading operations
prior to jacking up can commence. For an independent leg Jack
Up Unit, holding position is accomplished by going on location
with all three legs lowered so the bottom of the spud can is just
above the seabed. When the Unit is positioned at its final
location, the legs are lowered until they can hold
the rig on location without the assistance of tugs. Mat type Jack
Up Units are either held on location by tugs, or they drop spud
piles into the soil. These spud piles, usually cylindrical piles with
concrete fill, hold the Unit on location until the mat can be
ballasted and lowered.
JACKING
A mat Unit will jack the mat to the seabed in accordance with the
ballasting procedure. Once the mat has been lowered to the
seabed, the hull will be jacked out of the water. The Unit then
proceeds to Preload Operations . All Independent leg Units must
perform Preload Operations before they can jack to the design air
gap. Most independent leg Units do not have the capacity to
elevate the Unit while the preload weight is on board. For these
Units, the next step is to jack the hull out of the water to a small
air gap that just clears the wave crest height. This air gap should
be no more than five (5) feet. Once they reach this position, the
Unit may proceed with Preload Operations.
40. PRELOAD OPERATIONS
All Jack Up Units must load the soil that supports them to the full
load expected to be exerted on the soil during the most severe
condition, usually Storm Survival Mode. This preloading reduces
the likelihood of a foundation shift or failure during a Storm. The
possibility does exist that a soil failure or leg shift may occur
during Preload Operations. To alleviate the potentially catastrophic
results of such an occurrence, the hull is kept as close to the
waterline as possible, without incurring wave impact. Should a soil
failure or leg shift occur, the leg that experiences the failure loses
load-carrying capability and rapidly moves downward, bringing
the hull into the water. Some of the load previously carried by the
leg experiencing the failure is transferred to the other legs
potentially overloading them. The leg experiencing the failure will
continue to penetrate until either the soil is able to support the
leg, or the hull enters the water to a point where the hull buoyancy
will provide enough support to stop the penetration. As the hull
becomes out-of-level, the legs will experience increased transverse
load and bending moment transferred to the hull mostly by the
guide. With the increased guide loads, some braces will experience
large compressive loads. During normal preload operations it is
important to keep the weight of the hull, deck load, and preload as
close to the geometric center of the legs as possible, as this will
assure equal loading on all legs. Sometimes, however, single-leg
preloading is desired to increase the maximum footing reaction of
any one leg. This is achieved by selective filling/emptying of
preload tanks based on their relative position to the leg being
preloaded. Preload is water taken from the sea and pumped into
tanks within the hull. After the preload is pumped on board, it is
held for a period of time.
41. The Preload Operation is not completed until no settling of the
legs into the soil occurs during the holding period while
achieving the target footing reaction. The amount of preload
required depends on the required environmental reaction and
the type of Jack Up Unit. Mat Units normally require little
preload.
JACKING TO FULL AIR GAP OPERATIONS
Once Preload Operations are completed, the Unit may be
jacked up to its operational air gap. During these operations it
is important to monitor the level of the hull, elevating system
load and characteristics, and for trussed-leg Units, Rack Phase
Differential (RPD). All of these must be maintained within
design limits. Once the Unit reaches its operational air gap, the
jacking system is stopped, the brakes set, and leg locking
systems engaged (if installed). The Unit is now ready to begin
operations.
ELEVATED OPERATING CONDITION
When the Unit is performing operations, no particular
differences exist between the various types of Units.
Likewise, there are no particular cautionary measures to take
other than to operate the Unit and its equipment within design
limits. For Units with large cantilever reach and high cantilever
loads, extra care must be taken to ensure that the maximum
footing reaction does not exceed a specified percentage of the
reaction achieved during preload.
42. ELEVATED STORM SURVIVAL CONDITION
When the Unit is performing operations, the weather is to be
monitored. If non-cyclonic storms which exceed design operating
condition environment are predicted, Operations should be
stopped and the Unit placed in Storm Survival mode. In this
mode, Operations are stopped, equipment and stores secured, and
the weather and watertight enclosures closed. If cyclonic storms
are predicted, the same precautions are taken and personnel
evacuated from the Unit.
This is how a jack up rig is bought from the shore to the required
location for drilling.
43. ELEVATING SYSTEM
All Jack Ups have mechanisms for lifting and lowering the
hull. The most basic type of elevating system is the pin and
hole system, which allows for hull positioning only at
discrete leg positions. However, the majority of Jack Ups in
use today are equipped with a Rack and Pinion system for
continuous jacking operations. There are two basic jacking
systems: Floating and Fixed. The Floating system uses
relatively soft pads to try to equalize chord loads, whereas
the Fixed system allows for unequal chord loading while
holding. There are two types of power sources for Fixed
Jacking Systems, electric and hydraulic.Both systems have
the ability to equalize chord loads within each leg. A
hydraulic-powered jacking system achieves this by
maintaining the same pressure to each elevating unit within
a leg. Care must be taken, however, to ensure that losses due
to piping lengths, bends, etc., are either equalized for all
pinions or such differences are insignificant in magnitude.
For an electric powered jacking system, the speed/load
characteristics of the electric induction motors cause jacking
motor speed changes resulting from pinion loads, such that
if jacking for a sufficiently long time, the loads on any one
leg tend to equalize for all chords
of that leg.
45. UPPER AND LOWER GUIDES
All Jack Ups have mechanisms to guide the legs through the hull.
For Units with Pinions, the guides protect the pinions from
“bottoming out” on the rack teeth. As such, all Units are fitted
with a set of upper and lower guides. Some Jack Up Units, which
have exceptionally deep hulls or tall towers of pinions, also have
intermediate guides. These guides function only to maintain the
rack the correct distance away from the pinions and are not
involved in transferring leg bending moment to the hull. Guides
usually push against the tip (vertical flat side) of the teeth, but
this is not the only form of guides. There are also other forms of
guides such as chord guides, etc. Depending on
accessibility, some guides are designed to be replaced and are
sometimes known as “wear plates.”In addition to protecting the
pinions and hull, all upper and lower guides are capable of
transferring leg bending moment to the hull to some degree
determined by the design. The amount of moment transferred by
the guides to the hull as a horizontal couple is dependant on the
relative stiffness of the guides with respect to the stiffness of the
pinions and/or fixation system (if any).
46. DRILLING RIG COMPONENTS
1. Crown Block and Water Table
2. Catline Boom and Hoist Line
3. Drilling Line
4. Monkeyboard
5. Traveling Block
6. Top Drive
7. Mast
8. Drill Pipe
9. Doghouse
10. Blowout Preventer
11. Water Tank
12. Electric Cable Tray
13. Engine Generator Sets
14. Fuel Tank
15. Electrical Control House
16. Mud Pumps
17. Bulk Mud Component Tanks
18. Mud Tanks (Pits)
19. Reserve Pit
20. Mud-Gas Separator
21. Shale Shakers
22. Choke Manifold
23. Pipe Ramp
24. Pipe Racks
25. Accumulator
47. Crown Block and Water Table
An assembly of sheaves or pulleys
mounted on beams at the top of the
derrick. The drilling line is run over
the sheaves down to the hoisting
drum.
Catline Boom and Hoist Line
A structural framework erected
near the top of the derrick
for lifting material.
Drilling Line
A wire rope hoisting line, reeved on
sheaves of the crown block and
traveling block (in effect a block
and tackle). Its primary purpose is
to hoist or lower drill pipe or casing
from or into a well. Also, a wire
rope used to support the
drilling tools
48. Monkeyboard
The derrickman's working
platform. Double board, tribble
board, fourable board; a monkey
board located at a height in the
derrick or mast equal to
two, three, or four lengths of pipe
respectively.
Traveling Block
An arrangement of pulleys or
sheaves through which drilling
cable is reeved, which moves up or
down in the derrick or mast.
Top Drive
The top drive rotates the drill string
end bit without the use of a kelly
and rotary table. The top drive is
operated from a control console on
the rig floor.
49. Mast
A portable derrick capable of being
erected as a unit, as Distinguished
from a standard derrick, which
cannot be raised to a working
position as a unit.
Drill Pipe
The heavy seamless tubing used to
rotate the bit and circulate the
drilling fluid. Joints of pipe 30 feet
long are coupled together with tool
joints.
Doghouse
A small enclosure on the rig floor
used as an office for the driller or as
a storehouse for small objects.
Also, any small building used as an
office or for storage.
50. Blowout Preventer
A large valve, usually installed
above the ram preventers, that
forms a seal in the annular space
between the pipe and well bore
or, if no pipe is present, on the well
bore itself.
Water Tank
Is used to store water that
is used for mud
mixing, cementing, and rig
cleaning.
Electric Cable Tray
Supports the heavy electrical cables
that feed the power
from the control panel to the
rig Motors.
51. Engine Generator Sets
A diesel, Liquefied Petroleum Gas
(LPG), natural gas, or gasoline
engine, along with a mechanical
transmission and generator for
producing power for the drilling
rig. Newer rigs use electric
generators to power electric motors
on the other parts of the rig.
Fuel Tanks
Fuel storage tanks for the power
generating system.
Electric Control House
On diesel electric rigs, powerful
diesel engines drive large electric
generators. The generators produce
electricity that flows through cables
to electric switches and control
equipment enclosed in a control
cabinet or panel. Electricity is fed to
electric motors via the panel.
52. Mud Pump
A large reciprocating pump used to
circulate the mud (drilling fluid) on
a drilling rig.
Bulk Mud Components in
Storage
Hopper type tanks for storage
of drilling fluid components.
Mud Pits
A series of open tanks, usually
made of steel plates, through
which the drilling mud is cycled to
allow sand and sediments to settle
out. Additives are mixed with the
mud in the pit, and the fluid is
temporarily stored there before
being pumped back into the well.
53. Reserve Pits
A mud pit in which a supply of
drilling fluid has been stored.
Also, a waste pit, usually an
excavated, earthen - walled pit. It
may be lined with plastic to
prevent soil contamination.
Mud-Gas Separator
A device that removes gas from the
mud coming out of a well when a
kick is being circulated
out.
Shale Shaker
A series of trays with sieves or
screens that vibrate to remove
cuttings from circulating fluid in
rotary drilling operations. The size
of the openings in the sieve is
selected to match the size of the
solids in the drilling fluid and the
anticipated size of cuttings. Also
called a shaker.
54. Choke Manifold
The arrangement of piping and
special valves, called
chokes, through which drilling
mud is circulated when the
blowout preventers are closed to
control the pressures encountered
during a kick.
Pipe Ramp
An angled ramp for dragging drill
pipe up to the drilling platform or
bringing pipe down off the drill
platform.
Accumulator
The storage device for nitrogen
pressurized hydraulic fluid, which
is used in operating the blowout
preventers.
55. BASIC SYSTEMS IN JACK UP RIGS
Drilling Systems
Drilling system is the heart of the jack up rig. Drilling is carried out
at the drill floor which is at certain elevation from the main deck.
There is a mouse hole and rat hole in which the drill stems are
assembled. Once the stem is assembled it is placed concentric with
the rotary table which is either driven from the top or from the
bottom. Kelly bushing is provided to support the drill stem and
prevent it from buckling.
Mud Systems
These are of two types – high pressure mud system and low
pressure mud system.
Mud is a mixture of raw water, clay, bainite and some other
minerals. Working of the jack up rig depends upon the power of
the mud system. It is a cyclic procedure which is used to convey
the crushes from the bottom of drill bits to the top of mud tanks.
This is called high pressure mud system. when we punch through
the reservoir a high pressure builds up due to the presence of
gases, to prevent our system from blowing out high viscosity mud
is used to lower the pressure. When the mud returns from the BOP
and goes to shale shaker assembly, it is called low pressure mud
system
57. Power Generation Systems
Power is must to run a jack up rig. Jack up rigs are
installed in deep seas thereby we have no other provision
of getting power. Hence the internal combustion engines
are used to supply power. They are high capacity engines
which are located in engine rooms at the main deck. Once
the electricity suddenly shuts off then the crude oil in the
annular should be at the same level of the mud which is
present in the drill stem below the return line.
Cementing Systems
Cementing system is a provision which is used to cement
the sides of the well forbidding the soft soil to enter the
drill well.
In cement system there are four tanks which have an
accumulator on the top which mixes the cement
continuously with raw water. On both sides of the rig
there are centrifugal pumps which sucks marine water
from the sea and supply it to the tank which is used to
dilute the cement.
Living quarters & Landing Systems
Living quarters are provided for the officials and a
helideck is also commissioned for some external support
needed for the persons working on the rig. Complete care
of the people is taken to ensure a safe working
environment.
59. BOP & Well Completions
BOP stands for blow out preventer. It is the most important
part of the jack up rig. When the crude oil comes from the
well it at such pressure which can send a rocket to space i.e
about 10,000 psi so it can destroy the jack up rig in one blow
,hence to prevent the rigs we have BOP’s which cuts the line
when a limiting pressure value is reached , hence saving the
rig.
Well completion process, when the drilling and cementing is
done a plug is placed at the cement sleeve which at some
clearance from the sea bed. The Christmas tree is placed on
this plug and the plunger on the bottom of Christmas tree is
used for the production of oil.
Jacking and Hydraulic Systems
Care must be taken when positioning a new jack up rig at a site
previously occupied by another jack up because of the tendency of
the spud cans of the new rig to slip into the spud cans holes or “foot
prints” left on the sea floor by the previous rig. If there is an overlap
of a spud can over an old spud can hole, there is a tendency for the
spud can not to penetrate straight into the soil, but instead to slip
into the old spud can hole. This movement of a spud can, without a
corresponding movement of all the other spud cans in the same
direction, will impose a bending moment on the legs. This bending
moment can be quite severe and may damage the leg in the
preloading or jacking up process or it may reduce the allowable
storm environmental load of the rig due to the resulting bend of the
leg. When selecting a rig for a platform, it is always best to choose a
rig with the same leg spacing as a rig that previously drilled at the
platform.
60. However, the effect of previous spud can holes can be mitigated
if the new rig is positioned so that the centers of its spud cans
are positioned either at the center of the holes left by the
previous rig or about 1.5 spud can diameters away for the edge
of the holes left by the previous rig. If the rig selected for the
platform does not have the same leg spacing as a rig previously
at the platform and it is not possible to position the new rig so
that its legs either are centered over old holes or 1.5 diameters
away from old holes while still reaching all of the required
drilling positions, there are two techniques which can be used to
minimize the effects of old holes. These techniques are
“Reaming” and “Swiss Cheesing”.“Reaming” is a technique by
which the leg or legs are sequentially raising and lowering the
spud can in the hole left by the previous rig in an attempt to
wear away the side of the hole, thereby elongating the hole and
creating a new hole center location at the spacing of the legs of
the new rig. “Swiss Cheesing” is a method in which a number of
large diameter holes (24 to 30 inch diameter) are drilled at the
side of an existing can hole in order to degrade the strength of
soil at the side of the can hole, effectively enlarging the hole.
61. SOME SENSIVITIES OF JACK UP RIGS
LEG PUNCH THROUGHS
When a Jack Up is being preloaded, it is important to be
prepared to act in the event of rapid penetration of one or
multiple legs. Because of the increased demands on Jack
Ups (i.e., larger water depths and higher environmental
loads) resulting in higher elevated weights during
preload, the consequences of a punch through are
increasingly more pronounced. A typical soil’s bearing
capacity increases with depth. When a soil layer is underlain
by a weaker soil layer, there is a rapid reduction of soil
strength. When the spud can reaches this interface, the
weaker soil gives way and the support of the leg moves
downward at a faster rate than the jacking system is capable
of lowering the leg to maintain the hull level. As such, the
hull rotates, the legs tilt and bend, causing the hull to sway.
This results in a weight shift relative to the
supports, thereby increasing the required footing reaction
needed to maintain equilibrium. This process continues
until either the soil’s bearing capacity or any hull buoyancy
arising from the hull entering the water increase sufficiently
to reach equilibrium. Jack Ups of all design types experience
punch throughs and their resulting damages to
braces, chords and jacking units.
62.
63. The accidental loading resulting from a punch through can lead
to several types of leg damage including buckling of the
braces, buckling or shearing of the chord, punching shear and
joint damage. The extent of possible damage is dependent on the
magnitude of the punch through and, more importantly, on the
actions taken before, during, and immediately after the punch
through. Punch through is an extreme event; therefore, proper
management of this event is necessary. Modern rigs with a better
guide design along with a proper punch through
management system, can minimize some of the risks.
OTHER SOIL ISSUES (SCOUR, EARTHQUAKE)
There are soil issues other than poor bearing capacity to consider
when reviewing a Jack Up’s suitability for a given location. This
section presents just a few of the main issues. The first is the case
where the soil is extremely hard or calcareous. In these cases, the
penetration of the spud can will be minimal allowing only a
portion of the spud can bottom plate to be in contact with the
seabed. In this condition, only that part of the spud can structure
in contact with the soil will be supporting the environmental
loads, deadweight, and operational weight of the Jack Up. It is
extremely important to verify that such partial bearing will not
cause damage to the spud can structure. In cases like these, an
adequately reinforced tip on the spud can may be advantageous
compared to flat bottomed footings.
64. Scour is another problem that occurs in certain locations such
as areas with sandy bottoms and high bottom current. In this
case, the footing is originally supported over a certain portion
of its bottom area during the initial preload operation. Over
time, however, high currents may cause erosion under a
portion of the footing. When this happens, the bearing
pressure increases over the preload value due to loss of
support area. Depending on the bearing capacity of the
soil, additional penetration or spud can rotation may occur.
Additionally, if the footing is not structurally
adequate, structural damage may occur. Finally, if scour is
severe and over a large enough area, the footing may slide into
the depression created. Any of these scenarios can be
extremely severe, especially since they occur with the hull at
full air gap.
GUIDE / RACK TEETH WEAR
The legs are restrained in horizontal movement and in rotation
by the leg guides. Leg guides may also maintain the allowable
position of the elevating pinions with respect to the leg rack.
Over time, it is normal to experience wear in both the guides and
in the part of the leg that is in direct contact with the guides. This
wear in both the leg guides and the leg should be monitored.
When the leg guides are excessively worn, they should be
replaced. If leg wear should become excessive, the leg should be
repaired.
