University of Alexandria Dept.
of Naval Architecture & Marine Engineering
Faculty of Engineering Instructor: H. W. Leheta
CHAPTER 7 SUPPORT SYSTEMS
For offshore installations to perform their functions, support is required in various
areas to ensure continued safe operation. These installations must be supplied with
stores, assisted through towing and anchor handling and provided with crew. They
must also be covered by adequate fire fighting means during major repair operations
and must maintain good communications with shore and with vessels operating
nearby.
7.1 Types of Support Systems
The type of offshore operation being carried out determines largely the type of
support systems that are required. Whether an offshore installation is engaged in
drilling, production or pipelaying will dictate what and how much support will be
needed.
The principal types of support systems encountered in the offshore field are:
1. Supply.
2. Towing.
3. Anchor handling.
4. Maintenance and repair.
5. Well servicing and stimulation.
6. Crew transportation.
7. Backloading.
8. Pipelaying.
9. Surveying.
10. Diving support.
11. Heavy transportation.
12. Heavy cargo handling.
13. Rock dumping.
14. Accommodation.
15. Standby (rescue).
16. Other- such as firefighting, pollution control, communications, etc.
Several types of offshore support vessels make out the major parts of such systems.
These are shown in Fig. 46. A brief overview will be given of the support systems
mentioned above.
7.1.1 Supply
Supply is the primary support provided for offshore installations worldwide. It
involves the delivery of materials such as drilling pipe, casing, tubing and other
equipment. Also mud (liquid or bulk), cement, drilling water, potable water, fuel,
chemicals (e.g. methanol), and provisions are supplied.
The offshore supply vessel (OSV) is in almost daily attendance to the offshore
installations. The OSV is primarily designed to transport cargo. It is single decked
and has a high bow and forward accommodation in order to withstand the severest
of weather conditions and permit 360° of unrestricted vision from the wheelhouse. It
has a long flat wooden covered afterdeck (well) providing an ideal platform for the
stowage of containers, drill pipe, casing, anchors, etc. Below deck, a refrigerated
cargo hold facilitates the transportation of perishable food stuffs. Potable water, fuel,
cement, barite, etc. are carried in specially built integrated tanks.
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University of Alexandria Dept. of Naval Architecture & Marine Engineering
Faculty of Engineering Instructor: H. W. Leheta
Fig. 46
1 Multi-functional support vessels (MSVs) 10 Heavy transportation vessels
2 Accommodation unit 11 Anchor handling, tug, supply vessel
3 Fallpipe dumping vessel 12 Diving support vessel (DSV)
4 Pipeline bury barge 13 Well service vessel
5 Seismic survey vessel 14 Multi-purpose vessel
6 Flexible pipelay vessel 15 Reel pipelay vessel
7 Derrick lay barge 16 Pipelay barge
8 Pipelay ship 17 Standby vessel
9 Derrick and crane barge 18 Well stimulation vessel
Power, manoeuvrability and seakeeping characteristics are of prime importance. It
must have twin engines, twin propellers, twin rudders and bow thrusters. Fenders
are attached to the hull to minimize damage incurred in coming alongside or while
attempting to lay alongside a rig or platform during rough seas. Fig. 47 shows a
typical offshore supply vessel.
Fig. 47
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University of Alexandria Dept. of Naval Architecture & Marine Engineering
Faculty of Engineering Instructor: H. W. Leheta
7.1.2 Towing
Exploration rigs having no means of self propulsion require towing from one site to
another. Jack-up rigs are such an example. For long distance moves, they may be
loaded on a submersible barge and taken to a new location. Otherwise, they are
towed by tug/supply vessels to the new site. Similarly, semi-submersible drilling rigs
may require towing from one location to another.
The most important requirements of ocean towing are power and bollard pull,
directional stability and stopping ability.
Bollard pull is the force exerted by the towboat running full ahead while secured by a
long line to a stationary bollard. Effective bollard pull falls off as the speed through
the water increases. For directional stability, the arrangement of the towing vessels
around the towed platform is very important. One or more towboats must pull in the
reverse direction to maintain the required heading. The inertia of a towed structure,
especially a large one such as an offshore caisson, is tremendous. Thus, it tends to
keep moving ahead long after pull has ceased, so additional boats are needed to
slow the towed structure and stop it. Fig. 48 shows the positioning of tugs during the
installation of a large GBS.
Fig. 48
If a tow line breaks at sea, it is desirable that it fails at a known “weak link” so that it
may readily be reconnected, even in severe sea states. In Fig. 49, if the towline is
subjected to a high-impact overload, the short pendant between B and C breaks, the
shackle at B is pulled back on deck by means of a fiber rope pendant, a new
pendant fitted (BC), and the towline reconnected.
Fig. 49
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University of Alexandria Dept. of Naval Architecture & Marine Engineering
Faculty of Engineering Instructor: H. W. Leheta
7.1.3 Anchor Handling
An anchoring system is almost always used today for positioning of offshore
installations. Assistance is provided by boats having anchor handling capabilities that
must be able to pick and move anchors, even in a rough sea. Such boats must be
highly manoeuvrable with an open stern and armored so that wire or buoys can be
dragged in over the stern as required. A winch is needed at the forward end of the
well so that by means of a line, a wire pendant or buoy can be quickly dragged on
board. Wire pendants and buoys identify the position of anchors, see Fig. 50. Buoys
are pulled in first followed by the wire pendant and then the anchor is hauled on
board. AHTS (Anchor Handling Tug Supply) vessels are capable of towing and
anchor handling.
Fig. 50
7.1.4 Maintenance and Repair
All offshore installations need regular maintenance and repair during their lifetime. A
lot of inspection and maintenance work is carried out undersea. Hence, there is a
need for divers, diving bells and ROV’s equipped with cameras and different light
and heavy equipment (NDT, cleaning, wrenching, etc.). Inspection and maintenance
should cover offshore structures with their machinery and equipment, submarine
pipelines, risers, and subsea completion and production systems.
Offshore structures are exposed not only to the extreme conditions of the
environment such as wave slam, ice impact and fatigue, but also to accidental
events such as boat impact and dropped objects. Thus repairs become necessary.
During repair operations, safety of the installation must be ensured. Auxiliary
strengthening may be required before the damaged part is cut-out for repair. The
installation must be covered by adequate firefighting means before any repair work
can be undertaken.
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University of Alexandria Dept. of Naval Architecture & Marine Engineering
Faculty of Engineering Instructor: H. W. Leheta
7.1.5 Well Servicing and Stimulation
Wells need regular maintenance and as they get old may need stimulation to
improve hydrocarbon production. Depending on the size of the task, special purpose
built ships may be used. Typical well servicing operations carried out are the
cleaning of the production tubing, clearing blockages and removing sand. By well
stimulating, we mean improving the hydrocarbon bearing foundation near the well
bore. There are three main well stimulation processes: fracturing, acidizing and sand
control.
Fracturing involves the injection of oil or water based fluids at very high pressures
into the rock foundation to induce cracks and promote drainage through the rocks.
Acidizing involves injecting acid to dissolve deposits restricting the production tubing
perforations. Sand control involves getting rid of sand that may block casing
perforations by scooping out, screening or resin bonding of sand particles.
7.1.6 Crew Transportation
Crew boats are used to transfer personnel from shore to offshore operations
wherever sea conditions permit. The transfer of personnel from crew boat to platform
is a critical operation from the point of view of both safety and efficiency. Helicopters
are also used for personnel transfer to and from offshore installations. Great care
must be taken on approaching and entering the helicopter while the blades are
running.
7.1.7 Backloading
During transit, semisubmersibles float at a shallow draft, which makes it necessary to
unload pipes, casings, and supplies. This unloading operation is called backloading.
The duration of such an operation depends on the time required for the transit
operation. Also, additional buoyancy (buoyancy augmentation) may be required.
7.1.8 Pipelaying
Pipelines for the transmission of hydrocarbons, water and other fluids are basically
designed to lay on the seafloor or in a trench in the sea floor, with more or less
continuous support. Lines are buried beneath the sea floor in many areas of the
world to protect them from fishing trawl boards, from dragging anchors, and from
fatigue due to oscillation in a current. Many methods of pipelaying have been
employed. These will be discussed in Chapter 9.
7.1.9 Surveying
Adequate survey of the sea floor including any man-made objects such as pipelines,
cables, dropped objects and anchors must be undertaken before any installation
work can be carried out. Divers in shallow water and ROV’s in deep water are
capable of transmitting still and video images for their accurate detection. Acoustic
transponders assist in the location of jacket structures on the sea bed and for
controlling underwater operations.
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University of Alexandria Dept. of Naval Architecture & Marine Engineering
Faculty of Engineering Instructor: H. W. Leheta
7.1.10 Diving Support
Manned (diver) and machine (submersible) intervention is one of the most important
underwater tasks in offshore activities. The tasks that might be carried out include:
inspection and nondestructive testing (NDT), photography, underwater cleaning,
lifting, welding and cutting, explosive charging, etc. The diving support vessel (DSV)
provides the main support for diving and ROV operations.
7.1.11Heavy Transportation
A transportation operation may involve barges or other floating units. A jacket may
be transported to its installation site by cargo or launch barge. Heavy submersible
transportation vessels are sometimes used when the offshore structure is to be lifted
out of the water and then moved to a new site, such as in the case of a jack-up rig.
7.1.12 Heavy Cargo Handling
The installation of offshore structures usually includes the lifting and setting of
modules and other heavy loads on the platform. Derrick and crane barges can now
lift up to 10,000 tones. Vertical forces on lifting can include the favourable effects of
buoyancy, however full and partially submerged structures may pick up an added
mass component, see Fig. 51.
Fig. 51
7.1.13 Rock Dumping
Rock dumping vessels are designed to deposit large quantities of rock in localized
areas. Also, they are required to dump small quantities of rock to be placed over
pipelines. The three main rock dumping techniques are: side dumping, fall pipe
dumping and bottom dropping. These will be discussed further in Chapter 9.
7.1.14 Accommodation
Special purpose built accommodation vessels which can remain in close attendance
to fixed structures for months at a time are now being used. Transfer of personnel
may be by bridge or by helicopter.
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University of Alexandria Dept. of Naval Architecture & Marine Engineering
Faculty of Engineering Instructor: H. W. Leheta
7.1.15 Standby (Rescue)
Some countries require that all manned offshore installations have in attendance a
stand-by vessel. This vessel is to be capable of accommodating the entire crew of
the installation and of providing first aid facilities. One stand-by vessel may cover any
number of installations within a five mile radius. These vessels must be at least 11
meters in length, capable of a speed of 10 knots, and provide 360° unrestricted
vision of the surrounding seas from the wheelhouse. They must be of either twin
screw, or single screw assisted by an azimuthing thruster, and fitted with a bow
thruster. They must also carry two fast rescue boats (FRB) available for immediate
deployment.
7.1.16 Other
Other tasks may be required for support. These include firefighting, pollution control
and communications. Fire is one of the most important hazards to which an offshore
installation may be subjected. Special fast response fire fighting boats or supply
vessels equipped with monitors, provide support in the case of fire. A monitor is a
permanently fixed device for directing water or foam in jet or spray form onto a fire (a
water cannon). Special pollution control vessels are necessary if an oil spill occurs.
Such vessels must be capable of deploying booms to contain the spill, and have
skimmers to remove the oil or release chemical dispersents.
All offshore installations must be able to communicate with the home base, with
other nearby installations, supply vessels and helicopter. Types of communications
transmitted include:
1. Work progress to home base.
2. Instructions from home base.
3. Reports on materials, equipment and tools required.
4. Reports of supply vessel and helicopter moves and when to expect them and with
what materials.
5. Weather reports and forecasts.
6. Onboard communication and alarms.
7.2 Supply Bases
In areas of intense offshore activity, e.g. the North Sea, a number of supply bases
have been established. Supply bases should satisfy the following criteria:
1. Close to area of operation.
2. Good transport (roads, rail, etc.) and communication links.
3. Sheltered waters with access unrestricted by weather.
4. Adequate draft for all types of ships being supported during both low and high
tides.
5. Adequate berth and storage areas, and office space.
6. Good back-up facilities.
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