Typical Installation Vessels
Installation of subsea structures may involve the following vessels:
- Transportation barges and tug boats
- Drilling vessels including jack-up rigs, semi-submersibles, and drill ships;
- Pipe-laying and umbilical-laying vessels;
- Heavy lift vessels;
- Offshore support vessels, such as ROV support vessels, diving support vessels, and field support vessels.
Contents
Transportation Barges and Tug Boats
Subsea structures are normally transported from onshore to the offshore installation site by a transportation barge. Once at the offshore installation site, the subsea structure is transferred from the transportation barge to the drilling rig or construction vessels. Selection of the proper transportation barge and the arrangement of the structures on the deck depends primarily on the following features:
- Dimensions, weight, and center of gravity (CoG) height of the structure;
- Distance and transportation route;
- Schedule constraints;
- Cost;
- Ability to avoid bad weather.
Transportation barges are normally towed by tug boats from one location to another.
Drilling Vessels
Drilling vessels are mainly designed for drilling activities, but they are also used for the Subsea Production System (SPS) installation because of their installation capacities such as water range, lifting capacity, and positioning capacity. Drilling vessels normally include jack-up rigs, semi- submersibles, and drilling ships.
Jack-Up Rigs
The jack-up, or self-elevating, rig was first built in 1954 and rapidly became one of the most popular designs in mobile offshore drilling units. Jack-ups were popular for drilling activities in shallow-water areas-up to 360 ft (110 m). They provide a very stable drilling platform because part of the structure is in firm contact with the seafloor, and they are fairly easy to move from one location to another.
Semi-Submersibles
Semi-submersibles are the choice for drilling from a floating position, due to the deep water depths. Semi-submersible (“semi”) rigs float like a ship while being towed into position, where their pontoons can then be flooded, partially submerging the rig. The main part of the structure is beneath the sea surface, semis are not as susceptible to wave actions as drilling ships. Semis may be stationed using dynamic positioning or anchoring. This type of platform has the advantages of a smaller waterline plane, less susceptibility towave effects, good stability, long self-sustaining period, and a great working depth. Thus, they are suitable for installation of subsea structures ranging from 400 to 4000 ft (120-1200 m), and maybe deeper. For example, a spread-moored semi in 6152 ft (1875 m) water depth (WD) was set offshore in Malaysia in 2002, and a semi at a WD of 9,472 ft (2890 M) with a DP system was set in Brazil in 2003. A semi for a “taut line” mooring system was set in a WD of 8,950 ft (2730 m) in the GoM in 2003, which also set a record for subsea completions in 7,571 ft (2300 m).
Drill Ships
A drill ship is a vessel with a drilling rig and station-keeping equipment for drilling oil fields in deepwater. Drill ships have a larger payload capacity than semi-submersible drilling vessels, but face shortcomings in terms of their motion characteristics.
Pipe-Laying Vessels
Pipe-laying vessels can be categorized according to their pipe-laying methods based on site characteristics, such as water depth and weather. The three most common types are the S-lay, J-lay, and reel-lay types.
S-Lay Vessels
The S-lay method has been used for many years to lay offshore pipelines. An S-lay vessel has a broad deck operating area. The pipe joint assembly line is deployed in the middle or at the side of the main deck, which includes pipe conveyors units, welding, NDT inspection and coating stations etc. The stern of the vessel is constructed into a sloping slideway with a stinger, which is used to modify and control the configuration (stress/stress distributions along the pipeline) of the pipeline in an “S” shape down to the seabed.
J-Lay Vessels
The J-lay vessel uses a J-lay tower/ramp to install the pipeline instead of the stingers used in the S-lay vessel. The J-lay method differs from the S-lay method in that the pipe departs from the lay vessel at a near-vertical angle (e.g., 60 to 87). There is no overbend to maintain as in S-lay, thus no stinger is required. This type of operation is developed to cater to deepwater pipeline installations.
Reel-Lay Vessels
A typical reel-lay vessel usually provides an economical tool for installing long, small-diameter pipelines (typically smaller than 16 in. (0.406m)). The pipeline is made up onshore and is reeled onto a large drum(e.g., approximate diameter x width ¼ 20 m x 6 m)) on the middle deck of a purpose-built vessel in a spool base 66 ft x 20 ft. During the reeling process in the spool base, the pipe undergoes plastic deformation on the drum. During the offshore installation, the pipe is unreeled and straightened using a special straight ramp. The pipeline on drum will be unreeled accompanying with vessel speed at e.g. 12km/day (usually 3-15 km depending on pipe diameter). The pipe is then placed on the seabed in a configuration similar to that used by S-lay vessels, although in most cases a steeper ramp can be used and overbend curvature is eliminated as with a J-lay. This kind of pipe-laying vessel is easily manipulated with simple pipe-laying devices; in addition, it has a good pipelaying speed.
Umbilical-Laying Vessels
Umbilicals can be installed either by a reel-lay vessel or a carousel-lay vessel. Reels provide easy load-out. The maximum installed umbilical length is 1.9– 9.3 mile (3 to 15 km), depending on the diameter of the reels. The critical gross weight of reels with umbilicals is about 250 ton (226 metric ton). However, a carousel allows for a longer length of umbilical to be installed (>62 mile (100 km)), and it can avoid field splices. It does, however, require a dedicated vessel and longer load-out times of, for example, 22-33 ft (6 to 10 m)/min.
Heavy Lift Vessels
A heavy lift vessel (HLV) is a vessel with a specific crane that has a large lifting capacity of up to thousands of tonnes. Large and heavy subsea structures such as templates may require HLVs to perform lifting. The lifting capacities of most HLVs range between 500 and 1000 ton (454 to 907 metric ton), whereas crane capacity on normal construction vessels is below 250 ton (226 metric ton). An HLV’s stability and sea-keeping abilities are its most important characteristics.
Offshore Support Vessels
Offshore support vessels are special vessels that provide support for field drilling, construction, decommissioning, and abandonment. The support vessels normally include survey, standby, inspection, and installation assistance (e.g., monitoring). The following types of offshore support vessels may be utilized:
- ROV support vessel (RSV);
- Diving support vessel (DSV);
- Survey ship;
- Offshore supply ship or field support vessel (FSV).
An ROV support vessel is a platform with specialized equipment and spaces to store, deploy, and support ROVs for their subsea intervention. A diving support vessel is a platform with specialized diving equipment, such as diver-to-surface communication system, submersible, on-site diving hyperbaric chamber, compression chamber, and so on for subsea interventions by professional divers. A survey ship is a platform with specialized instruments and laboratories for the study of the ocean physics, chemistry, geology, topography, aerography, and hydrology required for SPS installation. A field support vessel is a multipurpose vessel that can provide transportation, supplies, and rescue and diving support.
References
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