Installation of subsea hardware requires the vessel to keep its position well so that the subsea hardware can be placed onto the target foundation with the required accuracy. The positioning operation includes surface positioning and subsea positioning. Surface positioning or vessel positioning refers to positioning that maintains the vessel at the correct position at all times during installation. It is the first step of subsea production system installation because it brings the hardware to be installed near its ultimate aiming position. Subsea positioning refers to positioning to monitor and control the equipment underwater relative to the installation vessel and seabed target area during the process of lowering equipment through the water and landing or locking.
A surface positioning system can be divided into these components:
- Power system, supplying power to all of the following systems;
- Position reference system, normally using DGPS a hydroacoustic measuring system such as USBL, SBL, LBL, and a taut wire system;
- Controlling system;
- Station-keeping system such as a mooring system including anchor gear, anchor lines, and anchors for positioning.
Once the surface vessel has been positioned, subsea hardware will be deployed from the vessel through water to the target location on the seabed. During the lowering and landing process, the hardware will be tracked with a hydroacoustic unit (e.g., transponder) for position measuring and a gyrocompass (and ROV) for inclination detection (roll, pitch, and heading), which were tied to the hardware on board before lowering. Two methods are widely used for subsea hardware installation: the guideline (GL)method and the guideline-less (GLL)method.The GL method uses guidelines (normally four tensioned wires) to deploy subsea hardware to the seabed. The subsea positioning for this method is quite convenient, but has been limited by the guidelines. It also consumes time and costs money for deepwater installations. The GLL method deploys subsea hardware in deepwater without guidelines. The subsea positioning for this method is relatively complex for hardware landing and requires limited installed tolerance such as inclination and position bias in the x, y direction. It also requires heavy structures on all equipment to be in the proper orientation. The hardware heave motion should be strictly limited during its landing process, so active or passive heave compensator systems have been introduced into the lifting system of installation vessels used for subsea deployment.
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