As field developments move into deep water, numerous subsea intervention tasks have been moved out of reach of human direct intervention. Remote operated vehicles (ROVs) and remote-operated tools (ROTs) are required to carry out subsea tasks that divers cannot reach. An ROV is a free-swimming submersible craft used to perform subsea tasks such as valve operations, hydraulic functions, and other general tasks. An ROT system is a dedicated, unmanned subsea tool used for remote installation or module replacement tasks that require lift capacity beyond that of free-swimming ROV systems [1,2].
The term subsea intervention refers to all activities carried out subsea. An intervention philosophy needs to be built before designing the interface for a subsea production system (SPS). The intervention philosophy normally focuses on the following questions:
- What kind of tasks will be done subsea?
- What methods will be used to complete these jobs?
- What are the requirements to complete the intervention activities?
The first question will be discussed in this section. For the second question, two intervention methods are commonly used in the subsea engineering:
(1) ROVs for inspection, cleaning, and so on, and
(2) ROTs for module replacement and subsea tie-in. Generally, ROVs are used for these issues, which are detailed in the following sections:
- Site survey;
- Drilling assistance;
- Installation assistance;
- Operation assistance;
- Maintenance and repair.
A site survey has to be carried out before offshore activities such as drilling and installation to obtain the seabed’s precise bathymetry and properties. Detailed seabed mapping through precise bathymetry may be performed by a seabed reference system with differential pressure sensors and acoustic data transmission, which may be deployed and retrieved by an ROV. A sub-bottom profiler (SBP) for sub-bottom profiling may be used to assess the quality of seabed properties for offshore installation foundation.
Drilling activities for production drilling and completion normally include:
- Deployment of acoustic units such as transponders or beacons by an ROV for surface or subsea positioning;
- Bottom survey by visual observation from a ROV with video and still cameras;
- Structure setting and testing (if needed) of permanent guide base (PGB), temporary guide base (TGB), Xmas tree, BOP, etc.;
- As-built (bottom) survey by ROV visual observation with supplemental equipment.
During the entire process, the observation tasks with video cameras (often with scanning sonar as supplemental “acoustic observation”) make up the majority of ROV drilling assistance. Tasks include conducting the bottom survey, monitoring the lowering of the structure and touching down, checking the structure’s orientation and level with a gyrocompass and bull’s-eye, respectively, and performing an as-built survey. Some necessary intervention work may have to be done with ROVs or ROTs during structure setting and testing:
- Acoustic transponder or beacon deployment and recovery;
- Debris positioning and removal from seabed and tree, including dropped objects;
- Structure position assistance with ROV pull/push;
- Guide wire deployment, recovery, and cutting during emergency conditions;
- Rigging (e.g., shackle connection and disconnection);
- Cement cleaning on guide base with brush or water jet;
- Valve operation with hydraulic torque tool or hydraulic stab-in;
- ROV-operable guide posts, replacement, and pin pull release;
- Control pod replacement if suitable for ROV (otherwise ROT);
- Anode installation by clamp and contact screw.
The installation of a subsea production system from the water surface to the seabed can be divided into two parts:
- Subsea equipment installation (e.g., manifold deployment, landing);
- Pipeline/umbilical installation (e.g., initiation, normal lay and laydown).
The installation methods for subsea equipment may be divided into two groups. Large subsea hardware with weights over 300 tonne (metric ton) can be installed by a heavy lift vessel where the crane wire is long enough to reach the seabed and the crane is used to both put the equipment overboard and lower it. A soft landing to the seabed may be required using an active heave compensation system with the crane. Alternatively, it may be installed with a drilling tower on a drilling rig, which can have a lifting capacity up to about 600 tonne. For smaller subsea hardware (maximum approximately 250 tonne), a normal vessel equipped with a suitable crane for overboarding the hardware may be used.
The vessel normally would not have a long enough crane wire to the seabed, so the hardware is transferred from the crane wire to a winch with a high capacity and a long enough wire for lowering the equipment to the seabed once the hardware passes through the splash zone. In both installation groups, ROVs are used for observation and verification and for engagement and release of guide wires and hooks. Subsea structures are widely positioned underwater using the long baseline (LBL) method in which transducers used for position measuring, a gyrocompass for orientation measuring, a depth sensor for depth measuring may be mounted onto structure by package(s) that will be retrieved by the ROV.
The orientation controlmay be assisted by theROV, and theROV has to verify via camera that the structure is aligned and level before the structure’s final setdown. ROVs may also be used to install chokes,multiphasemeters, and subsea control modules. For seal pressure tests, ROVs can be used for hot stabbing. ROVs can be used to assist in the installation of a dead anchor for pipeline/umbilical laying initiation. They can also be used to connect the pull-in line for J-tube or I-tube initiation. During normal installation and pipeline/umbilical laydown, the touchdown point is often monitored with ROVs in front and behind. The connections between subsea production equipment and flowlines and subsea equipment and umbilicals may be completed through flying leads from the umbilical termination assembly (UTA) to the tree/manifold, well jumper from the tree to the manifold, jumper from the manifold to the PLET. The flying leads may be handled and pulled in by an ROV directly. Jumpers can be deployed from a vessel with spreader bar(s), and then positioned and connector actuated with the assistance of an ROV.
Main production activities normally include:
- Flowcontrol by chokes and valves operated by hydraulic actuators through control pods and umbilicals or externally by ROVor ROT intervention;
- Monitoring of flow temperature and pressure by relevant measurement meters;
- Chemical and inhibitor injection for corrosion, waxing, and hydrate formation resistance;
- Flow separation of liquids, gases, and solids (filtering);
- Flow boosting by pumping;
- Flow heating or cooling.
During the operation phase, ROVs are normally not required except for noncritical valve actuation and possibly intermittent status checks, taking samples, etc.
Inspection may be needed on a routine basis for the structures expected to deteriorate due to flowline vibration, internal erosion, corrosion, etc. Inspection includes:
- General visual inspection, including cathodic measurements and marine growth measurements;
- Close visual inspection additionally requiring physical cleaning for close visual inspection, CP measurements, and crack detection by means of nondestructive testing (NDT);
- Detailed inspection including close visual inspection, crack detection, wall thickness measurements, and flooded member detection;
- Routine pipeline inspection including tracking and measurement of depth of cover for buried pipelines, which is also applicable for control umbilicals and power/control cables.
Crack detection may be performed by an ROV with magnetic particle inspection (MPI), eddy current, alternating current field measurement (ACFM) methods, etc.
Maintenance and Repair
Maintenance activities include repair or replacement of modules subject to wear. Maintenance is normally performed by retrieving the module to the surface and subsequently replacing it with a new or other substitute module. Retrieval and replacement have to be anticipated during subsea equipment design. Some modules such as multimeters, chokes, and control pods are subject to removal and replacement. A completed replacement may have to be carried out due to the significant wear on or damage to nonretrievable parts of subsea equipment. Due to the difficulty and expense of maintenance and repair, the operation may be continued with regular monitoring if the damaged module is not readily replaced and does not prevent production.
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