Alternative devised for deepwater connection
with well clusters and non-layaway developments
The attachment of flowlines subsea has required divers on the seabed or surface support vessels with winches and remotely operated vehicles to pull together and mate the connections. Both methods were costly, and as water depths increased beyond diver depths, the outlays began to occupy a large segment of the installation allocation. Also, the ideal surface support vessels were not always available.
One of the systems developed to alleviate the surface support vessel requirements is Sonsub's diverless flowline connection system (DFCS), which uses a remotely operated vehicle (ROV) to winch the two components together and make the connection. First use of the system will take place this month in the South China Sea in tests conducted by Amoco and Coflexip.
The system can be used for flowlines, smaller pipelines, and virtually all umbilicals. The immediate benefits of the system include the following:
- No cables, other than that for ROV support, are required from the surface.
- The system eliminates pipelay, drilling, or winch support vessels.
- The process is effective for single or bundled lines up to 18 in. OD.
- The DFCS work skid can be attached and detached easily to virtually any work class ROV.
Components
The DFCS system consists of an aluminum skid frame, winches, stab-in anchors, tensiometers, guillotines, slide tubes, a docking probe, a multi-port hot stab, clamping arms, and the telemetry system. Additional optional components include an air bag inflation system, a clumb weight winch, a direct-deployment frame, a torque tool with vertification, an equipment basket, an external seal test system, a slide tool, a flowline protection cap, and a flowline/umbilical interface.
Providing the connection between the ROV and the DFCS skid is a docking probe. Also, the hydraulic multi-port hot stab connects the ROV hydraulics to the DFCS hydraulic system. If the ROV hydraulic system malfunctions, failsafe disconnection occurs. There are no electrical systems in the DFCS system.
The two winches, which can hold 75 meters of 12.5-ton breaking strength Kevlar rope, can be controlled separately. The retrievable stab-in anchors attach the winch ropes to the subsea structure. Each winch line has a tensiometer to provide tension feedback to the operators at the surface. Guillotines mounted near the tensiometers can sever the winch ropes in the event of an emergency.
Procedure
Connection contractors have a choice of two methods - one featuring placement of terminations on the seabed structure and winching from the flowline, and the other with an opposite arrangement. Larger flowlines can sometimes be handled better with winching from the seabed structure, since the flowline can be suspended above the seabed and payed out by the lay vessel on the surface. Because it takes less time, especially for smaller flowlines, the flowline winching point is preferable.
The first step in the mating procedure is to place the flowline or pipeline within a defined target range of the seabed structure (wellhead, well cluster, manifold, etc.). At the surface, the DFCS skid is attached to an ROV, and the ROV is flown to the seabed.
The ROV-DFCS then stabs the DFCS stab-in anchors into the female receptacles mounted on each side of the flowline connector in order to anchor the ends of the two winch ropes. The ROV flies backwards, as the DFCS pays out the twin ropes. As it reaches the flowline, the ROV clamps onto the flowline. Mechanical indicators verify engagement. Winching then begins. Tensiometers monitor the process. Alignment can be aided by two processes:
- Vertical: Air bags can be deployed to lift the flowline into position. In the case of larger flowlines, air bags are needed to reduce flowline weight and friction.
- Horizontal: If the flowline is not horizontally aligned with the connector, the surface support vessel can deploy a seafloor winch with a clump weight anchor. Using hydraulic power provided by the ROV, the winch can pull the flowline into position.
Within 400 mm of the connector, slide tubes take over the mating approach. The flowline protection cap is removed from the pipeline by having the ROV manipulator arm pull release pins. Next, the manipulator arm installs the seal ring onto the end of the flowline hub.
Upon mating, the ROV-operated torque tool closes the clamping mechanism. To verify integrity of sealing, the hot stab is pressurized in an alternative position and the pressure monitored. If the test fails, the seal can be replaced by having the DFCS disconnect and back off of the connection. If needed, the flowline connector can be lifted to the surface for repair.
The entire flowline connection system with ROV can be deployed by virtually any vessel of opportunity on the surface, including a drilling unit. Only one ROV is required for both work and observation, since the ROV can disconnect completely from the DFCS at any time. All steps in the connection program are reversible.
Copyright 1996 Offshore. All Rights Reserved.
