Subsea Control System 

When running subsea equipment in deepwater, depending on the size and weight of the equipment a running tool will be required, the forces required to force the mating parts together, and to latch the equipment in place.

A major component such as the subsea control module or smaller component such the choke insert require both such a running tool. This can be a simple relatively low cost purpose built running tool for one particular application, or a more complex and expensive tool called a Multi-Mode Running Tool (MMRT). An MMRT can run and retrieve subsea control modules and subsea accumulator modules, and can also be designed and used for other activities such as running and retrieving subsea chokes, multi-phase flow meters, etc.

A large work class ROV is required for this activity.

The single purpose built minimum fit running tool is not described, as it is project specific.

The multi-mode running tool can be deployed on a lift wire from a surface winch or ROV.

Hydraulic power required to operate the running tool is provided by the ROV through hot stabs. For more complex applications, control can be on-board local control with power and data through a discrete electrical umbilical or through the ROV umbilical.

In lift wire mode and using guide wires, the running tool is lowered using a heave compensated winch onto the API 17H interface. The running tool will have a soft landing device to protect the SCM from shock damage. The running tool is heavy in water. The running tool uncouples the SCM or SAM and retrieves it to the surface. The component is exchanged at the surface, re-deployed to the subsea interface, and reinstalled.

When deployed by ROV (this technique is rarely used in deepwater application), the MMRT, the ROV, and the component have to be neutrally buoyant in water. The tool uses transfer weights of equivalent weight to the SCM or SAM to be recovered, and has buoyancy added to balance the weight requirements.

The ROV deploys to the interface, retrieves the SCM or SAM, and leaves the transfer weights in the vertical tubes of the interface, and the SCM or SAM is retrieved to the surface. The component is exchanged at the surface, re-deployed to the subsea interface, and reinstalled. The transfer weights are recovered before the ROV is retrieved.

Figure 25.1 - PXT SCM recovery - MRT with ROV

Alternatively, to eliminate a trip to the surface by the ROV, the MMRT can be lowered to the seabed in a cage that carries the new replacement component and a second set of transfer weights. The ROV docks onto the MMRT, and guide it onto the interface of the component to be replaced. The SCM or SAM is retrieved by the tool, and the transfer weights are left at the interface.

The ROV moves to the cage and deposits the SCM or SAM in bay 1 of the cage, and then picks up the second set of transfer weights. The ROV then moves to bay 2, picks up the replacement component and deposits the second set of transfer weights.

The ROV moves back to the job, docks at the interface and installs the replacement component. The ROV then retrieves the first set of transfer weights back to the cage.

An SCM or SAM can be deployed using a single guide wire. The guide wire needs a winch with a lift and lock tool, and a high quality heave compensation mechanism to prevent damage to the SCM on landing.

The SCM or SAM is lowered into a guide funnel as directed by the ROV, an orientation helix or location spigot(s) may be used to orientate the SCM correctly onto the mounting base. The deployment wire is uncoupled and the ROV latches the SCM onto the mounting base.

This method of deployment dispenses with the MMRT, but a mechanism is still required to exert down force and to mate and latch the SCM onto the SCMMB.

When a drilling rig is on station, it is possible that there could be a preference to run an SCM using a drill string. This is not a preferred and controlled method from an engineering viewpoint, but this method has been used in the past but nowadays this technique is not used anymore.