Accelerating subsea workover programs improves safety, saves money

In West Africa, subsea well workovers are traditionally carried out from whichever drilling rig the operator has available. This scenario is not likely to change anytime soon until a viable regional spot market for specialized well intervention vessels has been established. The total daily operating cost for any drillship/semisubmersible suitable for deepwater operations and all of the support services normally on board can be high and naturally leads operators to consider possible options for taking some tasks off of the rig.

The operators of most West African deepwater subsea developments generally retain a permanent FSV with a permanent ROV spread that costs relatively little by comparison to a drilling rig, but at the same time, generally has limited capabilities simply because of the economics and the presumed roles of the FSV.

Although each operator justifiably takes a different view of the role of the FSV, once an operator agrees to explore innovative new ways of increasing the role of the FSV and a decision is made to explore new ways of improving the utilization of the FSV to reduce rig time, it is necessary to turn to a solid subsea engineering foundation to demonstrate the new approaches are thoroughly proven from all angles prior to sanction.

A simulation carried out by Fugro engineers shows spool disconnection and rotation. (Images courtesy
of Fugro)

Recently procedures have been developed for tree recovery and innovative flow spool (pipeline jumper) wet parking from an FSV. The combination has resulted in major gains in both safety and cost efficiency in a Nigerian deepwater field by executing tasks traditionally done with a drilling rig.

West African field support
Fugro was contracted by a major operator to provide ROV, diving, and subsea intervention engineering services in a deepwater field offshore Nigeria. This contract provides Fugro Subsea Services an excellent platform to demonstrate the full extent of the company’s life-offield capabilities with its subsea support services including:
• Subsea intervention engineering and tooling supply;
• Construction support engineering;
• Inspection maintenance and repair;
• Drilling support; and
• Offshore surveys.

The EngSim engineering simulator was used to design the complex seabed rigging operations.

The field is an extensive FPSO-based subsea development South of Lagos in approximately 3,280 ft (1,000 m) water depth.

The field consists of a complex subsea network of 35 production wellheads and five manifolds with tiebacks to a permanently moored FPSO. Reservoir pressure is maintained with 16 subsea water injection wells. Produced oil is stored on the FPSO for tanker offloading via a CALM buoy, while gas is exported through a 16- in. pipeline to shore.

ROV spread works from an FSV
The offshore element of the subsea field services is based upon a Fugro 150HP FCV3000 workclass ROV spread that is permanently mobilized on board an operator-chartered FSV. The vessel is fully manned to provide intervention services as required.

Fugro is responsible for engineering, management, and supply of all of the subsea intervention activities carried out from the FSV. The scope of the work includes:
• ROV and Subsea Engineering. Besides routine operations and maintenance, Fugro provided conceptual and work method studies and detailed execution procedures for engineering solutions to provide intervention services using Fugro’s ROVs to inspect, maintain, and repair the deepwater subsea facilities, including drill support and construction activities associated with the establishment and tie-in of new deepwater subsea infrastructure. Also included is an ongoing requirement for survey activities, including metrology and pipeline inspection when required.
• Inspection and data recording. Offshore activities also require compiling and routine reporting of intervention/inspection data. The nature of the work generates significant volumes of digital data which need to be collated, indexed, and stored in a secure but accessible manner. Fugro fills the role of managing and filtering the key data being generated at the work site which is then summarized in a monthly HTML report.
• ROV tooling support. In addition, Fugro’s staff in Nigeria manages and maintains an extensive inventory of operator-owned ROV intervention tooling used in the field.
• Diving services. From time to time, diving services are conducted from the FSV. These activities are primarily air diving at the floating and fixed structures in the field.

A typical Fugro life-of-field support vessel can more efficiently carry out functions previously performed by more expensive conventional alternatives.

Since mobilization in April 2009, the Fugro team has had approximately 4,500 operational man-days without a lost time incident. The team has executed a wide variety of subsea intervention work tasks from the FSV, including routine visual, corrosion and wall thickness inspection; control module replacement; valve movement and override; subsea tree installation and recovery; tubing hanger spool installation and recovery; choke change-outs; electrical flying lead installation and recovery; hydraulic flying lead installation and recovery; operating deep-set wellbore valves; well jumper metrology; and well jumper installation.

In addition to ongoing life-of-field construction, maintenance, and inspection activities, the operator and Fugro subsea engineering team have faced extraordinary challenges, including a well kill, subsea wet parking of a jumper spool, tree recovery to the FSV, well stimulation, and subsequent tree re-installation and hookup. These are unusual tasks that rarely have been undertaken from an FSV because of the complexity and difficulty.

Well kill and rigid flow spool jumper wet parking
This challenging subsea intervention involved killing a water injection well with heavy brine, bringing the subsea tree to the surface, and disconnecting the rigid flow spool to allow for a well intervention to increase water injection and thereby production flow rate.

This operation required removing the christmas tree, jumper spool, and associated wellhead equipment to allow the drilling rig access to install a blowout preventer (BOP) onto the guide-base prior to well intervention. Safety concerns with recovery of the rigid flow spool to a heaving vessel and the soft nature of the seabed ruled out both the recovery of the spool or alternatively the conventional wet storage of the flow spool on the sea bed.

Therefore, a decision was made to investigate the viability of rotating the tree end of the flow spool suspended from the vessel heave compensated crane and attaching it to pre-installed buoyancy located at a safe distance from the wellhead, sufficient to allow tree removal and BOP installation without danger of clashing.

Fugro provided engineering analyses of all new design conditions and load cases of this procedure. This included analysis of all stages of recovery, landing, and re-connecting the jumper for structural integrity; stability of the jumper for loads imparted from the vessel during the lifts; and various engineering solutions for the wet parking configuration, culminating in the preferred option.

The final solution for wet parking involved using a clump weight and buoyancy module. All rigging items and lifting components were designed or selected on the basis of the analyses within the time constraints of the rig arrival.

The flow spool connection at the tree end was released by the ROV and only unlocked at the manifold end. The spool was then rotated to be clear of the tree for wet storage in an upright position above the seabed. This operation requires a well-proven crane with active heave compensation. The BOP could then be safely run and landed from the drill rig without risking collision with the flow spool, allowing the well to be worked over.

On completion of the workover, the BOP was recovered, and the wellhead equipment was re-installed. As soon as the FSV was next available, the spool was then rotated back into position and the connections closed and tested.

To help meet the deadline, some of the more complex subsea intervention processes were undertaken on Fugro’s engineering simulator, EngSim to help design the complex seabed rigging and ROV operations. EngSim also was used to identify potential clashes to prove the viability of each stage of the intervention process. The results of this work were used to verify the methodology of the intervention to be used.

Benefits
This innovative solution, devised jointly by the operator and Fugro subsea engineering teams, resulted in significant safety and financial benefits over more conventional methods. Parking a flow spool during workover as well as tree recovery and re-installation from a typical West African FSV can lead to a substantial savings in rig time.

The ability of the FSV to park the flow spool and recover the tree and to return later to reverse the process not only reduces risks by omitting the usual step of recovering the spool to the deck, but also leads to significant time savings.

Fugro Subsea Services is increasingly requested to examine subsea intervention challenges for which there are no existing proven solutions and often within the constraints set by the vessel assigned long term to provide intervention field support.

The immediate future of ongoing field maintenance relies on being able to safely expand the envelope, undertaking work normally carried out by drilling rigs and/or construction grade vessels through this type of innovative cost-effective subsea intervention work from field support vessels that, at first blush, may appear incapable of completing the tasks in a risk-free manner.

In time, larger and more capable field support vessels arriving in the market will take over this role and push the envelope further as operators begin to see their value in saving costs.