Having severed the wellhead, the SWAT removes it to the back deck of the vessel for recycling. (Image courtesy of OIS)
Each subsea well decommissioning project provides its own distinct set of challenges. But there is a common thread running through most of them, and that is the requirement for strong project management and coordination.

The value of management and coordination became apparent in a North Sea abandonment project that encompassed several wells on the Scott field, operated by Nexen Petroleum UK in Aberdeen.

The operator approached OIS (which had recently taken over from WellCut Decommissioning Services) to help with a survey and four-well abandonment program. One of the primary objectives for this project was to establish a skilled and focused group of service providers capable of working together as an effective team.

Part of the appeal of OIS was the company’s experience in conducting decommissioning projects, from site assessment through to final reporting. Nexen tasked the company first with planning the program, which was to be based on a previously undertaken well categorization study, and second to direct the work. To carry out the project, OIS had to assemble all of the necessary players, define their roles and responsibilities, and perform a thorough risk assessment of the entire operation.

Well survey, decommissioning

The project began with a 10-well survey using a dynamically positioned (DP2) vessel and a workclass remotely operated vehicle (ROV). At this early stage of operations, the opportunity was taken to remove entangled fishing nets from the wells before the four-well abandonment phase of the offshore program got under way.

For the well survey activities, OIS employed Trident and Claxton, both Acteon companies, along with four other specialist contractors. Trident provided the survey equipment for the vessel and marine support services during the work and mobilization phases. The survey activities were managed by OIS using the Island Offshore vessel the Island Vanguard and ROVs supplied by Oceaneering International. Claxton carried out wellhead cap removal, drifting the wells, and net clearance.

One of the critical components of the abandonment operation was a suspended well abandonment tool (SWAT) that was refurbished and supplied by Claxton. A second component was abrasive cutting equipment that NCA provided for use in abandonment operations on four of the wells.

Two of the wells were category 1; the others were 2.1 and 2.2. These categories are based on the Oil & Gas UK (ex-UKOOA) guidelines, which state:
• Category 1 well abandonments require the wellhead to be removed from 10 ft (~3 m) below the seabed, using a vessel.
• Category 2 well abandonments require cement plugs to be set, using a vessel. Due to vessel limitations, the present depth limit for setting cement plugs is 100 ft (~300 m) below sea level. Category 2.1 indicates the cement plug is to be set in one annulus, and category 2.2 indicates the cement plug is to be set in two annuli.
• Category 3 well abandonments require cement plugs to be set, using a drilling rig.

Trident supplied the necessary survey equipment and marine support services, subcontracting the supply of transponders and subsea positioning equipment to Seatronics. Meanwhile, Claxton provided offshore operations personnel, and Claxton provided SWAT-tooling and offshore operations personnel. InterAct delivered the well classification; the preliminary procedures; and the health, safety, and environment submittals.

Once the project was under way, Nexen added to the work scope by requesting assistance in severing an additional well in the field, one that had been recently suspended using a semisubmersible after encountering equipment problems during the abandonment phase.

Following an engineering review that was presented to Nexen for approval, the rig was moved off location, and a chartered vessel was brought in to carry out the intervention and severing. The operation was completed, with considerable overall cost savings, within eight hours of the rig leaving the location.

When the wells had been cut, the wellheads were recovered to the back deck of the vessel ready for component recycling and environmentally friendly disposal, which fulfilled the agreement to return the seabed to its original condition.

Key challenges

The project presented a number of challenges. One of them was managing and optimizing the performance of new abrasive severance technology. The NCA abrasive cutting system, which had not been used at the water depth required for this project, was required to interface with equipment from other subcontractors. OIS arranged a series of interface engineering meetings and managed them on behalf of Nexen.

Another complication was the fact that the vessel used on the Scott field was new. This meant that there was a shakedown period during which minor problems was addressed and overcome.

Finally, but not least challenging, the team of subcontractors selected for this project was based in different places; so effective communications were vital to optimize the overall performance.

Despite the obstacles that had to be overcome, the project was performed as a smooth operation in which all personnel and equipment reached the vessel in good time and worked together as planned.

Performance

Effective well decommissioning of this sort requires precise planning, careful project and contract management, and a positive working environment where the emphasis is on safety and teamwork.

Interface engineering and project and operations management logic were vital to the smooth running of the operation. OIS handled specific key contracts and provided risk management supervision to keep all of the parties working to the highest standards of offshore safety and to ensure that the quality of the work delivered was maintained throughout the project.

The success of the well-abandonment campaign at Scott field required skills and technology drawn from across

the Acteon group of companies and beyond. Contributions from all of the subcontractors were necessary to allow Nexen to achieve its aims, but the most critical component of the team’s success was effective project, contract, and operations management.