Shell Nigeria Exploration and Production Co. (SNEPCO) made the first major deepwater discovery in Nigeria at Bonga in 1995. When development of the field began four years later, Shell took steps to ensure that intervention equipment and procedures were in place to support Bonga during commissioning, start-up and operation.

SNEPCO’s preparations enabled the delivery of complex interventions in advance of first oil in

Planned intervention management lowered costs at Bonga. (Photo courtesy of Shell)
November 2005. It eliminated the need for a rig mobilization, avoided the considerable disruption to operations that would have resulted from undertaking the work from the floating production, storage and offloading (FPSO) vessel, and contributing significantly to the rapid ramp-up of the Bonga facilities. Usage of the remotely operated vehicle (ROV) for these tasks is now the base case plan for the Bonga Expansion Project.

Intervention philosophy
The Bonga project adopted an intervention philosophy at the earliest stages that reflected the normal industry practice of routine intervention limited to minimal inspection tasks, with significant intervention only in response to equipment malfunction. The subsea equipment was designed on this basis. It was assumed from an early stage, however, that because of the remoteness of the location and the limited availability of intervention capability in the region, SNEPCO would have to take action to ensure that capability could be delivered when, or if, required. It was not a project deliverable to provide that capability on an ongoing basis.

SNEPCO made the decision to provide a field support vessel (FSV) and ROV broadly following the model that had been adopted by Shell Philippines for the Malampaya field. In the run-up to first oil, it was recognized that the subsea equipment was potentially vulnerable to failure. It had lain dormant on the seabed for a lengthy period, and the limited well cleanup to the rig meant there was the additional risk of debris production and choke blockage. Project construction vessels could not be retained in anticipation of the commissioning and start-up of the field. Therefore, it was essential that SNEPCO’s subsea support resources should be available to provide intervention capability on what might be a more routine basis at least through the immediate start-up period. With field start-up having been delayed so long, the continuity of production following start-up was considered critical.

Intervention capability
Having decided that a dedicated intervention resource should be provided, it was necessary to identify the potential activities and define the capability of the equipment required. In 2003, SNEPCO issued tenders for an FSV and for ROV and diving services.

As first oil was then still expected in 2004, it was an ambitious contracting timetable. SNEPCO’s marine group undertook the process of specifying and procuring the necessary vessels to support Bonga. The subsea support engineer in the SNEPCO operations team began the process of specifying the ROV, diving and engineering support services that would be required.

Following the Malampaya model, where the FSV is primarily a methanol supply vessel and the ROV is deployed periodically to it from standby onshore, the Bonga FSV also was specified to handle methanol as well as the flushing and replacement of oil offtake hoses. As the contracts were developed for Bonga, it was recognized that in order to support the subsea maintenance tasks and to address what was a potentially large scope of future construction support work for the Bonga Phase 2 wells and other infield development opportunities, there would need to be significant enhancements to the vessel.

The capability of the vessel was therefore increased with the addition of a 100-ton heave-compensated crane and a 12-ton crane, both capable of deploying loads to 4,921 ft (1,500 m). In addition, a helideck rated for Super Pumas was installed, and also a HIPAP positioning transponder and uprated communications. Having the construction capability on the FSV avoids either undertaking these tasks from the drilling rigs or bringing in intervention vessels from the spot market. The vessel contracted is the Bourbon Emerald. The vessel was built in 2004, and initially worked as a supply vessel in the North Sea; it was modified as indicated above for this contract. The vessel is 4,200 dwt, 298 ft (90.7 m) overall length, 62 ft (18.8 m) beam, and 16 ft (5.8 m) maximum draught. The vessel has accommodation for 50 people and a clear deck area of 6,889 sq ft (640 sq m).

The ROV contract was written to cover the supply of ROV services, shallow diving services (required to inspect/maintain the FPSO and SPM) and associated engineering support. The nature of the anticipated tasks dictated that a powerful workclass ROV would be required. Following the tender process, a contract was awarded to Oceaneering International AG. Oceaneering provided a brand new Millennium Plus ROV (Millennium 25). This is a 225 HP ROV with twin manipulators and fiber optic umbilical and tether.

Circumstances of the tender process meant that the availability of both the FSV and ROV became critical activities in the run-up to first oil. The ROV was delivered from Morgan City, La., just in advance of Hurricane Katrina.

The conversion work on the Bourbon Emerald meant it could not be delivered in Nigeria until October 2005. To ensure that a potential Sept. 15 date for first oil could be supported, the ROV was initially mobilized to the Bourbon Hidra, which was also being supplied to SNEPCO as a FSV.

Intervention engineering
With the necessary equipment in place, an important part of the preparations was to ensure that fully detailed procedures were in place for what were considered to be the routine tasks. Rapid response would not be possible if procedures needed to be written for each job as they arose. The project team had already made a start by producing generic procedures for key tasks (using an earlier contract with Oceaneering Intervention Engineering in Aberdeen). These needed to be checked and revised to reflect the actual resources being used as well as amended to reflect the potentially live hydrocarbon status of the system. There were also some tasks that had already been undertaken by the project installation contractors, for which procedures were available.

The key intervention tasks on the Bonga subsea system were identified as:
• routine inspection;
• control module replacement (significant quality issues had been identified with the Bonga modules, and there was an ongoing program of recovery/rebuild and replacement);
• choke trim replacement (limited cleanup of wells to the rig led to concern that debris would block chokes);
• hydraulic flying lead replacement; and
• valve movement/override.
The future construction tasks for the vessel in support of Bonga Expansion Project were identified as:
• tree installation;
• THS installation;
• well jumper metrology and installation; and
• hydraulic flying lead installation.

With plans in place for the predictable tasks, the challenge was dealing with the out-of- the-ordinary issues that always develop. Bonga was faced with a couple of activities in the start-up process that have required a degree of innovation to resolve and needed the combined efforts of Shell’s global EP Technology team, the SNEPCO subsea team and their contractors to resolve.

Isolation valves
The water injection (WI) wells on Bonga were fitted with full-bore isolation valves (FBIV) downhole. These valves are opened by pressurizing and depressurizing the well, causing them to ratchet open. The pressurization is usually done by the drilling rig. At Bonga, however, the drilling rig had departed and was not expected back until 2006.

There was therefore a need to find an alternative methodology. It was recognized that the valves could be opened by pressurizing the well from the FPSO through the WI system. However, this would have the disadvantage of requiring each well subsequently to be shut in while further wells were pressurized in turn. Shutdown of WI wells was undesirable, as early build-up of water injection was a high priority and there was a potential for gas build-up at the tree during shut-in. In addition, two of the Bonga water injectors had been left with the FBIV valves open following injectivity tests. Instruments indicated that gas had migrated up past the tubing retrievable sub-surface valve (TRSSSV) and accumulated under the injection master valve (IMV). There was concern that when water injection commenced, the risk of hydrate formation was high. The subsea project team was therefore asked whether there was a way to remotely access the wells and undertake the operation of the FBIV using an ROV.

It was identified that for both tasks it should be possible to develop an interface to the top of the Christmas tree by modifying a tree cap. Using pumping skids mounted on the ROV, it would then be possible to apply the necessary pressure to open the FBIV by pumping raw seawater into the well or, alternatively, to bleed off accumulated gas and inject a hydrate inhibitor. A team in Houston worked with Oceaneering International to develop the skids to fit the Millenium 25 and to produce an ROV interface plate that could be fitted to a Vetco tree cap. Vetco Gray in Aberdeen worked on modifications to the tree cap while Oceaneering International in Aberdeen prepared detailed intervention procedures for the task. The subsea Intervention team in Lagos worked with operations to assess the isolation and well control issues for incorporation in the associated hazop and procedures.

Conclusion
The end result was a suite of equipment comprising two ROV-mounted skids, methanol/glycol storage tanks, interconnecting hoses and a pair of modified hydraulic leads that enabled a methanol supply to be directed from the main Bonga umbilicals via the ROV to the WI trees. The system was successfully deployed and was used to open four FBIV valves and undertook the remediation of the B5 and B8 wells, as well as the injection of tracer chemicals into two of the wells.

The SNEPCO subsea team in Lagos has established itself rapidly and is now able to provide competent and capable intervention support both to ongoing operations and forthcoming projects. Since first oil in November 2005, the level of subsea intervention required has fallen short of worst-case predictions. Due to the procedures and resources in place, however, we have been able to intervene quickly to replace two blocked chokes, investigate and identify two leaking HFLs, and replace one of them.