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Going where no riser repair has gone before

Written by  Pål Magne Hisdal, Andre Midtun and Ivar K. Hanson Sunday, 01 October 2017 00:00

Connector Subsea Solutions and Hydratight completed the world’s first deepwater riser repair at 1250m depth for BP off West Africa. Pål Magne Hisdal, Andre Midtun and Ivar K. Hanson explain.

Image from the installation phase. Images from Connector Subsea Solutions.

The Greater Plutonio field offshore Angola has been producing since 2007. A single hybrid riser tower connects the field’s floating production, storage and offloading vessel to the subsea flowline and control systems.

Two, 4in gas lift risers had damage at the base of the hybrid riser tower, at 1300m water depth. To repair the damage, BP considered replacing the entire gas lift riser or local repair and bypass of the damaged section. The latter option was taken.

In the past, pipeline operators have been reluctant to execute deepwater repair of risers due to the complexity and lack of availability of solutions with track record.

Working with Hydratight, Norway’s Connector Subsea Solutions developed a solution to make remote repair of rigid risers a cost-effective alternative to riser replacement.

The solution comprises a novel midwater connector installation system that is temporarily fixed to the riser and used to align, install and activate the repair connectors, and a mechanism to ensure that the risers were not overstressed or damaged in any way, even for small pipe diameters. The repair connectors are equipped with flexible jumpers in order to bypass the damaged sections on the two 4in gas lift risers.

Work started on the design of the system in April 2015. After going through an extensive testing, onshore and in water, the new deepwater riser repair system was field-proven, by repairing the two Greater Plutonio gas lift risers. The project was completed in April this year.

The repair work involved pulling the risers out of the tower, cutting and then preparing the risers, using Connector Subsea Solutions’ ROV-operated deepwater coating removal tooling, and installing Hydratight’s repair connectors with flexible jumpers to bypass the damaged location. The deepwater coating removal tooling reduced complexity and operation time, compared to a water jetting system is significant.

The whole operation was executed at 1250m depth, about 50m above the seabed, utilizing only ROVs and crane lifting support from the subsea construction vessel.

For the project, we did not have the “luxury” of having the sea-bed as a support when operating the tools and equipment needed. The ROVs were “flying” the equipment to and from the repair location, while other equipment and infrastructure were secured by cranes from the construction vessels, in which case access under the leaning riser tower became a key design criteria.

Restraining system for the tie-in location

The connector installation frame and anchor gripper unit, onshore assembly. 

Once the repairs were completed, a restraining system was installed to support the tie-in location and to avoid any potential fatigue related issues over the remaining lifetime of the risers.

One challenge in developing the restraining system was that an adjacent 12in riser was the only structure available for affixing the tie-in location. However, this riser was already highly utilized and embedded in the riser tower. As a result, there was no access to install the structural restraint clamp.

The solution was to develop a bespoke Buoyancy Removal Tool that was used to mill away the buoyancy around the 12in riser to create a space envelope sufficient for subsequent coating removal and the remote installation of the structural restraint clamp. Challenges around this operation included the toughness of the glass/epoxy composite and machinability, limited relevant machining experience worldwide and optimizing tool rigidity versus lightweight for ROV installation.

Since there was no previous experience from deepwater buoyancy milling, and thus no existing data regarding the behavior of either tool nor buoyancy during such an operation, it was satisfying to witness the operation being performed as planned.

The development of the technology and the successful operation with a satisfactory repair was confirmed by BP’s Subsea Engineer Wadih Malouf: "Hydratight and Connector Subsea Solutions have proposed innovative solutions to a complex riser repair project. They prepared a robust testing program which provided confidence to successfully perform the repair on a live system. The delivery was underpinned by very good project planning and execution.”

Project execution and installation

The project scope called for development and delivery within an extremely short timeframe, with project start in April 2015 and equipment delivery in December 2016. Since most of the equipment was bespoke and novel solutions were applied, this was a major challenge.

Achieving the delivery milestones within the time and budgets were made possible using our operation model of seamless design execution between our three engineering locations in Norway, Bosnia and Croatia. This enabled effective design processes and mobilization of the full engineering force on joint tasks. Another key to execution is having product champions that follow the equipment supply process from concept development, through detail design, testing and finally supporting the operation of the equipment in the field.

BP’s Project Lead, Neil Forster, praised the work, saying: ‘’Hydratight and Connector Subsea Solutions provided an excellent technical solution to a very challenging problem. Project management and engineering was excellent throughout, with elements of the project delivered against a very tight time frame. The testing regime was well planned and managed and the tools and hardware worked as per design, achieving the functional requirements without re-work. This transferred through to the field, where the tooling and connectors performed very well, achieving first time leak tight connections on both connectors’’

Having a field proven riser repair system, the risk of executing such operations has been significantly reduced, and we already have several inquiries to use our deepwater riser repair system for similar operations.

However, no project is similar. The next project may have completely different challenges, but the experience and knowledge gained from this recent project will certainly be a valuable asset we bring with us into the next challenge.

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