Disconnectable transfers critical to drilling vessel conversions

Early detection and treatment of corrosion can help the optimize safety, life cycle cost and uptime of new and converted offshore facilities. However, life-cycle cost approaches must align with the differing commercial drivers for the project and operational teams.

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Economic alternative to purpose-built FPSOs in harsh environments

Rob Anderson,FES International

Lower oilprices in recent years have resulted in fewer new oil fields where the services of drilling vessels are required. Many of these hugely expensive, often high-spec assets have been sitting dormant but in the new, more optimistic oil and gas environment operators are beginning to see the benefits of converting them to FPSOs.

There are a number of uses for a converted drilling vessel including:

• Well testing – to ensure an oil and gas reserve has economic viability

• Early production allowing for a revenue stream to be available while awaiting a longer-term solution (i.e. a platform) to be constructed

• Development of marginal oil and gas reserves where it is not economically viable to construct, install, and maintain major new offshore infrastructure

• Locations where technical and economic constraints have ruled out exploitation of a field’s full potential, for example, due to the marine environment

• A flexible, permanently moveable asset that can be quickly relocated to produce different oil and gas reserves.

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View of the disconnectable mooring system through the moonpool. (All images courtesy FES International)

Converting a drillship to an FPSO can be significantly more cost-effective than building a newbuild floating or fixed platform. Drilling vessels already contain most of the characteristics required for an FPSO such as a moonpool, dynamic positioning, oil processing equipment, and an offtake system. And the potential for speedy relocation allows producers to unlock substantial economic value far quicker than with permanent production installations. However, various factors must be considered for the conversion process, notably the requirements for connecting the vessel to, and transfer from, offshore pipeline infrastructure.

How does a DTS work?

Disconnectable transfer systems (DTS) were originally developed for use in the North Sea in 1999 to access marginal fields and were subsequently deployed offshore Australia and the Gulf of Mexico, both areas subject to hurricanes and cyclones. The configuration allows the vessels to quickly disconnect from the subsea infrastructure and leave the field until the weather event has safely passed. The DTS conversion consists of installing the systems in the drilling vessel’s midship moonpool. They are designed to be installed in a straightforward way without the need for major modifications to the existing structure.

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Shallow-water configuration.

The DTS supports various flexible risers and subsea control umbilicals from the wellhead, keeping the riser pattern static while allowing the vessel to weathervane unrestricted through 360°. Rapid and safe connection and disconnection of the flexible risers and control umbilicals, without any environmental issues, means that operators can work efficiently in challenging environments, with less danger to life. DTSs have also allowed certain vessels to leave the field for a major upgrade without the need for costly offshore repairs. This is because when disconnected, the DTS supports the subsea infrastructure above the seabed and is impervious to weather impact on the sea surface, dispensing with the need to dismantle subsea infrastructure for a disconnection.

The system works in both shallow water and at significant depth. The DTS can accommodate differing riser patterns and water depths, allowing for the re-use of equipment from one field to the next, continuing the active life of the converted drilling vessel to an FPSO.

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Deepwater configuration.

Helix conversion

In 2008, Helix Energy began converting theHelix Producer 1 into an FPSO for use on the Phoenix oil field in the US Gulf of Mexico. The company commissioned FES International to design and deliver a disconnectable transfer system that would connect to the side of the FPSO, with the dynamical positioning system maintaining the vessel’s position on the field without causing damage to the risers attached to the seabed oil well. In the event of an oncoming storm, the company was asked to ensure that the system would allow for a quick disconnection of the vessel from the risers, with the well closed off at speed so that the FPSO could leave its location and return once the weather had improved.

A controlled disconnect will usually take longer - around four to five hours - as flowlines are flushed and the buoy is lowered for safety. However, in an emergency, where an immediate response is needed, the DTS/turret can be quickly disconnected within minutes, isolating the flowlines, with no spillage of hydrocarbons into the surrounding waters. The DTS/turret swivel allows the FPSO to weathervane around the turret without risking damage to subsea infrastructure. The DTS incorporates a quick connect disconnect connector which allows for the safe and simultaneous disconnection of multiple flowlines and wellhead control umbilicals, all inspected and approved by the relevant regulatory bodies.

After FES International delivered the DTS/turret and the vessel was about to start producing oil on the Phoenix site, theDeepwater Horizon disaster hit the region. Because of the Helix Producer 1’s bespoke DTS/ turret system, it became a crucial part of the BP recovery operation as it was the only vessel locally that could quickly disconnect and travel to the Macondo site. •

THE AUTHOR

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Rob Andersonis Managing director of FES International, a subsea engineering firm that designs, manufactures and supplies subsea products to the oil and gas and offshore renewable energy markets.

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