Summary
This paper describes the development of a novel design of smoothbore riser
for dry gas applications. It has been developed for use on the Statoil operated
Åsgard field, located approximately 200 kilometres off mid-Norway.
Shortly after startup of gas export from what was then the world’s largest
semi-sub, Statoil's Åsgard B platform, a serious noise and vibration phenomenon
was identified in the gas export system. This system consists of a manifold on
the platform topsides connected to parallel flexible risers linked to a subsea
Export Riser Base which feeds the gas into the 42-in. export pipeline to
Norway. The noise, essentially a high-pitched whistling, and resulting
vibration caused two fatigue failures of smallbore piping fittings on the
topsides manifold, leading to gas leaks on the platform. There were also
concerns related to the subsea piping. The source of the noise, in this dry-gas
system, was related to vortex shedding of the gas flow past the internal
carcass in the flexible risers.
Initially, two roughbore risers were installed; this was subsequently
increased to a total of four, including one spare. The increase to three
operating export risers had been planned from the start of the project to
reflect increased export volumes from Åsgard. However, while it was now
possible to manage the gas flow such that the noise and vibration was
controlled, this could not be achieved without unacceptable pressure losses
along the length of the flexible risers. This problem was related to longer
term operation of the Åsgard Transport System as the export volume through it,
and hence the required operating pressure increased with the inclusion of
export from new fields, such as Kristin.
After comprehensive work with export system monitoring, dynamic simulations,
and assessment of alternative export arrangements, Statoil invited Technip to
validate the concept of a flexible smoothbore structure for the Åsgard B gas
export riser system. This paper describes the background for, and special
measures implemented in, the design of the world's first smoothbore flexible
riser for gas export. This design is such that noise from the risers is
eliminated while, at the same time, a smoother internal bore significantly
reduces the pressure losses along the riser, allowing the full potential of the
Åsgard Transport System to be achieved. The first smoothbore gas export riser
was installed in late Autumn 2006 as a replacement for one of the existing
standard roughbore risers.
Hence, the new application of smoothbore riser technology to the novel gas
export riser design for Åsgard gives several benefits:
- Noise and vibration issues are eliminated.
- Pressure losses are reduced.
- Environmental benefits are achieved from the subsequent reduction in need
for compression.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
5 February 2007
- Meeting paper published:
30 April 2007
- Revised manuscript received:
22 August 2007
- Manuscript approved:
24 August 2007
- Version of record:
20 December 2007
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