Fatigue Challenges Facing FPSO in the North Sea

The fatigue scenario facing an FPSO in the North Sea is way more demanding than in most other ocean regions. This creates challenges in projects such as converting tankers into production units.

“The weather situation also varies in the North Sea, but in general you can say conditions are more extreme on the Norwegian Shelf than in most other ocean areas – perhaps with the exception of Canada and the Shetland Isles,” says the technical manager at Inocean, Thomas Eckey.

“In the Gulf of Mexico or on fields offshore Africa the norm for significant wave height (the average of the top third largest waves measured in a given period) is 7-8 metres. But it’s not uncommon in parts of the North Sea to experience a significant wave height of 16-17 metres. Added to that, extreme waves are more frequently generated in the North Sea. We are well experienced at working to ensure FPSOs achieve a desired life expectancy under such strains. A vessel uses up its life expectancy much quicker in the North Sea than in calmer waters,” asserts Eckey.

Stringent demands

Inocean is a stickler when it comes to hulls in North Sea conversion projects. Because of the punishing climate and weather, the authorities, oil companies and classification societies require particularly strict safety margins and detailed documentation from FPSO owners. There should be no cracking or damages caused by the stresses of operating offshore, whether it be for 10 or 20 years operation.

This puts demands on the conversion candidate and the condition it’s in. Inocean has developed tools and methods for identifying the vessels that are best suited for conversion and which give a firm foundation for estimating necessary reinforcements and repairs. The next step is goal-oriented design work combined with detailed and advanced analyses – to ensure that specifications are met and that the conversion job is no more extensive than necessary.

Advanced analyses

“In our work we try to minimise the stress conditions a hull is subjected to in the harsh North Sea,” says Eckey. “Adding more steel isn’t necessarily the solution. At worst this can lead to inferior details and a shorter life expectancy. Our efforts in such cases are channelled into designing more sophisticated solutions. This requires advanced analysis work. Which is something we excel at after doing the math on more than 20 FPSOs, many of them intended for operations in the North Sea,” says Eckey.

[mappress]

Shipbuilding Tribune Staff, June 13, 2012; Image: bp