Extreme Wave Effects on Deep Water TLPs - Lessons Learned from the Snorre A Model Tests

In the last few years there has been an increasing focus within the shipping and offshore industry on extreme wave events and abnormal wave conditions. Hurricanes Rita and Katrina last year confirmed the importance of these extreme waves for all types of offshore structures. When they are not properly taken into account in the design, they can threaten the integrity and safety of a platform. Based on model tests and simulations on the Statoil operated Snorre A, the present paper discusses the following issues:
• Why should we focus on accidental wave events?
• How should we analyse them?
• How do we carefully take into account the inherent randomness of extreme waves?
• What are the available measures we can take in the design?
• What are the lessons learned for deepwater TLPs in hurricane conditions?

The paper discusses these issues based on an extensive series of model tests with the Snorre A. The airgap, wave loads and platform response were measured. The TLP was subjected to a range of wave height-wave period combinations and long time durations of storms were used to get sufficient insight in the statistics of the extremes.

Further numerical simulations were carried out with a Volume of Fluid (VoF) method coupled to a time domain simulation model. The VoF method computes the hydrodynamic loads, which are subsequently applied as external force in the motion model. The numerical simulations give a lot of insight in the loading and response process of TLPs.