Wave-in-Deck Impact Load Measurements on a Fixed Platform Deck

Abstract

New methods for estimation of extreme wave crest heights have resulted in an increase of the estimated 10,000 year crest height. At the Norwegian Continental Shelf this increase is typically 2 to 4 m, resulting in a crest height of 22 m to 24 m in the Central & Northern North Sea and the Haltenbanken area. As a result several fixed platforms designed prior to 2000 may experience negative air gap if being hit by the 10,000 year wave crest height. Numerical methods have been used for assessing wave-in-deck impact loads. The model tests discussed in this paper were conducted to be used as verification of the numerical codes. For the model tests two sea states along the 10,000 year contour line were considered. Several 3-hour (full scale time) realizations were calibrated in order to capture the natural variability of the most extreme crest heights. For wave deck impact problems, one is merely interested in the few very large wave crests out of a 3-hour simulation. A more efficient test scope would, therefore, be to generate only the largest wave groups of the realizations. For this reason the most extreme crest(s) per sea state were identified and most wave-in-deck tests were conducted by generating only the part of the time series containing the large crest(s). The wave calibration results were discussed in a previous paper, see [1]. For the wave-in-deck model tests, an existing North Sea jacket was built at scale 1:60 and instrumented in order to measure the global loads on the platform deck independently from the loads on the jacket itself. In this paper the model test setup as well as the measured wave-in-deck impact loads are discussed and compared to a simplified load prediction model. The presented results show that the simplified loading model, with wave properties based on Stokes 5th order wave theory, underestimates the measured horizontal deck loads.