Breaking Wave Impacts on Platform Columns: Stochastic Analysis and DNV Recommended Practice

To predict the characteristic impact pressure due to breaking waves on platform columns corresponding to an annual exceedence probability of 10⁻⁴ model test data with the Sleipner A gravity based structure (GBS) are subjected to a stochastic analysis. The analysis is based on the environmental contour line approach. In addition, the procedure recommended by Det Norske Veritas (DNV) for calculating shock pressures due to breaking waves is used for comparison. Time histories of pressure and wave elevation for the most severe measured impacts show that the measured pressures are induced by breaking waves. However, a difference between the resulting characteristic impact pressures based on the two approaches can be observed: The stochastic analysis results in much higher pressures than the approach recommended by DNV. These findings are supported by the analysis of data that were collected during model tests with the Gjo̸a semi-submersible and the Snorre A tension leg platform (TLP), where the difference between the results was rather large as well. For the semi-submersible and the TLP, one reason for the difference is bias in the fitment of the stochastic model. Furthermore, dynamic amplification effects in the force sensors have to be considered. However, this bias is less significant for the GBS and dynamic amplification effects are not present since different force sensors were used. For all three model tests, an important source for the different impact pressures is the size of the force sensor area, which varies between 2.25m² and 10.89m². Large areas may smoothen the pressure whereas small areas are overrating the impact. Further model testing is required to clarify this effect. If the difference is still present, the recommendation of DNV has to be altered to ensure a reliable prediction of the characteristic impact loads.