Abstract
A novel method is introduced to determine the long-term extreme value distribution of variables with an associated short-term distribution. The method is applicable to e.g. the extreme value with a return period of 10.000 years for wave height, crest height of waves as well as wave-induced response and loads on offshore wind turbine monopiles. The extreme values are determined by a combination of a joint probabilistic description of the met-ocean environment and of a recently developed database of fully nonlinear wave kinematics computations. For two benchmark monopile structures, representative for the conditions at Dogger Bank and the German Bight in the North Sea, the one-hour max value of crest height, inline force and overturning moment are computed via the Rainey slender-body force model. Convolution with the joint probability of the sea state parameters in the nondimensional space of wave steepness and Ursell parameter leads to the marginal probability distribution for these parameters up to a 10.000 year return period and properly accounts for the uncertainty of the extreme value parameters in also the short-term distributions. It is found that the best extreme value fit is generally obtained with a third-order polynomial fit within the space of steepness and Ursell parameter. Compared to the Forristall crest height distribution, the inclusion of full nonlinearity leads to larger crest heights at low return period levels. However, due to its realistic treatment of the breaking limitation, lower crests are predicted at the 10.000 year level for the two positions considered in the North Sea. With further verification of the wave kinematics in combination with load models and a thorough comparison to present engineering practice, the proposed methodology provides a robust future solution for directly estimating extreme value distributions of loads and response of offshore wind turbine monopiles.
Highlights
Offshore wind turbines may collapse or be damaged beyond repair due to excessive wind or wave loading
The wave load design format used in these codes are rooted in the deepwater oil and gas industry and is based on either nonlinear regular waves or linear representations of irregular waves (HSE RR087 [3] )
An underlying assumption in the codes is that there is a deterministic relationship between wave height and wave load
Summary
Offshore wind turbines may collapse or be damaged beyond repair due to excessive wind or wave loading. This is usually expressed in terms of an annual failure probability. The largest load is assumed associated with the highest wave and the long-term distribution of wave loading is deduced from the long-term distribution of individual wave heights. The kinematics of these extreme waves is usually represented by a single, nonlinear stream function wave [4]. The acceptable probability of failure is supposedly achieved by applying partial load
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