Extreme responses of flexible risers: Fid-Bau Portal

Extreme responses of flexible risers

Trim, A.D.

AbstractSeveral approaches to predicting long-term extreme responses of offshore structures are considered, including development of the averaged long-term peak distribution and variations on a design storm approach. Assessments are made for characteristic responses having a linear or quadratic dependence on wave height and with Rayleigh or exponentially distributed peaks. The quadratic response is found to be considerably more sensitive than the linear response to extremes of wave height, and exponential peaks produce significantly greater extreme responses than Rayleigh peaks.A series of time domain simulations is carried out for a Lazy-S flexible riser, from which response statistics, including extreme responses, are estimated. It is concluded that a three-parameter Weibull distribution is significantly more accurate than a Rayleigh distribution for representing the short-term response peaks, especially for more energetic seas where the response distributions tend to decay slowly. It is also concluded that allowing for the total duration of different storms throughout the projected design life gives considerably improved accuracy compared with the assumption of three hour storms or the use of simulation maxima. Estimation of extremes on a storm-by-storm basis is a convenient alternative to use of the averaged long-term peak distribution.