Estimating Distributions for Extreme Response of a Catenary Riser

The paper deals with the challenge of predicting the extreme response of catenary risers, a topic of both industry and academic interest. Large heave motions introduced at the upper end of a catenary riser can lead to compression and large bending moments in the region immediately above the touch down area. In the worst case, dynamic beam buckling may occur. Results from long nonlinear stochastic simulations of many seastates with varying environmental and operating conditions may be combined to describe the long-term response statistics of a nonlinear structure such as a catenary riser (So̸dahl, 1991). However, this theoretically straight-forward approach is very demanding computationally and ways to limit the extent of nonlinear stochastic simulations are therefore sought. Previous work by the authors (Passano and Larsen 2006) focussed on understanding the riser behaviour in extreme, low-tension response and on establishing suitable strategies for concentrating the nonlinear simulations to where they are most useful. The research presented in this paper is a continuation of this work. The clear trends found between the prescribed axial velocity at the upper end and response quantities in the touch down area (TDA) are used to estimate individual response values and complete samples of response minima / maxima. A method for estimating the underlying response distributions is also presented.