Estimation of Extreme Response and Fatigue Damage for Colliding Risers

Abstract

For design of deep-water riser arrays, consideration must be made of the possibility for mechanical contact between the different riser pipes. Both the anticipated frequency of collision and the resulting stresses in the pipes need to be estimated. Such an assessment need to cover a certain range of conditions regarding environmental loading and surface floater motions. The present paper outlines a procedure which admits the most “critical” conditions to be identified based on an iterative approach. For each “load case”, which corresponds to a certain combination of environmental actions and surface floater motion, the corresponding contact stresses are computed. Furthermore, the accumulated damage for each load case (referred to a certain duration) is estimated. The numerical procedure for external load calculation is based on Computational Fluid Dynamics (CFD). For a given riser spacing, interpolation is performed during the response simulation in a pre-established data base. Contact between pipes is checked at each time step by looping through the nodal coordinates of the Finite Element Mesh which represents the pipe geometry. By assembling the response and accumulated damage which correspond to all the different load cases, the long-term probability distributions and weighted damage can be calculated. The procedure is applied to a particular example riser configuration. The effect of varying current direction (relative to the symmetry axis of the floater and riser configuration) on fatigue damage is focused upon in particular.