Flexible risers are key components of floating production systems for oil and gas. Flexible pipes are also required in many other applications, as hoses in drilling equipment, flowlines, etc. Due to low installation costs and the possibility for reuse, the flexible pipe is a viable alternative to a steel pipe also for static applications. The design procedure for response calculation of flexible risers is often based on simple regular wave analysis, i.e. a deterministic wave description. Unfortunately, this approach has been demonstrated to have significant shortcomings for tensioned steel risers, /1/, and this approach is not likely to give improved results for flexible risers. On the other hand, methods based on stochastic dynamic time domain simulations are the most sophisticated and accurate at present, /2, 3, 4, 5/. However, these methods are often prohibitive for general design analysis, due to extensive computer effort. Consequently, there is a specific need for more rational and consistent methods. This paper outlines a method based on stochastic dynamic frequency domain response calculations, in combination with a limited number of time domain simulations. A theoretical description of the frequency domain method is presented. The procedure is employed for estimation of lifetime and extreme response for a specific riser system.
stiffness properties at the static equilibrium configuration of the riser form the basis for subsequent dynamic analysis. The static analysis is based on a finite element model and a nonlinear incremental solution procedure. The formulation allows for large displacements and rotations. Cubic interpolation polynomials are employed for bending in the transverse directions, whereas axial displacement and torsional rotation are interpolated linearly. The tangential stiffness relation for each 3-dimensional element may be written as (equation 1 shown in paper).
