Stochastic analysis of Workover Risers

Abstract

The goal of this thesis is to investigate the properties of dynamic analysis of slender, top tensioned risers when using both regular and irregular waves. The goal was to discover properties in the response that correlates to a parameter that can be found in both methods. The approach is to use a model of a riser in open water connected to the sea bed, and top tensioned by a semi submersible rig subjected to the dynamic load of waves and currents.

The theories that hold the basis of dynamic analyses using finite elements are outlined, and different methods of solving the dynamic equilibrium equation are discussed. Relevant wave theories and their statistical properties are investigated and outlined, followed by a clear methodology for performing the case study.

The data from the case study is based on a large number of irregular analyses, and one regular wave analysis for each sea state. The extreme values is extracted from the irregular simulations and fitted to a Gumbel extreme value distribution. The distributions are extrapolated to return periods of 1, 10 and 100 years, and compared to the extreme values from the regular wave analyses.

The main results from the case study are:

- The extreme response of an irregular simulation does not seem to correlate with the largest wave height in the simulation.

- The bending moment seems to correlate weakly to the displacement and velocity of the rig.

- The bending moment seems to correlate well with the displacement of the riser pipe. More in surge and pitch than in heave.

- The bending moment seems to correlate well with the velocity of the riser pipe. More in surge and pitch than in heave.

The results from the case study are used in a discussion on how to fine tune a regular wave analysis to be used in a consistent way to define the safe operation limitations for a top tensioned work over riser.