Statistics of bending moment in the splash zone of a tensioned riser

Abstract

Marine drilling riser extends from the drilling platform to the sea bottom, it experiences loads and motions from the environment and the vessel such as wave and current. In order to avoid the ultimate state failure, extreme loads should be correctly simulated. This is the main purpose of this thesis, to find out the extreme bending moment for a tensioned top riser at design lifetime.

For simplicity and practical purpose, hydrodynamic loads in tensioned riser are calculated by Morison's equation with acceptable accuracy in engineering in the wave zone. As it is know that the responses of tensioned riser are non-Gaussian due to the nonlinearities introduced by the hydrodynamic loading. For nonlinear system, full long term dynamic analysis can provide a relatively accurate result. However, it is almost impossible for complicated systems, since it is so timeconsuming for computers even nowadays. Hence simplified long-term statistic (SLTS) method proposed by professor Larsen is studied in this work to simplify the extreme estimation of slender structure.

The main idea of SLTS method is to find a design storm that the extreme responses of a structure in its design lifetime can be obtained from this sea state, instead of performing full long term time domain dynamic analysis. The design storm is found in an approximate long term response analysis, establish by the results from frequency domain analysis. The estimation of maximum bending moment can thus perform only in a short term simulation, since the design storm is expected to cause the maximum bending moment in lifetime.

Simulation uncertainty is an important factor that influences the accuracy of the extreme response estimation. In this thesis, uncertainties such as simulation length, number of data in tail function and phase angels were studied.