Statics and dynamics of a tension leg platform in intact and tether damage conditions

Deb, Manas Kumar (1986) Statics and dynamics of a tension leg platform in intact and tether damage conditions. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (30MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

The aim of the present investigation is to study the static and dynamic behaviour of a tension leg platform in the intact condition and after partial or total loss of tether at any corner. The exercise involves formulating the statics and dynamics of column stabilised structures with taut moorings and implementing the same in appropriate computer softwares. Formulations are also made to calculate the equilibrium tensions after a loss of tether. -- The computer program for hydrostatics is capable of generating the standard hydrostatic information as well as locating the equilibrium configuration of the floating body under a given set of external static loads. This is used to assess the statical stability of the tension leg platform in damaged cases and compare the same with its various other configurations. -- Under the category of hydrodynamic analysis, two major types of approaches, namely 'frequency domain’ and 'time domain' are employed. In the frequency domain approach, wave amplitudes and all displacements are assumed to be small. Also, integration of the hydrodynamic forces is carried out over the mean wetted surface of the body. The wave excitation forces are calculated via a 'Morison type' formula and an iterative scheme is adopted to handle the nonlinear drag term. This analysis is used to compute the 'response amplitude operators' (RAOs) in intact and damaged conditions. Significant responses are also computed using these RAOs and Pierson-Moskowitz wave spectra for chosen wind speeds. In the time domain analysis, though assumptions similar to frequency domain analysis are made regarding the incident wave. However, no restriction is put on the motion of the body. The excitation and the reactive forces are calculated over the instantaneous wetted surface and at the displaced position. Nonlinear equations of motion are integrated using Adam's method. This analysis is used to obtain time-series data of motion and tension responses in order to study the variation of steady state amplitudes in intact and damaged conditions as well as transients following a loss of tether. -- A special study of the occurrence of Mathieu type dynamic instability in tension leg platforms is also undertaken. A formulation to calculate unstable combinations of wave height and frequency is developed based on Floquet theory for periodic systems. The effect of various pertinent parameters including loss of tether on instability is also investigated. -- Furthermore, a 1:100 scale model of a representative tension leg platform is designed and fabricated. Experiments are conducted in regular and irregular waves in both intact and damaged configurations and the experimental observations are compared with those predicted by the theoretical models. -- It is seen that a complete loss of tether at any corner can reduce the statical stability of a tension leg platform drastically. A loss of tether, in general, is found to increase the platform motions in vertical planes as well as the static and dynamic tensions in the remaining intact tethers. Total loss of tether at one corner induces snap loads in one or more corners. Depending on the wave height and frequency, these snap loads could be high enough to trigger further tether failures. With regard to Mathieu type dynamic instability, even a 25% loss of total axial stiffness of the tethers is seen to have insignificant effect on the occurrence of such instability.

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