Motion Response Simulation of Damaged Floating Platforms

This thesis describes a computer based method and a procedure to simulate the motion response of a damaged platform under wave, wind and current effects. The aim of the study was to develop an analysis procedure which could be a useful tool to designers and certifying authorities in assessing the safety of mobile platforms in extreme environmental and damaged conditions. The thesis begins by explaining the benefits of using floating structures in developing oil fields. Basic stability requirements for floating production vessels are summarised. Recent and past damage simulation studies in the literature are reviewed. Some information about the number of accidents involving floating offshore platforms operated world-wide is presented. A few of the disasters occurring in recent years are given as examples to emphasise the importance of the subject. The Morison approach and 2D source-sink distribution technique are reviewed, and calculations of wave forces acting on a semi-submersible are carried out in order to make comparisons between the two methods. Theoretical derivations of wave forces in the frequency domain based on the Morison approach are carried out in detail for a twinhulled semi-submersible. The development of computer programs based on both methods is summarised. A general method for calculating wave forces and moments on circular cylinders of offshore structures is derived. By using the developed method one can calculate the wave loading on cylindrical members of fixed or floating offshore structures orientated randomly in waves. This method also provides a basic tool for determining the wave forces and moments that a floating structure is subjected to as it experiences large amplitude oscillations in six degrees of freedom. A general method is established in this chapter to calculate the hydrodynamic loading due to the rigid body motion of the platform. The calculation of restoring forces is discussed: a detailed description of the methods used to calculate hydrostatic forces, mooring stiffness coefficients and wind forces is given in the appendices. The calculation of inertia forces and moments defined from Newton's second law is introduced as part of a general calculation procedure. The derivation and the solution of motion equations in the time domain are presented. Details of model tests carried out to validate the non-linear large amplitude motion calculation procedures are presented. A description of a circular twin-hulled semi-submersible model and the loading conditions is given. The test setup and instrumentation are presented briefly. Test procedures for inclining, natural period and motion tests in waves are discussed. Methods of analysis of motion response measurements in six degrees of freedom in intact, transient and damaged conditions for head and beam seas are given. The results of motion response measurements are presented in time histories. In order to validate the numerical prediction procedures and the software based on these procedures, the physical test conditions are simulated numerically and a comparison of test results with numerical predictions is presented. Simulation studies based on the non-linear motion equations are presented with the aim of providing comparisons to illustrate the effects of non-linearities in wave and motion induced forces. A summary of the systematic study carried out to illustrate the effects of non-linear terms on the solution of the motion response equations is given. The results of the parametric studies to investigate the effects of flooding rate and of size of damaged compartment on motion response characteristics are also discussed. The other aspects of roll motion such as the effects of non-linear drag force, first order wave elevation, different wave heights and GM's, and non-linear added mass and damping forces on the motion behaviour and the steady tilt of semi-submersibles are investigated. The variations of GM and GZ values as a function of heel angle are also presented