Structural Condition Assessment Of A Semi-submersible Platform Subjected To Wind And Waves

As a main equipment used for deep-water oil and gas exploitation,the dynamic responses of a semi-submersible platform are always complicated under the complex ocean environmental conditions,especially random wind and waves.The structural strength and fatigue damage of key points will immediately affect the life-cycle safety of the platform.In order to guarantee the operating safety and reliability of the platform,it is vital to evaluate the dynamic response of the platform under the random wind and waves.Taking a 6^th generation semi-submersible platform as a study object,this paper studies the theoretical and numerical simulation methods for random wind and waves and the dynamic response of the platform.The in-depth investigations are conducted on the dynamic effects of random wind and waves on the motion response,structural dynamic response and fatigue damage.The main contents in this paper are summarized as follows,The theorectical and numerical methods for simulating random wind-wave are studied.The energy transferring mechanism of the wind generating waves is studied for depicting a relationship between the average speed of random wind and the significant wave height of random waves.Power spectral density functions are utilized to establish and describe the stochastic process of wind and waves.Three new numerical simulation methods are proposed,namely the modified Fourier method,the mixture simulation method and the improved mixture simulation method,improving the disadvantages of two traditional numerical simulation methods-the weighted amplitude wave superposition method?WAWS?with much time cost and the autoregression method?AR?with less accuracy.Thus,the present study can provide the accurate and efficiency numerical simulation methods for simulating random wind-/wave-time history curves.The wind field and wave field analyses of a semi-submersible platform are conducted.Firstly,the wind field investigations of the platform structures above the still water surface are carried out to reveal the regulations of wind pressure distribution and resultant wind force under different wind directions and platform inclined angles.And the time series of wind pressures on the structural surfaces are extracted from these anlyses.Then,the wave field investigations of the platform structures below the still water surface are investigated to explore the effects of the random waves on the towing performance and hydrodynamic performance of the platform.And the time series of wave pressures on the structural surfaces are extracted from these anlyses.The structural dynamic responses of a semi-submersible platform under random wind and waves are studied.A parametric modeling method for different types of the semi-submersible platforms is developed by the formex algebra and automatic replication technique.Then a combination of ANSYS/APDL and VC++package is applied to build the finite model of this platform.A series of structural dynamic response analyses are conducted by APDL to explore the relationship between structural stress and wind-wave incident angles and the relationship between the structural stress and the platform inclined angles.Then,the effects of a damaged horizontal connection on the dynamic structural responses are studied further.The fatigue damage evaluation of a semi-submersible platform under random wind and waves are conducted.Introducing the modified rain-flow counting method and multi-sea-state combination analytical method into the traditional time-domain analytical method?TDA?,the improved time-domain analytical method?ITDA?is proposed to depict the effects of wind-wave incident angles and platform inclined angles on the structural fatigue damages.This analytical process also demonstrates that ITDA is a reliable,accurate and efficient method.Then,integrated with sub-modeling technique,the fatigue damages of the structural key zones are further investigated to express the fatigue damage distribution on the key zones more finely.An integrated software for structural anslyses of a semi-submersible platform is provided with the independent research and development.Considering the above-mentioned specific problems for the structural analyses,this software is developed by the VB6.0 invoking MATLAB,ANSYS/Fluent and ANSYS/APDL,etc.It aims at modeling,simulating the external loads and conducting structural analyses under wind alone,wave alone or combined wind and waves,which is wished to improve the calculating and design efficiency of different types of the semi-submersible platforms.