Researches On Redundancy Strength Of Semi-submersible Platform

The main focus in this paper is on the redundancy strength of semi-submersible, the so-called redundancy strength is the strength when some of the platform components have been destroyed. In the service cycle of the platform, it will bear many kinds of load, such as environment load, permanent load; variable load, and various accidental load. In this cycle, the platform may get damaged. Evaluating its redundancy strength in damage condition is a very important theme. The redundancy strength is enough or not sometimes directly affect its life. The famous semi-submersible Alexander.Kielland collapsed just after one of its braces failure. The research object here is the 3000m deep semi-submersible of newest generation in our country ,which will be put into use in South Sea.The main work studied here are: ultimate strength of the semi-submersible pontoon; time variant ultimate of the pontoon considering corrosion; overall strength of intact platform in ultimate wind and wave condition; overall strength of platform in ultimate condition when a brace damaged.The overall strength calculation is processed following these steps: Firstly, 6 characteristic load cases are determined from the feature of the structure of the semi-submersible platform; Secondly, on the basis of design wave theory design wave analysis is made, from which design wave parameters are got; thirdly, load transfer analysis is carried out using the design wave parameters got before and wave load in each load case is known; lastly, through FEM analysis overall structure responses of the platform under wave loads are got, and evaluation of platform strength, in intact and damaged condition are made by comparison between allowable stress and the results.Another important part of this paper is about ultimate strength of pontoon. At first, nonlinear FEM is used to calculate the ultimate strength of intact pontoon in sag and hog condition; And then Weibull function corrosion model is chosen to estimate the corrosion process of the pontoon in its service cycle; At last ultimate strength of pontoon in different years are calculated considering corrosion influence. through the results of FEM calculation a formula estimating time variant ultimate strength of pontoon is proposed.