Ultimate Strength of Offshore Structures (Mainly Jack-up Rigs)

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Based on the Idealized Structural Unit Method, an ultimate strength analysis method was developed for the safety check of earthbound offshore structures. By this method the behaviors of the structures reaching ultimate strength under various loading conditions were investigated with consideration of the effects of local rigidity and strength.The object structure was divided into such elements as (1) joint, (2) pipe between two adjacent joints and (3) substructures composed of joints and pipes.T-joint consisting of cord and brace was assumed as the basic joint. Systematic theoretical and experimental studies were carried out on the rigidity and strength. As a result, simple estimating equations for the rigidity and ultimate strength were formulated.With this information, a new idealized structural unit, tubular joint element, consisting of springs, rigid bars and beam element was developed for T, Y and K joints in order to efficiently treat the local rigidity and strength in the whole structure analysis.Idealizing the elastic-plastic large deflection behavior of a pipe, an idealized pipe element was developed. Also, a similar element was developed for dented pipes.Using the above three idealized structural units, ultimate strength analyses were conducted for the whole structure of a jack-up rig and a jacket rig. Obtained were the following conclusions. (1) When the whole structure undergoes shear collapse due to horizontal load, ultimate strength is attained by yielding of the joints or buckling of the braces under compression depending on relative strength between the joints and braces. (2) The joint rigidity greatly influences the whole structural rigidity and ultimate strength. In addition, performing analyses of ultimate strength of a jack-up rig under survival and punch-through conditions, the safety of the structures in these cases was checked.