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

在理想结构单元法的基础上，提出了一种用于陆上海洋结构安全校核的极限强度分析方法。用该方法研究了考虑局部刚度和强度影响的结构在不同荷载条件下达到极限强度的行为。将目标结构划分为(1)接头，(2)两个相邻接头之间的管道，(3)由接头和管道组成的子结构。假设由拉索和支撑组成的t型节点为基本节点。对其刚度和强度进行了系统的理论和实验研究。从而建立了简单的刚度和极限强度估算方程。在此基础上，针对T节点、Y节点和K节点，提出了一种由弹簧、刚性杆和梁单元组成的管状节点单元，以便在整体结构分析中有效地处理局部刚度和强度。将管道的弹塑性大挠度特性理想化，建立了理想管道单元。同样，一个类似的元件被开发用于凹痕管道。利用上述三种理想结构单元，对自升式钻机和夹套钻机的整体结构进行了极限强度分析。得到以下结论。(1)当整个结构在水平荷载作用下发生剪力倒塌时，根据节点与支撑之间的相对强度，通过节点屈服或支撑受压屈曲来获得极限强度。(2)节点刚度对整体结构刚度和极限强度影响较大。此外，对自升式钻井平台在生存和穿孔条件下的极限强度进行了分析，验证了这些情况下结构的安全性。

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.