The structural failures of jack-up rigs were mostly found in leg structures, and due to cracks in way of connections between tubular members of leg structures. Although, stress calculations of leg structures can be made by using "finite element techniques", it takes a lot of time to complete the calculations for the full cases of all working conditions because of many tubular members. Therefore, a simple method for the stress calculations for design purposes is strongly desired. This paper describes a simple stress calculation method of leg structures and shows a satisfactory results compared With the results obtained by a detailed calculation method.
A lattice structure assembled with tubular members is normally adopted for the leg structure in deep waters for reducing wave forces and current forces on structure to a minimum, whereas a pure cylindrical pile structure may be considered for areas with comparatively shallow water depth. The triangular column lattice structure mentioned above for a jack-up rig consists of 3 chords, horizontal braces and six diagonal braces. The strength of this structure is investigated here. There are two conditions of the rig for Which the leg structure should be investigated the condition where the leg is grounded to the sea bed, and the towing condition. For the towing mode, the leg is lifted up to the free surface. In the former case, the structure is subjected to the fluid forces due to waves, currents and winds, and loads due to the platform, leg weight and loaded weights.