A framework model of the leg of a jack-up rig is analyzed using realistic loads according to appropriate standards (e.g. ABS, DNVGL). Then the diagonals of the legs are replaced by special hollow sections, so called shaped pipes. Shaped pipes are characterized by varying outer or inner diameter or wall thickness, respectively. Thus, the geometry of the tube section can be adapted to the stress distribution without the need of butt welding. It is figured out that with using shaped pipes, the framework can be optimized and the total weight of the structure can be reduced.
For the construction of the legs of a jack-up rig several thousand tons of hollow sections are used. The task in designing the framework of a jack-up leg is, that is has to withstand static and fatigue loads from dead and life loads of the hub, as wells as wind, waves and current loads during transit and in elevated condition. And for a mobile unit, the weight has to be as low as possible. This is why for the construction of the legs of a jack-up high strength steel grades are used. Another possibility to reduce rigs weight is to adapt the sections to the load distribution, as already done in bridge and mobile crane constructions for example. The problem is here, that circular hollow sections (CHS) of different diameters or wall thickness have to be welded together, which causes additional efforts and notches. This brings up the new sections of Vallourec, which are hot-rolled seamless CHS forged in special forms. For the so-called shaped pipes it is possible to vary both the wall thickness and the diameter over the length within one section.
In the following, a framework model of a jack-up leg is calculated and the results are compared to a leg with shaped pipes in the diagonals.
