Buckling Analysis of Offshore Jackets in Removal Operations

In analysis of the removal of offshore jackets an important failure mode is buckling. In current practice, a buckling check involves manual determination of the buckling lengths of each frame member. It is estimated that 5 to 10% of the man-hours in structural analysis of removal projects is spend on checking and correcting buckling lengths. Fortunately, an alternative method is available that does not require determining buckling lengths. In this paper it is shown how this method can be derived from the NORSOK standard for tubular steel frame structures. The method is demonstrated in a removal analysis of an offshore jacket. It is concluded that this method can be successfully applied.

Numerical Modeling of Launching Steel Jackets With Consideration of Water Entry Forces

In this paper, a numerical model describing launching of offshore jackets from barge is developed. In addition to commercial software’s, water entry forces on jacket members and an implicit Newmark solution technique are included in the model. The results are in general agreement with other numerical software’s available (SACS). Special attention is paid to the fluid forces acting on jacket and the importance of each one. Also it is observed that water entry forces on horizontal jacket elements are very significant and may locally govern the design of these members. This force is more important for horizontal slender members near the mud-line, which do not experience significant environmental loading in operating conditions. Therefore the water entry impact force with large magnitude can cause over-stress and/or ovalling. It is also observed that taking water entry forces in account modifies the jacket trajectory in little extent.