Mechanical Responses and Stability of Jacket Platforms During Cutting in Decommissioning Operations

The jacket platform has been widely used in offshore oil and gas development during the past several decades and faces the problem of decommissioning now due to approaching the design life. During the decommissioning process of a jacket platform, cutting the pile chords is one of the most important steps for removing the jacket. In the process of cutting, the freedom of the bottom of the jacket increases, decreasing its stability and potentially causing structure damage or failure. In this paper, the influence of the cutting sequences (cross-circulation cutting and clockwise-circulation cutting), offshore environmental conditions, and the overall weight of the jacket on the mechanical responses of the jacket platform during the cutting operation was investigated by using the commercial finite element package, SACS. The numerical results show that (1) during the circular cutting process, there is a negative correlation between the unit check (UC) values of the diagonal leg chords: the UC value of the leg chord at diagonal positions decreases by approximately 10%, and the final round of cutting is critical because the jacket platform has a high risk of failure with the UC value being likely to exceed 1.0; (2) the UC value of the piles downstream is 0.2 or much larger than that of the piles upstream, which controls the stability of the jacket during the cutting process; (3) the UC value at the skirt pile of the jacket roughly increases linearly with the weight of the jacket.