An Investigation into the Failure of Pinion Holding Capacity, Joint Cans of a Leg Structure, and Preload Tank Capacity of a Jack-Up Rigby Using Finite Element Method

Abstract

The joint can assessment analysis has been performed to investigate the failure of the leg lattice structure of the legs of a Jack-up Rig which is being converted from Mobile Offshore Drilling Unit (MODU) to Mobile Offshore Production Unit (MOPU). The primary objective of this study is to check whether the existing joint cans of the legs of a jack up rig can reliably support the increased elevated load when converted as production platform.

The Finite Element Method is used to determine the stresses and strains in various joint sections of the leg lattice by modelling the entire rig structure in SACS modelling and FE analysis software which is extensively used in offshore industry. A detailed penetration and foundation stability analysis is subsequently carried out by using the leg reactions of elevated analysis accounting soil fixity. This present study addresses the overturning stability of the unit, preload requirements and the structural integrity of the legs and the leg holding system (pinions) under storm loading. It is found that the preload tank capacity and pinion holding capacity are in-sufficient to support the required preload reactions due to the increased load after conversion as a production platform and suitable remedial measures have been suggested by considering the overall overturning stability and based on site specific conditions and increased air gap between the water level and the bottom of pontoon of the rig.