Investigation on the Overturning Stability of a Jack up Mobile Offshore Production Unit by Site- Specific Analysis to Find a Solution to Avoid the Failure of Its Leg Lattice Structure
Journal Title: International Journal for Research in Applied Science and Engineering Technology (IJRASET) - Year 2016, Vol 4, Issue 10
Abstract
The site-specific assessment analysis has been performed on a three legged drill rig with spud cans 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 failure of legs is quite common because of the increased elevated loads for a MOPU and also being a fixed platform and cannot be regularly jacked up & down due to severe wave conditions. Unless a proper assessment and analysis is done, it is found dangerous to put the vessel into operation. The authors in this paper have enunciated a proper procedure for the assessment of failure and a system and correct analysis procedure has been shown by taking into consideration the overturning stability of the unit, preload requirements and the structural integrity of the legs and the leg holding system (pinions) under storm loading. The hull strength has been considered to be completely rigid since the jack-up MOPU under the study has been continuing to maintain the Class Certificate. The research has been done for the jack-up MOPU in elevated condition without giving consideration to the relative displacements with respect to the adjacent fixed platform structure with which the MOPU will be connected with a walkway bridge. 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 very cost effective solution has been identified of changing a portion of the leg with a high strength material and increased thickness to encounter for the failure at higher wind loads. Comparative studies have been done with existing rig being converted, after modifications as suggested and taking entirely new rig with higher cost, indicating that the proposed solution is very versatile, cost effective and time saving solution by meeting the fundamental axiom of finding the solutions to the yesterday’s problems with tomorrow’s technology.
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