International Journal of Innovative Research and Development

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Study on the Failures of Leg Lattice Structure when Offshore Drill Rigs are Being Converted from Mobile Offshore Drilling Units (MODU) to Mobile Offshore Production Units (MOPU)


Affiliations
1 Department of Marine Engineering, Andhra University, Visakhapatnam, India
2 G. Naval Science and Technological Laboratory, Visakhapatnam, India
 

The Rig Owners intend to convert the age old offshore drilling rigs which are basically designed for drilling of oil and gas (called as Mobile Offshore Drilling Unit - MODU) into Mobile Offshore Production Units (MOPUs). While doing so, the important element to be noticed is the air gap between the water line to the bottom of hull vis-à-vis the operating depth at site specific conditions. The air gap which normally increases compared to when the same has been used as a MODU, due to the reason that it is a fixed platform and regular jacking operations cannot be performed whereasrules call for the supporting structure to withstand the 100 years of wave and wind data in which case the wave height can be expected upto 14m and wind speeds upto 100 knots, the bottom of hull should be elevated to a height of at least 1.5m more than the highest wave that can occur including the surge, swell etc. In such a conversion the important parameter that is being missed out is the lack of leg strength that will not be able to n withstand the 100 years wave and wind criteria. This paper shows the study conducted on the failure of leg lattice structure of the Jack-up Rig Legs by conducting the Global Strength Analysis.

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. The analysis have been done on the leg structure of a three legged drill rig with spud cans which has been undergoing conversion process from Mobile Offshore Drilling Unit (MODU) to Mobile Offshore Production Unit (MOPU) for installation at Mumbai High. The failure analyses have been done for two different locations of installations with different water depths and soil conditions with 25 years wind data, 50 years wind data and 100 years wind data at the site specific conditions.


Keywords

Jack up Rig, Mobile Offshore Drilling Unit (MODU), Mobile Offshore Production Unit (MOPU), Stress, Displacement, Chord Pipes, Bracing Pipes, Leg Penetration, Air Gap, Wave Theory.
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  • Study on the Failures of Leg Lattice Structure when Offshore Drill Rigs are Being Converted from Mobile Offshore Drilling Units (MODU) to Mobile Offshore Production Units (MOPU)

Abstract Views: 140  |  PDF Views: 1

Authors

Bh. Nagesh
Department of Marine Engineering, Andhra University, Visakhapatnam, India
I. N. Niranjan Kumar
Department of Marine Engineering, Andhra University, Visakhapatnam, India
Manu Korulla
G. Naval Science and Technological Laboratory, Visakhapatnam, India

Abstract


The Rig Owners intend to convert the age old offshore drilling rigs which are basically designed for drilling of oil and gas (called as Mobile Offshore Drilling Unit - MODU) into Mobile Offshore Production Units (MOPUs). While doing so, the important element to be noticed is the air gap between the water line to the bottom of hull vis-à-vis the operating depth at site specific conditions. The air gap which normally increases compared to when the same has been used as a MODU, due to the reason that it is a fixed platform and regular jacking operations cannot be performed whereasrules call for the supporting structure to withstand the 100 years of wave and wind data in which case the wave height can be expected upto 14m and wind speeds upto 100 knots, the bottom of hull should be elevated to a height of at least 1.5m more than the highest wave that can occur including the surge, swell etc. In such a conversion the important parameter that is being missed out is the lack of leg strength that will not be able to n withstand the 100 years wave and wind criteria. This paper shows the study conducted on the failure of leg lattice structure of the Jack-up Rig Legs by conducting the Global Strength Analysis.

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. The analysis have been done on the leg structure of a three legged drill rig with spud cans which has been undergoing conversion process from Mobile Offshore Drilling Unit (MODU) to Mobile Offshore Production Unit (MOPU) for installation at Mumbai High. The failure analyses have been done for two different locations of installations with different water depths and soil conditions with 25 years wind data, 50 years wind data and 100 years wind data at the site specific conditions.


Keywords


Jack up Rig, Mobile Offshore Drilling Unit (MODU), Mobile Offshore Production Unit (MOPU), Stress, Displacement, Chord Pipes, Bracing Pipes, Leg Penetration, Air Gap, Wave Theory.