Informatics Publishing Limited

Bh. Nagesh ¹, I. N. Niranjan Kumar ¹, Manu Korulla ²

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

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.

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Sunny Verma , Kareem Khan , P. C. Praveen , Manu Korulla , P. K. Panigrahi

Abstract

Trimaran is the most promising hull form for naval vessels because of its excellent performance characteristics notably reduced wave making resistance, larger deck area, good sea keeping and ability to perform optimally in a range of speeds. In its most generic form it consists of a slender centre hull and two side hulls. High slenderness ratio, differential breadth to draft ratios, stagger/separation of the hull forms gives an edge over equivalent monohulls/catamarans in minimising wave making resistance. However it's furher possible to reduce the wave making resistance by means of advancements to the existing design which can include bulb & wave piercing forms, stern wedge/flap/interceptors, asymmetry in side hull configuration, lifting bodies etc. The present paper investigates the qualitative performance of trimaran hull form modified to various configurations using Shipflow.

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Shiju John , Kareem Khan , P. C. Praveen , Manu Korulla , P. K. Panigrahi

Abstract

Hydrodynamic model studies were carried out at Naval Science & Technological Laboratory (NSTL), Vishakhapatnam, India to estimate the resistance contribution of appendages. This paper discusses categorization of ship hull appendages from a hydrodynamic viewpoint. An attempt is made to qualitatively assess the resistance contribution of each of these categories of appendages to the total ship resistance. A relative appraisal of the resistance contribution of appendages on a naval surface combatant, a submarine, a tanker, a container, a dredger and a single screw ship is made to emphasize the large appendage drag on naval ships. Results obtained from hydrodynamic model experiments conducted at High Speed Towing Tank (HSTT) facility at NSTL on a fast displacement hull form used for naval applications (FDHFNA) are discussed at length as a case study. A criterion for rating appendages is proposed.

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