Study on the failures of raw water tower structural joints when the air gap of the jack up rigs is increased

Abstract

It is quite common to convert the existing offshore installation from one form to another mode, depending upon the next charter requirement. 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 whereas rules call for the supporting structure to with stand the 100 years of wave and wind data in which case the wave height can be expected upto 14m and wind speeds of upto /exceeding 100 knots. As per rules 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 Raw Water Tower whose length also to be increased to meet the increased water depth and as such it will not be able to withstand the 100 years wave and wind criteria. This paper shows the study conducted on the Raw Water Tower (RWT) of a rig which is under conversion. The study is done by conducting the Global Strength Analysis. The Finite Element Method is used to determine the stresses and strains in various joint sections of the RWT by modelling the same in SACS modelling and FE analysis software which is extensively used in offshore industry. The structure is analysed as a discrete element model using beam elements. The stiffness analysis for the Raw Water Tower includes the computation of resultant stresses in members due to bending, shear, tensile & buckling stresses and checking it against the respective acceptable values.