The effect of sea induced motion on offshore process equipment

Abstract

The performance of offshore process equipment on floating production platforms may be reduced through imposed sea motion. Fluid sloshing inside primary separators and non-segregated storage tanks may lead to oil/water mixing. The aim of previous work into sloshing has been to prevent damage to LNG tankers and increase the stability of space rockets. Work into oil/water sloshing appears limited. A computer controlled motion simulator was developed to conduct experiments with two rectangular vessels filled with air, refined oil and water. Two single sinusoidal forcing motions were applied, pitch and surge at various amplitudes and periods. Additional experiments were conducted with combined forcing motions pitch/roll and pitch/surge. Air/water interface profiles were measured and analysed using a computer based data logging and processing system. Air/oil and oil/water profiles were recorded using high speed video equipment. Also studied were the effect of baffles in reducing interface amplitude and the effect of forcing on oil/water transfer. A linear theory was derived to predict natural frequencies of three fluid systems and a numerical model was developed to predict near resonant behaviour. Air/water experiments and numerical model showed a coupling of natural and forcing frequencies in the free surface frequency spectrum. Favourable comparisons were also seen between the numerical model and oil/water experiments. Additional experiments indicated that oil/water mixing is promoted by resonant forcing in an unbaffled vessel. The presence of baffles reduces interfacial breakup and hence reduces oil content of water.