Analysis of wave exciting forces on a floating rectangular barge at zero forward speed

Abstract

Surface waves have significant effects on the hydrodynamics of offshore bodies or structures on a fluid of finite depth. Wind, moving vessels, seismic disturbances of shallow sea floors (tsunamis) and the gravitational disturbances of the sun and the moon are factors responsible for generation of waves. Their influence is very crucial in engineering analysis, design, and optimization. Many researchers in the field of hydrodynamics have analyzed the effect that surface waves have on bodies with cylindrical cross section. However, little has been done on structures that have rectangular cross-section especially on incompressible fluids. This study, therefore, focused on the analysis of wave motions acting on rectangular offshore structures. The analysis of surface waves characteristics arising from incident wave potential was carried out. This is because the ability to predict offshore structure behavior begins with the study of the nature of ocean in which the vessel operates. These characteristics included: incident wave potential, incident wave elevation, wave velocity and acceleration. The influence of water depth on the wave characteristics was also investigated. Consequently, the wave exciting force resulting from the interaction of such waves on a rectangular floating barge was investigated and analyzed. Boundary integral method together with Green functions in its series form was used to obtain the radiation potentials. These radiation potentials were used to solve diffraction and the Froude-Krylov forces for the rectangular floating barge. The forces aforementioned were used in the calculation and the analysis of wave exciting forces. Research finding shows that, the vertical wave acceleration and wave velocity were quite high leading to high vertically induced motions. Acceleration are used in the determination of cargo loads and also in the predictions of sea sickness. Change in water depth was also found to have adverse effect on the wave properties and consequently on the hydrodynamic forces, this was in accordance with shallow water effect. For the surge exciting forces it was evident that the forces increased up to a certain level and then they were radiated away to avoid interference at far field. Moreover heave wave exciting forces was found to be inversely proportional to the wave frequency. Heave motion is the limiting factor in drilling of oil. To reduce the heave forces acting on a body it was observed that there was need to increase the distance of the body from the free surface, that is, the draught length. The results obtained would be of great importance to Kenya in the prospects of exploitation of oil on the southern coast and indeed also in the sand harvesting process that is taking place at the Kenyan coast for the construction of the standard gauge railway.