Wave Drift Forces On Two Ships Arranged Side By Side In Waves

To economically produce and process the natural gas in remote offshorelocations, an exploitation system of Floating Production Storage and Offloading(LNG-FPSO) is considered and its performance in a severe natural environmenthas been studied. When LNG in storage tanks is offloaded to a LNG carrier, theLNG carrier is usually moored side by side to the FPSO due to relatively easyoperation of offloading. However, when two floating bodies are situated side byside with relatively small gap, hydrodynamic interactions between two bodies areexpected to be large and complex, which must be taken into account in the designof a mooring system. Owing to hydrodynamic interactions, two ships may collideand large repulsion and drift forces may be exerted, resulting in damage of themooring system. To evaluate hydrodynamic-interaction effects on oscillatory andsteady forces, accurate numerical computations must be performed not only forhydrodynamic forces and wave-induced motions of the first order in theincident-wave amplitude but also for time-averaged steady forces of the secondorder. The latter steady forces are called the wave drift forces and can becomputed only from quadratic of the first-order quantities. Accurate compute thesecond order wave forces with high efficiency for multi-body is a hot topic amongthe hydrodynamic research field in the world. The main contents of this paper are:1. To validate the developed code, the source distribution model, namedHess-Smith method, is used to compute the added mass of single body ininfinite fluid domain. The numerical result is coincided with the theoreticalvalue. I also analyze the influence of the result on the added-mass-coefficient,which is brought by the distance of two single objects in the infinite fluiddomain. 2. About the linear hydrodynamic problem of floating bodies in waves, the hydrodynamic coefficients of floating single body is computed and compared well with the published result [2]. The hydrodynamic interaction effect by two floating bodies distance is analyzed and discussed.3. It is well known that two different methods are available for computing the wave drift forces: the near-field method based on the direct pressure integration and the far-field method based on the momentum-conservation principle. The near-field method gives individual forces on each ship but computation are rather complicated. On the other hand, the far-field method is relatively easy to compute but gives only the total force on all ships that are included by a fictitious vertical circular cylinder located far from the ships. Since the prediction of the steady forces on each ship is prerequisite in the present study adopts, the near-field method must be used. Hydrodynamic interactions are studied between two floating bodies situated side-by-side and closely as a model of LNG-FPSO system. Main interest is placed on the wave drift forces which are of second order in the wave amplitude. To solve the first order hydrodynamic problems with hydrodynamic interactions taken into account, a linear boundary element method is applied to the wetted surface of two ships. The second order drift forces on each ship are computed by the near-field method based on a modified formula of the force on a floating body in waves. Numerical calculations are performed for the side-by-side arrangement of a rectangular barge and a modified Wigley model. The numerical results are in favorable agreement with the results obtained by the high order boundary element method.