Construction of Numerical Simulation Model and Behavior Evaluation for Slewing Motion of Single Point Moored Hull

Abstract

The tandem offloading operation is widely used in offshore oil and gas production industry. This system can ensure higher allowable weather condition than the side by side operation. But the slewing motion of the shuttle tanker and increasing of hawser tension induced by the motion affect availability of the operation.

Previous studies about the slewing motion mainly treated about the dynamic stability analysis of the single point mooring system. However, specific values concerning the motion and hawser tension are needed when evaluating the availability of the offloading operation. The purpose of this study is to construct simulation model that can estimate range of the slewing motion and the value of hawser tension under complex environment such as wind, wave and current. In addition, understanding on the behavior of the shuttle tanker under this complex environment is also the purpose of this study.

Model test and simulation were conducted to achieve the above purposes. A membrane type LNG carrier was assumed as the shuttle tanker. Production and offloading vessel like a FPSO or FLNG was omitted to simplify the problem. The effect of tug boat’s assistance was simply modeled by using the thruster attached at stern of the carrier.

As the result of the comparison between experimental and numerical results, good coincidence was confirmed in the situation that without astern force and all environmental forces come from head of the carrier.

Parametric studies were conducted using the constructed numerical simulation model. All environmental forces came from head of the LNG carrier and astern force was not generated. The result shows that increasing of wind velocity impact on the hawser tension. And it also shows that increasing of current velocity and hawser length impact on the slewing motion of the LNG carrier. A noteworthy result is that the motion was also increased at no current velocity. The maximum value of the motion exceeded the operational threshold at this condition. These information will be useful when evaluating availability and improving the offloading operation.