The purpose of present paper is to evaluate how the forces and moments acting on the cylinder or the cuboid payload change with the relative postures between the wave incident direction and the offshore crane payload, and to develop a PID controller for the payload to hold its posture in the presence of external wave forces. Aiming at this purpose, numerical results of regular wave interaction with a vertically suspending cylinder are presented first to validate the NWT model, and then a series of simulation experiments which offer results of the forces and the moments exerted by the regular waves on one fixed suspending cylinder and cuboid payload with different postures. With some certain posture, the suspending cylinder and cuboid payload suffer the minimal forces and moments. After that, the PID controller is raised and applied on a free heave decay cylinder for validation. Lastly, the same PID controller is applied on the cuboid payload subjected to wave forces. The results show the controller works well in keeping the payload with a certain posture. All of the present works provide the fundamentals for further research on safe control of offshore lifting or lowering.

INTRODUCTION

The offshore cranes, widely used to lift or lower payloads in the ever-increasing marine exploration and subsea resource exploitation, suffer from persistent disturbances induced by ocean waves. The large hydrodynamic forces could damage the payload or break the cable, which would further cause accidents and impair the safety of life and property(Veritas, 2011).

In order to lift/lower payloads on the sea safely and efficiently, the capability to estimate the hydrodynamic loads on payloads occurring in wave conditions is of vital importance.

For the static wave-structure interaction, many researchers have investigated this problem by using different methods, such as the potential theory based model(Bai and Taylor, 2007; Christou, Swan, and Gudmestad, 2008; Hunt and Baddour, 1981; Koo and Kim, 2007) or the CFD model which is based on the N-S equations(del Jesus, Lara, and Losada, 2012; Higuera, Lara, and Losada, 2014; Ji, Dong, Luo, and Soares, 2017; Kang, Lin, Lee, and Zhu, 2015; Lin, 2006). Both of them focus on the diffraction or the radiation around the cylinder or other shaped structure, and get the wave force and moment response in the frequency domain or the time domain.

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