Investigation Of Wave-payload Interaction Using OpenFOAM

With the policy of Marine Power Strategy and ever-increasing marine resource exploration,offshore cranes mounted on vessels and carry out lifting/lowering works have been widely used in marine operations.While working on the sea,offshore cranes suffer from persistent disturbances induced by ocean waves.During lifting or lowering,the payloads may be subject to large hydrodynamic forces,which could cause payload damages or cable breaks,this would further cause accidents and impair the safety of life and property.In order to lift or lower payload safely and effectively,the ability to model the interaction between wave and payloads is of vital importance.To investigate the interaction between the incident wave and the payload of the offshore crane in the process of offshore lifting or lowering,main works and contributions of the dissertation are as followed:Firstly,a three-dimensional numerical wave tank(NWT)is established with Open Source Field Operation and Manipulation(OpenFOAM)software and waves2foam library.Numerical results of the present model are compared with published data for validation.Numerical results of regular wave interaction with a vertically suspending cylinder are compared with published paper first,the results which including wave exerted force and wave free elevation in the time domain and corresponding FFT results in the frequency domain,show that the present model could correctly get the force and elevation when the payload is stationary.Then numerical results of wave interaction with a floating object,including linear motion and rotation radian of the floating object,demonstrates that the present model could correctly get the rotation and linear motion of the payload when the payload is dynamic.Secondly,investigation about how the force of the payload exerted by the wave change with the relative posture between the incident wave and the payload is given.Three different shapes of payloads which including a cylinder,cube,and cuboid are studied separately,all three payloads both take the situation when payloads rotating around three global axes individually into consideration,the cube and cuboid also investigate the situation when the payloads rotating around two global axes at the same time.A common conclusion is concluded here,for the rectangular payloads such as cuboid and cylinder could suffer drastically changed moment when rotating along the long side,the direction of the moment is same with the direction payload rotate about.The simulation results also show how the forces and the moments change with different posture angles.With some certain posture,the suspending payload suffers minimum forces and moments.All the numerical simulation results and conclusions provide the fundamentals for further research and safe control of offshore lifting or lowering.Thirdly,a PID controller based on the investigation how the force exerted by wave change with the relative posture angle is raised and applied in the OpenFOAMsoftware.Once the PID parameters and target position and posture information are given,the controller will restrain the payload to keep the target position and posture.To validate the present PID controller,the PID controller is applied for a free decay cylinder case,the cylinder will finally half be merged in the calm water after several free decays if there not any restraint,and the controller will speed the progress.After that the PID controller is applied for the floating object subject to wave,the floating object keeps the initial state with the help of the controller.Finally,a deep learning method is applied in the CFD domain,and a novel model to predict the wave exerted force of the payload is raised here.A large amount of time sequence data is obtained through the current NWT model,which including time-variant force,motion rotation information of payload.These data are treated as training data and provided to the GRU model of pytorch.The well-trained model is able to predict the force and moment of the payload.When given wave parameters,the rotation angle,linear motion and corresponding velocity and angular velocity,the force and moment payload suffer are predicted.