The growing interests of mankind towards offshore activities in order to colonize the ocean mostly for space, food and energy, with more innovative ideas and greater challenges than before are increasingly driving the research towards greater height.
Safe operation of offshore crane vessel is an integral part of all offshore activities starting from transportation, installation, maintenance and salvaging of various offshore structures. The stability of the floating crane in such situations is of outmost importance as it operates at sea and determination of the critical ranges of its operating conditions are quite essential as well, especially during the assembly of costly structures such as subsea devices. Moreover, for heavy duty lifting, the operations of crane ship in waves, even as the sea is relatively quiet, are often restricted by the unexpected and excessive motions of the payload underwater. Such disturbances of the system, which might arise during crane ship operations due to difficulties in positioning the submerged objects being handled accurately, can cause the collision between the load and the ship or other objects and thus cause loss of valuable asset and time.
My research focuses on providing an insight of the entire process of offshore installation via the evaluation of hydrodynamic performances of a crane barge and its payload in water waves. The main attention is given towards the comprehensive analysis of submerged cylindrical payload behavior and the fully nonlinear three dimensional time domain approach is used to solve the problem as this method is capable of handling the full nonlinearity compared to the frequency domain approach. The study is divided into several parts. At the beginning, a numerical model is developed to analyze the hydrodynamic performance of a submerged payload and validated against existing studies. The nonlinear wave radiation by a fully submerged vertical circular cylinder undergoing various forced sinusoidal motions in otherwise still water is simulated using this model and the response of the submerged cylinder is found to be dominated by the heave amplitude motion in most of the cases.
The next part of the research includes the investigation of the hydrodynamic feature of a submerged vertical cylindrical payload attached to a cable for constrained motions and moving towards the seabed at a constant speed in water waves. The crane barge itself is then simulated in various wave conditons and the simulation results are compared with published data for single floating barge. Finally, the submerged payload and floating crane barge problems are coupled and the combined analysis is performed for different physical conditions, for example fixed barge and fixed payload in head sea and beam sea, fixed barge and moving payload in head sea and beam sea etc. In each situation, comprehensive parametric studies are performed to understand the characteristics of the coupled system from the hydrodynamic point of view. It is found that, generally the moment acting on the cylinder increases with the increase of cable length irrespective of the various scenarios considered. Besides, phenomenal influence of shielding is observed in cable tension and angular motion of the payload in terms of slow varying low frequency responses for head sea and the beam sea scenarios. The payload in upstream beam sea case is also found to be facing a very large mean drift motion which arises because of the shielding effect as well. A further finding of this study is the influence of low frequency on cylinder motion exists even after the cylinder moves towards a deeper position equals to its length.
Overall, the study carried out generates the understanding on the dynamic behaviour of the submerged payload of a crane barge. The results of the analysis can be used for safety considerations during the offshore installation process and would be beneficial to the researcher working on designing active damping devices to extend the operating range of crane ships by means of controlling the motion of the submerged payload.