Hydrodynamic Analysis of a Heavy Lift Vessel during Offshore Installation Operations

Abstract

As the raising demand of energy nowdays, offshore transportation and installation using heave lift vessel (HLV) becomes more common in offshore industry. This thesis pays attention to the hydrodynamic performance of a given heave lift vessel under selected working conditions covering from seakeeping analysis to zero speed operations cases. After an introduction about hydrodynamic problems under potential flow theory which is the basis for all the following analysis, we start to investigate the transportation cases, and the seakeeping simulation are perform with GL-Rankine, a 3-D diffraction/radiation programme from DNV-GL. Both Rankine source method and zero-speed green function method are used in simulations considering different forward speeds as well as wave heading angles. The results from GL-Rankine are compared with the existing numerical database for this type of vessel. A small discussion for the difference between Rankine source method and 2-D strip method is also conducted. Then, the following studies investigate the multi-body interaction cases. The frequent usage of two closely positioned vessels during offshore operations makes this an important topic. Again, GL-Rankine software is used as the solving tool here. After a verification study for a simple multi-body system in limited water depth, the research focus on the hydrodynamic performance of the HLV/ barge system in frequency domain. Some typical results i.e. RAOs and mean drift forces from multi-body interaction simulations are stated by comparing with the single body data under same environmental conditions. Besides of this, the influences from special factors, i.e. limited water depth, asymmetric geometry and resonance trapped waves, are also discussed in this chapter. The last part of this thesis focuses on the time-domain analysis during the first two phases of offshore installation: Lifting-off and lowering through splash zone. Due to time issues, only several selected cases are studied, with some simplifications to be assumed for modelling with Orcaflex software. In addition, the methodology and the theory background is also included. For lifting-off process, the study is concentrated on the influence of lifting speed during cargo transfer process and coupling between the two ships is introduced through a spring connection to restrain the horizontal movements. For the lowering operation, attentions are paid to the hydrodynamic loads on lifted object. The dynamic forces in slings and crane wire during both cargo transferring and the lowering operation are obtained for selected cases. Based on this study, a more understanding of the dynamic performance of the offshore operation is obtained, therefore it will help to estimate the possible response under given wave conditions, select safer operation window as well as avoid the potential risk.