‘Wet Handshake’: Workability Study of an Offshore Thruster Exchange Operation

Thruster exchange operations are performed when an azimuth thruster on a DP operated vessel needs to be replaced for repair or maintenance purposes. At present these operations are performed either during a dry-dock call or in sheltered waters with the assistance of a shearleg or heavy lift vessel moored alongside the DP operated vessel. In order to reduce downtime of the DP operated vessel a clear trend is observed of operations being carried out more offshore and in deeper waters by means of a wet handshake between a heavy lift vessel (HLV) and a DP operated vessel. To get insight into the workability of such offshore thruster exchange operations it is important to study the dynamic interactions between the bodies involved. This paper describes the development of a methodology which accurately determines the workability of an offshore thruster exchange operation. The methodology is developed by BigLift Shipping, which has performed several thruster exchange operations in sheltered waters in the past few years. Furthermore the results of a workability analysis of a thruster exchange operation offshore Ghana are presented. In order to determine workability, first the hydrodynamic behavior including interaction effects of the coupled configuration of the DP operated vessel, the heavy lift vessel and the azimuthing thruster is calculated for a range of seastates, which results in the response spectra of the vessels. Then for each seastate the response is compared with the defined operational criteria to determine whether this seastate is workable, critical or non-workable, which results in a workability diagram. The workability can be quantified by combining this workability diagram with a wave-scatter diagram of the location of interest. In general this is a thoroughly studied topic. In order to achieve a higher level of accuracy of the workability prediction the focus of the methodology described in this paper has been pointed on two aspects that can be of significant influence. • First the persistency of a certain seastate is investigated. In the approach described above any variation of the environmental conditions over the duration of the operation is not considered. The effect of changing weather in time can be of significant influence on the overall workability. In order to take into account persistency information a numerical dataset of 3 hours statistics over a period of 10 years is used. Combining this information with the duration of all consecutive steps of the operation, results in a more realistic workability prediction. Similar approaches in seakeeping of ships are shown by Dallinga et al. (2004) [1], Naito et al. (2006) [2]. • Secondly the influence of a frequency domain approach compared to a time domain approach is analyzed. The nonlinear effects that occur in the coupled configuration of multiple bodies can not be taken into account in the frequency domain approach. Therefore the motion analysis is performed in the time domain. As an example case a thruster exchange operation offshore Ghana has been studied. The results of the study show that the workability prediction based on scatter diagram metocean data is influenced substantially when persistency information of metocean data is taken into account. The effect of the duration and criteria of independent operational steps on the workability are clearly visible in the persistency approach. This enables a to-the-point approach in improving the workability. Although time domain analysis is taking into account nonlinear effects, the difference between time and frequency domain analysis can be neglected for the presently studied configuration.