Station keeping is required for many offshore operations, for instance when transferring crew with a gangway, but also for drilling, dredging and pipe or cable laying. Increasingly, these operations are performed with vessels on dynamic positioning (DP) due to its advantages for mobility and for operating in deeper water.
A vessel operating on DP is a complex system, as the thrusters work together to compensate the forces due to waves, wind and current. To determine whether a vessel can stay on its place despite the environmental forces, DP capability plots are calculated. These are static force balance calculations, where the mean forces are used and it is determined whether the vessel stays on its position on average.
However, the system is not static and the forces are not constant at all. Our research into the wave drift forces has shown that the force can build up to four or five times the mean value during some periods. This can lead to large offsets of a vessel from the position that is kept on average. When transferring crew from a ship to a structure, due to these offsets, the risk of collision (or the opposite: drifting too far from the structure) is present. Even though the capability plot shows that the vessel will stay on its place.
An example of data of a real vessel operating on DP is shown above, split into the low frequent and the wave frequent motions. It can be seen that on average the vessel stays on the same position, but the low frequent motions lead to considerable offsets.
For safe operations, it is necessary to estimate the expected maximum offsets in a certain time interval, known as the footprint. To do so, currently multiple simulations are carried out and from this the most probable maximum is used as the footprint estimate. Because this is a time consuming and complex process, it is often not done in practice. One can imagine that if an estimate is necessary for the next hours for example, running simulations of multiple hours are out of the picture. This means that for planning purposes before a vessel has sailed out of port, it is not known if an operation can safely be performed. Only when the vessel arrives at the location, it can be determined if the operation is feasible.
DP footprint in frequency domain
Computing the footprint in the frequency domain may be a solution. It can lead to very fast estimates and therefore it can be used on board during and just before operations, to give insightful footprint estimates. To use a frequency domain approach, some challenges have to be overcome. One is constructing a model of a vessel that can be used to calculate the offsets of a vessel accurately. Another is finding the statistical relations between the offsets and the expected extreme behaviour.
In the past year we have progressed rapidly in developing an algorithm to determine the footprint estimates using a frequency domain approach. Hydrodynamic and statistic research have proved the feasibility of the developed methods, and further research is planned. The next steps toward implementation will be started soon in cooperation with MARIN and Acta Marine.
In the future, these methods may lead to a better and safer way of working on DP, eliminating uncertainties about expected offsets when the stakes are high.