Dynamic positioning in the initial design
One of the key aspects for any operator of a vessel with Dynamic Positioning (DP) capability is to evaluate the ability to stay in position in case of failure of one or more units.
Some of our recent DP projects include large shuttle tankers and semisubmersibles. The assignments range from initial numerical predictions using our holding capability tool DPLab over DP simulations to full-blown DP model tests in our large towing tank.
The challenge in the initial design
Embarking on a new project with a dynamically positioned ship or platform immediately poses the following three questions once the site has been determined: Is station keeping possible under the anticipated conditions? What sizes of thrusters are required, and where do we install them to maximize our station keeping? How are the thrusters best utilized?
These three questions do not, of course, have independent answers; rather a common solution solving all three must be found. In 2009 FORCE Technology developed the holding capability software tool DPLab. A new version, 2.0, has been released including the following features:
Calculation of basic DNV requirements and advanced evaluation with respect to critical heading and real data for the vessel.
Environment scenarios allow the user to compare holding capability for different wave heights, different currents etc. within the same analysis.
Import of externally calculated riser forces
The option of importing the riser forces and their direction for a given analysis has been added. Some customers calculate the riser forces externally and have a more accurate value for the riser force than the simple model calculated by DPLab.
Forbidden zones for thrusters
The option of setting forbidden zones for thrusters is included in the optimization so that the calculations are based on the thrusters not being used in specified directions.
Feasibility calculation/powerThe feasibility calculation indicates the total amount of power required to keep the vessel at its position in a specific environment. In the calculation, the wind is given, and the power is increased until a stable situation is obtained. The result of a feasibility analysis is a power demand percentage rosette or alternatively a thrust utilization plot.
ParallelizationTo better utilize modern PC hardware which can often run several threads simultaneously, DPLab 2.0 has been modified to include parallelization. This allows the application to run multiple calculations simultaneously, thereby significantly reducing the computation time.
The new user interface has been updated to provide a better flow in the procedure and to be more intuitive to use.
Holding capability basics
The tool performs static calculations, balancing the available thrust and the environmental forces. The holding capability is defined as the limiting environmental forces (arising from wind, current and waves) that can be balanced by the vessel using its propulsive systems in an optimal way.
The key to an accurate investigation of a vessel’s dynamic positioning is to establish the environmental impact on the vessel, therefore our approach is to base the DPLab software on accurate wind tunnel test data for the wind and current, and 3D diffraction code for wave drift forces. These drift force data can be imported from either our own seakeeping tool or from industry standard tools in a specified format.