Over the past years more heavy lifting operations are executed on DP, the installation of modules off shore using a dynamically positioned vessel has various advantages but also several disadvantages. The advantages are mainly financial or resulting in a financial advantage such as the reduction of time for the operations.
A major disadvantage is the ‘horizontal mooring stiffness’.
Heavy lifting operations include three different scenarios, first with the heavy lifting vessel, HLV, with the module hoisted. The HLV with the module fixed and the HLV without the module, the first and last scenario are most understandable, besides the draft, trim and wind surface not so many factors will influence on the positioning. But during the moment where the HLV is fixed through the module the disadvantages start.
For example, if a module is installed on a jacket, the moment the module is mounted on the jacket, the HLV is moored to the jacket via the crane wires. At that specific moment, the load is not released and the change from HLV loaded with the module to HLV empty is undergone. The trim and draft will change and the DP system will continuously keep the vessel on position. But the DP system is not setup for a moored vessel and the change in trim may result in the deviation of position with the crane not being precise above the load.
This change of the crane top position in respect to the load will result in a horizontal force, the so called horizontal mooring stiffness. This force is significant in many heavy lifting operations and until this moment there is no better solution than not to use DP in the scenario with the module fixed or reduce the gain of the DP system.
Both result in instable positioning and reduced response to the change of environment forces, exactly during the most critical part of the operation with the closest distance to a monopile of jacket.
As long as no feed forward force systems are created or another industry solution is provided the best we can do is, work on the solution, prepare the best procedures to reduce the risks and create a high level of situation awareness under DPO’s and engineers.
