A workability study on floating installation of monopiles using the MUST

Abstract

Adapting the size of upending cradles according to the ever-increasing dimensions of offshore wind turbine monopiles is costly and requires structural modifications to the associated Heavy Lift Vessels (HLVs). This study therefore investigates the potential of the Monopile Upending Smart Tool (MUST), which is a platform suspended in the HLV crane, on which a winch is installed. Two grommets of constant length connect the platform with two trunnions attached to the monopile, and the winch cable suspends the bottom of this structure. Unwinding the winch cable allows for in-crane upending of the monopile. This way, the dependency on the size of the available cradle is circumvented. The focus is laid on determining the workability of the application of the MUST in the crane of the Seaway Strashnov and identifying limitations in its design. Moreover, the potential of systems that increase the workability or reduce the limitations is investigated.

The installation of monopiles using the MUST comprises three phases: the barge mooring / lift-off, upending / slewing and lowering / driving phase. Based on the results from a qualitative critical event analysis, the lift-off phase is expected to be limiting. Hence, a hydrodynamic model is developed to quantitatively analyse this phase. Experimental simulations, which are performed in parallel with the model development, result in the preliminary conclusions that multiple pendulum effects influence the results to a limited extent, while viscous roll damping and hydrodynamic interaction effects can be strong determinants of the resulting responses.

The relative z-motion between the lifted monopile and the barge and the barge roll response are identified as governing parameters. To reduce the first limiting factor, a system that allows for instantaneously increasing the vertical clearance between the monopile and the barge is proposed. The effectiveness of this system is tested for two barge loadcases. For the first case, the average workability increase for the optimal heading at a typical location is calculated as 8.3%. For the second, the increase is marginal, as the barge responses are more limiting. Furthermore, is it found that a Passive Motion Compensator (PMC) can reduce the probability of the introduction of snap loads in the winch cable, and therefore allows for system optimisations. A PMC with 10% of critical damping can reduce the required winch capacity with a factor of 2.3 w.r.t. the uncompensated case.

It is recommended to perform follow-up studies into the system performance during upending / slewing and lowering / driving. Also, it is advised to evaluate the effect of a larger barge and a PMC on the workability. To balance the associated investments and workability increases, the logistical models developed in a parallel study can be used. Finally, for iterative calculations, it was found to be beneficial to make an estimate based on a fast simplified model and to subsequently feed the results back into a more detailed, but slower model.