Experimental study on the in-place stability of a self-installing jack-up type 10 MW offshore wind turbine during leg lowering

A 1:25 scale model test was conducted to evaluate the motions, green water risk, and motion-induced interruption (MII) during the leg lowering steps of in-place installation for a self-installing jack-up type 10 MW offshore wind turbine. For three leg lowering steps, in which the draft, center of gravity and submerged leg length change simultaneously, tests were carried out under combined wave, current and wind conditions at head sea and bow quartering incidence. Heave was insensitive to environmental changes owing to the large waterplane area, whereas the current tended to reduce the pitch response. The wind had little effect on heave and pitch, whereas roll was selectively amplified under bow quartering incidence. The green water risk was concentrated at the single windward pontoon tip selected by the wave incidence direction and was most severe at the step with the smallest freeboard. The MII satisfied the NORDFORSK and Graham criteria with a substantial margin at all work locations, indicating that green water occurrence at the windward pontoon tip rather than the MII was the governing operational constraint. The wave incidence direction was the primary governing factor, hence heading control is key to safe installation and to defining heading-dependent allowable sea states.