The increased consciousness of the negative impact of human-induced climate change has resulted in rapid developments within the offshore wind industry. With Wind Turbine Generators (WTGs) becoming increasingly larger and available shallow water sites becoming scarcer, new offshore wind solutions must be developed to accomplish the goals set by today’s policymakers. The new deep water sites available for offshore wind are not applicable for the current benchmark of monopiles and jack-up vessels. New state-of-the-art applications, such as Floating Offshore Wind (FOW), come into play in the global offshore wind market. To meet this demand, Heerema is developing installation methodologies for floating to floating installation applications. One of these methodologies is the RNA+ method (Rotor Nacelle Assembly + Tower), lifting a fully assembled WTG module in one lift from the vessel deck on top of its foundation to limit the number of critical lifts. Operability studies are required to examine whether such operation is feasible. As the FOW industry is the new kid on the block, little is known about FOW’s configuration and other influences. Therefore, this research aims to determine the effect of design parameters of the three key components of an offshore floating to floating installation on its operability. These key components are the WTG, floater, and installation vessel. For this research, the IEA 15 MW reference turbine with an altered tower, a Tension Leg Platform (TLP) provided by Intecsea, and Heerema’s Semi-Submersible Crane Vessel (SSCV) Sleipnir were used as WTG, floater, and installation vessel respectively.

A numerical base case model containing these key parameters was built to address the influence of the design parameters. This base case models the free-hanging stage of a complete WTG module suspended in the SSCV’s crane 3 m above the TLP in a water depth of 150 m. The numerical model was analyzed in the Frequency Domain (FD), considering only first-order effects. With respect to this base case, all parameter variations were compared. A mean JONSWAP spectrum was used as wave spectrum.

The results show that this installation method is sensitive to long wave periods ( > 8 s). The clearance between
the nacelle and crane-boom is deemed the most governing limiting criterion. The relative vertical Z-tip motion between the tower bottom and TLP top and the side-lead angle of the crane hoist wire are the secondary governing limits. Design parameters that influence the static clearance between the nacelle and crane-boom have the most impact on the total operability. With the current design parameters world’s largest SSCV has a limited operability for installing the modified version of the IEA 15 MW reference turbine with a single crane lift. Alterations to increase its crane boom reach and clearance are needed to perform this single lift installation. The hub height and nacelle casing size of the WTG limit the operability significantly. Furthermore, due to its relatively small size, stability and stiffness in heave direction, the TLP hardly affected the operability.