Access to semisubmersibles ‘could be a challenge’ for floating wind turbines

The report, Floating offshore wind access strategies and considerations: public summary report, published by the Offshore Renewable Energy Catapult (ORE Catapult), suggests that although safe and reliable access is essential to the maintenance and upkeep of floating wind turbines, there are concerns around safety, some transfer methods and a lack of standardisation.

The authors of the July 2025 report said that – with commercial-scale floating windfarms on the horizon in several countries – all potential means of access, such as from service operation vessels (SOVs), using gangways and/or daughter craft; crew transfer vessels (CTVs); and helicopters; have a number of concerns in common, and concerns that are type specific.

The challenges include relative motion between the turbine and a transfer vessel complicating transfers; a lack of regulation and standardisation in the floating wind sector; substructure design that has a significant and direct impact on access methodologies and challenges; harsher sea state conditions and farther-from-shore locations that will adversely affect workable hours; and inconsistent technician training and knowledge gaps.

Overall, said the report, SOVs with walk-to-work gangways are considered the most likely and workable option, due to far-from-shore locations and harsher weather conditions, which SOVs are able to handle better than CTVs. However, state the authors of the report, there is a risk of loss of positioning and a gangway losing contact with a turbine, and "the largest concern with motion compensation is gangway movement during transfer, where technicians risk getting trapped between moving parts," an issue that has recently been highlighted by the UK Health & Safety Executive in the bottom-fixed market, where transfers are easier.

CTV use is expected to be restricted for closer to shore or individual repairs when an SOV is not active in the field. This is due to their stricter weather limitations and the long transfer times expected for floating wind locations. The cost and material requirements for landing platforms is also higher than that of walk-to-work gangways. Methods for increasing CTV use include the addition of a clamping mechanism; use of a hoist on the turbine; and using electrification to reduce the response time and help maintain contact with the turbine.

To date, transfer using helicopters has not been considered for standard transfers. Despite having the highest operable weather limits, they are generally reserved for use only during walk-to-work weather limits and for emergency or specialised personnel transfer. "Their high cost, including fuel use, limited space for technician and materials make for an unlikely regular transfer method in the floating wind market," said the report, noting that "transfer generally occurs using a winch system, winching onto either an SOV or onto the nacelle, though it has been suggested that transfer could occur onto the substructure, due to the high movement of a nacelle making helicopter access improbable."

The helicopter analysis focused on the movement of the nacelle, which predominately moved in the horizontal plane in the direction of the wind and waves. There is no current guidance on assessing helicopter accessibility for a moving nacelle, so a heli-hoist design was used to set motion limits for access. Using this limit, the tension leg platform (TLP) foundation was accessible in almost all sea states considered, while the semisubmersible was accessible in only the most benign sea states. This difference was caused by the much greater movement of the nacelle of the semisubmersible. However, the limit for the range of motion was set by the minimum landing area; if the landing area’s width and length increased, the limits would correspondingly increase, leading to a higher accessibility. "It should be noted that the issue of velocity would persist… which was shown to be significant on a semisubmersible, with the nacelle reaching a maximum velocity of 1.2m/s in 2.5m Hs."

Results from a CTV analysis suggested that access would differ significantly across the different floating foundations. In cases where the wind and waves were aligned with the access direction, the TLP foundation’s access was very similar to the fixed foundation, but the access to the semisubmersible was significantly reduced. Consideration of the access process suggested this was due to the comparatively increased surge and heave motion of a semisubmersible, as these motions would negatively affect the friction interaction set up between the fender and the landing bumper. Although the semisubmersible created an element of shielding, this sheltering was only present for waves of certain wavelengths, and thus was only observed for modelled sea states with a wave period (Tp) of 6s.

SOV models suggested access was possible up to 2.5m Hs for all foundations, but that access was contingent on the wave direction and heading. Direction in particular had a very significant impact on access, as the force of the wind and waves on the beam of the vessel and the roll response severely reduced the time that a gangway was able to maintain a connection. Additionally, shorter wave periods reduced access, as they presented an increased challenge to the station keeping of the DP system. This was particularly pronounced for the semisubmersible foundation.

Overall, the ORE Catapult’s modelling suggested access to floating wind turbines might be reduced compared to fixed turbines, depending on foundation design and chosen method of access. In particular, if using a helicopter or CTV, the semisubmersible might be significantly less accessible than the current fixed foundations. If using an SOV, access for a semisubmersible would be more comparable to fixed foundations, except in low wave states. Modelling suggests that access to a TLP foundation would be very similar to current bottom fixed offshore wind turbines when using an SOV.

The study carried out by the ORE Catapult also found the impact of substructure design on accessibility varies according to location, and that transfers onto semisubmersible floating foundations will be more limited than TLPs, which are similar to bottom-fixed wind in availability and weather days.

"The similarity between TLP and bottom-fixed wind regarding weather days, number of transfers, and availability, are encouraging for maintenance expectations," the authors of the report concluded, noting TLP maintenance planning could draw closely on that used in the bottom-fixed market. "Regardless of substructure type, designs should take into consideration stability and motion mitigation methods to increase accessibility," the report concludes.

"These findings point to the importance of considering wave period in addition to wave height when planning for maintenance activities. The direct relationship between considering wave period and a loss of availability and increase in weather days highlights the potential for project interruption due to poor understanding of weather conditions."