THE CHALLENGES FACING THE EXPANSION OF THE OFFSHORE FLOATING WIND INDUSTRY
One of the greatest challenges facing the offshore floating wind sector is the launch of the Floating Foundation Structures (FFS) or floaters that are used to support the wind turbine when in deeper water. Why are they a challenge? Well, as the size of the turbines grow so too must the size of the required FFS. These currently range from 40m wide up to 115m wide for some of the largest tri-floater designs, and this trend is only getting bigger as the industry pushes to maximise the size of the wind turbines. This article will focus on some of the challenges faced by the growth of the offshore wind industry.
Currently most large offshore structures are either built in dry dock, launched from slipways or rolled out and floated off from semi-submersible heavy lift vessels. The growth in this sector has resulted in different projects competing for limited infrastructure,
and consequently the suitable drydocks and slipways will soon be at capacity, with large scale civil infrastructure investment required to provide additional capacity.
The industries current focus is on securing leases for offshore sites and the detailed design and optimisation for the floaters, however the largescale fabrication and launch strategy of these projects has not yet been fully addressed. Many commercial ports are engaged in partnerships with the fabricators for suitable quayside space to fabricate the floaters, however this does not solve the launching challenges.
Design and Fabrication
Floaters are designed with hydrodynamic stability and seakeeping in mind. This leads them to have deep draughts and large displacements for enhanced stability. To achieve these displacements some are made from steel using conventional offshore structural methods combined with water ballast to increase the draught that they float at, thus increasing their displacement. Steel fabrication is typically more time consuming, costly, and labour intensive when compared to reinforced concrete fabrication. With the demand for high numbers of large floating structures the shift towards concrete or at least part concrete construction methods is proving favourable amongst designers. Some of the larger concrete floaters can be anywhere from 10,000 to 20,000 Te.
The main disadvantage of concrete fabrication is that they cannot be suitably deballasted to lower draughts and displacement, making the launch and regular maintenance significantly more challenging.
Towage vs Local Fabrication
Most ships are built in shipyards in the far East which can cater for the fabrication of large floating structures, however floating wind foundations typically cannot be wet towed long distances and are a challenge for dry towage even on some of the world’s largest heavy lift vessels. As a result, they need to be built in close proximity to the location that the wind farm is to be installed at. This will mean several new large scale fabrication facilities will be required around the world to accommodate this need, each with its own launch solution.
Large-Scale Production
Large tri floaters will likely be built in sections with each sub assembly being fabricated, and then transported to a suitable launch site, whether this be a dry dock or quayside facility. Once in position the final assembly will be carried out and the complete unit will be ready for launch, however this will still be limited by drydock capacity and other port infrastructure.
Semi-Submersible barges and Heavy Lift Vessels
Submersible barges offer an alternative means of launch floating structure by load out and float off. Floaters built on land can be moved to the quayside by SPMT (self-propelled modular transporters), and then rolled onto a semi-submersible barge. The semi-submersible can then be submerged, and the floater floated off. This process is currently underway for some of the small-scale demonstrator units. The current global fleet of semi-submersible barges are undersized for the full-scale floaters as the maximum beam of a typical semi-submersible barges is 36 to 38m. The largest semi-submersible heavy lift vessels have a maximum beam of 60-79m, which is bigger than some of the largest semi-submersible barges available on the market. However, even the larger barges will struggle with the widest floaters and unmanned barges will offer a significantly more attractive day rate compared with the largest heavy lift vessels.
Purpose Built Newbuild Floating Dock
Some developers have issued proposals for use of a purpose built wide floating dock capable of loading out and floating off the larger floaters. A purpose-built floating dock, with beam in excess of 100m, would require a large dry dock or slipway of its own. It would also be limited in where it can be built, located and moved as it would be beyond the limits for the Suez Canal and other major shipping routes.
Barge Coupling
By coupling 2 or more large semi-submersible barges together it will create a large platform that would be capable of supporting the wider tri floater types. This could either form a dual barge for some of the smaller floaters, or a triple barge for the larger types. Dual barge float offs have been used for launching a number of large offshore structures and can make use of a variety of existing semi-submersible barges. The use of barge coupling introduces complex multibody interactions which are not present when using a single launch vessel.
Buoyancy Modules
There are a number of proposals for use of modular buoyancy tanks that can be ballasted to launch the floaters. These come with their own challenges as a high cargo centre of gravity (CoG) on deck and no waterplane area for the buoyancy modules once submerged, the stability will need to be justified purely through the upwards reaction force on the floater. Designs are still underway for this method with a working solution still to be developed. The longitudinal strength requirements for a load out of a large structure by skidding or SPMT will also add further complexity to a modular coupling method.
This article has sought to cover some of the identified challenges surrounding the fabrication and launch of the FFS, teamed with an insight into the current potential solutions.
