Trivane - Floating Offshore Wind (FOW) Platform
A Semi-submersible Turret-moored Trimaran
Offshore wind energy is a massive resource. Wind speeds are generally higher at sea than they are on land, and the speeds tend to increase further away from land. Also, the air flow is smoother because, compared to many sites on land, the sea is relatively flat.
Wind turbines located offshore UK, parts of Europe and elsewhere, are now quite a common sight. They closely resemble their onshore counterparts, the tower of each one being mounted on a cylinder or frame that is supported on the seabed. This is feasible in water depths of up to about 60 metres, but the next horizon is to develop economical ways of supporting large wind turbines on floating platforms that can be installed almost anywhere and in any water depth.
Most designs of floating platforms for wind turbines have one or more cylindrical hulls,, with ballast to provide stability. There are also tension leg designs that rely on tension in the mooring system.
Designed from First Principles: The Evolution of Trivane
Trivane is a trimaran that weathervanes about its turret mooring in accordance with the combination of the effects of the prevailing wind, seas and any current. It carries a single wind turbine.
During its evolution, many potential designs were initially considered, including a single hull and a catamaran
A single hull is impracticable. A catamaran is feasible but it appears to have inefficiencies in design, compared to a trimaran, because neither the tower nor the turret are supported on a hull; they are on frames between the hulls.
Attention then turned to a trimaran consisting of a simple long centre barge and two outer barges, with none of them submerged. This is simple to build and performs well in most sea states but its motions are questionable in some extreme seas.
Trivane is now still a trimaran but is a semi submersible, with the centre part of the centre barge partially submerged. This greatly reduces motions in extreme seastates
Any standard tower and turbine is supported on the stern of the centre hull. An OEM underlined the importance of Trivane mounting a standard certificated turbine and tower. This should include for allowing the nacelle to rotate in accordance with wind direction so as to remain standard and to maximise power production
The turret is connected to the forecastle which is the forward part of the centre hull. This particular turret is a column turret having a plain or journal bearing near to the keel, these having been proved to be maintenance free bearings. And a roller or plain bearings at the top. Chains or wires are connected to a chain table which is beneath the column but is above keel level. The designer has had considerable experience of these column turrets via work at LMC
High voltage power swivels have been in use on FPSOs for a long time. These are often in open air because they are located high above still water level. On Trivane there is an export power swivel on the top deck of the forecastle, which is 10m above still water level, so that the swivel rarely if ever risks any green water impact. However it will be in a ventilated housing
The length of the centre barge affects the natural period of pitch. So it is set so that this natural period exceeds the period of any possible incident seastate. The length must be sufficient to ensure that Trivane weathervanes about the turret
The outer barges provide transverse stability
Trivane operating offshore at 20 metre draft
The Many Advantages of Trivane
LOW DRAFT : Unlike most other designs, Trivane can be assembled and towed out from most places, because of the very low lightship draft of 6 metres during assembly and tow out from port. The draft is increase to 20m offshore, by ballasting with sea water
Trivane at 6 metre draft for Assembly and Tow
EASE OF CONSTRUCTION : The three hulls are of the most basic hull construction, in flat stiffened steel plate. These stiffeners are a grid consisting of parts of the transverse web frames of the hull, and its longitudinal stringers. The assembly of these stiffened plates constitutes a high proportion of the entire fabrication: maybe 60%. And this is all done under cover, in a controlled environment, in the fabrication shop
OPERATES IN HARSH ENVIRONMENTS : A key driver in the design of FOW devices is to maintain extreme angles and accelerations within values that are acceptable to the wind turbine and its tower. Trivane, carrying a 15 MW turbine, can safely operate in all year extreme conditions in, for example, the Celtic Sea: (Hs = 13.3m Tp = 17 seconds)
Its performance in higher seastates is currently being checked
SHARING THE WORK: Any shipyard or fabrication facility can make these stiffened plates, at a low cost per tonne. The work can thus be shared because fabrication facilities, perhaps even inshore, can fabricate stiffened plates and transport them to the assembly yard by barge, or even by road or rail. In contrast, cylindrical designs require specific facilities, which are not available everywhere, to roll the outer plates of the hull. On Trivane, the tower support is the only rolled cone and cylinder.
RELATIVELY SMALL CHAIN OR WIRE SIZES : Some FOW devices require mooring chains or wires of very large diameter. On Trivane a) the weathervaning and b) the ship-shaped form of each hull , combine to give low wave loading, The mooring line loads are thus minimised and hence the size and cost of the mooring lines and anchors is minimised. Conversely the projected area, to waves, of cylindrical structures, from any direction, is higher.
EFFICIENT CONNECTIONS: The barges are connected by simple box beams and tubulars.
STABILITY AND DAMAGE STABILITY : Trivane has good static stability and also readily allows damage stability to be provided, with the centre hull being divided into 7 watertight compartment. This is the ability to remain safely afloat with some of the barge compartments accidentally flooded. It is wise to have good damage stability since FOW installations are unmanned and any ingress of water may go unnoticed until it may be too late.
DECK SPACE AND VOLUME: Large deck space is available. The large internal hull volumes on Trivane provide space for batteries and other technologies to be accommodated, as these become more widely used onboard floating wind platforms.
BOARDING: Since Trivane weathervanes, it can be boarded relatively easily, via an access platform below the tower, astern.
LOW TOW FORCES: The ship-shaped form of the hulls means that tow forces are low and tow speeds are relatively high. This all reduces costs
MAINTENANCE ASHORE: If may be that major work needs to be done during the design life, for example an upgrade to the turbine, or changing the blades. Offshore work at height is expensive and It may be most economical to disconnect the export cable and mooring chains, tow Trivane to shore, entering port at 6m draft, and doing the work whilst tied up alongside a quay. The chain table can even be inspected above water by trimming Trivane slightly by the stern
Dimensions (Version for 15 MW Wind Turbine)
Length
The length of the centre barge is 88 metres excluding the turret
Hull Form and Beam
Each barge is ship-shaped, with the beam set to 15m to 12m (varies along its length) for the centre barge and 12m for each outer barge, to minimise wave forces and mooring loads.
Drafts
The lightship draft of each barge is 1.5 metres and the draft of Trivane is initially only 6 metres
Overall beam
72 metres, to provide good stability and to minimise roll.
Fabrication, Tow-out and Installation Offshore
The hulls can be launched, lifted, or assembled in dry dock according to the fabricator’s facilities. The hulls can be connected to one another whilst alongside a quay, either floating or grounded on a pad. The tower, nacelle and blades are then lifted onboard.
The mooring lines and anchors can readily be pre-installed using standard anchor handling vessels, which is a cost - effective option. The anchors are pre-laid and pre-tensioned.
Connection to the pre-laid mooring is a standard procedure, only requiring anchor handling vessels.
Investors & Partners
In September 2021 BEIS provided generous funding of up to GBP 3.2 million to support development of Trivane.
The design is being refined. There will be 1:50 scale model tests early in 2023
Investors and partners are now invited to join Trivane in building and deploying a Demonstrator and Commercial versions.