Designed from First Principles: The Evolution of Trivane

LOW DRAFT : Unlike most other designs, Trivane can be assembled and towed out from many ports, with the tower and turbine onboard. The draft is increased to 20m offshore, by ballasting with sea water

CONCRETE AND STEEL : in Europe, construction in concrete is about half the cost of building in steel, and locally sourced materials are used when building in concrete. The centre pontoon and tower support of Trivane are also built in prestressed concrete. The prestress tendons ensure that the concrete is in compression, thus preventing cracking. The transverse pontoons are also in concrete. The three ’Floaters’ are built primarily in stiffened flat steel plate.

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: There are many competent contractors that can build the concrete pontoons.

The steel parts of the floaters are made in flat stiffened plates, at a low cost per tonne.  The work can be shared because fabrication facilities, perhaps even inland, can prefabricate 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.

RELATIVELY SMALL CHAIN AND POLYESTER 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, in other designs, the projected area, to waves, of cylindrical structures, from any direction, is higher.

EFFICIENT CONNECTIONS: The concrete longitudinal centre pontoon and the two transverse pontoons are cast as one structure. The steel floaters are connected to them using large pins, which is quick and reliable.

STABILITY AND DAMAGE STABILITY : Trivane has good static stability. Damage stability 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.

The centre pontoon is divided into 7 watertight compartments. Coffer dams are provided in the flaoters, above and below the waterline

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. On Trivane the export cable and mooring lines can readily be disconnected an laid on the seabed, Then it is towed shore, entering port at low draft, and the work is done whilst tied up alongside a quay.