‘Forklift’-like lifting solution eliminates motions and combines the use of low-cost jack-ups and barges
Recent months have seen a spate of new designs for Jones Act-compliant turbine installation concepts that would enable international vessels to install turbines on US offshore windfarms. Most require the use of a high-spec, high-cost wind turbine installation vessel. Many also include the use of motion-compensation systems, to enable components to be lifted safely from a feeder vessel or a barge.
They are required because the Jones Act requires any vessel transporting cargo between US ports to be built and flagged in the US and, for the purposes of the Jones Act, a bottom-founded wind turbine is considered a US port.
A non-Jones Act wind turbine installation vessel can install wind turbines but cannot transport components from a port to the foundation, hence the need for feeder vessels or barges.
At the project site, a wind turbine installation vessel would lift components from a feeder vessel or barge and then install them, an operation made complex and challenging by the relative motion between the jack-up and barge – hence the motion-compensation systems for feeder vessels and barges that have been proposed.
Contrasting with concepts such as these is one from Friede & Goldman, a company that has decades of experience designing mobile offshore drilling units and jack-up vessels for the offshore oil and gas industry, which is branching out into offshore wind.
In June 2021, Friede & Goldman unveiled a Jones Act-compliant solution that combines the use of low-cost barges and relatively inexpensive jack-ups that could be built at low-cost yards outside the US. The solution also does not require the use of motion-compensation systems.
The company believes that not only will it be much less expensive than a purpose-designed Jones Act jack-up of the type Dominion Energy in the US has ordered – which has a price tag of approximately US$500M – it will also be less expensive than using jack-ups specifically designed to install turbines and a barge with a motion-compensation system.
“You can’t offload cargo if barge motions exceed very low limits and this often leads to downtime,” says Friede & Goldman. Its solution, BargeRack, eliminates relative motions between a barge and the jack-up by lifting the barge – loaded with turbine components – out of the water as the jack-up is elevated.
This means that, once the jack-up and barge are out of the water, there is no relative motion between the barge and the vessel and lifting operations can be conducted efficiently and safely.
“There is no need to build specialised feeder barges with high-tech equipment,” says the company. “You don’t need expensive motion-compensation equipment if the barge is attached to the jack-up.”
The BargeRack solution, which is suitable for newbuilds and potentially for conversions, uses an L-shaped truss installed on the aft end of a jack-up to lift the fully loaded barge. Friede & Goldman compares the concept to a forklift.
“The barge comes in, fully loaded, with the forks below the waterline. It berths on the rack and the jack-up lifts it up out of the water,” the company explains. “We can do this with a single set of components for a 20-MW turbine. Once the components have been installed, the barge is lowered back into the water and departs, the jack-up moves to the next location, and another barge comes in with components for the next turbine.”
Friede & Goldman says a jack-up of the type that could undertake this kind of work comes in at around 100 m overall with a breadth of 55 m and depth of 11 m. It would have a leg length of 130 m and be capable of jacking up with the combined load – including barge and components.
It says the fact that a jack-up like this could be built at many different yards “would be very advantageous from a cost point of view.”