Suction piles have anchored offshore oil and gas production for years, but a new type - called a bucket foundation - is being promoted for offshore wind turbines in an effort to reduce costs.
In February, Forewind, a consortium planning a huge turbine farm in the North Sea, started testing the bucket foundation system, using it for a meteorological station at Dogger Bank site where the farm is to be built. Universal Foundation, a Danish affiliate of Fred. Olsen of Norway, originated, designed and patented the new foundation type. Harland & Wolff of Belfast is the manufacturer.
Universal’s bucket is one of four innovative foundations being pushed by the UK Carbon Trust’s Offshore Wind Accelerator research and development program, which aims to cut offshore turbine construction costs and make wind power more competitive.
The other three OWA-endorsed designs are a tri-bucket system from SPT Offshore, a twisted jacket from Keystone Engineering, and a gravity base from GBF. All four, including Universal’s bucket foundation, aim to cut costs by simplifying installation.
The Universal design is the closest of the three in appearance to a monopile, although it has a much wider base. It amounts to a giant upside-down bucket with a shaft rising from the center of its upturned bottom.
The shaft extends above the water and is topped by a distinctive integrated transition piece onto which a wind turbine or other topside structure can be installed. Usually, adding the transition structure is a separate step, Universal says.
Universal says the bucket foundation can work in waters out to 50m deep, the relative shallow water depth at which most of the world’s wind turbines are installed.
Like traditional suction piles used in the oil and gas industry, the bucket foundation fills with water when placed into the sea at the start of installation. Pumps then suck out the water, creating a pressure gradient that, along with the structure’s weight, pushes it into the seabed, like a cookie cutter slicing into dough.
Unlike suction piles, the bucket foundation’s skirt has controllable, water-emitting jets or nozzles lining the lip where the skirt cuts into the seabed. Varying the force of water shooting from the jets into the seabed helps ease the bucket into place in a level orientation.
“The rim of the skirt is equipped with a unique system for distribution of water pressure,” says Soren Nielsen, Universal’s technology director. “The bucket structure will, by these means, be steered vertically, allowing precise location within the inclination tolerances.”
Using this approach, the monopilelike structure extending above the water ends up almost perfectly vertical, without grouting or other subsea work. The typical wind turbine inclination limit is 0.25°. Universal can achieve installation within 0.1 degree of vertical, promoters say.
As for stability and strength after installation, the bucket foundation can resist the large forces exerted on it by wind and waves and remains in place because it combines the benefits of a gravity structure and a monopile, Nielsen says. The design also prevents scour, avoiding the need for anti-scouring measures.
“The stability of the foundation is ensured by a combination of earth pressures on the skirt and the vertical capacity of the foundation,” he says.
There have been questions about the bucket foundation: Installation in seabed that is too rocky presents challenges; manufacture requires a lot of welding; delivery and installation methods remain to be perfected.
Universal has answers for all of the questions: If installers encounter unexpected rocks, the bucket foundation is easy to shift slightly; the relative shallowness of its penetration into the seabed, compared with traditional suction piles, also helps when installing in potentially problematic seabed.
As for the welding in manufacturing, promoters say it is a benefit because it enables the cost savings associated with unitization.
As for hauling and installation, existing jackup vessels have the capability needed for delivery, and other hauling options are being studied for larger projects; installation has been proven at Dogger Bank and will be optimized in future projects, Universal says. But the bucket foundation has to do more than work effectively to be competitive. It has to cost less.
The Carbon Trust’s head of offshore wind, Phil de Villiers, warned a recent UK conference that offshore wind power development could stall without more powerful turbines, lower-cost foundations and full-scale demonstrations that the technology can be cost effective.
Cost is clearly an issue; especially with offshore turbines costing as much as four times onshore turbines, in the range of £3-4 million per MW of capacity.
Universal is happy to talk about cost. They estimate the design can shave 30% off the cost of traditional subsea foundations (the old-style monopoles, jackets and gravity bases).
First, the bucket foundation is much lighter. It uses less steel. “Compared to monopiles, we expect to demonstrate a 25-30% reduction in weight,” says Universal spokeswoman Michelle Maria Langkilde.
With the transition piece already attached, installation requires fewer steps. Importantly, the design avoids the grouted connection that can lead to future problems.
“The traditional foundation relies on a grouted connection, and in recent years, failing grout connections have gained the attention of industry experts and could result in huge upgrade investments for already operational wind farms,” Langkilde says.
Universal points to the bucket’s comparative ease of decommissioning and removal, which allows it to be reused or recycled, cutting costs even more.
Complete removability also is an environmental benefit. When a site is decommissioned, pulling the bucket completely out returns the seabed to its prior condition. Traditional monopoles often are cut off at the seabed with their bottoms left embedded.
Promoters also tout the lower environmental impact of installation. Unlike conventional foundations, Universal’s requires neither pile drivers nor vibration machines that disturb marine life. Two other Universal bucket foundations have been installed; one in 2002 to support a 3-MW demonstrator turbine at Frederickshavn (harbor), Denmark, and a second in 2009, to carry a meteorological station at the Horns Rev 2 wind turbine site, also in Danish waters.
Two other bucket foundations are under contract for met stations; one at Dogger Bank, and for the Seagreen turbine site in Scotland’s Firth of Forth, Langkilde says.
It is too early to say which foundations will be used for the wind turbines at Dogger Bank as that decision will be made by the lead operators of the various parts of the sprawling development, a Forewind spokeswoman says.
But Forewind General Manager Lee Clarke notes the bucket’s advantages, saying that it was chosen for two met masts because it uses less steel than conventional piled foundations and its design removes the need for pile driving, seabed preparations, scour protection and a transition piece.
“We have taken our requirements for met masts to look beyond the standard approach and instead use the opportunity to demonstrate a new, and potentially very exciting, technology with possible benefits well beyond just the Dogger Bank development,” he says. OE