Jacket required for harsh marine locations

WORLDWIDE: Developers of future UK offshore wind farms have a number of challenging issues to address.

En route… A barge carrying two four-leg jackets is being towed by tug to the Ormonde site off the coast of Cumbria in the UK (pic: Vattenfall/Ben Barden)
En route… A barge carrying two four-leg jackets is being towed by tug to the Ormonde site off the coast of Cumbria in the UK (pic: Vattenfall/Ben Barden)

These include the vast scale of planned projects, the large amounts of money involved, tight schedules and intense competition for the offshore supply chain from other wind-power developers as well as the oil and gas industries. In addition, they will be working with bigger turbines characterised by more complex load profiles, in deeper waters with harsher sea and weather conditions. All these factors make projects more hazardous and expensive.

To minimise risk and cost, a successful foundation design must not only be robust, but also quick and cost-effective to manufacture and install. Quality is paramount because any mistakes or defects could be replicated in a great number of foundations.

Beyond monopiles

Most offshore wind farms to date have been built near shore in shallower waters using monopile foundations. Monopiles are relatively simple to design and manufacture, and the industry has built up plenty of experience in using them. But they have not been without their problems, principally the failure of the grouted connection between the monopile and the transition piece connecting to the turbine tower.

However, the industry has no experience of handling designs suitable to carry 5MW-plus turbines in 35-metre water depths, which means there is no proven low-risk or low-cost solution. Installation is also an unknown in terms of pile-driving extremely large structures that require an immense amount of impact energy to drive them into the seabed. This is before one considers the potential environmental consequences associated with the noise of such an operation, or the difficulties in keeping a structure with the requisite diameter vertical during this process.

Gravity-base foundations (GBF), have been the next most popular foundation type, mostly used to support smaller turbines in shallow near-shore projects with rocky seabeds. The use of GBF in the only completed project with larger turbines and deeper waters — Thornton Bank Phase 1 using six Repower 5MW turbines in depths of up to 19 metres — proved more complex than envisaged by the developer. RWE switched to four-leg jackets to complete the remaining two phases of the project, which used bigger 6MW turbines, the last of which were installed earlier this year.

Jacket types

There are many different types of lattice-frame jacket foundations: three-legged, four-legged and even one variety with six legs. Some have vertical legs, others have angled legs and one has twisted legs. The perception of these jacket foundations is that they are comparatively expensive to fabricate and install.

The twisted jacket has few joints to produce, which saves on costs, but the sections are heavier. It uses no less steel and the twisted leg structure requires twisted piling. It also needs a centre pile-driver that must be vertical as the jacket sits over it. The twisted jacket has been trialled with a met mast but is yet to pass the insurance industry's proven technology test for a wind turbine foundation.

The hexabase jacket uses standard pipe sections in the lattice structure. Their use cuts the cost of steel by up to 40% according to weight, but a six-leg structure is much more complicated and costly to make. Low-cost manufacturing is all about reducing set-up costs and this solution may require more such operations. It also has six piles and hence six piling operations with, potentially, six times the amount of piling noise.

The four-leg jacket has the largest market share and can rightly claim to hold a proven track record. However, it still has one more leg than a three-leg jacket, so brings with it 33% more of that expensive set-up time and piling.

The original four-leg jackets were installed with skirt piles and mud mats. This meant the jacket could be installed and then piled as two separate operations by vessels suited to each task. However, this flexibility of installation was not realised, as the same expensive installation vessels were used to do the installation and piling. As a result, the industry has moved to pre-installed "pin piles", which can be installed ahead of the jacket and avoids the need for mud mats. The downside of this approach is that the pile installation vessel must be fitted with an expensive template to ensure the piles are driven into the seabed in exactly the correct position.

Standard footprint

Installers demand that only one or two templates are required for wind-farm development, meaning the installer essentially "fixes" the jacket footprint. But the water depth at a wind farm will undoubtedly change from turbine to turbine, so the jacket geometry must also be different for each depth, which somewhat limits the possibilities for standardising jacket production and reducing set-up time.

Many of the same points also apply to three-leg jackets, but they have fewer joints to make and fewer risky piles to drive, which in turn means less noise. And when it comes to holding level while the grout sets between the jacket and the pile, three legs are also better than four. Have you ever tried to level a wobbly table in a pub or restaurant with a beer mat? Three-legged stools never have that problem.

But a fear with the three-leg jacket is that if one leg fails it becomes highly unstable, a concern that goes back to the oil and gas sector, where the consequences of such a failure are considerable. The wind industry, which would not suffer as dramatically, should focus on finding a solution to eliminate this problem.

Jacket foundations have been used by the offshore oil and gas industry for many years and require fabrication of many welded node joints between the tubular elements of the lattice.

These complex joints have largely been constructed by manual welding methods — a long and costly process. If the wind industry is to use them in large numbers, such manual manufacturing methods cannot continue, neither in terms of cost nor capacity. There are simply not enough large facilities in Europe to meet the demand, nor can the industry afford such expensive production methods. It must move to a more industrialised and mass production approach.

Sweet spot

This industrialisation must start with the design of the jacket, with low-cost production in mind. It is not enough to produce elegant designs that use steel efficiently, nor ones that are cheap to install. They must also be easy and cheap to manufacture in large numbers. The winning jackets will be those that hit the sweet spot between design, manufacture and installation.

Standard designs will have additional advantages for transportation and installation. Sea transport grillages can be kept the same, handling rigs can be identical.

All these peripheral items can add substantial costs to an offshore wind project.

The basics of mass production have been used in other industries for many years. We just need to learn the lessons, adapt them and apply them to the offshore wind industry. To reduce the levelised cost of energy to acceptable levels, we need to attack the cost of wind projects at all levels. There is no single silver bullet to save the day. It is much more likely that we will achieve our target by a thousand cuts.

Jon Wren is programme manager for the Triton jacket design at OGN North Sea


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