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Gravitational pull

Written by  Elaine Maslin Monday, 30 October 2017 00:00

Two offshore wind firsts have been achieved this year, both relating to use of gravity-based foundations. Elaine Maslin reports.

While gravity-based wind turbines are not new, they’re far from the most common form of foundation for the offshore wind industry.

To date, the most common foundation used for offshore wind turbines has been the monopile. According to trade body Wind Europe’s 2016 report, 81% of substructures installed were monopiles, with just 7.5% gravity foundations (the same as in 2015, at 315). The rest were jackets (6.6%), tripods (3.2%) and tripiles (1.9%).

This year, however, has seen two new innovations in the gravity foundation space for offshore wind. At the Blyth Offshore Wind Demonstrator in the UK, gravity-based foundations (GBFs) were built then floated out to site and submerged – a first. Meanwhile, in Finland, GBFs designed for ice-impact have been installed at the Tahkoluoto project.

The Vole au Vent vessel. Image from Jan de Nul Group.

Tahkoluoto

An offshore wind farm setting out a few global firsts is Tahkoluoto, Finland’s first offshore wind farm (not built on an island) – and the first offshore wind farm designed to withstand ice loading.

Tahkoluoto, which was completed this September, is a 10-turbine farm, using Siemens’ 4.2MW turbines, on top of GBFs.

What makes Tahkoluoto different is that it’s sited 0.5-3km offshore Tahkoluoto, in the Gulf of Bothnia, near Pori, on Finland’s west coast, in up to 8-15m water depth.

Finnish waters offer a different challenge to the North Sea, says Xavier De Meulder, marine operations manager, Tahkoluoto Offshore. The country’s Baltic Sea coast sees ice flowing down from the north west into its shallow waters. The seafloor is strewn with boulders, beneath which is rock bed interwoven with mud, “making it tricky to install anything,” which led to the use of GBFs.

Despite the challenges, project owner Suomen Hyötytuuli Oy is already considering an expansion of the offshore wind farm, with Tahkoluoto 2, adding 100MW after five years, in more like 25m water depth, says Toni Sulameri, managing director, Suomen Hyötytuuli.

Suomen Hyötytuuli has taken a step-wise approach to its Baltic wind farm ambitions, however. It started with an initial design for a GBF, which was built and installed in a one-turbine pilot in 2010, with a 2.3MW Siemens turbine, 1.2km offshore in 9m water depth, also at Tahkoluoto.

Designed for ice

The pilot used an ice-strengthened steel shell GBF, based on principles used for ice-class vessels, which is filled with rock for ballast once sited. It has a ring footing and a conical top to withstand ice ridges, which can drift along the coast.

For Tahkoluoto, the same GBF has been used, but optimized and three similar but separate designs created, according to site specific characteristics. These weigh 450-500-tonne, largely due to differences in steel thickness.

The designs were developed to withstand the highest ice conditions, based on meteorological data going back 50 years, says De Meulder, and taking into account wave and wind conditions, as well as subsoil, which all impact how fast the ice moves, and therefore the impact it would then have on the structures.

Unlike traditional wind turbines, where grout is used to position and fix the transition piece in place offshore, the transition pieces on the Tahkoluoto GBFs, built by Technip Offshore Finland, are pre-welded in place before being installed. The main turbine column is then placed on the transition piece.

While it was easier installing the transition piece in the shipyard, however, this means tighter tolerances during installation – the seafloor has to be level, the structure has to be built with tight margins, and the lifting and positioning margins are also tight, says De Meulder.

Preparation and Installation

Enerpac’s Synchoist X-frame. Images from Suomen Hyötytuuli Oy.

Work on seabed preparation for Tahkoluoto started in April 2016, to avoid the October-April winter season, and involved digging foundation pits, which were then filled with crushed rock, which was then compacted to create a level seabed.

In June, Jan De Nul Group’s Vole au Vent jackup vessel completed the installation of the 10 GBFs. While it might have been a bigger vessel than possibly required, it was the right tool and meant no waiting on weather, says De Meulder. The careful positioning operation was done with the help of Enerpac’s Synchoist load positioning system. The system was used below the crane hook during lowering through the splash-zone and positioning on the seabed, to ensure the foundation remained as close to vertical as possible to prevent damage to the levelled seabed surface. The wirelessly operated system, comprising a lifting frame with four SyncHoist, self-contained PLC-controlled, double acting, push-pull hydraulic cylinders at each corner, and diesel hydraulic powerpack with battery back-up, also support the installation of the turbine towers.

The system was able establish the center of gravity for each foundation, on the vessel, just above the water, 3-4m into the splash-zone, and 5cm above the foundation. The first foundation took 12 hours to install, later foundations took eight hours as the installation team became more proficient, Enerpac says.

Each GBF was then filled with ballast and scour protection added over the ring base. The Vole au Vent jackup then installed the turbines – a more routine job, compared to the GBF installation – before demobilizing to work on the Blyth project. Finally, about 14km of a 30kV undersea cable was laid in trenches, which were then backfilled. Because the wind farm is so close to shore, it is connected via four cables to an onshore substation.

Operations
While weather conditions haven’t been as bad as they used to be – the last bad winter for serious ice conditions was in 2011-12, says De Meulder. Changing weather patterns have made weather prediction harder, making day-to-day operations trickier. In winter, especially, operations vessels will have to navigate lumps of ice in the water. Because of this, an aluminum crew transfer vessel is used in summer, but a steel-hull former naval vessel is used for access in winter.

In addition, the boat landing ladder on each GBF has a heating element, to stop ice forming – as happened on the pilot turbine foundation, which meant the ice had to be chipped off to gain access. Now, the four main pillars are heated to prevent icing.

For future projects, Suomen Hyötytuuli will want to bring in more expertise relating to the seabed, says De Meulder. “It’s an important element of installing the wind farm. If we don’t get the bottom right, then you could lose a lot of time and costs. We were good on this project, but it’s just 10. The next part we really have to be careful.”

It’s likely the GBF design will also be further adapted, in consideration of the larger turbines being used today – up to 8MW, for offshore. But, whether larger or smaller turbines are used is yet to be decided, as it could influence the size of the foundation in such a way that larger vessels are needed, making operations less economical.

Blyth spirit

For Blyth, GBFs were chosen because of the chance to use the float and submerge method, reducing the need to use heavy lift vessels, as well as reducing the impact on the environment from installation operations in other methods (eg. Piling).

Blyth Offshore Demonstrator cable works. Photo from EDF Renewables.

Using GBFs also suited the ground conditions, which comprises a thin veneer of gravelly and/or muddy sands, on top of a bedrock, which would make piling very difficult, says operator EDF Energy Renewables.

The wind farm, 6.5km off the Northumberland coast, comprises five, MHI Vestas V164 8.3MW wind turbines (another first, for a GBF) with a total generating capacity of 41.5MW, connected using 66kV rated cables from JDR (another first).

Fully installed, each 60m-high GBF (from seabed to access platform) is made up of more than 1800cu m of concrete and weighs more than 15,000-tonne. All five turbines were installed in late September.

EDF Energy Renewables acquired the rights to develop the project in October 2014, having taken it over from NAREC, now the Offshore Renewable Energy (ORE) Catapult.

Part of the attraction to using GBFs for the project was that, unlike steel foundations, self-buoyant GBFs can be mass-produced locally using conventional civil engineering construction skills. They’re thought to be economical in 35-60m water depth, depending on ground
and/or environmental conditions.

The GBFs comprise a concrete caisson and a steel shaft. Designed and built by Royal BAM Group, the GBFs were constructed in the Neptune Dry Dock on the River Tyne, northeast England, with the shafts fabricated in the Netherlands and shipped to Newcastle to be installed into the caissons. Once complete they were floated down river to the Port of Tyne, where extra ballast was added before being towed offshore to be submerged.

While piling isn’t required, the seabed at each foundation location had to be dredged and two layers of different size gravel placed to create a level surface and suitable depth. Once the GBFs were on site, Dutch contractor Strukton Immersion Projects used a specialist vessel to pump sea water into the foundation as ballast to lower it to the prepared sea bed. Once in place, the water ballast was replaced with a sand ballast to keep the foundation on the sea bed.

Once all five GBFs were in position, VBMS laid 66kV inter array and export cables to connect the wind farm to a new substation being constructed at Blyth.

The turbines were than installed using the Vole au Vent jackup. It is anticipated that the turbines will start generating power by the end of the year.

“The project has provided an opportunity to optimize the design of the GBFs by combining two technologies – one that makes the floating concrete structure and the other which uses a monopile steel shaft,” says EDF Renewables. “This challenge was overcome successfully with a fully certified design from DNV GL.”

As part of the project, the ORE Catapult designed a sensor system in two of the five GBFs. This is the first time that sensors have been installed in a GBF to analyze the performance of the foundations in the challenging conditions to which they are exposed out at sea.

The project was conceived as having maximum of 15 wind turbines across three arrays, with the first five turbines being array 1. However, EDF says: “There are currently no immediate plans to develop the other two arrays, but the option to develop them at a later stage will be kept under review.” 

 
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