Four installation sequences, one jack-up

The Saint-Nazaire offshore wind farm sits on the Banc de Guérande, 12 to 20 kilometres off the Loire-Atlantique coast, in 12 to 25 metres of water. France named Eolien Maritime France (EMF) the winning bidder in 2012, and construction was launched in September 2019. The farm became fully operational in November 2022: 480 MW from 80 GE Haliade 150-6MW turbines, each on a monopile with a 7.0-metre outer diameter below the transition piece. EDF describes it as France’s first offshore wind project.

The bank beneath it is calcarenite and dolomite, and its thickness is the problem. EDF Renewables’ published account gives a range from 0 metres, in a former valley of the Loire and in depressions, to more than 30 metres. The top of the bank is an eroded surface of holes, ridges, cliffs and overhangs, and its base degrades progressively into sand. That variation shaped both the monopile design and the way each pile went in. Driving into intact rock was ruled out because of the risk of pile buckling.

The answer was not one method but four, matched to the rock thickness at each position: fully driven, at seven locations without rock; drilled and grouted, where the calcarenite was more than about 22 metres thick; a hybrid of drilled, driven and grouted between about 10 and 22 metres; and a hybrid of drilled and driven with no grout where the rock was thinner than about 10 metres. For that last case, EDF Renewables notes that the design case is equivalent to a fully driven pile in water deeper by the thickness of the rock.

The 73 drilled positions used the 350-tonne Offshore Foundation Drill that DEME and Herrenknecht designed together, with a drilling method based on vertical tunnelling technology. It cut a 7.7-metre hole for the 7.0-metre pile, leaving a 0.35-metre annulus for grout where the sequence called for one; Herrenknecht’s record gives drilling depths of 6 to 24 metres. Around the drill stood MODIGA — Monopile Offshore Drilling Installation and Grouting Aid — nearly 60 metres high. It enclosed the drilling and installation work and lowered the drill under control through a liner, which doubled as temporary casing to hold the borehole open and was used to grout the bottom plug and the annulus as it was lifted out.

All 80 turbine foundations went in from one jack-up, DEME’s Innovation, and the vessel itself was adapted to the ground. Rock tips were fitted beneath its spudcans in dry dock, seabed preparation was considered at some positions, and a seabed impact assessment set the weather limits for jacking onto the rock.

Choosing the sequence for each position could not rest on the ground model alone. The seismic picture of the bank was degraded by shallow water, a hummocky rock surface and a base that faded into sand. It could not delineate calcarenite thinner than 5 metres, and EDF Renewables states that confidence was not sufficient to design the hybrid installations. So the final site investigation drilled a borehole at each monopile location, to establish the base of the calcarenite and determine the end of the drilling operation.

The borehole came before the sequence.

The published record runs as follows. The substation foundation, four piles drilled and grouted with a sacrificial top casing, was installed by Sea Installer on 21 April 2021. The first turbine foundation followed from Innovation in May 2021, and in June 2021 the first drilled XL monopile went in, which the trade press reported as a first for an offshore wind farm. Forty foundations were in by November 2021, Herrenknecht records the final drilling on 24 May 2022, and DEME and Eiffage Métal reported all 80 foundations installed, ahead of schedule, that month. The wind farm was inaugurated on 23 November 2022. Two years after installation, EDF Renewables reports a measured first natural frequency within 1.5% of the digital-twin prediction, excluding corrosion.

For the wider context of monopile campaigns, see the Dogger Bank case study and the offshore wind sector hub. For another case where the installation vessel entered the design, see Issue 14.