Offshore wind design

Early stage bidding carries considerable risks. Using local soil, bathymetry, metocean and wind loading data in a Dogger Bank monopile and foundation concept design study for Norwegian national renewable energy company, Statkraft, LICenergy UK analysed detailed design data to identify optimum dimensions that would support an 8MW wind turbine in water depths of 20m to 35m. An initial 96 models were whittled down to 12 for full extreme and fatigue loading analysis.

( Text content written by Twenty6 for Luke Fussell, LIC Energy )

Client challenge

To support the early stages of a potential bid to develop a proposed wind farm at the Creyke Beck B site on the North Sea’s Dogger Bank. LICenergy UK was asked to undertake a concept design study focussed on the sizing and cost of monopoles.

A key aim of the project was to remove commercial and technical uncertainties common in early stage bidding.

Our response

We began by developing a design basic document for the Creyke Beck B site using local soil, bathymetry, metocean and wind loading data. Circa 100 initial models bracketing the most probably site conditions were subsequently reduced to the 12.

A full analysis was then run on this selected group to identify the most appropriate monopile dimensions.

Site challenges: – These included site water depths varying from 20m to 35m, plus the need for monopile foundations able to support a new generation large 8MW wind turbine mounted on a 6.5m diameter tower.

The client also specified a grouted connection be used to join the MP and TP.

After assessing different approaches, we selected a conical transition making it possible to accommodate a wider, and therefore stiffer, MP diameter with a higher loading capacity.

This meant that pile penetration could be reduced, leading to lower potential steel costs. However, there is a fine balance between lean piling steel and the increased weight of larger MP diameters.

Comprehensive analysis: – We ran 96 different computer models to investigate a complete range of depths and soil conditions. Natural frequency analyses whittled this down to a final 12 covering all combinations of three different depths and four different soil profiles.

A full analysis was then run on this final selection to examine extreme and fatigue loading. The resulting realistic cost model was fed into the client’s CfD (contract for difference) preliminary bid work.

LIC’s in-house NONLIN software, which runs multiple structural models to make development swift and efficient, was used to optimise final MP designs.

Benefits

Bidding for offshore wind farm contracts often involves a massive investment of finance and time.

On this project, LICenergy UK used its extensive design experience to quantify and reduce unknowns that can often generate expensive risks during the opening phase of large projects.

The more accurate the initial design and cost information available, the greater the opportunities to reduce potentially expensive uncertainty. High data quality and interpretation are important.