Ensuring structural integrity for first US offshore wind farm

The Challenge

The offshore wind farm design comprised five 6 MW wind turbines in the Atlantic Ocean, just under four miles from Block Island, off the coast of Rhode Island, US.

Keystone Engineering’s optimised design was based on four-pile jacket substructures. Compared with traditional monopile approaches, this reduced steel requirements by 15 % and installation costs by 20 %.

Keystone was acutely aware that the commercial viability of this trailblazing project would determine the future of offshore wind development in the US. It was vital to:

• Optimise the pile-driving operations;

• Monitor noise levels, above and below water;

• Monitor the degree of scour caused by the velocity of flow around the pile.

The Solution

Pile-drivability analysis

We started reviewing the existing geophysical, geotechnical and laboratory data in May 2014. Combining all the third-party data from eight borings enabled us to improve the definition of the soil parameters and recognise continuous strata across the site – mainly hard clay and very dense sands, which we are highly experienced in working with.

We applied all this knowledge to produce a generalised ground model that was based on strata, rather than boring locations. We then generated the pile design data for Keystone, including soil pile interaction: axial lateral load analyses; and axial capacity in tension and compression for the piles.

We also studied the effectiveness of four different pile-driving hammers. Keystone acted on our recommendation to use hydraulic hammer (the largest of the four).

Pile-driving plan

We created a robust and innovative driving plan that limited the impact of the hammering activity on pile fatigue and noise pollution (in-air and in-water). It included optimised hammer blow counts that we were confident would be sufficient to drive the piles to the required penetration (approximately 57 m).

Block Island is known for its complex weather systems, so our pile-driving plan included clear parameters for decisions about if and when to halt operations if a storm were forecast. Fortunately, the sun shone during the three days of piling for each jacket and the 20 piles were installed as designed – on schedule, with no refusal and no damage incurred. The farm started operating in December 2016.

Scour monitoring

We installed two Fugro acoustic beam scour monitoring systems on a pile jacket. They generated a continuous record of the elevation of the sea floor at four specific distances from the jacket during a 14-month period. The resultant data confirmed that the extent of the scour is lower than predicted.

Innovative highlight

We optimised the hammer type, size and stroke, to minimise pile fatigue damage and noise pollution.

Our innovative hammer technique involved starting off with a low stroke and increasing it incrementally. This ensured that the hammer operated at full stroke only as it approached the final penetration depth.

The plan included carefully calculated hammer blow counts for the lower bound coring and plugged cases. The hammer drove the piles to their target depths, with no refusal or damage.

Impact

Our involvement enabled Keystone to:

• Benefit from our considerable experience in working with hard clays and dense sands;

• Achieve substantial cost-savings – we were able to use existing third-party data, avoiding the need to perform additional site investigations;

• Enhance the definition of the soil structure and produce a reliable ground model for design;

• Optimise hammer type, size and stroke;

• Minimise pile fatigue damage, to enhance the long-term integrity of the structure;

• Minimise in-air and in-water noise during the installation process;

• Gather an entire year of scour data, which will be used to develop a realistic model for scour management.