April 2005

The Lynn and Inner Dowsing windfarm sites lie off the Lincolnshire coast, approximately 8km east of Skegness. For this development of 60 wind driven turbines the resources of two Fugro companies, Fugro Engineering Services (FES) and sister company Alluvial Mining Ltd, were employed by Offshore Windpower Ltd to determine the ground conditions for the foundations of these wind turbine structures.

Fugro Engineering Services Limited operated two jack-up platforms and also performed a separate shallow site investigation along cable routes and at some of the turbine locations, each jack up was fitted with a drilling spread to obtain deeper borehole data for mono-pile foundation design.

The Lynn and Inner Dowsing sites cover areas of approximately 2.5km by 4km and 2km by 5km respectively, with water depths between 6m and 13m below LAT. Soils encountered included Holocene sediments above Boulders Bank Formation sands and sandy clay, overlying cretaceous chalk.

The investigation involved several phases. The shallow site investigation was performed from the survey vessels Voe Venture and Telco Baltic, Alluvial Mining deployed an 8-tonne thrust seabed-mounted CPT and a 6m vibrocorer to obtain geotechnical data at 16 locations on the Inner Dowsing site, 25 on the Lynn site and 20 locations along the cable routes. The Voe Venture was used for the cable routes and the Telco Baltic for the remainder.

The two jack-ups were used to drill twenty-nine boreholes to maximum depths of around 50 metres. Cable percussion techniques were employed in the upper cohesive soils, supplemented by rotary coring with a wire-line Geobore S system in the underlying chalk.

In the upper cohesive soils a number of Standard Penetrometer Tests (SPTs) were completed and open-tube 100mm samples recovered and sealed to preserve their natural moisture content.

In the rotary part of the boreholes, FES conducted further SPTs within the weathered chalk to correlate with the core descriptions. High-pressure dilatometer tests were also carried out in six boreholes to measure the in-situ strength properties of the chalk.

FES subsequently completed a series of geotechnical soils tests at their UKAS-accredited laboratories; these were supplemented by chemical analyses at an independent specialist laboratory.