Navigating ground risks in an intertidal zone for UK wind farm development

Challenge

In order to connect the Hornsea 4 Windfarm to the grid, Ørsted needed to understand the feasibility for construction of the new cable route through the intertidal zone.

The ground conditions at Hornsea, a complex mix of rock and sand, required specific equipment to be utilised so that our geotechnical team could conduct the onsite ground investigation (GI). The tidal conditions were also an important factor to consider whilst on site and required careful planning of crew changes and setup procedures.

One particular challenge within the investigation area was delivering robust characterisation of the Rowe Chalk Formation. Testing included strength, degree of weathering, special chalk properties, fracture characteristics and thermal conductivity, to allow for the design of the cable route using horizontal directional drilling (HDD) techniques.

Solution

Fugro provided expertise on HDD landfalls and utilised state-of-the-art equipment to provide Ørsted the best quality Geo-data in this complex site. To better inform the site investigation (SI) phase, our consultancy team ran a geotechnical desk study to advise Ørsted on the best locations for the investigation.

Feasibility assessment and desk study

Utilising a database that captures all recent and historical SI in the area (from 1987 to 2012), we were able to rapidly appraise detailed geological and geotechnical conditions, as well as the risks, to a depth of 35 m.

This Geo-data included:

• an enhanced interpretation of the local geology

• associated potential geohazards

• mapping of beach exposures

• geomorphological mapping to infer ground conditions situated away from existing data points

Ground model

Ørsted received an enhanced understanding of ground conditions for their cable landfall options and a visualisation of this data through a conceptual ground model. The ground model was used to help shape geotechnical parameters and provide a hazard matrix, benefitting Ørsted with comprehensive insights into the complex, variable and potentially risky geology in this section of the North Sea.

Ground investigation

For Ørsted to receive the optimal design parameters for development of the cable landfall route, the scope of works included:

• Jack-up barge situated in intertidal zone

• 4 boreholes using a combination of cable percussion and triple tube rotary coring

• 4 cone penetration tests (CPTs) using piezocones to measure pore pressure and thermal cones to determine the thermal conductivity along the cable route

• A total of 136 m of drilling

• Laboratory testing for geological description including CIRIA grading and offshore and onshore laboratory testing

Ørsted were able to access real-time data through VirGeo®, Fugro’s cloud-based data engagement platform, which was continuously updated over the two weeks of onsite activity. Geotechnical data was provided to the project designers and changes in scope could be assessed swiftly.

Rowe Chalk Formation

During the project, it was determined that the Rowe Chalk Formation was far more fractured than anticipated. This enabled us to make recommendations to Ørsted for mitigating the impact this would have on HDD techniques, saving potential delays and allowing for a more accurate assessment.

Innovative highlight: thermal CPTs

The use of thermal CPTs is becoming more common with cable landfall projects because of the need to know the thermal properties of the ground. Fugro’s thermal CPTs provided insights into the heat flow characteristics of soil, such as ground temperature, temperature gradient, thermal conductivity and volumetric heat capacity.

The thermal conductivity of the soil surrounding a cable, especially when it is in a land or nearshore setting, can determine its efficiency and safety. This is an important factor to understand, as the hotter the cable becomes, the less efficient it tends to be at transporting power into the grid.

Knowing the thermal resistivity of the ground at the early stages of the project allows for adjustments to designs and installation plans, removing potential risks, and reducing the possible impact on operations in the long run.

Impact

The detailed characterisation of the ground properties at potential landfall locations allowed Ørsted to have a greater understanding of the site and more certainty around the cable route design.

All works were delivered by Fugro completely in-house, removing multi-tiered sub-contract risk and maintaining full control of the deliverable timelines. Ørsted had a single point of contact with us so we were able to ensure the focus was wholly on quality deliverables and not the management of multiple contractors, enabling the work to be completed on time and within budget.

This investment in site investigation at the beginning of a project allows for better risk management and reduced uncertainty going forward, particularly in intertidal zone sites which can have increased risk.