Installation-Driven Foundation Selection

Author: Ben Andrew – Director, 2H

Offshore wind turbines are becoming larger and installed in deeper waters, which increases the challenges faced by engineers designing and installing the foundations. Turbines that are currently under construction will generate up to 15MW of energy, and developers planning for the next decade and beyond are assuming the availability of turbines with capacities of 20MW or more. This, coupled with increased water depths, means that the loads placed on foundations will only increase, requiring longer (100m+), larger (12m+ at base, 10m+ at top), and heavier (2000Te+) foundations that can make installation more challenging.

Site-specific ground conditions are also critical to selecting the most appropriate foundation type and design. Sands, clays, and rocks can be present in the seabed environment, and a risk assessment must be performed to determine the level of confidence in the ground conditions based on the detail and extent of the geotechnical survey. It is essential to understand the installation technologies, challenges, and risks when selecting the foundation type and design and hence these must be assessed early in the project life cycle.

Monopiles have been the preferred foundation type for offshore wind turbines, but as turbines get larger and move into deeper water, handling and installation can become more difficult. Increased risks of both refusal and pile run can also present unexpected challenges during piling operations. In addition, anvil sizes are reaching their limit and hence less efficient piling arrangements are required which adds to the mounting challenges. Furthermore, the use of larger impact hammers without careful hammer selection and operation can lead to increased noise that can be damaging to marine wildlife.

Solutions to meet the challenges of larger turbines in deeper water

To mitigate some of the challenges faced during the installation of large foundations in uncertain soil conditions, pile-top drilling can be used as part of a drive-drill-drive (or ‘3D’) programme provided by LDD, a marine foundations brand in Acteon’s Engineering, Moorings and Foundations division, to overcome the early or expected refusal, or even to reduce required hammer energies and noise levels.

Installation-Driven Foundation Selection Fixed Assets

Piled jacket foundations

Piled jacket foundations are an alternative to monopiles, but they come with an incremental cost and their own installation requirements. Increased installation vessel specifications (hook height, lift weight) are often needed together with additional temporary works such as pre-piling or pre-drilling templates. As a result, this foundation type is generally considered undesirable compared to monopiles. There are other foundation types such as suction bucket jackets and gravity base structures that can be good solutions under the right conditions.

The foundation selection decision is not purely driven by installation considerations. Design, manufacturability, transportation, environmental impact, operation and maintenance, and decommissioning all play a role. Appropriate weighting should be applied to each of these considerations, and this may vary between different localities and regulatory frameworks. 2H, part of Acteon, provide assessments that use a risk-based scoring matrix aiming for independence and transparency to ensure that the most appropriate foundation selection solution is reached. It considers the parameters mentioned above as at a minimum but can be customised to specific requirements.

The increased size and weight of foundations, coupled with varying ground conditions means that decisions on foundation type and design will be increasingly driven by installation considerations, rather than just the in-place design. To successfully design and install foundations for these turbines, engineers must carefully consider site-specific ground conditions, understand the installation technologies and risks, and be prepared to use innovative but still technically viable installation techniques.

Pile-top Drilling

Through 2H, Acteon can provide this high-level technical expertise. Acteon is also independent of vessels or concepts, which allows for practical efficiency and adaptability to each project’s circumstances.