Two piling techniques used widely onshore have shown promise for potential use in the offshore wind industry, Heerema reports
Heerema has confirmed that DNV GL has awarded a statement of feasibility to two ‘silent foundation’ concepts it has been developing.
The aim of the development projects is to find ways to install foundations without the high levels of underwater sound propagation associated with pile-driving during monopile installation.
The concepts the company has been investigating are helical or ‘screw piles’ and so-called push-in piles.
Heerema says despite the challenges associated with 2020, the development programme continued with support from University of Dundee, where an extensive testing and modelling programme was undertaken.
As part of a technology qualification process, the results of the tests were discussed with DNV GL, and several workshops were conducted to review the foundation concepts.
Heerema says the statements of feasibility are the first formal step towards qualifying the technology for use on offshore structures.
A paper published by The Royal Society, Helical piles: an innovative foundation design option for offshore wind turbines, B W Byrne and G T Houlsby say the concept has “many advantages.’
These include “very good tensile loading characteristics” and that it can potentially be used in a wide range of soil conditions, as demonstrated by their use for onshore applications.
The authors of the paper say they are also “reusable” in the sense that they can easily be “unscrewed”, and this will aid the eventual decommissioning of structures.
“The torque motors that could be used to drive the helical piles into the ground will be relatively quiet, at least when compared with conventional piling, and they could be sited over water or underwater,” says the paper. “Importantly, the measurement of the torque during installation will provide a quality assurance on the pile capacity, as good evidence exists that relates torque resistance to vertical capacity.”
However, the authors of the paper note that “a considerable development of the technology is required,” not least because helical piles for offshore wind turbines would be significantly larger than those used onshore and will require an installation torque that is up to two orders of magnitude greater than that required onshore, although broadly comparable to the torque required for offshore drilling.
“A range of structural engineering issues will need to be resolved including the design and fixing of the helical plates as well as fatigue. However, such issues can be resolved using standard fabrication methods, and indeed it should be possible to develop a modular foundation system, based on the helical pile technology,” the authors of the paper say.
The final area where further work is needed relates to practical installation activities including the provision of the reaction torque, although the maximum torque is not reached until full penetration of the pile.
According to a paper published by International Journal of Environmental Studies, An investigation into the use of push-in pile foundations by the offshore wind sector, by David Igoe, Kenneth Gavin and Brendan O’Kelly, push-in piles are proven to provide an environmentally friendly alternative to driven piles when used in urban settings, where noise and vibration generated by pile driving is unwelcome.
In addition to environmental benefits offered by a push-in systems, the authors of the paper found that piles installed using minimal load cycles may also develop higher load resistance with “greatly enhanced stiffness at the pile base when compared with driven piles.”
As highlighted above, helical piles are not a new concept onshore but have not been used offshore. What is new about the concept being examined by Heerema is the size of the piles and the use of a large rig that will make it possible to screw the pile down. Also under consideration is an option to use a pile with different diameter along its length.
DNV GL says the main uncertainty relating to the adaption of the concept for use offshore for large piles, relates to its use in soil types other than those in which it was tested. Heerema is updating results for other soil profiles.
The class society says technology qualification can continue with an assessment of possible failure mechanisms and their cause.
The push-in pile replaces a conventional tubular pile with a cluster of four smaller-diameter open tubular piles.
Using a number of strokes, each of the piles in the cluster is pushed into the soil, with two or three piles in the cluster providing the uplift resistance required to push in a third pile, with a tool gripping the ‘uplift’ piles and pushing down onto the pile that is penetrating.
By sequentially pushing-in each of the piles while holding the others, the cluster as a whole penetrates into the seabed.
Analysis demonstrates cost-effectiveness of gravity tripod
BVG Associates (BVGA) in the UK recently completed a cost analysis of a gravity tripod foundation that demonstrated the commercial competitiveness of Offshore Wind Logistics and Construction’s (OWLC) concept. The analysis was used to demonstrate to potential investors the merits and commercial possibilities of the foundation.
The gravity tripod is designed for mass manufacturing, fast assembly and easy installation. BVGA says the concrete substructure has several advantages over conventional foundations. It has a low seabed bearing pressure, so it requires less seabed preparation than other gravity-type designs. It can also be assembled quickly and requires no piling for installation.
OWLC describes the gravity tripod as “a componen-based structure that benefits from the best aspects of other foundation concepts. It is a hydro-dynamically transparent structure which doesn’t require piling, is manufactured from low-cost tubular sections in a rapid assembly process and is installed with minimal seabed intervention.”
The two components of the gravity tripod act together to ensure the structure has an extremely long design life and is insensitive to turbine loads. The structure is able to accommodate turbines from 3 MW to 16 MW. In addition, the structure has a low bearing pressure on the seabed and so is capable of accommodating a wider range of sediment types, with less seabed preparation, than other gravity-base designs.
Other features of the design highlighted by OWLC include: reduced construction time and risk; a long service life; it is insensitive to turbine type; and reduced manufacturing costs. It is suitable for use in water depths of 20-65 m and with a range of seabed types. The company believes it is 20% less expensive than its competitors when installed.
To achieve commercial success, the foundation needs to be technically validated and prove it can be economically competitive with conventional options. To achieve this, OWLC has progressed the engineering and certification work, and asked BVGA Associates to carry out an independent commercial review; including cost comparison and levelised cost of energy analysis.
The analysis considered a commercial, 500-MW offshore windfarm with 12-MW turbines and compared the results to windfarms using monopile and jacket foundations at the same size and date.