Alternative foundation installation

◀ Back to projects overview P201102-010-BN

In 2010 Ballast Nedam performed a study for a concrete drilled monopile, subsidized by Vattenfall, for the Kriegers Flak project which is located in the Baltic Sea.

As an outcome of this study further development and proof of feasibility of a (concrete) drilled monopile required two major issues to be investigated:

- the (P-Y) calculation method for the pile-soil-interaction, which is presently used for designing driven piles, should be validated for drilled piles as well, and:

- the drilling technique should be made suitable for drilling monopiles for wind turbine foundations as such that disturbance of the surrounding soil can be controlled as such that the conditions of the P-Y methodology are met.

As a side effect the drilling method as installation method for steel monopiles is also considered, for reason that pile driving in Europe is facing more and more limitations caused by “precaution-regulations” around underwater sound and the effect on the environment (e.g. harbour porpoises).

This Alternative Foundation Installation (AFI) project concentrates on the technical issues concerning the installation of monopiles by drilling (regardless the fabrication material of the pile, steel or concrete), and more specific the fact that after installation the surrounding soil is (still) suitable to support the pile, horizontal and vertical.

In another (FLOW) project (Concrete Drilled Monopile (CDM), P201305-005) the concrete pile itself will be considered, including feasibility of design and fabrication, transport and handling and total costs, etc.

Approach and results obtained
This resulted in the research proposal: Alternative Foundation Installation (AFI) within the FLOW program which targeted to solve the constraints for two alternative techniques of installing monopiles: drilling and also vibrating. The latter as this is as well a ‘sound friendly’ installation technique for monopiles.

The desired outcome of that research was a confirmation that the drilling and vibration techniques can be controlled as such that no unacceptable distortion of the soil surrounding the pile will occur. Secondly the desired outcome of this research was that certifying bodies will confirm that the present design rules for monopiles can also be used for piles which are installed by drilling or vibrating, including possible adaption of certain parameters.

The research was originally split in three phases:

Taking a certifying body ‘on board’ and agree the scope of the research project as such that the outcome of the tests can enable the certifying body to validate the present design rules also for a drilled or vibrated monopile
Seek cooperate with wind farm developers and owners as acceptance of design rules for the said techniques is expressly in their interest,
Organize onshore scale tests comparing 3 hammered, 3 drilled and 3 vibrated piles to be dynamically tested in order to validate and/or correct the design assumptions.

Phase C of the proposal was explicitly open to be redefined during the course of the project.

In March 2014, it was reported that:

“During EWEA OFFSHORE 2013 [November, Frankfurt] the project was presented for a selected audience and parties (Utilities, contractors, consultants, certifier, etc.) have been approached to participate in the project. The reason for this is that in principle the whole industry has an interest in the certification of alternative installation methods and thus should (and ís) be interested to contribute to this project. Nevertheless at the end of the reporting period there was not enough external funding to justify a go-ahead for the final phase [C] of the project.
A positive phenomenon was that several other projects (mainly: VIBRO and PISA) have been set up in parallel of the AFI project and presently serious investigations are made to explore the possibilities of combining (parts of) these projects.”

In May 2014, it was reported, in addition that:

“Specifically the tests performed in Berlin by two German companies are close to the setup of this project, although the purpose was different. Presently the results of this project are studied to judge if they could serve as a basis to prove that the P-Y method could also be used for calculating drilled monopiles.
[…] the project will be completed as follows: Letter of Approval DNV plus Engineering report and appendices is the first deliverable followed by a study, […], in which the Berlin results will be used as a basis to prove that the P‐Y method can be used for designing drilled foundations, which was the original purpose of this project.”

The ultimate aim of this AFI research project is the validation of design rules for drilled (and vibrated) (mono)piles. Initially full scale onshore drilling tests were envisaged in Phase C. But after the initial design stage (Phase A and B) it was found out that several other tests (VIBRO, PISA and Berlin) were set up by other parties with in principle other purposes than in the AFI project, but which other tests seem well suitable as basis for the validation of the parameters and design rules as set in the AFI project. The present and positive conclusion is that the AFI Engineering report can well serve as a standalone result and complies with the targets as set for Phase A of this project. It also can serve as a basis for follow up in the earlier mentioned foreign projects.

For this reason this FLOW Project Alternative Foundation Installation concludes with:

- DNV Verification Comment Sheet and Letter of Approval (Sept. 2013),

- Engineering Report “Background for P-Y determination in onshore scale tests” (Feb. 2014), and

- Assessment of the Berlin tests compared to “Background for P-Y determination in onshore scale tests” (Dec. 2014).

The vibrating technique is tested in 2014 (on steel piles) by other parties and stays for that reason out of focus as it has also always been a ‘nice to have’ option of this FLOW project.

All other research efforts are compiled in the Ballast Nedam Offshore, Engineering Report: “Alternative Foundation Installation of a Concrete Drilled Monopile”, issued 18 Dec 2014, which report aligns the two studies AFI and CDM, but is primarily the end report for the CDM project only.

Thus Phase A and B are completed and Phase C redefined in an assessment on the Berlin tests.

The final result of this project is that validation of design rules for drilled monopiles is classified by DNV as feasible and secondly that assessment of the Berlin tests prove that getting approval from a certifying body for the implementation of the P-Y theory (without reduction) for the drilling method is feasible.

Contribution to cost and risk reduction of far offshore wind energy
According to the FLOW cost model the savings of vibrating the monopiles for the LCoE for a complete wind farm are 2,3% compared to a wind farm with hammered piles. The savings for the LCoE for a wind farm with monopiles with only drilled (steel) monopiles is 0,2% compared to a wind farm with hammered piles.