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Slip joint

Public summary


Nowadays some Offshore Wind Parks have failing grout connections of the wind turbine generator (WTG) foundation. To avoid these problems the industry is looking for alternative connections between the monopile and transition piece. The slip joint is a conical, metal to metal connection, simplifying the fabrication and installation of the foundation, and eliminating the risks and problems related to a grout connection..
The objective of this research project is to prove that the slip joint is a beneficial joint to connect a monopile with a transition piece as part of an offshore tubular wind turbine foundation. This alternative contributes to a lower Levelized Cost of Energy (LCoE) by reducing fabrication cost due to a simple design, a more simple installation method and therefore reducing risks and installation time.

This project aims to develop a technically and commercially feasible solution of a slip joint connection according the following design parameters/ aspects:

  • Design shall be according existing design codes and methods.
  • Loads of WTG, wind and waves shall be based on existing data and information.
  • Manufacturing shall be according the existing fabrication methods and tolerances.
  • Installation shall be with existing installation vessels.
  • The need of additional installation and removal tools shall be investigated.

The project is divided in two phases and six Work Packages (WP), where each WP is a preparation for the next WP.

Phase 1:

  • WP 1: Design Criteria (Parties: TUD, WMC and Van Oord)
  • WP 2: Modelling Slip joint (Parties: TUD, WMC)
  • WP 3: Measurements and Testing (Parties: TUD, WMC and Van Oord)

Phase 2:

  • WP 4: Design and certification
  • WP 5: Prototype
  • WP 6: Monitoring

Only he first phase of this project has been awarded and executed under FLOW program and is reported in this report.

The first three work packages have resulted in:

  • Design criteria have been defined, and the design method has been reviewed and commented by a Certifying Authority, Det Norsk Veritas (DNV), and was found in line with the design code.
  • Recommendations related to corrosion, tolerances and fatigue have been received as a result of the review by DNV.
  • A conical section of an actual monopile from the Teesside OWP was scanned. The data was used to set up the geometric models.
  • Numeric models of a slip joint have been set up and simulations have be executed with North See Metocean data and the numeric model of a NREL 5 MW reference Wind Turbine.
  • Several scale models (1:10) have been fabricated and tested with static and dynamic loads.
  • The scale models of the TUD have been used to test the influence of the installation tool on the behaviour of the connection and the materials.
  • Results of the tests  have been published in reports.

Risk and Cost reduction
By having a Slip Joint the use of a grouted connection is omitted. It therefore reduces the risks that come with the grouted  connection. The reduced and eliminated risks are:

During design and certification phase

  • conditional certification
  • certification of grout seal and redundancy
  • application of grout in Germany (Zustimmung im Einzelfall)

During installation

  • no hardening of grout during offshore conditions or cold conditions
  • no risk of grout tubes getting plugged
  • reduction of items to be taken on board of the vessel
  • no risk of failing grout seal
  • no mitigations when grout connection fails
  • Due to the simple and fast installation, offshore time can be reduced which contributes to HSE aspects of the project.

The cost reduction compared with current standard foundation solutions is achieved by the following advantages:

  • no grouting activities;  reduction in vessel time
  • no secondary grout lines
  • no secondary landing points and levelling equipment
  • no grout seal
  • no grout material
  • saving in waiting time for grout hardening

The total contribution to the LCoE is an overall estimated percentage of 1.28%.

The results show that the slip joint is a feasible connection that can be designed and certified according the present design rules and can therefore contribute to the FLOW target of  4500 MW in 2023 (subject to solving the issues identified by DNV, like corrosion and fatigue). By omitting a grout connection risk is reduced, not only in the design and installation, but also in the operating phase of future offshore wind parks where the simplicity of the design contributes to lower maintenance cost.

Conclusion and Recommendation
The design method as stated in the design Brief has been found suitable for the design of a slip joint. The verifier has addressed the following attention points to be investigated in order to progress towards a fully certifiable concept:

  • corrosion fatigue in the slip joint (this shall include an investigation of the behaviour of the corrosion and the metal to metal wear)
  • coulomb friction coefficient
  • the influence of corrosion and tolerances in the ULS and FLS
  • choice of the S-N curve of the material

This must become part of the future phase 2 of this project.

Publicly available reports and/or publications

DOWEC report on Slip Joint Connection

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