New design practices for offshore wind turbine structures

Det Norske Veritas (DNV) and key industry players are today releasing a report on the design of offshore monopile wind turbine structures. This is the outcome of a joint industry project which was established to improve the basis for calculating the axial load capacity and to review current design practices.

During the autumn of 2009, DNV assessed the current industry practice regarding the design of offshore monopile wind turbine structures, and particularly the basis for calculating the axial load capacity of large diameter grouted connections without shear keys. It was then found that the existing design practices did not properly describe the physical behaviour of such connections.

Based on this new insight, DNV immediately established a joint industry project (JIP) involving key industry players to revise and improve the basis for calculating the axial load capacity and to review current design practices. The collaboration between DNV, owners, operators, grout producers and designers has involved carrying out physical testing in DNV’s laboratory in addition to structural analyses, field monitoring and the sharing of experience. The group, consisting of 12 partners, is now publishing a report on this project.

Background

A grouted connection is used to connect the transition piece to the monopile as shown in Figure 1. A transition piece is placed on top of the monopile, resting on temporary supports. During installation, the transition piece is then jacked up to the correct verticality before the grouting is carried out. After curing, the jacks are removed, leaving a gap of a few centimetres between the temporary supports and the monopile.

Settlement down to the temporary supports may result in a different force flow in the structures than that intended at the design stage. An unintended force transfer through the temporary supports has led to concern about fatigue cracking in the structures which would lead to repair needs.

Grouted connections

The joint industry project has concluded that a cylindrical shaped design of grouted connections without additional support arrangements for axial load is not recommended. The main reasons for this are that the axial capacity is found to be lower than that previously assumed due to the effect of large diameters, the lack of control of tolerances that contribute to the axial capacity, and the abrasive wear of the grout due to the sliding of contact surfaces when subjected to large bending moments from wind and waves.

However, where cylindrical shaped grouted connections are already used offshore and where the design is such that settlement may be expected, additional support arrangements should be considered for the transfer of axial loads. Such mitigation methods for existing installations have been developed and are already implemented on several structures.

Grouted connections with shear keys

The new knowledge is also expected to influence the design of large diameter grouted connections with shear keys. Shear keys are circumferential weld beads on the outside of the monopile and the inside of the transition piece in the grouted section. The shear keys’ purpose is to increase the sliding resistance between the grout and steel so that no settlement occurs. The existing design standards for such connections are based on limited test data for alternating dynamic loading. Before this solution can be recommended, a design practises for shear keys should be developed and properly incorporated in design standards. DNV has therefore initiated a complementary joint industry project with the aim of updating existing knowledge of and design practices for grouted connections with shear keys.

Conical shaped connections

Based on the JIP, a design practise to account for large dynamic bending moments on monopiles has been developed using conical shaped connections. According to this, the monopile and transition piece are fabricated with a small cone angle in the grouted section.

If the bonds between the steel and grout are broken during in-service life, some slight settlement of the transition piece will occur. This settlement will introduce compressive contact stresses between the steel and grout which, together with some friction, will provide sufficient resistance against further settlement.

Participants

Acknowledgement is made to the JIP partners for their support and contribution to this work:

Ballast Nedam Engineering, BASF Construction Chemicals Denmark A/S, Centrica Renewable Energy Limited, Densit A/S, DONG Energy, DNV, GustoMSC, MT Højgaard a/s, Per Aarsleff A/S, RWE Innogy GmbH, Statoil ASA, Statkraft AS and Vattenfall Vindkraft A/S.

Updating standards

The DNV-OS-J101 Design of Offshore Wind Turbine Structures standard has been partly amended throughout the JIP period and a new revision will be issued during the second quarter of 2011.

A summary report of the JIP results may be requested online at DNV’s webpage www.dnv.com/windturbinestructures

Source: Det Norske Veritas

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