Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Philippe Thibaux (1) F P Marc Vanderschueren (1) Jeroen Van Wittenberghe (1)
(1) OCAS NV, Zwijnaarde, Belgium
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Efficient fatigue testing of jacket nodes
The design of the structure of offshore wind turbines is pre-dominantly fatigue driven, welded joints being often critical. Any improvement in the fatigue resistance of the structure would result in decreasing the mass and the cost of the structure. However, any improvement needs to be demonstrated experimentally. In the present investigation, a fast test method for fatigue testing of tubular joints of jackets has been developed.
It was investigated by numerical simulation if it would be possible to excite the node of a jacket close to its resonance frequency. After some iteration, a suitable geometry was found, based on an X-node. The proposed test method was then applied on a small component as a demonstration. Results of the testing of the demonstrator are compared to finite element computations.
Main body of abstract
Eigenfrequency computations showed that, by choosing the adequate parameters, it is possible to achieve a resonance frequency between 20 and 40 hz for X-node. Furthermore, the resonance frequencies of the in-plane bending and out-of-plane bending modes are close to each other (differences of less than 1hz) and the position of the resonance nodes are also almost identical. Based on these findings, a patented fatigue testing method was developed, based on the excitation of an X-node with an eccentric mass.
The test method was then demonstrated on a X-node with braces of OD219mm WT 5.59mm welded to a leg of OD368mm WT 25mm and 500mm length. The geometry was modified in order to obtain similar stiffness in OPB and IPB. It was then possible to obtain similar resonance frequencies for the OPB and IPB. The behavior of the system was also computed by the finite element method, giving a good agreement of local measured strains and computed values.
The present investigation has demonstrated that it is possible to test in fatigue a tubular joint at frequencies higher than 20hz by applying a rotating bending moment principle. By choosing adequately the dimensions of the specimen, it is possible to obtain a resonance frequency below 40hz, with little difference between the OPB and IPB mode.
This test method can then be applied to deliver in short time a test result for the resistance of a tubular joint. It will then be possible to validate the fatigue performance of nodes with improved performance.
Measuring the fatigue life of welded tubular joints will open the door to the improvement of their fatigue resistance, and ultimately to reduce the costs of jackets. The present method found a way to simulate 10 million cycles in one week of testing.