Civil Engineering Association

Long-term fatigue analysis of multi-planar tubular joints for jacket-type offshore wind turbine in time domain

Long-term fatigue analysis of welded multi-planar tubular joints for a fixed jacket offshore wind turbine
designed for a North Sea site in a water depth of 70 m is performed. The dynamic response of the jacket
support structure due to wind and wave loads is calculated by using a decoupled procedure with good
accuracy (Gao et al., 2010). Hot-spot stresses at failure-critical locations of each reference brace for 4
different tubular joints (DK, DKT, X-type) are derived by summation of the single stress components from
axial, in-plane and out of plane action, the effects of planar and non-planar braces are also considered. Both
a 2-parameter Weibull function and generalized gamma function are used to fit the long-term statistical
distribution of hot-spot stress ranges by a combination of time domain simulation for representative
environmental conditions in operational conditions of the wind turbine. A joint probabilistic model of
mean wind speed Uw, significant wave height Hs and spectral peak period Tp in the northern North Sea
is used to obtain the occurrence frequencies of representative environmental conditions (Johannessen,
2002). In order to identify the contributions to fatigue damage from wind loads, wave loads and the
interaction effect of wind and wave loads, 3 different load cases are analyzed: wind loads only; wave loads
only; a combination of wind and wave loads. The representative environmental condition corresponding
to the maximum contribution to fatigue damage is identified. Characteristic fatigue damage of the selected
joints for different models is predicted and compared. The effect of brace thickness on the characteristic
fatigue damage of the selected joints is also analyzed by a sensitivity study. The conclusions obtained in
this paper can be used as the reference for the design of future fixed jacket offshore wind turbines in North

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