Abstract
Fatigue assessment of fixed offshore platforms is essentially important to ensure their structural safety. Because of the small-scale leg diameter, these structures are often drag dominated, and linearized wave force is often utilized for the fatigue estimation. To evaluate the influence of nonlinear effect on fatigue damage, the nonlinear drag force is directly incorporated into the computation of the wave force spectrum based on autocorrelation function. The features and the contributions of nonlinear wave force are first investigated in the frequency domain. Then, structural fatigue damage associated with the linear and nonlinear wave forces is compared using a time-domain fatigue assessment method. Investigation indicates that the influence of nonlinear drag force is closely related to the characteristic diameter of the structure. The nonlinear drag force can contribute up to 10% of the total wave force and lead to about 10% structural fatigue life reduction when the characteristic diameter is small.
Highlights
The offshore structures are designed to resist continual wave loading which may lead to significant fatigue damage on individual structural member, even though those applied loading may be below the structure resistance capacity.[1,2] The fatigue damage has become a major cause of damages occurred on the offshore structures
Structural fatigue assessment considering the nonlinear drag force is investigated using a series of mono-pile platforms with different diameters
Based on the wave force autocorrelation function, the effect of nonlinear drag force is directly incorporated into the wave force power spectral density (PSD) computation
Summary
The offshore structures are designed to resist continual wave loading which may lead to significant fatigue damage on individual structural member, even though those applied loading may be below the structure resistance capacity.[1,2] The fatigue damage has become a major cause of damages occurred on the offshore structures. One of them is the fracture mechanics approach based on Paris crack propagation criterion, which is usually applied to predict the propagation life from initial crack or defect.[3] Another one is the traditional fatigue curve (S–N curve) method on the basis of Palmgren–Miner (P-M) linear damage hypothesis (the method used in this article) which is intended for application at the design stage. Due to the difference in the theoretical basis, the latter approach can be divided into three categories, namely, deterministic method, frequency-domain method (spectra based method), and time-domain method, respectively.[4] For the deterministic method, the random wave process is represented by the regular wave with a deterministic wave height and period and neglects the stochastic characteristic of the sea waves.[5] the structural fatigue damage may be underestimated in a large extent with this method which is not appreciated in offshore engineering
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