Nonlinear Effects on Fatigue Analysis for Fixed Offshore Structures

Fatigue analysis for fixed offshore structures is an important practical issue. These structures are often drag dominated, which makes the deck response a non-Gaussian process when it is assumed that the irregular waves are Gaussian. Incorporating nonlinear and non-Gaussian modeling in the fatigue analysis can be a complicated issue, cf. work of Madhavan Pillai and Meher Prasad [2000, “Fatigue Reliability Analysis in Time Domain for Inspection Strategy of Fixed Offshore Structures,” Ocean Eng., 27(2), pp. 167–186]. The goal of this paper is to provide evidence that for drag dominated offshore structures it is, in fact, sufficient to perform linearization in order to obtain accurate estimates of fatigue damage. The latter fact brings fatigue analysis back into the Gaussian domain, which facilitates the problem solution. Beyond straightforward linearization of the exciting wave forces, this paper employs two different approaches accounting for nonlinear effects in fatigue analysis. One is an application of the quadratic approximation approach described in the work of Naess and co-workers [1997, “Frequency Domain Analysis of Dynamic Response of Drag Dominated Offshore Structures,” Appl. Ocean. Res., 19(3), pp. 251–262;1996, “Stochastic Response of Offshore Structures Excited by Drag Forces,” J. Eng. Mech., ASCE, 122, pp. 155–160]. to the stochastic fatigue estimation of jacket type offshore structures. An alternative method proposed is based on a spectral approximation, and this approximation turns out to be accurate and computationally simple. The stress cycles causing structural fatigue are considered to be directly related to the horizontal excursions of the fixed offshore structure in random seas. Besides inertia forces, it is important to study the effect of the nonlinear Morison type drag forces. Since no direct method for dynamic analysis with Morison type forces is available, it is a goal to find an accurate approximation, allowing efficient dynamic analysis. This has implications for long term fatigue analysis, which is an important issue for design of offshore structures.