RIASSUNTO
Abstract
The accurate assessment of fatigue damage is a crucial issue for the design of marine pipelines. In this study, we modified the approach of Zheng et al. (2007) to simulate the random wave elevations for the fatigue assessment of marine pipelines applied to intermediate seas, e.g., the Persian Gulf. The cumulative fatigue damage due to the bending stresses and the linear and nonlinear acting wave forces was estimated on the basis of the finite element program for free-spanning sections. The results showed that the fatigue damage is highly dependent on both the wave characteristics and the modeling approach for the irregular wave.
Introduction
The unburied installation of marine pipelines, which is the most common technique of construction, may result in free spanning due to the current or wave action. The free spanning induced by waves in rather intermediate seas, e.g., the Persian Gulf, threatens the total safety of marine pipelines by creating a random cyclic load for several sea states with different heights and frequencies (Bai, 2001). This cyclic load makes the structure unsafe by increasing the risk of fatigue damage; hence, the oscillatory acting loads and their effects on the fatigue damage to the marine pipeline need to be calculated during the design process (Kanegaonkar and Haldar, 1987).
To investigate the fatigue damage, a series of information should be taken into account, e.g., the hydrodynamic loads, seabed formation, and material properties of the pipe and its coating. In the assessment process, the probabilistic and/or spectral analyses are used to estimate the sea wave characteristics in terms of either the time domain analysis or the frequency domain analysis. Traditionally, however, the spectral analysis is commonly used to predicate the dynamic response of the pipeline in which the wave loads are assumed to follow the Gaussian random process. This approach leads to the linear forces acting on pipelines laid on the seabed and the employment of both the Morison equation and the linear wave theory. On the other hand, Rice (1945) proposed two formulations for modeling electronic signals as Gaussian stationary processes from energy spectra, namely the deterministic and nondeterministic spectral amplitudes known as the Wave Superposition Methods (WSMs).