Suspending pipelines between two points over an uneven seafloor is called free-spanning which induces more vortices behind the pipe especially when the pipelines are exposed to the hydrodynamic effects of waves and currents. These influences cause a significant increase in the dynamic acting drag forces. In fact, the oscillatory acting drag forces, as a result of changes in the net pressure and velocity filed around the pipeline, increase the probability of fatigue damage. Since the fatigue life of offshore pipelines has a major influence on the useful life of a project, studying it for offshore pipelines laid on uneven sea beds has become very important. Both the consequence of vortex shedding as a result of wave and current interaction with the pipeline, and the selected suitable approach in assessing fatigue damage has made the design process sophisticated. In the present study, it is attempted to employ a proper finite element model with the capability of coupling the dynamic drag forces induced by wave and current conditions with estimating cumulative fatigue damage methods. The wave profile is simulated using Non-deterministic Spectral Amplitude in Gaussian sea state and using linear wave formulation to calculate wave -induced forces. Moreover, the current force is estimated by using the conventional Morison Equation as well as employing a new approach of estimating current -induced dynamic drag forces obtained by a numerical model that includes the VIV effects. Finally cumulative fatigue damage as a result of acting those forces is investigated for various conditions of laying pipeline on the seabed.

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