In an offshore petroleum production system, subsea pipelines have an important function in the production of oil and gas from subsea wells. They are primarily horizontal steel pipe laying on, near or beneath to the seabed. Usually, a subsea pipeline is used to transport gas or oil from a satellite subsea well to a manifold as well as in the transferring process of the petroleum production from the offshore facility to onshore installations. Frequently this pipeline is buried in the seabed, and sometimes it passes over uneven seafloor creating freespan lengths. The pipeline structure will need to withstand environmental forces caused by soil, current and waves. In the free spanning portion, for the in-line direction, which is defined as the direction parallel to the ocean current flow direction, drag and lift effects usually are present. The cross-flow, or cross-flow direction, the presence of vortex induced vibrations (VIV) forces and self-weight are observed. Free-span lengths of pipeline can be a concern to the overall pipeline system due to material fatigue resulting from induced dynamic behavior. Previous investigations have shown that vortex induced vibrations are important element in the reduction of life-time service due to fatigue. In the present work, hydrodynamic forces are calculated by a semi-empirical formulation based on the Morison type equations, for inline and cross-flow directions. Ocean current and wave forces are considered for calculations of the free spanning pipeline behavior. Fluid particle velocity and acceleration are calculated based on the 5th order Strokes and Airy wave theory. The pipeline is modeled as an Euler-Bernoulli beam. Different boundary conditions have been investigated in the present study and the results are discussed. Discussions are carried out in terms of the behavior of the free-spanning pipeline, through time domain simulations.

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