Dynamic Analysis of a J-lay Pipeline

Oil and gas reserves are moving towards deeper waters day by day and it has become increasingly important to construct structures and subsea pipelines in deeper waters to transport the hydrocarbons for the users. The J-lay technique has become a better alternative to the conventional S-lay technique for installing subsea pipelines in deep waters. Here the pipeline leaves the vessel in a near vertical position rather than the horizontal position and acquires the J-shape as it reaches the seabed. This method offers several advantages over the conventional S-lay method such as minimal bending and reduced suspended length of pipeline leading to reduced tension and reduced thruster power requirement, precise pipeline positioning and better vessel control. This paper considers a simplified J-lay pipeline numerical modelanalysed using ORCAFLEX. The model consists of 0.6 m diameter steel pipeline being laid at a water depth of 2000 m. Dynamic responses namely effective tension, bending moment and maximum von-Mises stress of the pipeline are studied under the action of waves with and without vessel interaction and under the combined action of waves and currents with vessel interaction. Vessel interaction and presence of currents induces additional stresses in the pipeline being laid and the increase in the maximum values of effective tension, bending moment and maximum von-Mises stress due to the dynamic effects is observed as 36%, 64% and 47.7% respectively.