Finite Element Analysis of Flexible Riser Impact

Flexible risers and pipelines are complex structures composed by several layers of steel and plastic sheaths. Many of these layers are intricate structures; having complex shape and self interaction. Take for instance the helically cross winded tensile armor or the interlocked pressure armor. Each of these components are individual structures which interact by internal contact and friction. Prediction of cross sectional riser response caused by external loads is therefore not straight forward. This is especially the case if flexible pipes are subjected to impacts and accidental actions. The article presents analyses and basic results from a riser impact study performed by Reinertsen AS and NKT Flexibles I/S. The study is highly relevant for impact scenarios such as: Flexible risers subjected to actions from dropped objects, and riser clashing against offshore structures or riser to riser clashing. A detailed model of a flexible production riser is generated and analyzed using the non-linear explicit finite element (FE) software LS-DYNA. The focus of the study is twofold: First, the cross sectional indentation and damage resistance of the riser is investigated. This implies that the riser/pipe response to local deformation is established. This is very similar to the dropped object impact scenario. Next, the local deformation response is coupled with global deformations. In other words, the riser is allowed to deflect with a bending mode during impact. This involves a model which allows local cross sectional deformations as well as global beam-like deflection, e.g. as will be the case during riser clashing with offshore or subsea structures. All analyses are performed dynamically and involve nonlinear-geometry, material response and contact interaction. The impact response of flexible pipes is discussed in details. Focus is on cross sectional deformation and how this is distributed through the structural layers. In addition the effect of global riser response is discussed.