Impact - Response Behaviour Of Offshore Pipelines

Abstract A method for analyzing the dynamic behaviour of marine pipelines subjected to impact loads or sudden forced movements is outlined. Inertia forces (also from hydrodynamic mass), hydrodynamic drag forces as well as friction and lift effects for a pipe at the sea bottom are accounted for. An extensive nonlinear formulation is used for the pipe itself; it includes large displacements and elasto-plastic material behaviour. Aspects of the numerical formulation of the problem and the solution of the nonlinear dynamic equations are discussed. The examples show computed dynamic response for pipelines lying on the sea floor and for a pipe section freely submerged in water when subjected to various force and displacement histories. Introduction Pipelines in marine environment may be subjected to impact loads of various sorts. One type of incident that has been considered is when trawl gear from fishing vessels (trawl doors, beam trawls) hit pipelines on the sea floor. An extensive program of laboratory and full scale testing of such situations has been carried out by the Norwegian Hydrodynamic Laboratory in Trondheim. 1,2,3 It was concluded from these studies that the trawl gear normally hit and skid over the pipe on the sea floor, giving it a short impact. However, tests on 16" pipes also showed that, under special conditions, the trawl gear may actually get hooked under the pipe and move it along with the trawl equipment, leading to a very severe loading situation. Several types of accidental loading conditions are also feasible. For instance, heavy objects (e.g. anchors) may be pulled along the sea floor and hit a pipeline. Similarly, submerged pipes with free spans may accidentally be hit by heavy objects. Explosions in the sea generating shock waves do also constitute a threat to pipes. The purpose of the present study is to develop a computational model for numerical simulation of the dynamic behaviour of marine pipelines subjected to impact loads. A main advantage by using a computational model is that it gives quick answers, and it allows for parametric studies and investigation of the various effects contributing to the total behaviour. The information obtained this way are useful for practical applications as well as for adjustment of experimental set-ups. Important Effects The class of problems considered here is limited to pipelines that are loaded and deformed in one plane. This plane is spanned by the x - y system where the x-axis follows the axis of the pipe and the y-axis is the direction of transverse displacements, see Figure 1a. The x - y plane must be parallel to the sea floor for pipelines lying on the bottom of the ocean. In addition to the inertia forces Fi from the pipe itself, inertia effects from the surrounding water are also considered, see Figure lb. These hydrodynamic inertia forces act in opposite direction of the transverse acceleration u, and may be expressed through(Mathematical equations) (Available in full paper) Forces due to friction between the pipe and the sea bottom are also considered.