Self-lowering of pipelines has for many years been considered as a feasible and cost effective way to achieve pipeline stability with reduced weight coating and to reduce trenching costs. In the Joint Industry between 1991 and 1994, the numerical model PIPESIN was developed for predictions of free span development and pipeline lowering. The model simulates the pipeline seabed interactions in the time domain for the period from installation until a stable pipeline seabed configuration has occurred. All important physical processes are modelled integrally. Nevertheless, the PIPESIN model has a number of limitations in practical applications. Firstly, in the model the pipeline is assumed to be placed on a horizontal seabed and the global seabed level is assumed to be constant, The model is therefore not able to describe the behaviour of pipelines on (stable or migrating) sand waves. Secondly, the model simulates the pipeline behaviour from the moment of pipe laying. The model does not enable predictions of the behaviour of an existing pipeline and the development of an individual free span. These limitations can be important in practical applications, for example, if free spans are observed in an annual survey, we would like to predict the further behaviour in order to assess the requirement for remedial measures. Further, validation of the PIPESIN model has been Limited to pipelines with large diameters. This paper presents an update of the original PIPESIN model which overcomes the above limitations. The following improvements have been carried out. Actual pipeline levels and seabed levels can be specified in the model. They can vary along the pipeline. This enables simulations to be started at any time during the pipeline operational phase. For example, the model can simulate how an existing free span will develop. Another major improvement is that the global seabed level may not only vary along the pipeline but may also vary in time. This enables simulations of interactions between offshore pipelines and migrating sand waves. The model has further been calibrated with additional field data from both large and small diameter pipelines. The paper will present a few examples of practical applications.
