Offshore Pipeline Flotation during Sand Backfilling with a TSHD

Abstract

Offshore pipelines represent major items in the oil and gas industry nowadays. These submarine pipelines are usually covered with backfill for protection and a well-known method for this is sand backfilling with a trailing suction hopper dredger (TSHD). Although pipeline installation with a TSHD is a developed technique, there are still some challenges regarding uplift movements of the pipeline during the backfill procedure. This action is called pipeline flotation, which is a frequently occurring problem during the sand backfilling process and the costs for repair measures afterwards are very expensive. Accurate modelling of the sand backfilling process over a pipeline can lead to significant cost reductions by optimising the sand backfilling technique. Pipeline flotation is induced by the development of upward buoyancy forces from the sand-water mixture on the pipeline. If the weight of the pipeline is not sufficient enough, it will experience uplift movements during the backfilling process. The purpose of this research is to develop a better understanding of the mechanism of pipeline flotation. Due to a lack of available field data, it is anticipated that a set of laboratory experiments will provide a better insight into the parameters involved in pipeline flotation during sand backfilling. This research builds on the study Yang (2020), in which a small-scale set-up and 2D calculation model are designed to perform experiments concerning pipeline flotation during sand backfilling. A total amount of 50 experiments is performed with Geba Weiss sand in four different stages: with horizontal discharge, vertically upward discharge, elevated initial pipe positions and increased pipe specific weights. The set of experiments have provided better insight into physical processes that influence pipeline flotation during sand backfilling. The discharge of the sand-water mixture is dominated by its sedimentation and dispersion, so the model is based on empirical equations and the sedimentation theory. The main conclusion regarding the static force balance on the pipe is that the degree of pipe embedment influences the magnitude of an additional frictional force working downwards on the pipe body. A new approach for assessing this frictional downward force, due to the presence of sand that has deposited, is described and added to the model. In the small-scale experiments, this influence plays a significant role in pipe flotation triggering. An undesirable result from the small-scale experiments are the additional hydrodynamic effects from the fluid in the experimental tank, which are induced by the geometrical constraints. A pipe flotation limit was found in the parameter of sand volume concentration in the surroundings of the pipe. This flotation limit was defined at a domain concentration of approximately 7.5% for the experiments with a pipe specific gravity of 1.03. The boundary of flotation for heavier pipes was not achieved in this research, but evidence points towards a significantly higher flotation limit. The discharge flow rate and the total discharged sand volume are the key factors for development of domain concentration in the small-scale experiments. With the acquired data as a strong foundation, additional research regarding pipe flotation during sand backfilling is recommended. As there is still a strong variation in several important input parameters in the small-scale experiments, a Computational Fluid Dynamics (CFD) analysis is suggested to relate the domain concentration to the input parameters at every moment in time. Additional 3D experiments on a larger scale are also recommended to optimise the sand backfilling process and validate the CFD analysis. From this analysis, a prediction can be made regarding the boundary of pipeline flotation on the seabed when discharging with a TSHD. Moreover, additional research regarding the stress and drainage conditions underneath the pipe is proposed to validate the cause and accuracy of the relatively high friction coefficients. Finally, a model could be built to examine the friction force generated on the pipe due to its embedment. With this method, the suggested approach of the frictional resistance due to pipe embedment could be further analysed.