Numerical Modelling of Offshore Pipeline Flotation during Sand Backfilling

Abstract

Offshore pipelines are considered the arteries of the offshore oil and gas industry and transport hydrocarbon products as well as other fluids. During the installation process of an offshore pipeline, the pipeline is most often located in a trench and covered with backfill material. The burial of the pipeline ensures onbottom stability as well as mechanical protection. With a Trailer Suction Hopper Dredger (TSHD) sand can be deposited in the trench, thereby covering the pipeline in a relatively controlled manner. One major risks associated with this backfilling method is the risk of vertical upward displacement of the pipeline during the backfilling process. The vertical upward movement of the pipe is referred to as pipeline flotation and may result in unprotected or damaged pipes. With the rising use of small diameter and lighter pipelines this risk has become more prevalent.

Pipeline flotation is induced by augmented buoyancy which originates from the presence of the water-sand mixture around the pipeline. The particle concentration of the water-sand mixture and the embedment rate of the pipe are leading in the assessment of the buoyancy force acting on the pipeline. The weight of the pipeline is the major force counteracting buoyancy force. In addition, a friction force, resulting from the contact between the pipe and the new formed sand layer around the pipe, counteracts the buoyancy force.

The aim of this research is to develop and validate a simplified numerical model for the analysis of offshore pipeline flotation during sand backfilling with a TSHD. The numerical model has been validated against the small-scale physical experiments of Eikhout (2021). The small-scale physical experiments have been developed and used by Yang (2020) and Eikhout (2021) to simulate a simplified sand backfilling process.

The numerical model is developed to simulate the sedimentation process and has been developed in the finite element software COMSOL Multiphysics. The model is capable of effectively simulating the simplified backfilling process from the small-scale experiments by Eikhout (2021). Moreover, the sedimentation model is able to simulate the inflow of material over time in the physical domain. The sedimentation process has been modelled with a convection equation in which the settling velocity of the water-sand mixture is described with the hindered settling formulation proposed by Metha (1986). The hindered settling formulation describes the velocity of the suspension as a function of its local particle concentration. The numerical data has been processed and used in a force balance which is able to predict the occurrence or absence of pipeline flotation.

The parameters in the hindered settling formulation as well as the numerical settings have been described from a theoretical perspective as well as their practical impact on the numerical solution. After validation against the small-scale physical experiments the numerical model has been used to simulate a more practical scenario. In addition a simplified, spreadsheet friendly, calculation method is proposed.