The function of a pipeline is normally to receive fluid in a given condition and transport this fluid to a remote destination while ensuring that it arrives in a state that is acceptable for the receiving facility. In the offshore environment, pipelines often transport hydrocarbons in a multiphase form with liquid being the dominant phase for oil and gas phase for natural gas, this fluid is normally compressed and at a temperature significantly higher than ambient temperature. The distance over which the product within the pipeline is to be transported can vary from tens of meters to hundreds of kilometers.
The pipeline must perform its primary function for an extended period of time, perhaps up to tens of years in a safe manner. In this time, it will be subjected to a range of effects within the environment in which it is constructed. Some examples of these effects include extreme wave and current action, the effect of commercial activities, such as fishing and changes in the mechanical properties of the seabed on which it is placed.
Interfaces with Geotechnical Engineering
From a geotechnical engineering perspective it might appear that the items of interest in the functional description of our pipeline system are somewhat limited. If we inspect each of the requirements in further detail however, we can find opportunities to influence and optimise the global system design:
1. Flow Assurance: The process of satisfying the need to ensure that the fluid carried within a pipeline will arrive at the host facility in an acceptable condition. This requires that the heat transfer out of the pipeline system (so called U value) is maintained at an optimum point. The ability of soil to act as an insulating material is sometimes a useful subject to ponder in this case, it is particularly so for passively insulated single-bore flowlines. For a quick primer on hydrate and wax formation see this OTC paper.
2. Global Stability: Ensuring that the pipeline system remains stable for the in-service load condition throughout the life-cycle of the system. The axial forces developed within a pipeline due to pressure and temperature induced expansion can be significant (say 200Te for our pipeline). These forces can cause the pipeline to buckle in the horizontal and vertical planes. The soil resistance mobilised during a buckling event is significant and often forms a major component of an engineered buckling-mitigation scheme. You can find more information on global stability assessment in DnV RP F-110.
3. Temporary Stability and Tie-in: Maintaining a stable condition during the different phases of the construction process. This includes pipelay, tie-in, pre-commissioning and trenching operations. The pipeline may be subjected to alternative load cases, rate / magnitude of loading or may even temporarily contain bore fluids that reduce the effective pipe weight. Pipe-soil interaction is a significant consideration in each of the operations highlighted, with the soil resistance being as significant for successful installation as it is for longer term global stability.
4. Protection Philosophy: All pipelines are required to be capable of resisting impact and snagging forces that may be imposed during the life cycle of the pipeline system. In the North sea, it is normally accepted that snag loading from fishing gear can be ignored for pipelines greater than 16” (0.41 m) in diameter. The pipeline is still required to resist impact loading from objects dropped from sea level. For the case where a pipeline is not protected with concrete weight-coat this can lead to a requirement to bury the pipeline. The cost of burying a marine pipeline is non-trivial and so a rational basis for selecting a target burial depth is required. This can be arrived at by considering the probability of an impact occurring and ability of seabed soils to dissipate energy from an impact. Clearly, the consequence of an impact can be mitigated through the use of sufficient cover for a given set of soil conditions.
Over the next few blog posts, we will cover each of the four key “interfaces” identified above. I’ll attempt to discuss some of the interesting challenges we encounter and the limits of existing methods and approaches as seen from my perspective.