Pipeline hydrodynamic stability is one of the most fundamental design topics which are addressed by pipeline engineers. In its simplest form, a simple force balance approach may be considered to ensure that the pipeline is not displacing laterally when exposed to the maximum instantaneous hydrodynamic loads associated with extreme metocean conditions. If stability can be ensured in a cost efficient way by applying a minimal amount of concrete weight coating only, this method when applied correctly, can be regarded as a robust and straightforward approach. However in many cases pipeline stabilisation can be a major cost driver, leading to complex and costly stabilisation solutions. In these circumstances, the designer is likely to consider more refined methods in which the pipeline is allowed to displace under extreme conditions. This paper discusses various design approaches and acceptance criteria that are typically adopted in pipeline stability design. Both force balance methods and calibrated empirical methods which are typically defined in modern design codes, are discussed in terms of their applicability as well as their limitations. Both these approaches are based on the assumption, directly or indirectly, that lateral displacement is a Limit State in its own right. It will be argued that this assumption may lead to unnecessary conservative design in many circumstances. It will be demonstrated that even if relatively large displacements are permitted, this may not necessarily affect the structural integrity of the pipeline. An alternative stability design rationale is presented which is based on a detailed discussion of the Limit States pertaining to pipeline stability. This approach is based on the application of advanced dynamic stability analysis for assessing the pipeline response. Pipeline responses obtained through advanced transient finite element analyses is used to illustrate how a robust design can be achieved without resorting to strict limits on the permissible lateral displacement.

One of the aspects of pipeline design is ensuring pipeline stability on the seabed under the action of environmental loads. During the 1980s, significant efforts were made to improve the understanding of hydrodynamic loads on single pipeline configurations on the seabed (Reference 1). The stability of piggyback (bundled) pipeline configurations is less well understood, with little quantitative data readily available to the design engineer for practical application in engineering problems (References 2–6). This paper describes an extensive set of physical model tests performed for piggyback on-seabed and piggyback-raised-from seabed (spanning or lifting pipeline) configurations to determine hydrodynamic forces in combined wave and current conditions. The piggyback is nominally in the 12 o’clock position. The well-established carriage technique was used, in order to obtain data for use in full-scale stability modelling. The model tests are benchmarked against existing test data, to confirm the validity of the test method. Key findings are presented in terms of non-dimensional coefficients, and force time histories for the vertical and horizontal forces. A brief interpretation of the hydrodynamic load behaviour of the Piggyback System is provided by considering the physical flow mechanisms causing the force time history variation; furthermore the influence of the seabed separation on the piggyback loads is also discussed.

Several design approaches can be used to analyse the stability of subsea pipelines [1]. These design approaches vary in complexity and range between simple force-balance calculations to more comprehensive dynamic finite element simulations. The latter may be used to more accurately simulate the dynamic response of subsea pipelines exposed to waves and steady current kinematics, and can be applied to optimise pipeline stabilisation requirements. This paper describes the use of state-of-the-art transient dynamic finite elements analysis techniques to analyse pipeline dynamic response. The described techniques cover the various aspects of dynamic stability analysis, including: • Generation of hydrodynamic forces on subsea pipelines resulting from surface waves or internal waves. • Modelling of pipe-soil interaction. • Modelling of pipeline structural response. The paper discusses the advantages of using dynamic stability analysis for assessing the pipeline response, presents advanced analysis and modelling capabilities which have been applied and compares this to previously published knowledge. Further potential FE applications are also described which extends the applicability of the described model to analyse the pipeline response to a combined buckling and stability problem or to assess the dynamic response of a pipeline on a rough seabed.

The objective of pipeline drying during pre-commissioning is to remove residual water left in the pipeline after dewatering and desalination operations. Removing the residual water mitigates corrosion and hydrate formation and aids quicker delivery of product to required dryness. The common pipeline drying methods are vacuum drying and convection drying. The convection drying method blows dry air through the pipeline to remove the residual water. Its disadvantages are an inability to adequately dry complex-shaped pipeline networks, significant equipment footprint and expelling air noise during the convection drying operation. The vacuum drying method can achieve low dewpoints particularly for complex-shaped pipeline networks and the equipment footprint can also be smaller than for the convection drying method. Therefore, it is advantageous when facing space restrictions for equipment. This paper introduces a dynamic integrated model to simulate the pipeline drying operation. This model considers vacuum pump performance and gas saturation condition in the pipeline during the drying operation. The modelling results can be used to determine the vacuum drying suitability, predict the drying operation duration and identify opportunities to improve the pipeline drying efficiency, such as vacuum pump performance, dry gas injection and convection dry air flow rate. It also demonstrates where vacuum drying is unlikely to be feasible, i.e. low ambient temperature conditions, and methods for identifying such. An optimisation case study is also presented. The drying duration can be reduced significantly by integrating vacuum drying with dry gas injection. This combined methodology can thus significantly improve the pipeline vacuum drying efficiency, which reduces the project cost and improves and de-risks scheduled and simultaneous operations.