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Pipeline embedment into the seabed is a key consideration for offshore oil and gas developments with high-temperature fluids. To date, the mechanism of steady-state heat transfer from partially and fully buried pipes has been modeled predominantly through analytical and numerical approaches. The current study focuses on making detailed measurements of heat-transfer characteristics. A laboratoryscale experimental apparatus imitating a subsea pipeline partially or fully buried into the seabed is created. Hot flow of hydrocarbons inside oil and gas offshore pipelines and the cold external flow of seawaters are simulated by means of 70 C and 5 C water flows from two separate water tanks, respectively. The experiments are carried out for seven different burial depths representing a range of various burial configurations, from fully exposed to fully buried pipes. The temperatures measured on the external surface of the pipe are analyzed, and the overall heat-transfer coefficient of the pipe is calculated. The effect of burial depth on the overall heat-transfer coefficient is compared with analytical formulae.

A differential-algebraic system is presented to model unstable two-phase flows in pipe-riser systems. Equations derive from the space integration of an isothermal drift-flux model assuming quasi-equilibrium momentum balance. A linear analysis of this system gives a new stability criterion for gas-liquid flows in pipe-riser systems. This criterion is validated by laboratory experiments. Then, a nonlinear analysis shows that the severe slugging phenomenon is a hydrodynamic instability coming from a supercritical Hopf bifurcation.

It is increasingly recognized that the state of the seabed surrounding an on-bottom pipeline may change during the operating life of the pipeline. For seabed sediments that are soft and fine-grained, the strength may vary through episodes of pipeline movement due to consolidation effects. For seabed sediments that are mobile due to waves and currents, the burial state and the adjacent seabed topography may vary due to sediment transport and scour. These changes in the strength and topography of the surrounding seabed alter the exposure of the pipeline to hydrodynamic loads and ambient cooling, as well as the level of geotechnical support and insulation provided by the seabed. The design relevance of these changes in seabed condition is amplified by modern design approaches in which the pipeline itself can be tolerably mobile — for example in a dynamic onbottom stability approach or through engineered schemes of global buckling and axial walking. This paper illustrates the interactions between the geotechnical and sediment transport processes and the resulting global pipeline behaviour. Two interactions are considered: the long-term axial walking behaviour on soft soil, and the long-term insulation and temperature profile on a mobile seabed. The examples highlight the potential for over or underestimation of various inputs to a pipeline design when these temporal changes in pipe-seabed condition are overlooked. Emerging analysis methods for pipeline-seabed interaction that incorporate these temporal effects can lead to more reliable and cost-effective design.

Flow Assurance (FA) engineers usually produce a range of technically feasible solutions in response to a particular challenge. Often these solutions are cost neutral. In order for a development manager to select the optimum solution he/she examines the risk profiles associated with the options. To progress a marginal development, explicit understanding of financial risk is imperative, needing a step up from qualitative “gut feel” assessments to a more reasoned and auditable quantitative evaluation of FA risk. As part of Woodside's Next Generation Flow Assurance Strategies initiative, Woodside embarked on a Joint Industry Project (JIP) called RiBFAT, standing for Risk Based Flow Assurance Toolkit. The main objective of this JIP is to develop quantitative risk profiles for FA solutions to the wide range of FA challenges. Phase 1 of the JIP produced a Proof of Concept for a risk based approach to hydrate management, one of the main FA challenges of cold ambient temperature field developments, typified by deepwater. Phase 2 was the conversion of the agreed methodology into fully functioning non-commercial software; completed by the end of 2014. This paper discusses the methodology developed and illustrates how this can be applied to a field development concept in order to compare the risk profiles for alternative FA strategies for hydrate management. The risk profiles cover the life of the asset including additions / modifications to facilities as required to optimise the development of the reserves. Ongoing use of the methodology results in the production of a database of knowledge for each FA challenge, which may be interrogated for benchmarking or assurance purposes, allowing the user to develop greater confidence in a particular proposed strategy. The methodology for addressing risk described in this paper will be extended in the next phase of the JIP to other FA challenges such as slugging, scale, wax and corrosion, building up to a comprehensive toolkit for FA risk assessment, including the database. Concurrently, we plan to document this methodology as the first ever global FA risk assessment recommended practice.