At OMAE in 2008 the ‘state of the art’ in pipeline on-bottom stability engineering was summarized, providing an overview of the current available knowledge for addressing pipeline stability. The aim of that work was to summarise key aspects of the pipeline stability design process and to include some historical perspective. The paper discusses the advantage and shortfalls of the different design approaches with a view to consolidate understanding, rather than to provide a ready-made solution to a complex design problem [1]. Since that time, a decade of research and further methodology refinement has extended the boundaries of the industry’s knowledge and understanding of the behaviour of subsea pipelines and cables, including geotechnics, hydrodynamics, oceanography and structural response modelling. In particular, progress has been made in: • The response of pipelines to sediment transport and scour; • Understanding the behaviour of small diameter pipelines and cables within wave and current boundary layers; and • The behaviour of cables on rocky seabeds in high energy marine environments. This paper summarises these innovations to enable the application of new paradigms in engineering practice and improved outcomes for initial project capital cost, reliability and operational integrity, as well as better models to predict the long-term behaviour where pipes are decommissioned in-situ. While a relatively widely studied field of engineering, there remain areas of active ongoing research to improve our understanding and ability to model and predict subsea pipeline on-bottom behavior, with a summary of the anticipated future opportunities proposed.

On-bottom stability design of subsea pipelines transporting hydrocarbons is important to ensure safety and reliability but is challenging to achieve in the onerous metocean (meteorological and oceanographic) conditions typical of large storms (such as tropical cyclones, hurricanes or typhoons). This challenge is increased by the fact that industry design guidelines presently give no guidance on how to incorporate the potential benefits of seabed mobility, which can lead to lowering and self-burial of the pipeline on a sandy seabed. In this paper, we demonstrate recent advances in experimental modelling of pipeline scour and present results investigating how pipeline stability can change in a large storm. An emphasis is placed on the initial development of the storm, where scour is inevitable on an erodible bed as the storm velocities build up to peak conditions. During this initial development, we compare the rate at which peak near-bed velocities increase in a large storm (typically less than 10 −3  m s −2 ) to the rate at which a pipeline scours and subsequently lowers (which is dependent not only on the storm velocities, but also on the mechanism of lowering and the pipeline properties). We show that the relative magnitude of these rates influences pipeline embedment during a storm and the stability of the pipeline.

The on-bottom stability design of subsea pipelines is important to ensure safety and reliability but is challenging to achieve, particularly in Australia due to onerous metocean and seabed conditions, and the prevalence of light gas pipelines. This challenge has been amplified by the fact that industry design guidelines have given no guidance on how to incorporate the potential benefits of seabed mobility, which can lead to lowering and self-burial of the pipeline on a sandy seabed. In this paper, we review the learnings of the STABLEpipe Joint Industry Project (JIP), which was initiated with the aim of developing new design guidelines to assess the on-bottom stability of pipelines on mobile seabeds. The paper summarises the new research undertaken within the STABLEpipe JIP to better predict sedimentation and scour, pipe-fluid interaction and pipe-soil interaction. New design methods to assess the on-bottom stability are also outlined, which have been developed based on the new research. These methods have been adopted in a DNVGL guideline authored by the JIP researchers in collaboration with DNVGL and presently available for use by the JIP participants. Finally, applications of the STABLEpipe JIP outcomes and focus areas for further work are discussed.

Accurate assessment of lateral resistance is critical to ensure the on-bottom stability and integrity of subsea pipelines and cables in the oil–gas and marine renewable energy industries. However, on rocky seabeds recommended practices provide limited recommendations on pipe–seabed interaction, suggesting only a single value for the friction coefficient of 0.6. This paper reports on a programme of physical experiments and theoretical modelling investigating the lateral resistance of pipes on rocky seabeds. It is shown that the peak and mean effective friction can significantly exceed the interface (or Coulomb) friction coefficient when the pipe diameter (D) is similar to the median rock diameter (dn50). Only when the pipe diameter becomes large compared to the rock size does the mean effective friction approach the interface friction. The effective friction coefficient was found to vary with variability in rock size and shape, as well as the length of pipe relative to median rock diameter. Each of these findings is reproduced well using the theoretical model. Collectively, the results demonstrate that the effective lateral friction coefficient may be higher than 0.6 for mean friction, and significantly higher for peak friction. This implies that inaccuracy may exist in current design, which may be rectified using the theoretical model.

In this paper a beam bending model is combined with existing predictive formulas for pipeline scour to study changes to pipeline stability during scour and lowering. The model is introduced and demonstrated for a range of simplified conditions, including scour-induced lowering of a pipeline resulting from multiple uniformly spaced scour initiation points. The model is then used with a synthetic seabed generated with a variety of length scales. In this simulation the pipeline is ‘laid’ onto the seabed, leading to the formation of ‘natural’ initiation points for scour. The distribution and spacing of the initiation points (which are a function of the pipeline bending stiffness, tension and seabed roughness) lead to different rates of pipeline lowering and stability. The resulting model may be used within a probabilistic framework to estimate changes to pipeline stability resulting from sediment mobility and scour.