Pipeline and Cable Stability: Updated State of the Art

Abstract

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.

The offshore renewables industry may be better served by new bespoke design guidelines than by automatic adoption of recommended practices developed for oil and gas infrastructure: A recommendation illustrated by subsea cable design

“…The COREstab (Cables On Rock Enhanced stability) approach has been developed to address these considerations and has been described in Griffiths et al (2018b); Griffiths et al (2018c) and Griffiths (2022). The meso-scale approach consists of four steps:…”

“…In practice no pipes are ever prismatically uniform, however that is profoundly so for cables placed onto rocky seabeds where the vast majority of the cable is suspended above the seabed with only occasional localized points of contact occurring. In the case of the MeyGen cables Griffiths et al (2018b) found that less than 1% of the cable length was in contact with the seabed. Under these conditions, the limit state proposed by DNV in Eq (1 above) only makes logical sense as a length-averaged condition, recognising that a natural consequence of this longitudinal averaging is that some intermediate pipe movement may occur as a result, for example at each spanning section between touchdown points.…”

“…An updated summary of the research contributions made over the last decade in this field has been provided by Griffiths et al (2018a). These works point to a broad body of expertise and industry understanding gained from the use of existing recommended practices in design, such as the commonly-used F109 (DNV, 2021a), and the less-well-used but still-relevant AGA/PRCI design methods (AGA, 2002).…”

“…The plot shows the general trend that the required SG for stability tends to infinity as diameter gets small, with the existing design guidance for on-bottom stability based on hydrodynamic research using pipes greater than 0.2 m in diameter. Example images of power cables on rocky seabeds (Images courtesy Simec Atlantis, Griffiths et al, 2018b) showing the subsea power export cables from the MeyGen tidal stream energy project. Key features to be observed are the size/scale of the rocky boulders/outceops compared to the diameter of the cable and the resulting wedging of the cable into crevices at the points of contact between cable and seabed, between which the cable is in span above the seabed.…”

“…It has been recognised that on rocky seabeds the local profile of the seabed surface, at length scales comparable to the cable diameter, can have a very significant influence on the behaviour of subsea pipes and umbilicals, as demonstrated by the MeyGen cables shown in Figure 2. Where the rugosity of the seabed includes length scales of a similar order of size as the cable diameter, both the lateral resistance and hydrodynamic forces are dramatically altered, as documented by Griffiths et al (2018b); Griffiths et al, (2018c). In order to correctly predict the behaviour of seabed cables, it is necessary to be able to model the meso-scale roughness elements which are often too small to be resolved by conventional MBES survey methods.…”