Lateral resistance of pipes on rocky seabeds - Comparison between measurements and models based on synthetic seabeds

The behavior of pipelines, cables and umbilicals on rocky seabeds has to date received little research attention. This is despite the marine renewable energy and oil and gas industries relying on these 'pipes' to cross a variety of rocky seabed types in the presence of extreme metocean conditions. Present design solutions are challenging and costly, yet there remains a track record of in-service failures. This paper forms part of a wider research effort being undertaken by the University of Western Australia (UWA) into pipe behavior on rocky seabeds. This work includes the effects intermittent gaps have on hydrodynamic forces, the effect of seabed roughness on enhanced boundary layer thickness and the validity of existing hydrodynamic force models for small diameter cables. In this paper, the lateral resistance of pipes on rocky seabeds is investigated using both physical and numerical testing of model pipes over artificially-created rocky seabeds. Four model pipes of varying diameter have been displaced laterally over 1 m square model rocky seabeds, with a range of pipe to rock diameters. The lateral resistance of the physical pipe tests were recorded using load cells and a digital data-logger. Analysis of the physical test results has enabled comparison to (and refinement of) numerical models as well as improved understanding of the importance of different parameters. Our results show peak frictions above 6 arising under conditions where interface friction is only about 0.3, which contrasts dramatically with the friction value of 0.6 nominated in F109. This work contributes towards generation of new design methods suitable for application to field conditions.