Surface-laid pipelines and cables in areas of sand waves and seabed mobility: quantitative analysis from a decade of field observations

The embedment and spanning of surface-laid offshore pipelines and cables can change during their operational life due to seabed mobility. Understanding these changes is important as it affects their operational response. However, systematic interpretation of long-term observation data across different projects and in different seabed environments is lacking. This study analyses the survey data of three different pipelines with diameters ranging from 4 to 42 inches, installed in two different sand wave fields on the Northwest Shelf of Australia. These surveys were carried out from the as-laid (initial) condition to a maximum period of 13 years after installation. The results quantify how the proportion of the pipeline length that is in span reduces over time, and identifies approximately 2- to 3-fold embedment increases in certain parts of the sand waves. Notably, the data reveals that current design practice significantly underestimates as-laid embedment by a factor of approximately ten and that temporal changes are not usually considered. Quantitative insights due to these changes were further assessed, including geotechnical aspects, structural aspects, hydrodynamic loading and stability, and the progression of scour. These findings illustrate the value of a multi-disciplinary approach to evaluate the long-term integrity of cables and pipelines. • A decade of field survey data shows pipeline embedment and span evolution in two static sand wave fields. • As-laid embedment prediction requires soil stiffness over 10 times lower than design guidelines. • Temporal changes of embedment and spanning result from soil–structure interaction and seabed mobility. • A practical approach is proposed to estimate bending moments and seabed reactions from survey profiles. • Results support performance-based design of pipelines and cables considering long-term seabed mobility effects.