An investigation of lay effects on as-laid embedment of offshore pipelines in carbonate silty sand

Accurate prediction of as-laid embedment is critical for assessing pipe–soil interaction after installation, yet current design guidance remains limited for coarse-grained carbonate sediments which are common on the North West Shelf of Australia and present unique challenges due to their high compressibility, crushable particles and variable drainage characteristics. This study investigates how pipe lay parameters influence embedment behaviour in carbonate silty sand, with particular focus on the combined effect of cyclic (sweeping) lateral movement and variable vertical load. Centrifuge model tests were conducted at 30 g using a pipeline element, with testing performed using sediment sourced from the North West Shelf of Australia. In addition to two monotonic (push in) tests, 15 cyclic tests were performed with vertical loads from 2 to 8 kPa and lateral amplitudes from 2 to 50% of the pipe diameter. An empirical two-parameter model was subsequently developed to capture the progression of embedment under cyclic loading, with the model fitted to single-packet test data and validated against multi-packet loading sequences, achieving reasonable predictive accuracy across varied loading histories. The model was then integrated into an iterative finite element framework to simulate the touchdown zone during laydown – with parametric studies revealing that lateral amplitude and bottom lay tension are the dominant factors influencing embedment depth. These findings support improved site-specific embedment prediction in carbonate sediments with drained response and offer a practical approach for optimising lay parameters in offshore pipeline design.