Stability of Concrete Mattresses

Concrete mattresses are widely used in the offshore industry for a variety of purposes, including to stabilise pipelines, flowlines and umbilicals, provide supports for span correction or crossings over other services and for scour mitigation. Despite their widespread use, the stability of mattresses under hydrodynamic loading is not very well understood, an issue which is particularly apparent when the purpose of employing mattresses is to provide secondary stabilisation for a pipeline, flowline or umbilical against hydrodynamic loading. The complexity of the mattress shape does not easily lend itself to analytical solutions based on data presented in existing codes and papers, with the result that a conservative analysis of mattress stability is regularly adopted in project environments. Where the project requirement is for a small number of mattresses, this is not a significant issue, however where large numbers of mattresses are required, such conservative approaches can result in unnecessary project costs. This paper presents the outcomes from two programs of physical model testing of mattresses specifically aimed at improving understanding of mattress stability under hydrodynamic loading, and the peak resistance that mattresses can provide to prevent flowline lateral movement when both the mattress and flowline are under hydrodynamic loading. While the physical model testing programs performed were tailored to specific project requirements, the testing results and means of application for those projects provide valuable insights to mattress stability under hydrodynamic loading, and a pathway to a less conservative approach to defining mattress requirements in such scenarios.