Design of Direct On-Seabed Sliding Foundations

Abstract

This paper describes a new framework for the geotechnical design of pipeline-related foundations (e.g. foundations for PLETs and Tees) designed to slide directly over the seabed during pipeline operation. This approach can present considerable savings in terms of fabrication and construction costs because of reduced foundation sizes. Over the design life, a pipeline is likely to endure many thermal/ pressure load cycles due to product and flow rate variations during operation. These cycles result in the foundation sliding back and forth across the seabed within a footprint. These loads and corresponding motions impose cyclic shear stresses on the soil that can (i) degrade foundation bearing capacity and (ii) cause additional foundation settlement. Often the key design consideration is whether or not the cumulative settlements will eventually compromise the integrity of the pipeline system to which the PLET and its associated foundation are attached. In addition to consolidation and creep, two key mechanisms are shown to control cyclic foundation settlement: (i) bearing mechanism induced burial and (ii) cyclic shear stress driven soil volume reduction. Their relative significance depends on the soil conditions (soil state) and input pipeline movements. The paper presents key aspects required for the design of direct on-seabed sliding foundations, including the soil parameters and associated testing required. Validation of the design approach is illustrated by comparison to laboratory model tests performed on carbonate soils. The impact of soil properties on potential foundation performance is illustrated with a design example and the importance of conducting site specific soil testing and settlement analyses is emphasised. It is also illustrated that close integration of the pipeline, structural and geotechnical analysis is necessary to reliably quantify system performance of these novel foundations.