On-bottom pipelines are usually designed to buckle and move in a tolerable range to reduce the axial forces. However, large amplitude cyclic displacements of a pipe change the topography and strength of the seabed. The goal of this research was to shed light on pipe-soil mechanical behaviour during cycles of pipe displacement and to find a reliable method of predicting the soil response during lateral breakout and at the residual conditions after buckle initiation for undrained conditions in clay.
To achieve the research goals, centrifuge model tests were performed on kaolin clay, involving realistic simulations of pipe-seabed interaction supported by detailed characterisation of the surrounding soil. These tests showed that soil response was different at different parts of a buckle depending on the possible operational conditions during the operational life of a pipeline. The importance of considering realistic patterns of cyclic lateral movement (rather than movement between fixed ‘goal posts’) was presented to the industry and similar test programmes have already been conducted for specific offshore developments.
Back analysis of large displacement swipe tests using finite element (FE) limit analysis resulted in an expression quantifying the softening of soil elements in the berm. The shear strength of each soil element at an arbitrary pipe location is a function of the distance travelled by the soil element, the pipe embedment at the given location, and the sensitivity and brittleness of the particular soil. The average berm shear strength is the convolution of shear strength of its constitutive soil elements.
Finally, berm-pipe geometry was modelled using FE limit analysis for three idealisations: (i) a square soil block representing the berm, (ii) a partially remoulded zone below the mudline and (iii) an infinitesimally thin fully remoulded zone at the mudline. The last two zones were introduced following detailed study of the berm formation mechanism in model tests with particle image velocimetry and close range photogrammetry, supported by back calculation of another set of tests using upper bound plasticity analyses. This geometry was used in FE limit analysis to analyse probe tests. The match between the calculated and measured soil resistance and pipe trajectory was satisfactory for tests with as-laid embedment values greater than 15% of the pipe diameter, which is the lower range of as-laid embedments in soft clay seabed soils.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - Sep 2015|