In the last few years some ‘long’ HPHT (high pressure high temperature) offshore pipelines have been required to be routed across challenging seabed terrains such as salt diapirs and deep trenches with relatively large OOS (out of straightness) and steep slopes. This has given rise to a combined design concern of lateral buckling with pipeline walking. Typically anchor points in the form of caisson piles as well as buckle sites in the form of sleepers or buoyancy modules maybe required in the vicinity of the seabed features to release the compressive forces associated with the high pressure and temperature loads in a controlled manner and to prevent the pipelines from migrating down slope during operation. The instigation of buckle sites can essentially divide any ‘long’ pipelines into a number of short sections with different walking behaviour due to varying seabed slopes and varying pressure and temperature gradients at locations along the route. In addition pipeline anchors influence the development of any planned buckle site and thus limit the extent of cyclic feed-in and/or pull-out. This integrated mechanism is termed buckling walking interaction within this paper.
The prevailing methodology and numerical modelling of pipeline walking usually evaluates ratcheting end expansions by means of ‘short’ and overall ‘long’ pipe lengths which ignore or consider the existence of buckle sites along any pipeline route respectively. This paper presents a general-purpose study on the phenomenon of buckling walking interaction considering a typical pipe route configuration with anchors and buckles along its length. Aspects such as the cyclic evolution of mode shapes of buckles and the effect of soil berm resistance on buckling walking interaction are explored. Finally the design application of buckling walking interaction for ‘long’ cyclically constrained pipelines with buckles and anchors is discussed.