Uplift Resistance of Buried Pipelines at Low Cover-Diameter Ratios

Abstract Reliable estimates for the maximum available uplift resistance from the backfill soil are essential to prevent upheaval buckling of buried pipelines. The current design code DNV RP F110 does not offer guidance on how to predict the uplift resistance when the cover:pipe diameter (H/D) ratio is less than 2. Hence the current industry practice is to discount the shear contribution from uplift resitance for design scenarios with H/D ratios less than 1. The necessity of this extra conservatism is assessed through a series of full-scale and centrifuge tests, 21 in total, at the Schofield Centre, University of Cambridge. Backfill types include saturated loose sand, saturated dense sand and dry gravel. Data revealed that the Vertical Slip Surface Model remains applicable for design scenarios in loose sand, dense sand and gravel with H/D ratios less than 1, and that there is no evidence that the contribution from shear should be ignored at these low H/D ratios. For uplift events in gravel, the shear component seems reliable if the cover is more than 1-2 times the average particle size (D50), and more research effort is currenty being carried out to verify this conclusion. Strain analysis from the Particle Image Velocimetry (PIV) technique proves that the Vertical Slip Surface Model is a good representation of the true uplift deformation mechanism in loose sand at H/D ratios between 0.5 and 3.5. At very low H/D ratios (H/D 0.5), the deformation mechanism is more wedge-like, but the increased contribution from soil weight is likely to be compensated by the reduced shear contributions. Hence the design equation based on the Vertical Slip Surface Model still produces good estimates for the maximum available uplift resistance. The evolution of shear strain field from PIV analysis provides useful insight into how uplift resistance is mobilized as the uplift event progresses. Introduction Pipeline networks are instrumental for transporting hot crude oil from offshore platforms to onshore refineries. At shallow water sites (water depth =15 metres), the trench-and-burial method is typically adopted for pipeline laying projects, and the excavated material during trenching is used as primary backfill. The purpose of burial is three-fold: Firstly, the soil cover shields the pipeline from physical damages such as collisions, trawl-gear and anchor operations; Secondly, the surrounding soil provides additional thermal insulation, and helps maintain the high temperature in the containment required for low-viscocity crude oil flow; Last but not least, the overlying soil resists any tendancy of the pipeline to move vertically upwards, the result of a phenomenon known as upheaval buckling (UHB). UHB is a thermally induced structural effect. The operating conditions of high temperature and large internal pressure, which are significantly above the ambient seabed conditions at first laying, lead to thermal extensions of the pipeline. These axial movements are restricted by the frictional forces at the soil-pipeline interface. Consequently, large compressive forces are developed throughout the pipeline profile, which can cause it to buckle globally if restricting forces are inadequate. At locations where the pipeline profile features an over-bend, the easiest mode of buckling is for the pipeline to heave upwards through the backfill soil, hence the name upheaval buckling. For safe design at these locations, the upward heaving force generated by UHB must be adequately balanced by the downward resistance provided by the cover. Wherever this resistance is inadequate, an additional layer of rock dump will be required as secondary backfill.