Simple solutions for downslope pipeline walking on elastic-perfectly-plastic soils

Abstract

Pipeline Walking is a phenomenon that occurs when High Pressure and High Temperature pipelines experience axial instability over their operational lifetime, and migrate globally in one direction. Existing analytical solutions treat the axial soil response as rigid-plastic but this does not match the response observed in physical model tests. In this paper, the authors develop a new analytical strategy using elasticperfectly-plastic axial pipe-soil interaction, which leads to more realistic walking rate predictions. The new analytical methodology is benchmarked with a series of Finite Element Analyses (FEA), which constitutes a parametric study performed to test the proposed expressions and improve on the understanding of the influence of axial mobilisation distance.

Effect of Gain in Soil Friction on the Walking Rate of Subsea Pipelines

“…Note that this is denoted as the global axial displacement of a pipeline per operational loading cycle in this study (more details will be described in the following section), which is consistent with the description of Carr et al [1]. Many other studies have been carried out examining the most influential factors in pipeline walking, such as the transient variation in flow velocity and temperature [6], the temperature gradient [7], the friction between the inner pipe and the sandwich layer [8], the weight variation along the pipeline [9], and the mobilization distance of soil resistance [10]. These studies provided fundamental and useful insights into pipeline walking for the purposes of practical design.…”

“…The coupling effect of the parameters evidently exists. The results of pipeline walking rate captured from finite element analysis reported in [10,28,37] are also demonstrated in Figure 5. The parameters adopted in each case are listed in Table 2.…”

“…Hence, reasonable adoption of soil friction in the design of subsea pipelines is necessary in order to capture a reliable estimation of walking rate. The concept of equivalent friction coefficient is employed, which is commonly used in the design code of subsea pipelines, such as DNV GL-RP-F114, and other research related to pipeline stability, such as [1,10,[25][26][27][28][29]. Following their descriptions, the equivalent friction coefficient is the production of axial soil resistance F divided by pipeline submerged weight W per unit length of pipeline (µ = F/W ).…”

“…The two soil property parameters are initial soil friction coefficient µ 1 and its gain η. Note that the ranges for each parameter tabulated in Table 1 represent typical conditions in offshore pipeline design, and refer to [1,2,9,10,18,25,26]. For instance, the length of the pipeline is taken as 1 to 4 km, since the pipeline cannot be too long to reach a condition of full constraint, which is traditionally less than 5 km [25].…”

Solutions for Downslope Pipeline Walking on a Seabed With a Peaky Trilinear Soil Resistance Model

“…Costly and time demanding finite element analyses are used to confirm walking behaviour and to generate a reliable walking rate. However, emerging academic research (Castelo et al, 2019;Castelo et al, in press b) demonstrates that, if more realistic soil behaviour is considered in the analytical formulae, the requirement for time demanding and expensive finite element analyses can be reduced.…”