Abstract
The method of using residual curvature during pipeline installation, primarily for the purpose of lateral buckling control, has caught an increasing amount of attention over the past few years [1], [9]. The use of residual curvature sparked a particular interest after positive experiences from a 26 km long pipeline on Statoil’s Skuld project (2012) in the Norwegian Sea [7]. As such, a range of technical papers elaborating on the topic have recently been published [6], [7], [9]. Some of this work has identified some particularly novel applications for the residual curvature method including freespan mitigation to reduce the requirement for seabed intervention, allowing for direct pipeline tie-ins, use with s-lay installation and even for steel catenary risers [10], [11]. However, these applications are currently only identified and not yet proven successful in any published work.
This technical paper focusses on validating the use of residual curvature for the purpose of lateral buckling control in subsea pipelines installed by reel-lay. The residual curvature method demonstrates high buckling reliability without the use of subsea structures or additional installation equipment, with a controlled buckle response and favourable operational bending moments [1]. The residual curvature method has been shown less sensitive to some design parameters than other lateral buckling control methods [6]. However, published work also show that high strains will develop for short residual curvature lengths, high pipe-seabed frictions and for certain levels of residual strains [6].
Previous research has predicted the behaviour of residual curvature as a means of controlling lateral buckling in a deterministic approach [6], [7], [9]. However, performing a lateral buckling design with a probabilistic approach can offer a more realistic design and demonstrate higher reliability. There is a range of research on probabilistic approaches for lateral buckling design of subsea pipelines, but there is little published work on the same approach for residual curvature in particular.
For this reason, this paper suggests a method for determining the likelihood of buckling and the associated bending moments via structural reliability analysis (SRA). A numerical model combining Finite Element (FE) Analysis and a Monte Carlo simulation is applied. A similar approach has already been presented by others for a different lateral buckling control method, and involves forming a database of finite element solutions followed by multivariate interpolation for the stochastic variables [16].
The multivariate interpolation necessitates a permutation of the cases in an FE result database. In order to keep the simulation efficient, only a limited number of variables are treated as stochastic. The variables that are considered as stochastic are those that have been determined that the lateral buckling response due to residual curvature is sensitive to. The variations of the remaining parameters are also accounted for but in a simpler way.
The suggested SRA is used to assess the reliability of a pipeline that resembles the Skuld pipeline. The proposed SRA validates that residual curvature is a reliable lateral buckling control method irrespective of great variations in the design parameters that cannot be quantified easily, such as target residual strain. The proposed SRA also serves as a cost attractive solution in the qualification testing, by potentially relieving the installation contractor from the expensive exercise of performing an additional straightening trial.