Feasibility Study of Residual Curvature Method for Deep Water Pipelines

Abstract

Abstract A study on the feasibility of applying the Residual Curvature Method (RCM) as the primary global-buckle triggering mechanism for deep water high-temperature/high-pressure (HPHT) pipelines has been conducted using a Virtual Anchor Spacing (VAS) model on a flat seabed. RCM has only been implemented in shallow water projects and has not yet been used in deep water. Installation of a RCM feature in deep water has two major challenges: (1) Relatively high top tension immediately after the creation of the residual-curvature (RC) section can straighten the RC feature somewhat; (2) Due to the long catenary length, it is difficult to avoid excessive rotation of the RC section during the installation process. Thus, the RC section may end up in an inverted (crown-down) orientation during the touch down on the seabed, which may be undesirable. To determine the feasibility of the RCM in deep water, the effects of both challenges are investigated. VAS models are used to determine the optimal RC configuration, and the reasonable spacing between two adjacent RC sections, to reliably trigger acceptable global buckles. An optimal RC configuration and an acceptable RC spacing was first identified considering a typical symmetric as-laid RC shape with horizontal orientation and a commonly used 2D non-linear pipe-soil interaction (PSI) model (FRIC subroutine in Abaqus). To determine the feasibility of the selected RC configuration in a deep water setting, a 3D nonlinear pipe-soil constitutive model (UINTER subroutine in Abaqus) was included in the VAS model to investigate the effects of the following key parameters: bottom lay tension, top tension, PSI uncertainty, and inverted RC orientation during the touch down. The proprietary 3D nonlinear pipe-soil constitutive model, developed by Brunner & Bai [1], was used to capture the highly nonlinear pipe-soil interaction behavior. This affects the residual curvature section configuration and its orientation as it interacts with the seabed during installation, which in turn affects the pipeline configuration, stresses, strains, and fatigue life during operation. This 3D constitutive model is able to better describe the fundamental physics between pipe and soil by including uncoupled nonlinear lateral, axial, and vertical pipe-soil interaction properties. The model considers the variations in lateral and axial soil resistances depending on the change in penetration depth of the pipe into the soil. The effect of the inverted orientation, asymmetric contact, and the twist (rotation after contacting the seabed) of the RC section near the touchdown point has also been investigated to determine the asymmetric as-laid shape of the RC section and its lateral buckling performance in deep water. VAS analyses are carried out to investigate the buckle initiation reliability at the planned locations. They are also used for design criteria checks that include the maximum nominal longitudinal strain, LCC and DCC, and the fatigue limit state according to DNVGL-RP-F110 [7] and DNVGL-ST-F101 [8]. The analysis results show that, for a selected sensitivity range, all design criteria for the flowline are satisfied under different loading conditions. Therefore, the RCM was found suitable as a global buckle trigger solution for the deep water pipeline investigated in this study.