Methodology for Installation of Under-Straight Pipe for Lateral Buckling Mitigation

Abstract

The residual curvature (RC) method has been implemented by Subsea 7 for lateral buckling mitigation on various projects in recent years. The method involves deploying short sections of under-straight pipe in the catenary from a reel-lay vessel. The objective of this paper is to present RC results on bending moment and curvature and to show how these results can be combined with those from dynamic Orcaflex runs to predict the allowable sea-state for installation. ABAQUS models to simulate the reeling/unreeling/straightening procedure on one of Subsea 7’s reel-lay vessels are presented, together with a description of the settings adjustment to achieve under-straight pipe with a prescribed level of residual plastic strain. These models describe the pipe profile on the reel, free-span, aligner, passage through the straightener and finally within the catenary to the seabed. The model is used to simulate the deployment of an RC section and to quantify its tendency to rotate in the sag-bend. The dynamic loads are captured by running an equivalent RC section model in Orcaflex. Results for installation of a typical 14″ × 18″ pipe-in-pipe (PIP) are presented in 115m water depth (WD). Plots showing the spatial distribution of the bending moment and curvature of the RC section at various time increments within the catenary are provided. Attention is focused on the sag-bend. Stress-strain hysteresis loops are presented showing the multiple bending cycles experienced by the RC section from initial spooling-on until eventual deployment on the seabed. The impact that the RC length, residual plastic strain and ramp angle has on the bending moment/curvature is discussed. Orcaflex results are presented which show dynamic change in curvature at the sag-bend over a typical three-hour interval due to vessel motion. The most characteristic dynamic load (Ref. /15/) on the RC section is calculated to establish the loading envelope seen in the sag-bend region. Checks are also undertaken at other locations to ensure that the dynamic loads are not introducing any additional residual plastic strain in the RC section. The maximum bending moment seen by the RC section in the sagbend has an important bearing on the allowable sea-state for installation. The presented methodology for calculating the allowable sea-state (combining results of static and dynamic simulations) is a novel feature in the present work.