Abstract

Traditional pipe-in-pipe systems installed by reel-lay are straightened on the lay-vessel. The process results in the inner and outer pipes having opposite residual moment and the inner pipe is under-straightened, which typically means hogging residual curvature in the sag bend. This may lead to significant twisting rotation of the inner pipe whilst laying.

The residual curvature method for buckling control can be applied to pipe-in-pipe systems. As for single pipes, it is better if the pipe twists during installation, so that the residual curvature rotates into the horizontal plane. This increases as-laid horizontal out of straightness and reduces free spans which is important for in-place performance. Therefore, using predictive analysis at the design stage, parameters should be selected to promote rotation during installation.

Analysis for rotation of pipe-in-pipes is presented, which is more complex than for single pipes. The following features are included: simplified calculation of residual curvature for inner and outer pipes; separate rotation for each pipe; distributed contact force between the two pipes and interpipe friction. Results are presented from sensitivity studies on interpipe friction and the effect of route curves.

Introduction

The pipe-in-pipe (PIP) concept is characterized by two concentric pipes, the inner conveying fluid, and the outer providing mechanical support. Some form of spacer between the two pipes keeps the inner pipe centralised and the annulus is typically filled with insulation, see Figure 1. The concept has been used in subsea pipelines since the early 1970s (Bai & Bai, 2005) and has the following advantages: low heat transfer; high structural integrity; no/little extra installation cost if laying from a reel. These advantages help the designer overcome challenges for example with wax formation and trawl loads, and may prove to be more effective and cost-efficient than alternative measures such as trenching, burial or thick wet coating.

Other challenges for subsea pipeline design include the control of expansion and buckling. High pressure and temperature in conveyed fluids make the pipeline tend to expand axially, though restraint from seabed friction resists this and axial compression force builds up. If the axial force becomes sufficient, the pipeline may become unstable and buckle perpendicular to the axis leading to high local bending moments, stresses and possible failure.

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