A Novel Compression Armour Concept for Unbonded Flexible Pipes
Authors: Rytter Jan , Rytter Niels-J. , Nielsen Rishøj , Glejbøl Kristian 
Published: October 2008

This paper presents a novel concept for substituting the existing carcass in un-bonded flexible pipes with a special compression armour. The conventional carcass in flexible pipes is made from a folded steel strip thus forming an interlocking metallic layer, which provides collapse resistance. In the new concept the carcass is replaced with a special compression armour made from pre-fabricated steel profiles, which offers a far better radial compression capacity. Furthermore, the compression armour has the ability to sustain high axial compression loads, which is important for deepwaterapplication.

The compression armour concept can be used in the conventional flexible pipes thus expanding the present deepwater capacity. Furthermore, the compression armour can be used in combination with fibre-reinforced materials to construct an innovative ultra deepwater flexible pipe.

The compression armour has been analyzed using FEM models. The significance with respect to optimizing the geometry of the steel profiles for obtaining unique performance characteristics of the compression armour layer will be presented. To illustrate the high potential of the new armour concept a conventional pipe will be compared to a pipe system utilizing the new armour. Furthermore, a concept for a carbon fibre-reinforced pipe taking full advantage of the novel armour concept will be presented.

Introduction

In rough bore un-bonded flexible pipes, the inner most layer is a flexible metallic structure that prevents collapse of the polymeric barrier layer against external hydrostatic pressure and provides the pipe structure with crushing resistance during e.g. installation operations. When the un-bonded flexible pipes are used in deeper waters, the performance and properties of the carcass becomes increasingly significant as the hydrostatic pressure increases with the water depth. Furthermore, the increase in pipe top tension results in higher crushing loads from the installation equipment, e.g. caterpillars, and from the tensile armour during operation of a riser. Also, in deep waters the hydrostatic pressure may result in a reverse endcap effect, which combined with dynamic axial compression loads may result in a damaging axial compression of the pipe structure. This may cause birdcaging or lateral buckling of the tensile armour. Consequently, the decision was made to initiate the development of a novel compression armour concept, which will greatly improve the behaviour of deepwater flexible pipes.

The new compression armour concept

The conventional carcass in flexible pipes is made from a folded steel strip thus forming an interlocking metallic layer that provides resistance against radial loads, Figure 1.

Figure 1: Cross-sectional view of a conventional inter-locking carcass structure.(Available in full paper)

To improve the collapse and crushing resistance of the conventional carcass structure would require an increase of the strip thickness and total height of the inter-locking profile, or replacement of the conventional metallic materials with high strength alternatives. However, these high strength alternatives seldom possess the deformability required for the on-line folding operations. Furthermore, the interlocking structure does not possess the ability to support axial compression, afeature that will be required in the innovative deepwater flexible pipe design presented in later sections of this paper.