The increasing development of marginal offshore hydrocarbon deposits has resulted in production of increasingly corrosive fluids. This has increased the need for piplines capable of operating at elavated temperatures in the presence of high concentrations of H2S and C02 gases. Conventional pipelines require the use of stainless steels or corrosion resistant alloys which drives up the cost of materials and fabrication/installation of the system. The use of flexible pipe systems is assuming an important role in these applications where stainless steel and CRA carcass materials may be economically utilized.
This paper addresses the use of unbonded flexible pipe (figure 1) for aggressive environmental conditions. These conditions limit the use of rigid pipe to stainless or corrosion resistant alloys thus driving up the price of the pipe as well as expenditure for expensive corrosion inhibition systems (Hill, 1989, OTC 6114). In these types of environments flexible pipe can provide a cost effective and environmentally sound solution to field development problems.
A great deal of research has been completed over the past several years by the Force Institute and various material suppliers leading to an increased understanding of corrosion mechanisms in flexible pipe systems. The results of these studies are discussed later.
Figure 1-Typical Flexible Pipe Design (Available in full paper)
Each of the layers of unbonded flexible pipe is free to move relative to the others. This means that there is no composite action in bending or torsion. Each layer of the flexible pipe has a specific function to perform.
The primary purpose of the carcass layer is to prevent collapse of the flexible pipe. The collapse mechanism may occur due to either external hydrostatic pressure or the squeezing force of the helical armor wire layers under the influence of axial tension. For severe environments the material specified for carcass is generally 304L, 316L, Duplex or AL6XN alloy, however full-scale testing performed at the Force Institute, Copenhagen has shown that carbon steels are viable for a much wider range of process fluid corrosivity than previously believed. Under extremely severe conditions the carcass layer may be omitted and its role performed by the plastic barrier layer in conjunction with the steel hoop stress layer (McCone, 1989).
The carcass consists of an interlocked helix of steel strip which is pre-formed into an S-section (figure 2). The resulting structure is dimensioned to ensure stability and resist the greatest applied external pressure loads. When dimensioning the carcass a safety factor of 1.5 x the design external pressure is used.
Figure 2-Carcass Cross Section (Available in full paper)
The critical pressure for collapse is calculated from the classical formula for Euler stability of a solid cylinder with allowances included for ellipticity of the final formed shape as shown in figure 3 (B. Chen et al, FPT 92).