The practical applications and mechanical characteristics of flexible pipes(l used in offshore oil production systems have been widely reported. However, few if any reports have appeared concerning the problem of gas permeation in flexible pipes, despite its importance. Accordingly, in the present paper, we shall estimate the gas permeability of flexible pipes composed of a plastic inner pipe, a metallic armor layer and an external sheath, such as those employed for conveying gas and petroleum, and deduce the life of such pipes from the pressure of the permeating gas. Furthermore, as regards basic measures to cope with this problem, we shall discuss a low gas permeability flexible pipe.
Owing to their convenient storage, transport, installation, operational characteristics and retrieval, flexible pipes have come into wide used in offshore oil producing systems. Particularly in oil-producing systems in marginal fields, since the required capital investment is comparatively small and the time required for installation is short, flexible pipes are by far the most advantageous type.
As illustrated in Figure 1(Available in full paper), in order to provide an appropriate combination of flexibility and strength, flexible pipe is made with a composite structure of plastic and metal. More specifically, the components of this flexible pipe are as follows.
* A steel conduit with an interlocking structure, which provides high flexibility and crushing resistance. The steels selected for this internal layer normally have the good corrosion resistance characteristics appropriate for handling hydrocarbons. This conduit is also wear resistant and minimizes cross-sectional distortion (ovality). However, due to its interlocking structure, this component is not watertight.
* An inner plastic pipe, normally extruded over the steel conduit. This pipe essentially serves the purpose of making the pipe leak-proof, and it must be resistant to abrasion and degradation, as well as to fluid characteristics such as high sulfur content or high temperature. It is usually composed of nylon 11 or PVDF.
* Pressure reinforcing armor, normally composed of an even number of layers of steel wire, which serves to resist axial load and internal pressure. This armor also provides torsional strength. The pipes often have two sets of pressure reinforcing armor, one being wound contrahelically for axial load (resistance to maximum tension) and the other for resistance to internal pressure. Also, steel tape is sometimes used as an additional metallic armor to resist internal pressure.
* An external plastic sheath, normally extruded over the pressure reinforcing armor, keeps the wires together and protects them against external corrosion, abrasion or damage. This sheath is usually composed of nylon 11 or polyethylene.
As described above, flexible pipe consists of a plastic conduit reinforced with metallic components. When gases or gas-bearing liquids are passed through such a pipe, gas molecules permeate the plastic layer by absorption, solution and diffusion phenomena, and consequently gas accumulates in the interstices of the metallic armor between the inner pipe and the external sheath. This accumulated gas gradually increases with the passage of time and the pressure in the armor rises.
