Introduction to Stress Analysis of FRP Pipes
The fiberglass reinforced plastic, FRP, pipes are composite materials produced from glass fiber reinforcement, thermosetting plastic resins and additives. Regarding to some unique advantages, these pipes are used widely in different industries such as oil, gas, chemical, petrochemical, water and waste water etc…. Some of these advantages are as follows:
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The FRP pipes have high external and internal corrosion resistance in aggressive environments and are suitable for sour or salty fluid services.
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There is no need for cathodic protection or corrosion inhibitors in FRP piping systems and pipelines.
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The strength to weight ratio in FRP pipes is higher than metallic ones; therefore the handling, erection and installation costs are lower than metallic pipes.
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In comparison to ordinary metallic pipes, the service life of FRP pipes is longer which leads to a lower maintenance cost.
The first component of FRP pipes is fiber glass reinforcement filaments which are individual glass filaments drawn from a furnace of molten glass and woven around pipe matrix. The mechanical strength of FRP pipes depends on the amount, type, arrangement of filaments and directions in which the fibers are woven. Strength increases proportionally with the amount of glass fiber reinforcement filaments, see figure 1.
Figure 1: A typical FRP pipe structure
The second component of FRP pipes is resin system. Manufacturers choose a resin system based on chemical, mechanical and thermal properties. The three major resins which are used in FRP pipes are Epoxy, Polyester and Vinylester and consequently FRP pipes which are made of those resins are known as GRE, GRP and GRVE respectively.
Due to the lack of knowledge for stress analysis of heterogeneous materials, their behavior and failure modes are complex and hard to be understood; therefore most of manufacturers prefer to implement a variety of extensive stress testing on their components, such as hydrostatic and cyclic pressure, uniaxial tensile and compressive, bending, and combined loading tests. In the following codes and standards, some methods are developed to calculate flexibility and stresses of a FRP process piping system. The related manufacturer’s testing methods are presented in the standards: [13]
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BS 7159 Code (Design and Construction of Glass Reinforced Plastics Piping Systems for Individual Plants or Sites)
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UKOOA (Specification and Recommended Practice for the Use of GRP Piping Offshore being notable exceptions).
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ISO 14692-3 (Petroleum and natural gas industries —Glass-reinforced plastics (GRP) piping —Part 3: System design)
The main approach of these standards for stress analysis will be discussed in the following.