Nonlinear modelling and analysis of composite pipes for offshore oil and gas applications

Reinforced thermoplastic pipes (RTPs) are considered as prospective alternatives to traditional steel pipes in various offshore oil and gas applications due to their attractive properties such as better corrosion resistance, high stiffness and strength to weight ratios and low maintenance costs. However, as RTPs are relatively new products, their design and qualifications are governed by industry practice at present, which only cover limited loading cases.

In this project, studies of the mechanical behaviour of RTPs composed of pipe grade polyethylene (PE) and aramid fibres, and subjected to various loads are conducted using non-linear Finite Elements Analysis (FEA). 

Figure 10: Buckling of the reinforced thermoplastic pipe under bending.


            Figure 10: Buckling of the reinforced thermoplastic pipe under bending.

Since the spoolability of RTPs is essential in their installation process, their flexural behaviour is also analysed (see Figure 10). The analysis showed that the estimation of the minimum allowable bend radius of the RTP can be improved by using the proposed modelling approach. The minimum allowable bend radii of RTPs with different ply angles and diameter-to-thickness ratios are determined. Attributing to the material nonlinearity, the RTP with certain parameters can be bent to a smaller radius without buckling. It was found that the sufficient spoolability of RTPs can be also achieved by employing certain two angle-ply reinforcing layer systems. It has also been shown that the reel diameter could decreased by 24.29% based on the analysis performed. This will result in significant re reduction in the transportation and material costs during the installation phase of the pipeline. 

For the transport of hydrocarbons that may contain corrosive elements such as H2S, CO2 or other corrosive products, the steel pipelines are required to be lined with corrosion resistant alloys (CRA) liner for better corrosion resistance and service life of the pipeline. However, the CRA lined-pipes are special made-to-order products that can be quite expensive due to high cost of the liner and the manufacturing techniques involved. A promising alternative technology to CRA lined steel pipelines is the use of HDPE-lined steel pipe. This will improve the corrosion resistance and prolong service life of the pipelines. The plastic-lined pipe consists of a thermoplastic inner liner and a carbon steel outer pipe. One of the possible failure modes which could be considered in offshore pipeline installation is the stability of the liner under bending (see Figure 11). Non-linear Finite-Element (FE) models have been developed to study the liner behaviour under bending during the offshore installation phase. The outer pipe has been modelled using shell elements while the liner has been modelled using three-dimensional brick elements in Abaqus/standard. The effects of different types of liner fit (e.g., snug-fit, tight-fit), contact pressure and friction between the outer pipe and the liner on the stability of the liner have been explored. For large diameter lined pipe (24 in.), the minimum bend radius for the pipeline which could safely be applied during installation without the buckling and separation of the liner has been determined.

Figure 11: S-lay installation and pipe deformation at at (a) Bend radius = 30 m, (b) Bend radius = 10.71 m, and (c) Bend radius = 10.3 m.