Failure of a sag water pipe triggered by aging of the GFRP composite relining
Introduction
A feed pipe of a hydraulic power station failed, after the aged and corroded original steel pipe had been reinforced with a relining consisting of glass fibre reinforced polymer (GFRP) composite tubes. The leak which was detected due to slight water discharge at the surface occurred > 12 years after the centrifuged composites tubes had been installed, in the pipe located approximately 1 to 4 m below ground level (Fig. 1). As the failure did not occur immediately after insertion of the relining, which consisted of the centrifuged composites pipes, the failure had to be associated either to overloading or to some fatigue or aging mechanisms, and to material deterioration.
Centrifuged composite pipes are widely used in water supply and in wastewater systems. They can be used in new installations or for refurbishment and retrofit of existing old pipes, where the original pipe might be aged (risk of leakage) or underdesigned for increased external or service loads. Such composite pipes are commonly manufactured in a centrifugation process, where the resin and different filler and reinforcing materials are sprayed into a rotating drum. Their development started in the early to mid-70s [1], [2], [3], [4], and the technique was also applied to silos and tanks. The rotation assures a homogenous distribution of resin and reinforcing components, and a uniform layer thickness for a stacked layup of such pipes. A drawback of the process may be that the resulting layup is not very reproducible nor within a narrow specification. An alternative manufacturing process of fodder silos and highly loaded GFRP pipes is filament winding with continuous glass fibres. Long fibre reinforced pipes are suited for applications with higher loading (including harsh surrounding conditions), where consequently more severe failures are reported, e.g. in the oil and gas industry [5], [6]. In both processes the final product is prone to aging in harsh, humid, or alkaline environments which leads to relaxation and a loss in mechanical performance (stiffness, strength). Hancox provided a review including relevant standards for aging analysis [7], and more recent reviews [8], [9], [10] described the application of GFRP extrusion profiles and components in the construction sector. Many studies aim to quantify the change in properties of GFRP laminates and the long-term behaviour of engineering structures due to continuous loading and environmental effects [5], [11], [12]. Farshad and Necola performed ring creep compression tests on GFRP pipe segments to investigate the effect of an aqueous environment on the stress corrosion cracking and the creep strength of pipes made in a centrifugal casting process [13], [14]. Current studies attempt to accelerate aging tests especially on GFRP pipes [11], [15], and finally to reduce the required test times.
The main mechanisms procuring to aging of GFRP due to environmental effects are related to the type of exposition such as liquid uptake (e.g. water by direct contact or in vapour phase), thermal loading (constant or cycling), ultraviolet exposition, or oxidation in air (usually at high temperature). The combination of moisture and temperature usually accelerates the chemical reactions, fatigue and creep and hence most degradation mechanisms. The constituents of the composite are sensitive to different effects, as for example glass fibres show static fatigue especially in presence of moisture, or stress corrosion cracking in presence of loads and alkaline media. Thermoset matrices are usually sensitive to moisture absorption which may have a plasticizing effect inducing a reduction of the matrix dominated properties like compressive and shear strength, in some cases even reducing the stiffness of the composite. Finally the interface between fibres and matrix governs the load transfer between them and may be affected by moisture and temperature, too [7].
The goal of the present failure investigation was the assessment of the state of aging of the involved composite materials over material and coupon tests, and the evaluation of the structural strength of entire pipes by means of product or certifications tests. The strength assessment was complemented by means of non-linear, time dependent numerical simulations based on the Finite-Element-Method (FEM) which considered the interaction between soil, pipes and external loads.
Section snippets
Description of local situation and visual inspection on-site
The installed relining was part of a sag water pipe with varying inclination. The operating pressure of the pipe was nearly constant, except for maintenance and shutdowns which created certain stress amplitudes in the pipe shell (only a few incidents per year). A longitudinal section of the failed pipe is shown in Fig. 2. There were two observed cracks in 2 pipe segments (No. 54 and 78). The original pipe was made of steel, and the composite pipes had been inserted after several decades of
Materials and methods
The material of the affected GFRP composite pipes has a layered construction. The layup and each layer thickness are based on the specifications of the manufacturer, which are not published. The final pipe has to fulfil product standards defining a minimum ring stiffness, static strength, and burst pressure, respectively. How these values can be achieved is upon the decision of the manufacturer. Hence, no clear specification is available about the layup of the composite pipe shell, the layer
Material analysis
The determination of the filler content with the calcination method for individual layers and entire laminate sections provided the results summarized in Table 1. The fibre content in both armoured layers was relatively high, especially in the outer layer (cf. Fig. 7).
The 3-point bending tests on samples taken in circumferential pipe direction showed quite different failure modes, depending whether the samples were tested in convex or concave orientation (cf. Fig. 8). Consequently, also the
Discussion
The relevant findings with respect to material analysis are summarized in Table 1. As mentioned the difficulty with the present laminated pipe structure is the complex layup with different layers which are not clearly specified (Fig. 7). The layers vary in thickness, filler content and homogeneity. For a classical laminate the knowledge of the ply properties is important if the plies are used repetitively. The mechanical properties of the laminate can then be calculated from the ply data. In
Recalculation of the strength assessment (numerical simulations)
Aging was identified as a primary failure mechanism of the composite relining in the examined sag water pipe. The crack pattern with one longitudinal crack in the apex of the GFRP pipes made it suspect that the non-uniform bedding of the relining found in the top of the cavity between steel and GFRP pipe (Fig. 6) produced an additional (local) loading on the GFRP pipe and superimposed local hoop stresses in the composite shell. The spacers at the pipe's ends provided enough radial stiffness to
Conclusions
The investigation initially presumed an advanced aging of the polymer composite tubes (centrifuged GFRP composite layers with short random glass fibres) which were analysed with extensive material examinations. However, the results could not fully explain the observed crack patterns in the apex of a few composite pipe sections.
Hence, the original installation procedure and the behaviour of the whole system including pipe environment under variable load conditions were reanalysed. After all, the
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