Behaviour of infilled rehabilitation system with composites for steel pipe

Behaviour of infilled rehabilitation system with
composites for steel pipe

Fibre-reinforced polymer-based composites are found effective and suitable for steel pipe repairs because of their unique advantages such as high strength, lightweight, non-corrosive, and fast and easy to handle. In this study, the behaviour of a new type of composite repair system for steel pipelines underwater, which is the grouted composite sleeve, was investigated. This type of repair system relies on the effective transfer of stresses from the steel pipe to the encircling composite sleeve through the grout infill. Research was therefore necessary in optimising the material and geometric properties of each component of this system in order to have a betterunderstanding on its overall behaviour and its effectiveness in repairing steel pipelines.

An experimental study on the mechanical, thermal and shrinkage properties of five epoxy-based grouts commonly used for structural repair was conducted. Three grouts have compressive strength of more than 80 MPa and total shrinkages of only 2.77%, which were deemed applicable for structural repair of steel pipelines. Moreover, the glass transition temperature of these grouts ranges from 50 to 90oC. The inclusion of coarse filler significantly increased the modulus and compressive strength, and reduced the shrinkage, but also reduced the tensile strength. There was considerable reduction in mechanical properties due to hot-wet conditioning at 70oC. This is because of the plasticisation and weakening of the aggregate matrix debonding at a temperature close to the glass transition temperature.

The glass fibre – vinyl ester composites considered in this study have strength and modulus suitable for sleeve in the grouted sleeve repair system. The tensile strength, modulus and interlaminar shear strength of this composite was 427 MPa, 25 GPa and 30 MPa, respectively and its glass transition temperature was 110oC. It absorbed only 0.22% of moisture after hot-wet conditioning of 3000 hours. More importantly, there was almost no change in the modulus and it retained sufficient tensile and interlaminar shear strength needed for a composite sleeve repair system even after exposure to moisture and elevated temperature of 80°C. The glass transition temperature decreased to 97°C after conditioning and reached saturation indicating that, the composite was suitable for pipeline repair in continuous service at higher temperature.

A finite element analysis using a simplified 2D model was conducted to determine the effect of critical parameters on the behaviour of a grouted composite sleeve repair system. Grout modulus and thickness, and sleeve thickness were considered for the analysis. The results of the analyses indicated that the thinner and higher modulus grouts are more effective to transfer load from the steel to the sleeve than the thicker and lower modulus grouts. Similarly, a thicker sleeve reduced the level of stresses and strains in all the components of the repair. Based on the results of the analysis, it was recommended that a grout thickness of 20 mm with a modulus of at least 5 GPa was appropriate for an effective grouted repair system and for practical application.

A full-scale 3D analysis was carried out to determine the effectiveness of the repair system for steel pipelines with a range of localised metal loss. The results of the analyses indicated that the modulus of the infill grout governs the load transfer between the steel and the sleeve, but the tensile strength governs the functionality of the system. The cracking of the grout resulted in a significant increase in the stress level in the steel pipeline and composite sleeve. The repair system using grout with a higher tensile strength provided a higher pipe capacity by utilising the plasticity of the steel, while the grout with a lower tensile strength cracks even before yielding of the yielding at a low applied internal pressure. Moreover, a thicker sleeve provided higher capacity in the repaired pipe, and both the carbon and glass fibre composites were found effective for composite sleeve repair system. Most importantly, it was determined that the considered grouted composite sleeve repair system can effectively reinstate the capacity of the pipelines with a localised defect of up to 70% metal loss.

An improved understanding on the behaviour of the grouted composite sleeve repair system for pipeline with metal loss was achieved in this study, providing a base knowledge from which further research could continue. The results obtained provided important information on the optimal material properties of the infill grout and the composite sleeve for an effective repair system and the effect of different operating conditions on the overall behaviour of the repaired steel pipelines. These results are very valuable and will help researchers, engineers and stakeholders to consider the actual application and implementation of this new composite system in repairing steel pipelines