Behaviour of repaired composite steel pipeline using epoxy grout as infill material

Abstract

The use of Fibre Reinforced Polymer (FRP) composites together with infill grout has been proven effective for repairing damaged steel pipelines. The common understanding of the role of grout is to fill the damaged section and to transfer loads from damaged pipeline to composite wrap. The properties of grouts are important parameters used in numerical simulation or theoretical prediction on the behaviour of a repair system. However, relatively limited information on the behaviour and role of grout in composite repair system has restricted efforts to explore the contribution of grouts as a secondary load bearing component. Therefore, this study aimed to investigate the performance and behaviour of epoxy grouts in terms of load transfer mechanism and load bearing capacity of pipeline composite repair system through detailed material characterization, hydrostatic burst test and finite element analysis (FEA). Selected mechanical and thermal tests were carried out on ten different grouts, steel pipe coupon and FRP composite wrap. Four hydrostatic burst tests were conducted on non-defect steel pipe, defective steel pipe and two composite repaired steel pipes. FEA was then utilized to enrich the information of grout in terms of load transfer mechanism and load bearing capacity. The finite element (FE) models were developed to simulate all hydrostatic burst tests for sensitivity analysis purposes. Results revealed that Grout A with highest silica sand filler content exhibits the highest modulus under all loading conditions. In terms of strength, Grout A shows the best performance under compressive load but the lowest resistance under tensile, flexural and lap shear load. Modified grout with no filler content, Grout A (1:0), shows contradictory properties and behaviour. In studying the effect of different grouts on overall performance of composite repaired steel pipe, Grout A and Grout A (1:0) were used to repair two steel pipe segments. Both grouts have increased the burst pressure of the steel pipe by about 23% and 26%, respectively. All FE models were found to be capable of predicting the behaviour and burst pressure of experimental test with margin of error less than 8%. The grout has experienced relatively high tensile stress when compared with the compressive stress. The highest tensile stress of grout was found at hoop direction while the highest compressive stress was recorded at radial direction. In addition, sensitivity analysis revealed that repair using Grout B resulted in 8% decrease of burst pressure, while grout with high tensile modulus and strength increased the burst pressure by 11%. Thus, based on the experimental test and numerical analysis, it is proven that the role of grout is not limited to transferring load and filling the defect, as it also provides additional reinforcement. It was also confirmed that different properties of grout affect the overall performance of repair. For a low tensile strength grout, an increase of modulus shows little difference of burst pressure, while for high tensile strength grout, a similar increase in modulus has led to a considerable increment in burst pressure. The finding in this study is significant as it provides comprehensive understanding of the role and contribution of grout in composite repaired steel pipeline. This can serve as an initial step towards optimizing the current design, such as minimizing the usage of composite layers and subsequently design repair without composite layers.