Issue 46

S.M. Medjdoub et alii, Frattura ed Integrità Strutturale, 46 (2018) 102-112; DOI: 10.3221/IGF-ESIS.46.11 103 the continuity flow of the fluid and there is not hot work in order to avoid the risk of explosion. The use of the composite wrap as an alternative of the cracked replacement pipe often saves economical resources to immediately pay behind costs of repair [6-10]. After the realization of 2 to 65 composite wrap repairs on pipelines more than 300 mm of outer diameter can save 15.000 to 780.000 m 3 of natural gas by year. In choosing the composite wrap, they saved 4 .106 m 3 of gas during 5 years [11]. A study conducted by the US Department of Transportation showed repair costs can be reduced by 24% by using composite repair instead of welded steel sleeves. When compared to the replacement of the whole defective pipe section, the cost can be further reduced to approximately 73% [12]. The strength of the repair is governed by the thickness of composite wrap and the tensile stress at each layer of the wrap, where effective stress transfer initiates once plastic flow occurs in the repaired steel pipe [13,14]. Murad et al [15] developed an integrated structural health monitoring approach for composite-based pipeline repair. However, the cumbersome process of installing electrical strain gauges on the steel pipe prior to the application of the composite repair greatly limits its adoption in offshore subsea application. Numerical methods such as finite element method (FEM) have grown considerably in recent years. Several authors have used this method to analyze the performance of pipe repair by a composite patch [16-20]. The majority of these authors have used the linear mechanics of fracture approach to evaluate the reduction of stress intensity around the crack front by the composite wrap. Benyahia et al [18] calculated the stress intensity factor (SIF) at the front of repaired crack with bonded composite wrap in pipe subjected to internal pressure. They showed that the composite wrap repair leads to a significant reduction of the SIF which improves the service life of the cracked pipe. The same conclusions were made by Bezzerrouki et al [19] who studied the performances of bonded composite wrap on pipes subjected to traction. However, for pipe subjected to bending moment the repair efficiency is less significant according to Achour et al [20]. A finite element study of cracked steel circular tube repaired by fiber reinforced polymer composite (FRPC) patching is executed by Lam et al [21]. It was shown that the mode I stress intensity factor (KI) of cracked steel members was found to be reduced with the application of FRPC patching. Using the KI and Paris equation [22], the fatigue life of the cracked steel member was increased by 22 times with the application of FRPC patching. The various works conducted have showed that application of FRPC to reinforce structural element is a viable option. Experimental testing can be used as a means of determining the effectiveness of the repair or reinforcement [23,24]. The use of numerical modelling can be a more cost effective solution where accurate results have been shown to be attainable through numerous previous studies [25,26]. The design of bonded FRPC wrap for repairing damaged pipe line has not been studied enough in the literature. The use of fiber-reinforced polymer composite as a load bearing sleeve has emerged as a promising means of pipeline rehabilitation due to advantages such as high specific strength, high corrosion resistance, lightweight, do not require welding and are simple to install [27]. Erdogan et al. [28,29] studied on the cracked panels using an analytical formulation for the fracture parameters such as stress intensity factor. Other closed-form expressions for SIFs are presented by Zahoor [30], Sanders [31] and Forman [32] for cracked cylindrical pipes. Zárate et al. [33] presented a framework to update and predict crack length as a function of the number of cycles in structural elements subjected to fatigue. In all previous researches of bonded composite repair of damaged pipe, the optimization of the repair parameters was not made. The objective of this work is to optimize the repair parameters in the technique of repair of cracked pipeline with composite wrap. To achieve this objective, a finite element study is used to analyze the performance of cracks repaired with composite patches by calculating the crack stress intensity factors in elastic behaviour. The effect of the geometrical properties on the reduction of the stress intensity factor at the crack tip is also analyzed. The obtained results are analyzed by the methodology of the experimental design to develop a constitutive mathematical model which controls the stress intensity factor as a function of the combination of three geometrical parameters of bonded FRPC wrap: length, angle and thickness. This experimental design method was applied to optimize the FRPC wrap size and to find the most influencing dimension on the repair efficiency. M ODEL DESCRIPTION AND MECHANICAL PROPERTIES onsider material alloy often used in gas pipelines, API 5L grade X65 containing a longitudinal external semi- elliptical crack of length (2c = 15.4 mm) and depth (a = 2.8 mm), he is repaired with a FRPC wrap, the fibres are all oriented at 0° stuck around the entire circumference of the pipeline, the model is shown in Fig. 1. The C