# Stress Analysis in Damaged Pipeline with Composite Coating
## 1 Introduction  
Pipelines are the most convenient and cost-effective means of transporting oil and natural gas from production fields to processing enterprises. At the same time, the main pipelines are highly loaded structures, since at the design stage, for material-saving reasons, they obtain a relatively low safety factor compared to other industries. In addition, during construction and operation, pipelines may be exposed to extra loads [1, 2, 3] and various risks [4, 5, 6]. The prolonged operation of pipelines leads to damage to pipeline steel at the nano- and microscale and causes the appearance of macrodefects (caverms and cracks). Hydrogen embtitlement, degradation of the mechanical properties of the pipe material, corrosion, and fatigue cracking under the influence of operational loads are the main problems for pipeline steels with a long service life [7, 8]. Equally important is the issue of the safe operation of oil and gas pipelines in difficult mining and geological conditions [9]. It should be noted that the destruction of a single section of a pipeline can lead to catastrophic emergencies and, as a consequence, environmental problems and significant economic losses. Therefore, acute problems concerning the effective prevention of the appearance of defects of the pipeline wall (especially fatigue cracks), as well as the repair of damaged pipes, arise in relation to pipelines with a long history of operation. In particular, the issues of the anticorrosive protection of new pipelines and re-insulation of long-runningoil pipelines remain relevant for professionals in the field of pipeline transportation of hydrocarbons. Previous studies [10; 11] have proposed an innovative insulative composite coating for underground sections of pipelines based on fiberglass cloth-reinforced epoxy resin. According to the developer's plan, this coating, alongside its anticorrosive properties, should also prevent the growth of fatigue cracks and mechanically strengthen the pipe surface. Therefore, the motivation of our study was the desire to obtain estimates of the impact of this insulative composite coating on the strength of defective and defect-free sections of pipeline. There are many ways of preventing the appearance and growth of fatigue cracks in pipeline structures, but they are not always effective in long-term use. In particular, improving the quality of steel does not completely solve this problem [12; 13]. Well-known effective methods, particularly peening and burnishing, add compressive stress to the metal's surface, and this stress impedes crack growth. However, such methods of preventing fatigue destruction are difficult to apply to industrial and main pipelines. Another common method is to apply an insulation coating in track conditions over time to identify a number of disadvantages associated with the loss of adhesive strength during the long-term operation of pipelines (wrinkles, corrugations, folds, etc.) [14]. The scheme of winding a roll coating onto a pipe is shown in Figure 1. Recently, much attention has been paid to the development of new composite anticorrosive coatings that improve the corrosion resistance of insulation, but these coatings do not significantly increase the mechanical strength of the pipe [15]. Noteworthy are the projects that used multicomponent composite coatings with fiberglass cloth bandages [10; 11], which provide proper strength and adhesion of the coating to the pipe. The first step in the successful practical application and full use of the capabilities of such coatings is a theoretical study of their behavior under operating conditions. In terms of mechanics, a defect-free pipe coated with an insulative composite coating can be represented as a layered structure. Some authors have considered the mechanical effects in layered coatings under various temperatures [16; 17]. Estimated calculations for the strength and rigidity of layered structures under arbitrarily oriented loads were considered in [18].