Development of pipe repairs using bonded metal plate – Part I: Shape factor, stiffness and surface treatment

https://doi.org/10.1016/j.ijadhadh.2020.102594Get rights and content

Abstract

This work aims to study the resistance of repairs with bonded plates in pipes with holes subjected to hydrostatic pressure. It is a promising alternative for application in low-risk repairs (up to 1 MPa). Due to the differences between the circumferential and transverse stresses in the pipes, the influence of the bonded plate area in the joint strength was evaluated, checking whether the shape factor occurs. The effect of plate thickness in the repair strength, besides the addition of silane on the plate surfaces, were also studied. Finally, considering a control plate based in the performed experiments, the surface treatment influence on the tubes was evaluated in the face of rupture pressure of the system. High hydrostatic resistance values were found, demonstrating the efficiency of the procedures used in the repair. The studies show little influence on the repair strength due to dimensions variation in different directions, and high influence on the surface treatment, stiffness and bonded area increase. The work aims to initiate the development of a new procedure for contingency repair with low risk.

Introduction

Adhesive joints have been widely used in the areas of oil industry, aerospace, automotive and others in general. Advantages of bonded joints include: i) a more uniform stress distribution compared to conventional joints (screws, rivets and welding), ii) do not require heat input in their application, which implies a better weather resistance, iii) simple application and lighter structure [1,2]. These properties have motivated the making of many researches works on this subject.

Due to the great use of pipelines to transport liquids and gases, several studies directed to the maintenance of the failures have been carried out. Pipe repairs cover a variety of damage situations, corrosion being the main cause [3], but they also include dents, gouges, fretting, cracks, leaks and manufacturing defects, according to ASME PCC−2 [4]. The repairs may be temporary or permanent. The temporary repairs include bolted clamps or fixtures and can involve metal deposits, as in a sleeve-repair welding. Although this method usually results in good strength obtained in the final repair, it is possible that the pipe-wall melts through or that, martensite is formed, in addition to residual stresses in the pipe wall [5]. All these factors have welding as main cause. The temporary repairs are done for a fixed time and should be replaced later by a permanent repair, which include the replace of the entire pipe (requiring production stops, in addition to being expensive and slow), nonmetallic liners and wraps (composite materials).

Numerous papers have studied the repair of steel pipes with laminated composites, (e.g. Saeed et al. [6], Rohem et al. [7] and Budhe et al. [8]), however, few studies have been done related to repairs with bonded plates. In a recent work, Çitil et al. [9] investigated the pipe repair with bonded plates. Artificial cracks were created on the pipes and repairs were made using adhesive and galvanized steel patches with different overlap lengths and thickness. After subjecting the pipes to internal pressure, the study concluded that the larger the thickness of the patch the greater the internal pressure capacity. Regarding increasing overlap length, the internal pressure capacity did not change significantly. In addition, the experimental and numerical results data are very similar. The repair of small holes using composite patches was studied by Ayaz et al. [10]. The repaired pipes were subjected to internal pressure until damage occurred. The results showed that the greater the overlap length the greater the failure load, occurring a relationship approximately linear. The failure load also increased when the patches thickness increased. Small diameters (13.45 mm of internal and 16.85 mm of external diameter) were used in both works ([9,10]).

Due to the existence of several influence factors on the bonded joints strength, more detailed studies of some aspects such as substrate thickness and surface treatment are necessary. In the case of pipe repairs with bonded plates, studies to assess the effect of plate thickness in the strength of the joints are still rare. Obviously, thicker plates should lead to greater rupture forces, but other difficulties such as plate conformation - more difficult for thicker plates - should also be considered. Several studies have investigated the effect of surface treatment on bonded joints, besides pretreatment. The joints mechanical behavior depends fundamentally on adhesion at the interface between the adhesive and the substrate, thus adhesion-promoting molecules are often used to improve the bond strength between adhesive and substrates and or surface pre-treatments [11]. One of the most widely used adhesion promoters are those based on silane molecules, which significantly increase the strength at the interface and hence the durability of the adhesive joints [12]. Da Silva et al. [13] conducted experimental studies in single lap joints (SLJ) to investigate the effect of three surface treatment (one mechanical and two chemical conversion coatings) on joint strength. For the three adhesives used, the authors concluded that the surface treatments did not result in increased joint strength. Pereira [14] studied SLJ with aluminum substrates bonded with epoxy adhesive using five different surface treatments, and concluded that the shear strength of the test specimens increased with the decrease of the surface roughness. The influence of grooves in the preparation of bonded surfaces was studied by Da Silva [15]. The results showed that the surface pattern had an influence on the joint mechanical strength, and this effect was more pronounced on test specimens without surface treatment. The grooved specimens showed mixed (cohesive and adhesive) ruptures, while those without grooves showed adhesive ruptures (at the interface).

The works of Da Costa Mattos et al. ([16,17]) investigated and confirmed the shape factor in single lap joints and in corner joints, respectively. That is, even in adhesive joints with the same bonded area, but with distinct linear dimensions, the rupture strength is different. The theoretical consideration that circumferential stress is double the theoretical longitudinal stress in pipes [18], besides the cited works motivated new studies to obtain the shape factor of metal plate bonded with epoxy adhesive, that could be applied in contingency repairs to leaky pipes. Several works about different methods of pipe repairs have been made in the last years, however, most describe the pipe repair with composites and little research has been conducted on repairs with bonded plates, especially for diameter above 50 mm (2 in). The standard ASME PCC2 [4] considers two classes of risk in piping: a low risk (pressure up to 1 MPa), establishing as minimum pipe diameter required for the validation tests 100 mm; and a high risk (pressure above 1 MPa), establishing as minimum diameter required 150 mm. Hence, the importance of studying the repairs on pipes diameters over 100 mm.

This work aims to study the resistance to hydrostatic pressure of the repair with bonded plates (or sheets) with epoxy adhesive in pipes with a hole of 10 mm. The area shape factor, i.e., the effect of changes in plate dimensions in the longitudinal and circumferential directions on the joint strength, was investigated. A study about the influence of the plate stiffness on the system failure pressure has been done considering three plate thickness. Additionally, a surface treatment analysis has been performed in the plate, considering the silane addition. Finally, an investigation about three different surface treatments for the pipe was performed. The main contribution of this work is the experimental study of pipe repairs with bonded plates in 150 mm diameter pipes, using the NVT 201E structural adhesive, which has not previously been done.

Section snippets

Materials and methods

This section is divided in four specific parts – influence of: shape factor, plate stiffness, surface treatment of plate and surface treatment of pipe (2.1–2.4, respectively). In all cases, hydrostatic tests were performed, therefore, there are points in common to be described in the methodology, which will be presented below.

The specimens for the steel plates were obtained by cutting a low carbon steel galvanized plate and then calendaring them. The NVT 201E structural adhesive (also used in

Bonded area influence and shape factor

The results in terms of plate rupture force obtained in the hydrostatic tests and the standard deviations are presented in Fig. 6 for all studied groups, where five tests for each group were performed. The rupture force was obtained by multiplying the pressure that the leakage occurred by the hole area. The values obtained for Group 3 (control group) was taken as a basis for the stretch of the track that represents the proportional relationship between force and area. The leakage occurred in

Conclusions

This paper studied some important aspects with respect to pipe repairs with bonded plates. For the conditions used in this study, the greatest rupture forces were those of larger bonded plate area. The area shape factor was investigated in the plates. In general, there is no statistical difference on the rupture force of bonded plate when compared to the same bonded area, considering the plate length variations in the longitudinal and circumferential tube directions. Possibly, this occurred

Funding

This work was supported by Adhesion and Adherence Laboratory (LAA) – Nova Friburgo-RJ - Brazil.

Declaration of competing interest

None.

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