Effects of defect dimensions and putty properties on the burst performances of steel pipes wrapped with CFRP composites

Highlights

• The mechanical properties of the putty affect the burst pressure of the CFRP wrapped defective pipe.

• Failure mechanism and process of the CFRP wrapped defective pipe are discussed.

• Based on the failure process, a prediction method of burst pressure for the repaired pipes was proposed.

Abstract

Externally wrapped carbon fiber reinforced polymer (CFRP) composites are effective to repair a defective steel pipe. In the present study, the pipes of defects with different hoop lengths were repaired with CFRP and different putty was used to fill the defects. The effects of defect dimensions and putty mechanical properties on the burst performances of the repaired pipes were investigated through burst tests and finite element (FE) analysis. If the putty failed before the CFRP, CFRP continued to bear the load when the hoop length of the defect was less than 20% of the pipe diameter, while it was inactive when the length was greater. The rupture strain of the putty should be greater than CFRP to avoid the putty fails firstly. According to the burst performances of the repaired defective steel pipe with CFRP, the design methods based on allowable stress in the pipe substrate (LAStress) or allowable strain of the repair laminates (LAStrain) in ISO/TS 24817 were compared quantitatively. A method to predict the burst pressure of the repaired pipes was proposed based on the failure mechanism of the repaired pipes.

Introduction

In the petroleum and chemical industry, steel pipelines are essential elements in the production and transportation of liquid or gas. The total mileage of oil and gas pipelines in China was 120,000 km by 2017, and will excess 240,000 km by 2025 [1]. Steel pipes suffer from lots of damage scenarios as operating in harsh environmental conditions and transporting a wide range of high-pressure fluids, which may include coating damage, erosion, corrosion, or mechanical damage [2]. The primary defects come from external and/or internal corrosion, which will reduce the strength and service lives of the pipelines [3]. To repair such damaged pipes, various repair methods have been developed [4].

Traditional repair methods include replacing damaged sections or repairing with welding [2]. However, these methods require a plant shutdown and/or introduce open flames during construction, which will cause substantial economic losses and may bring in additional dangers [5]. Several investigations have been carried out on reliable, durable, and cost-effective methods for the rehabilitation of defected pipes [6]. Carbon fiber reinforced polymer (CFRP) composites possess a high strength-to-weight ratio, excellent corrosion and fatigue resistance, and high formability. As reported, CFRP can be used to repair defected pipes during operation without ignition [[7], [8], [9], [10], [11], [12]]. The use of CFRP overcomes the disadvantages of traditional methods, resulting in a significant reduction in repair costs, and an improvement in safety [13].

For the application of CFRP to repair damaged pipes, the defect dimensions on the strengthening effects were concerned [[14], [15], [16]]. The depth of the defect has the most substantial detrimental effect on the burst pressure of a steel pipe [14]. The axial length that denoted by for α ≥ 3.5 axisymmetric defect and α ≥ 4.5 for the narrow flaw, can be considered as of infinite length [17]. Here, α is a non-dimensional half-length of the defect and depends on the material property, defect thickness and pipe section. The burst pressure and stress state of a composite repaired steel pipe dependent on the depth and axial lengths of the defect. However, the hoop length shows a slight impact on the burst pressure, and only influences the stress state in the pipe substrate [18]. Except for the strength and stress state, the load transfer from steel substrate to CFRP is also affected by the corrosion and gouging size [4]. The combined contribution of composites, putty, and defect geometry in an in-filled composite repair system is a gap in the current knowledge.

For a CFRP wrapped steel pipe, the strains between steel substrate and putty, putty and CFRP are complex under internal pressure [19]. Prior to the yielding of the steel in the defect region, the strain of the steel and outside wrapped CFRP are equilibrated to bear the inner pressure loading. After the yielding of the steel, the CFRP start to bear the increased pressure loading with increased strain effectively [20]. The putty used to fill defects is considered to be essential for stress transfer from steel substrate to outside CFRP layers in the defect zone [21,22]. It is worth noting that limited attention has been paid to the effect of putty properties, e.g., tough or brittle [18,23]. As known, the toughness of putty or adhesive has been shown to play a key role in the CFRP – steel interfacial behaviors [24].

Numerical studies on the highly nonlinear burst pressure of piles were carried out using FE software with an explicit integration procedure [25]. An accurate burst pressure and failure mode of the strengthened steel pipe can be simulated by the FE model. Elastic, bilinear-compression, bilinear-tensile, and tensile elastic-perfectly plastic models were used to simulate the putty, and tensile elastic-perfectly plastic one is a good predictor [1]. Based on the assumption that the radial displacement at the interface between the pipe substrate and the composite is identical, theoretical methods were proposed to predict the failure pressure of the pipes wrapped with composite laminates [[26], [27], [28]]. These methods are dominated by the strength of the pipe substrate. However, for the pipes repaired with CFRP, the damage of CFRP leads to the burst of the pipe [18], and these prediction methods become inappropriate. The burst pressure prediction method of CFRP repaired pipes should be established based on the specific failure mechanism.

Several design methods for defective pipes strengthened with composite were proposed in the pipe reinforcement standards, such as ISO/TS 24817 and ASME PCC-2 [29,30]. The design methods limited by the allowable stress in the pipe substrate (LAStress) or allowable strain of the repair laminates (LAStrain) were proposed in the ISO/TS 24817. The calculated CFRP thickness with the former method is about 2.5–5.5 times higher than the thickness calculated with the latter one [19,30]. It is interesting to quantify the effectiveness of the methods for various repair requirements. The internal pressure of the pipe at the time of repair is considered in LAStrain (Eq. (1)), while the repair thickness has proven to be independent of the pressure through analytical and finite element analyses and an appropriate modification was proposed (Eq. (2)) [31]. The repair laminate thickness calculated using Eq. (1) underestimates the repair thickness when the internal pressure is not zero during the repair installation and the modified method (Eq. (2)) is safer than it.

$${\epsilon }_{\text{c}}=P_{\text{design}}D/2E_{\text{c}}{t}_{\min }-s{t}_{\text{s}}/E_{\text{c}}{t}_{\min }-P_{\text{live}}D/2\left(E_{\text{c}}{t}_{\min }+E_{\text{s}}{t}_{\text{s}}\right)$$

$${\epsilon }_{\text{c}}=P_{\text{design}}D/2E_{\text{c}}{t}_{\min }-s{t}_{\text{s}}/E_{\text{c}}{t}_{\min }$$

In the present paper, the effects of the length of hoop defects and the mechanical properties of putties on the burst performances of CFRP strengthened steel pipes were investigated through burst tests and FE methods. The strengthening effectiveness of design methods (LAStress and LAStrain) in ISO/TS 24817 was compared quantitatively. The stiffness degradation and failure of the pipe substrate, putty, and CFRP were considered in the FE models. A simple prediction method of the burst pressure was proposed based on the failure mechanism of the CFRP strengthened steel pipes.