This paper proposes a reliable and stable numerical method for homogenized axial elastic modulus prediction of RTPs using finite element models subjected to tension. Compared with the homogenization method, the differences were even less than 1%, which demonstrated that the homogenization method could be applied to prompt and accurate predictions of elastic responses of RTPs subjected to tension. The investigation on the effect of winding angles demonstrated the thickness and area ratios of each layer also play significant roles in the determination of homogenized axial elastic moduli. The effects of thickness-radius and stacking sequences are also discussed.

INTRODUCTION

In recent years, RTPs have been found an increasingly extensive utilization in ocean oil and gas industry in terms of its excellent performance such as corrosion resistance, high-pressure resistance, high strength, and low weight, etc (Bai et al.,2014; Bai et al., 2016; Toh et al., 2018; Liu and Wang, 2019a). As shown in Fig.1, liner and coating made of isotropic materials such as HDPE are the innermost and outermost layers of RTPs to protect the laminates from corroding due to the transported fluid and the external environment (Yu et al., 2015; Liu and Wang, 2019b). The middle laminates composed of HDPE matrix and carbon fibers (or glass fibers) wound at different angles are the principle load-bearing structures when RTPs are subjected to various loads. Adjacent layers should be bonded with good adhesion and no delamination for a quality RTP.

As the exploration of oil and gas moves into deep water, understanding the mechanical responses of RTPs under tension are essential to ensure the secure and reliable operation of offshore engineering constructions. However, due to the complicated material components, it's hard to predict the axial elastic characteristics of RTPs with high accuracy. Many pieces of research has been carried out in the past decades to push forward the utilization and promotion of RTPs. Based on the classical laminated-plate theory, Xia et al. (2002) proposed a displacement-based approach to analyze the stress-strain and deformation of pipes under pure bending. The results coincided with the results from the general method (Pagano,1972). By assuming RTPs as homogenous cylindrically orthotropic pipes, Sun et al. (2014a; 2014b) proposed the homogenization method based on stress approach to predict the homogenized elastic constants. Compared with other analytical methods for composite plates and shells (Sun and Li, 1988; Enie and Rizzo, 1970), the homogenization method can predict similar results and carry out the stress analysis quickly. On the other hand, the finite element method is an alternative method to analyze the composite cylindrical structures under different loading cases. Yoo et al. (2017) proposed a practical and stable method using 8-layer and 5-layer finite element models to conduct the ultimate-tension assessment and elastic response of unbonded flexible risers. He et al. (2014) predicted the collapse behavior of thick-wall pipes taking account in the effects of initial ovality, yield stress and anisotropy by Abaqus. Bai et al. (2016) simulated the collapse behavior of steel strip reinforced thermoplastic pipes subjected to external pressure using the arc-length method. Ren et al. (2013) studied the behavior of unbonded flexible risers subjected to axial tension based on Abaqus/Explicit. The above numerical methods can provide reliable predictions of ultimate strength and elastic responses, but it requires building a specific model in each case. Meanwhile, due to the geometric nonlinearity and material anisotropy, the numerical models of composite pipes generally occupy massive computation resources.

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