ABSTRACT

The unbonded flexible subsea pipes have been widely used for oil and gas transportation in offshore energy exploration and development. The inner layer of flexible pipe is formed by corrugated inter-locked carcass. As the erosion of smooth pipes has been extensively studied, the erosion of flexible pipes has rarely been investigated. In this paper, experiments are performed to study the erosion of full scale flexible pipes with inner diameter of 15 cm. The erosion tests are carried out using water flow at velocity of 2.5m/s containing sand particles of various sizes on straight and 90° bend pipe sections. Erosion test is performed with duration of 425 hours for bend pipe and 1275 hours for straight pipe. Thickness loss is obtained by measuring the carcass sections with a metallographic microscope. The results show the higher erosion rates appear at the leading edge of the inner carcass layer for both straight and bend pipes. The highest erosion rate is found at 20-35° along the bend pipe. The results from this paper could provide assistance in determining the service life subsea flexible pipes exposed to erosion risk.

INTRODUCTION

Unbonded flexible pipeline is being increasingly used for oil and gas transportation due to its advantage of flexibility and compression resistance. Typical flexible pipeline is composed of multiple layers with the innermost layer formed by corrugated inter-locked carcass, which mainly provides structural support to resist collapse, or crushing.

As sand particles usually inevitably appear while gas and oil is transported, the inner layer of flexible pipe then experience the risk of erosion, leading to the potential risk of pipe collapse or unlock of carcass. Regarding the erosion wear mechanism of pipeline, Finnie (1958, 1960) proposed the micro-cutting erosion theory of solid particle erosion of plastic materials by exploring the erosion wear of solid particles on plastic materials. Bitter (1963) found that there are two types of wear attacks which include cutting action and repeated deformation during collisions. Bai (2004) studied the material failure mechanism of particles under different impact angles, and the experimental results were consistent with Finnie theory and Bitter theory. Tilly (1973) proposed the theory of particle secondary erosion by considering the impact of particle impact on rebound and energy loss after rebound. Levy (1986) observed the surface morphology of materials after erosion and proposed the theory of extrusion deformation and shedding of materials surface.

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