Qualification of Polymer Materials for High Pressure CO2 Flexible Pipe Structures
Authors: Wang Christian , Rubin Adam , Von Solms Nicolas 
Published: June 2014

Abstract
In the oil and gas industry there is a growing demand for riser solutions dedicated to high partial pressures of CO2. The demand is primarily driven by deep water high pressure reservoirs with high CO2 content. In addition there is a growing interest in reinjection of CO2. The reinjection can be driven by a desire to reduce CO2 emission as in Carbon Capture and Storage (CCS) projects, but also as a method for Enhanced Oil Recovery (EOR). In all cases it is likely that CO2 will be present at high pressures and in its supercritical phase.
NOV Flexibles has in recent years developed dedicated riser solutions for a broad range of flexible pipe applications for use with high pressure dense phase CO2 also referred to as supercritical CO2 (SCCO2). An important part of the qualification program has been material qualification. For polymeric materials, the main challenge is the high solubility in some polymers of high pressure CO2. This leads to potential high swelling and a risk of mechanical damage (blistering) in explosive decompression situations. Furthermore, high pressure CO2 has a strong extraction power which may result in rapid loss of plasticizer in plasticized polymers.
This paper describes the latest status and achievements in the qualification program including a broad variety of tests qualifying against potential failure mechanisms to CO2 pressures in the range of 650 to 700 bar. The results show that peroxide cross-linked poly-ethylene (XLPE) in-line cured inner liner with a high degree of cross linking has superior resistance to high pressure CO2 on all tested parameters compared to other available liner materials. This makes it the preferred liner material for ultra high pressure CO2 gas risers and flowlines whereas PVDF is the optimum choice for production risers with temperatures above 90 °C and intermediate CO2 partial pressures.

Introduction
For decades unbonded flexible pipes have been used as key components in the offshore oil and gas production for transporting fluids between a sub-sea installation and the off-shore vessel/platform. Flexible pipes are often an economically as well as technically competitive solution alternative to rigid steel pipes. Applications of flexible pipes are to be found as flow lines and dynamic risers serving a multitude of functions such as production and export of hydrocarbon fluids, injection of water, gas and chemicals into an oil/reservoir, and service lines for wellheads. Some of the main advantages of using flexible pipes are faster installation times, better adapting potential for changes in the field and the excellent ability to function under extreme dynamic conditions as well as under harsh corrosive environments due to their good chemical compatibility properties compared to rigid carbon steel pipes.
In order to maintain a high mechanical flexibility unbonded flexible pipes are constructed from concentric unbonded layers of polymeric and steel materials. A generic construction of an unbonded flexible pipe is presented in Figure 1. The design of the unbonded flexible pipe depends on the functionality so in some cases an exclusion of some layers is seen and in others additional layers such as insulation or an additional outer sheath are applied. The functionality of the inner liner, also often referred to as the polymeric pressure sheath, is to make the pipe leak-tight. At the end termination of a flexible pipe all the pipe layers are anchored in an end fitting where gasket rings ensure the sealing between the end fitting structure and inner liner. The integrity of the inner liner layer and sealing gasket material is vital and critical for the life time performance of the flexible pipe. Small scale test and mid-scale test results of the inner liner material as well as of the sealing gasket material are presented in this paper.