Session: MF-24-02 Matls & Fabrication for Refining-Design & Fabrication Issues Affecting Design Life

Paper Number: 123239

123239 - Enabling the Use of Carbon Steel for Co2 Transport, Processing, and Intermediate Storage 

Carbon capture and storage (CCS) is essential for lowering carbon emissions and limiting climate effects due to anthropogenic CO₂ in the atmosphere. Most of the captured CO₂ is destined for injection in geological storage sites. To increase transport efficiency, CO₂ gas is brought into a denser phase. Depending on the mode and volume of transport, it is converted to a subcooled liquid phase under low to intermediate pressure, or to a dense phase under supercritical pressure at ambient or higher temperatures.

There is a large drive for capital efficiency on CO₂ storage projects. Therefore, carbon steels are the prime candidate for material of construction as they are the cheapest option. After drying and sufficient purification, CO₂ becomes an essentially non-corrosive substance. For this paper, internal corrosion has not been considered. The operating conditions are not particularly challenging from a structural integrity point of view. However, there are some incidental conditions that need to be addressed.
In this paper, strategies will be discussed for enabling the use of carbon steel for CO₂ liquefaction plants, compressor stations, intermediate storage, and transport piping. Specifically, the analyses below will be elaborated.

Firstly, CO₂ fluid can cool to very low temperatures approaching -80°C during uncontrolled depressurization due to the Joule-Thomson effect and evaporation. The concern is that toughness of carbon steels at low temperature is insufficient and brittle fracture might occur.
The first consideration that can be made is how much a volume of CO₂ can theoretically cool the surrounding steel mass. This approach may be different from more traditional materials selection based on lowest fluid temperature. But it makes sense not to invalidate materials for a failure mechanism that would not be physically possible.
Should it be shown that very low metal temperatures during depressurization are indeed likely, then various design options can be utilized. For example, credit can be taken for low applied stresses in the pressure containment due to low pressure coinciding with the low temperature. This works well for high pressure process piping handling supercritical CO₂. Next, a fitness for service (FFS) analysis often shows that for piping systems handling gaseous or subcooled liquid CO₂ at low and moderate pressures, the design is acceptable down to -80°C under full design pressure.
For large intermediate storage bullets or spheres with large CO₂ volume, additional analyses can be made to demonstrate their integrity at these low temperatures with a FFS analysis, taking credit for stress relief heat treatment and 100% non-destructive examination. Process based solutions for controlled depressurization should be implemented for large storage vessels.

Secondly, particularly for transport pipelines, damage from external factors like soil corrosion or excavators must be considered. For small holes or cracks which leak before break, CO₂ will escape and expand and cool. It must be assessed if and how much a small CO₂ plume can affect the temperature of the pipe steel and create a risk of progressive, stepwise brittle fracture. In case large cracks have formed by local brittle fracture or mechanical damage, a running brittle or ductile fracture might initiate.  The Battelle two-curve model for deriving fracture arrest toughness criteria is non-conservative for dense CO₂. With the use of a safety factor based on full scale testing most likely result in unrealistic high impact toughness, of more than 200 J, in combination with large wall thicknesses. Instead of focusing on the prevention of a running propagating fracture it is worth to focus on the prevention of the crack which is prerequisite of the running propagating fracture.

This paper will demonstrate that carbon steels are suitable for application in CO₂ process plants and transport pipelines.

Presenting Author: Jan-Willem Rensman Fluor BV

Presenting Author Biography: Mr. Jan-Willem Rensman is a Fellow for Metallurgy & Welding at Fluor BV in the Netherlands. He is a certified International Welding Engineer and has a Master of Science degree in Mechanical and Materials Engineering from Twente University in Enschede, the Netherlands. He is a welding engineer and specializes in materials integrity analysis in challenging conditions. Some special interests of his are fabrication technology, materials performance, and failure mechanisms of structural and pressure vessel materials.

Authors:

Jan-Willem Rensman Fluor BV
Alfons Krom N.V. Nederlandse Gasunie