Under Pressure Welding on CO2 Pipelines: The Effect of Thermal Decay on Mechanical Properties

Abstract

Whilst there is extensive industry experience of under pressure welding onto operational natural gas and liquid pipelines, there is limited experience for Carbon Dioxide (CO2) pipelines, either in the gaseous or dense phase. National Grid has performed a detailed research program to investigate if existing natural gas industry under pressure welding procedures are applicable to CO2 pipelines, or if new specific guidance is required. At IPC 2014 a paper was presented (IPC2014-33223) that dealt with the results from one part of a comprehensive trial program, which defined the cooling time from 250 °C to 150 °C (T250-150) in CO2 pipelines and compared them to the typical decay times for natural gas pipelines. The results from this part of the work identified that maintaining the pre-heat using the established guidance in T/SP/P/9 during under pressure welding on dense phase CO2 pipelines would be very difficult, leading to potential operational issues. The previous paper gave a brief summary of the effect that cooling time had on the mechanical properties. The aim of this paper is to present the findings of the T800-500 weld decay trials in more detail including the full testing programme, detailing the affect that variables such as CO2 phase, CO2 flow velocity and the welding parameters had on the weld and heat affected zone (HAZ) hardness. The main finding is that although there is an indication that a higher cooling rate measured in the weld pool (characterized by the cooling time from 800 °C to 500 °C) leads to increased hardness in the HAZ region, there are no clear correlations. No hardness values were recorded that were considered unacceptable, even for the dense phase CO2 case which delivered the fastest cooling time. A significant finding was the requirement for controlling the buttering run procedure. A discussion of the critical aspects, including the link between weld cooling time and hardness, is presented with guidance on how this essential variables need to be controlled. The paper is aimed at technical, safety and operational staff with CO2 pipeline operators. Read in conjunction, this paper and the previous IPC paper form a comprehensive review of this critical work that is contributing to the development of dense phase CO2 transportation pipelines and will facilitate the implementation of Carbon Capture and Storage (CCS)1 projects which is a critical part of the transition to a low carbon economy.