Carbon Equivalent and Sour Service
High-Strength Wind-Resistant Power Transmission Towers – Research and Development
January 31, 2026Carbon Equivalent and Sour Service
3. Chemical Integrity: Carbon Equivalent and Sour Service
For subsea pipelines, weldability is paramount. To ensure the heat-affected zone (HAZ) does not become brittle, the Carbon Equivalent (CE) is strictly limited.
We typically use the IIW Formula:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$$
For X65QO, $CE_{IIW}$ is usually kept below 0.39, ensuring that offshore welding can be performed with minimal preheating, speeding up the pipe-lay process.
Additionally, because many offshore reservoirs contain $H_2S$, these pipes are often tested for HIC (Hydrogen Induced Cracking) and SSCC (Sulfide Stress Corrosion Cracking). This requires extremely low sulfur levels ($\leq 0.002\%$) and calcium treatment for inclusion shape control (converting elongated sulfides into spherical shapes).
