Corrosion mitigation of instrument tubing for both new and existing facilities in simulated offshore environment

RIP2025-00022: Instances of leaking in stainless steel instrument tubing at wellhead platforms tend to increase as the facilities age. High-grade alloys offer excellent resistance to corrosion in offshore environments. However, achieving an optimal balance between cost and corrosion resistance is important. This study aims to select suitable materials for instrument tubing fittings for new facilities and to evaluate the anti-corrosion property of anti-corrosion coating products on stainless steel material.

The tubing materials candidates were SS316L, SS904L, and TPU jacketed tube SS316L, chosen based on field experience, their cost-effectiveness, and corrosion resistance in offshore environments. The investigation included the effect of sheath coverings at connecting joints and fittings by visual inspection and microscope. Localized corrosion profiles were simulated on plates to observe the corrosion progression beneath anti-corrosion coatings. The offshore environment simulation was conducted using a salt spray and xenon light chamber, following standard ASTM G85 (Annex A5) and ISO 16474-2, respectively.

After a 2,000-hour salt spray test, visual inspections demonstrated the absence of corrosion products on bared SS316L and SS904L surfaces. However, pitting corrosion was observed in both SS316L and SS904L under microscope. The variation of pit depth and size of SS316L is higher than SS904L. Significantly, in crevice areas, SS904L showcased superior corrosion resistance when compared to SS316, supporting findings in literature that indicate SS904L surpasses SS316L in terms of corrosion resistance. Conversely, the jacketed tube SS316L showed minimal changes in appearance post-testing. The jacket thus can generally prevent corrosion well. Nevertheless, if a jacket develops defects, corrosion can be occurred on the surface of tubes and fittings due to penetration of moisture. This finding aligns with field experience which reported no signs of damage or deterioration due to external corrosion in jacketed tube SS316L since its installation in 2015. Regarding anti-corrosion coatings, an enlargement in the size and depth of pits and crevices was detected under laser microscope. Reduction in pit depth was observed after applied anti-corrosion coating.

Our main concerns include cost-effectiveness, anti-corrosion efficiency, and installation difficulties. The results will inform future asset design and provide guidelines for treating both new and existing tubes. This study is incorporated into company standardized design and led to significant cost savings by switching from SS904L with ACT clamp to a combination of TPU-jacketed tube with conventional tube support and SS316L with ACT tube. This change is estimated to save approximately 5.34 million USD for 31 wellhead platforms and reduces non-value-added activities.