Are Drill Pipe Conventional Coatings Suitable for Subsea Early Production Risers?

Abstract

Drill pipe (DP) materials and corrosion protection methods have been designed for use in drilling operations characterized by specific stress levels, temperatures (high), and fluids (mud). Changing the functionality of DP from drilling operation to completion operation with early production in a subsea environment requires a good critical analysis because the typical internal coating designed for the abrasive and erosive actions of mud may not be suitable for exposure to potentially corrosive production fluids. Moreover, long-term operation with external cathodic protection may require different properties for the internal coating. This paper presents failures of the internal coating of drill pipe riser (DPR) systems that operated for 8 months in extended well testing (EWT) in Brazilian pre-salt fields at 1900-m sea depth. After two different EWT operations, blisters were seen through the internal coating of tubes that had been in service. Visual inspection was carried out on a sampling of the tubes that had been in service at different positions of the string along the water depth in order to characterize and to provide an assessment about the damage found on the internal coating. An investigation found that the coating disbondment was associated with blistering and that the intensity of damage increased with the water depth. Two hypotheses were pointed as potential sources for blistering:Hydrogen blistering: In this case, the hydrogen was generated by the cathodic protection from sacrificial anodes distributed on the wellhead stack-up frames. The hydrogen crossed through the wall of the tubes from the external to the internal side and emerged as bubbles under the internal coating. The incapacity of the bubbles to pass through the organic coating is related to its impermeability to hydrogen. In this case, the closer from the subsea equipment, the higher will be the hydrogen formation rate, explaining the worst damage in deep operating positions. The nascent monatomic hydrogen passes rapidly through the matrix recombining to molecular H2 at the coating interface; the hydrogen permeates through the coating at a much slower rate compared to the formation of the molecular gas; the blister occurs 1st before the hydrogen diffuses through the coating. This hydrogen recombination at the interface results in disbondment. A corresponding analogy is the recombination of nascent hydrogen to molecular hydrogen forming internal fissures and blisters at internal nonmetallic inclusions.Thermal gradient: This hypothesis suggests that the temperature gradient caused the blistering (cold wall effect). The temperature outside the pipe decreases with ocean depth while the temperature inside the pipe increases with depth. So the temperature differential increased with depth and the blistering of the coating on the pipe ID got worse in deeper zones.