Residual stresses in 3LPO external pipeline coatings - Disbondment and cracking
Additional Document Info
This is a US DOT (Dept of Transportation) sponsored research project with a team of pipeline coating experts participating in this research project to assess the integrity of 3LPO (3 layer polyolefin) external pipeline coatings. There are two major integrity issues with 3LPO coatings - disbondment and topcoat cracking. In the past few years, several incidents of coating disbondment at the FBE-steel interface of 3LPO and polypropylene (PP) topcoat cracking have been reported in the literature. These coating issues have raised concerns over the use of 3LPO. Generally speaking, 3LPO coatings use thicker polyolefin topcoat to provide more mechanical damage and water resistance. However, polyolefins have much higher thermal expansion coefficient than steel and consequently, higher thermal residual stresses are generated within the coating systems. High residual stresses cause disbondment, particularly at the cutback or any coating edges, where there is high stress concentration. Specially, if the steel pipe surface was not prepared properly to secure good and durable bond strength. Inadequate selection of FBE primer with thermo-oxidative degradation, will also contribute to the premature coating failure. At a high temperature service, the polypropylene topcoat embrittled by thermo-oxidative degradation. The embrittled PP topcoat may crack under the residual stress. This paper analyzes the residual stresses in the 3LPO coating systems and describes effect of the residual stresses on 3LPO coating disbondment and PP topcoat cracking mechanism. BHR Group 2009.