Polyurethane (PU) coatings are versatile as they can be applied in different thicknesses, using different chemistries and on site or in plant. They are typically applied over a 3-layer PE system. BS Coatings has been in the business of pipe coatings since 1945 when the company Bitume Speciaux was created by TOTAL, Vallourec and Pont á Mousson and the Carboplast coating was patented. To date it has coated more than 100,000km of pipe.
BS Coatings has compared PU coatings with epoxy for rehabilitation projects and found that it has a much lower drying time (1h vs 4h) and better mechanical properties. It has been tested in Subkha conditions: this is an Arabic term for coastal regions where soil has an 18% salt content and resistivity as low as 20 Ohm.cm. Specific electrochemical resistance was tested to Saudi Aramco specification 09-SAMSS-089. The material has also been tested for use in Russian cold climates.
Iran has over 30,000km of high pressure gas pipelines and there have been some issues with coating disbondment. Safa Rolling and Pipe Mills company applies a variety of pipe coatings from 3-layer PE to PU and bitumen enamel and has studied the phenomenon of cathodic disbondment. It had been assumed that the issue was due to problems in manufacture, but tracing the failed pipes to the quality control results at the factory did not corroborate this.
A variety of tests was carried out including an 80C hot water soak: disbondment occurred when the cathodic potential was high. The failure mechanism may be the accumulation of hydroxyl ions at the cathode, which react with the iron oxide on the steel surface to form hydroxide, increasing the pH on the surface in contact with the coating. This increases the stress in the coating, which is more critical for 3-layer PE as the PE layer is thicker and the thermal expansion differential is higher.
There are several measures that can be taken to reduce this problem including better pipe surface treatment and careful monitoring of the cathodic voltage in service.
As discussed by Safa Rolling Mills, the factors that adversely affect epoxy bonding include heat and humidity, the cathodic protection current and residual stress in the upper coating layers of 3-layer systems. The adhesion of FBE has been improved by Dow Coating Materials using new technology to address these issues.
Berry Plastics has been looking at methods of field joint coating for PP-coated pipes. Damage to pipe coating can occur during field joint coating and operators like TOTAL, Saipem and Petrobras have specifications that hold subcontractors responsible for disbondment and require preventative measures to be taken.
The options for joint coating include high temperature shrink sleeve, injection moulding, flame spray, fused tape systems and field film extrusion techniques. The Covalence shrink sleeve system includes a new dual adhesive system with one optimised for epoxy bonding and the other for polyolefin bonding. It has been used in the TOTAL Laggan Tormore pipeline and the MEW pipeline from Abullah WOC II Port to West Funaitees.
