Over the last few years the demand for higher operating conditions of pipelines has been challenging corrosion engineers to come up with suitable coatings’ systems. FBE coatings have been developed to satisfy part of this need.
Statoil, as long ago as 1996, required the ability to operate lines at temperatures as high as 140°C with the system of choice a multilayer polypropylene insulating system to maintain the temperature at this high level. In order to operate at such temperature new materials had to be developed.
For the FBE layer, standard FBE coatings exhibit glass transition temperatures in the range of 100 to 110°C. In practical terms this means that at temperatures above this the FBE film is softer, easily damaged and shows a tendency for high water absorbance which in turns lowers the glass transition temperature even more. The adhesive bond to the substrate is also put at risk as the cohesive strength of the film becomes greater than the adhesive strength.
As for the polypropylene part of the system there were some challenges to meet such operating conditions as the normal polypropylene adhesives start to soften at these temperatures and degradation is a potential risk.
Rising to the challenge, the industry co-operated in the development of a system. The essential components for the system developed were the Fusion Bond Epoxy and the adhesive.
The challenge for the epoxy industry was the development of a FBE that had a glass transition temperature above the operating temperature for the project, would adhere to a grafted polypropylene adhesive and be flexible enough for the lay operation which required reel barge operation.
The FBE developed by Jotun Powder Coatings had a glass transition of close to 150°C and an application window with the polypropylene which was easily acceptable to the pipeline coating applicator and retained enough flexibility to be reeled at Norwegian temperatures (ambient often below 0°C).
The initial projects were laid in 1998, have been carefully monitored ever since and are performing very well.
The FBE developed for this application was however not recommended as a stand-alone FBE, as compromises had to be made to obtain flexibility at an acceptable level for the lay operation.
These compromises utilised the properties of the polypropylene to compensate by limiting exposure of the covered FBE to moisture and oxygen.
This valuable experience did however provide a good basis for establishing stand-alone grades of FBE suitable for higher operating temperatures.
What are the likely failure characteristics for a stand-alone FBE coating operating above 100°C?
If the surface is truly above 100°C then the surface will not be exposed to water, as the water will be vapourised and driven from the pipe surface. In this condition, then the physical damage to the coating will become the issue.
Does the coating softens and allows penetration? Is adhesion maintained? Will soil damage the coating?
If this damage does take place, then the coating is compromised when it is in a cooler phase and corrosive attaches can take place at the damage
On pipeline close to 100°C, then rapid cycling of the surface can occur as the pipeline gets wet then that water is driven off. What are the effects of this thermal stress?
Considering initially the requirement for the coating to maintain its hardness and to maintain a cohesive bond at the operating temperature the glass transition temperature (Tg) is clearly an important property.
To confirm that the Tg gives an indication of suitability for purpose, dolly pull adhesion testing experiments were carried out to compare adhesion as temperature rises.
To set the base line, several proprietary standard FBE’s were compared. The strength of the adhesion and the mode of failure where noted.
It was clear that as the temperature of the test increases, the adhesion value falls. It was also clear that the mode of failure changes from cohesive to adhesive. This change occurred at close to the Tg values as determined by DSC and well below those determined by TMA.
With higher Tg grades the same changes are seen at the DSC determined Tg.
This clearly showed that the determined DSC provides the best indication on where adhesion failure mode changed and would therefore be a good indicator of performance in respect to elevated temperature, soil stress, and mechanical actions.
A similar determination of penetration resistance based on Shore “D” measurements.
This again confirms that the DSC determined Tg is the best indication of a significant change point in performance for penetration type incidents which soil and temperature stresses may induce.
In addition to the conditions at the elevated temperature any FBE protection system needs to perform at lower temperatures to cover periods, when an operating line might be shut down for maintenance, etc.
As one of the most widely used specification for FBE, the Canadian specification Z245-20.02 is a good measure of suitability for purpose.
The track record of systems in the field with H.O.T. projects
Exxon: Mobile Bay in 1999
Conoco: Sui Tu Den in 2003
BP: for Thunderhorse in 2003
Statoil: Kristian in 2003 (design temperature 150°C)
PTT: Bonkot Phase 3C in 2004
Bahr Essalam: Gathering and export lines in 2004
Saudi Aramco: Qatif project 2002 + projects in 2003, 2004 and 2005
AIOC pipeline: Azerbijan 2004