Norner has an ongoing Joint Industry Program (JIP) that covers service life prediction of elastomers in oil and gas sealing applications.
The service life expectation for such sealing materials is 20-30 years, and the temperature is typically up to 100-120°C with a fluid exposure of hydrocarbon fluid mixtures containing up to 10% H2S.
Such applications are not possible to test in real time, so the JIP test program is set up using temperature to accelerate the ageing. Both elastomers and polymers show more rapid degradation at higher temperature, and the typical rule of thumb indicates that a 10°C increase in temperature doubles the degradation of the material.
Polymers and elastomers are used for years without failure in numerous applications throughout many industries. The performance of the materials is well known since the application and conditions the material is exposed to is well known. Furthermore, many applications are such that replacing a material during service is a relatively straight forward exercise. However, this is not always the case. For many applications, across industries, service conditions that polymer and elastomer materials are exposed to coupled with the service life expectation together warrant rigorous testing and qualification efforts.
Ageing the material
When an application requires service life beyond half a year to a year, there is no good way of performing real time testing. It would take too long before the material was fully tested, and this proves too costly and ineffective for most applications and industries.
Arrhenius methodology is quite common for many applications that have issues of long term use of materials. However, many of todays applications do not take advantage of this type of extrapolation methodology, even though it could prove benefitial. The method, in very short terms, involves ageing the material under relevant accelerating conditions (fluid exposure, temperature or other as relevant) untill a clear degradation trend is found. Typically three accelerating conditions with varying degree of acceleration is used. When temperature is used for acceleration, three set temperatures are used, with at least 10°C between them. Same type of approach is relevant for all other accelerants. The key important features of Arrhenius methodology is to ensure that all degradation mechanisms are relevant for the actual application, and that the degradation mechanism(s) form a trend. If competing degradation mechanisms form competing trends, it is very difficult to generate extrapolation curves that have a small standard deviation.
The special nature of the elastomer and polymer materials is that they exhibit time dependent degradation behavior, and very often also exhibit more than one degradation mechanism. These degradation mechanisms are not always causing the same physical effects, and thus one degradation mechanism can for a while mask the effect of another. Examples of these are swelling and crosslinking. Swelling will result in a softer material that will exhibit a lower tensile stress and strain, and increased elongation at break. Whilst crosslinking will cause hardening of the material with increase in tensile stress and strain, and a shortening of elongation at break. If both mechanisms are happening at the same time, it is often difficult to distinguish these and see the true degradation over time unless testing is indeed carried out over a very long time. The current JIP test program covers testing for 2 years, at 3 off different temperatures in order to create sufficient data sets for high accuracy prediction of service life and material degradation.
Both elastomer and thermoplastic materials for use in mixed production fluid are typically high end materials. These types of applications see much use of HNBR, FKM and FFKM type elastomer materials and for thermoplastics, often PTFE, PEEK and PVDF are seen in use. Thus, chemical stability against a variety of chemicals and temperature is a key feature. This is also why such materials are so important to study for a very long time. They are inherently very stable in aggressive chemicals, and so it takes a very long time before chemical degradation is observed.
One additional feature is the study of how tensile measurements and compression measurements are able to capture chemical degradation as a function of temperature and time. As the application is sealing, studying compression is the best link to application behavior. However, ISO standards typically call for tensile testing. Tensile behavior is the most common behavior to map in qualification testing. It is easy to perform tensile testing, but tensile behavior is inherently difficult to directly relate to sealing in compression. Establishing acceptance criteria for tensile measurements is therefore difficult. Thus, studying compression directly is very important.
To compare with tensile testing of elastomers, Norner utilizes the technique of compression stress relaxation, where seals are measured in compression mode to capture the sealing force of the seal. It is a nondestructive methodology. Compression stress relaxation is viewed as an excellent test methodology to evaluate both material performance and sealing ability in a generic material setting.
This current JIP covers elastomer sealing materials for oil and gas applications, but the same technical approach can be used for all materials and applications that suffer the same issues of degradation over time where prediction of service life is difficult within current knowledge of chemical degradation of the material.
In addition to this current JIP on elastomer sealing materials for oil and gas, there is an upcoming similar Joint Industry Project for thermoplastic materials. The idea is identical to what is described above, only evaluating the thermoplastic materials instead of the elastomer materials. For this upcoming thermoplastic JIP, compression stress relaxation is not part of the scope, as thermoplastics do not exhibit the same type of sealing force behavior as the elastomers. Instead a different type compression test will be used, more suited to thermoplastic sealing behavior.
Norner has a large number of autoclaves to perfom long term ageing where chemicals can be used under pressure and temperature in an effective way to age the material. Both temperature and pressure are very effective accelerants for degradation in polymer and elastomer materials, and using the autoclave setup to perform long term testing of materials is a reasonable method to ensure safe and successful service of the material in its application.