Evaluation of subsea wet thermal insulation coating materials via exposure to seawater for a 12-month period at high temperatures and high pressures is presented in ISO(1) 12736:2014. This is a harsh exposure regiment which few materials have been evaluated against for such a long time period and the ISO(1) standard deliberately does not present acceptance criteria for the test results.
How to interpret and apply the results of this testing to the design and qualification of subsea wet insulation systems is presented, in addition to experiences and challenges faced whilst undertaking this evaluation program with various materials at temperatures up to 150 °C and pressures up to 30 MPa.
The question of product lifetime for subsea operation of polymeric insulating coatings is not straight forward. There are currently no accepted industry-wide criteria for determining lifetime. The difficulty is that, in order for a lifetime to be predicted, one must be able to achieve failures in an accelerated manner under conditions that can be applied to the true conditions.
Operation in a deoxygenated environment, as would be found subsea, is expected to have very long lifetimes. In the case of polypropylene, for example, minimal degradation is expected even at temperatures close to the melting point of the material. Polypropylene is very stable in aqueous environments and does not hydrolyze. Thus, acceleration of aging in a meaningful way is very difficult. Typically, accelerated aging utilizes the Arrhenius method and elevated temperatures; however, this is not possible in this case as elevated temperatures giving usefully short failure times would be past the melting transition of the polymer and thus not applicable to temperatures below the melt transition.
Even in situations where an Arrhenius model can be successfully produced to model product lifetime, it must be remembered that this prediction must still be validated under real conditions. This is very difficult when service lives are expected to be decades long and much shorter than the true product lifetime in applications where the product is rarely retrieved after its services has ended.