The fatigue design of the steel armour layers is an important factor in flexible riser design. Typically, the riser must be able to endure the loads of a field condition for a period of 20 years. In flexible pipe design, this is analyzed in global and local flexible pipe fatigue modeling, which needs to be supported by proper testing. One of the widespread test methods is the so-called in-plane bending fatigue test, which is one of the important qualification tests for flexible pipes.
API 17B references two purposes for performing in-plane bending fatigue tests. One is to demonstrate that the pipe can endure the loads which it is exposed to during its lifetime (service simulation). The other is to validate the design methodologies (service-life model validation).
The first purpose calls for test conditions being as close to reality as possible. However, testing in conditions close to reality does not necessarily call for field environment conditions being present during the in-plane testing itself. This is because field conditions are covered with a combination of small-, mid- and full-scale tests according to the recommendations given in API 17B.
The second purpose can be, or is even best fulfilled with simplifications compared to field conditions.
This study will go through the options and discuss the pros and cons of simplifications versus a more complicated test. The discussions in the paper are based on the author’s experience and can be supported with examples from actual testing.