THESIS BY Vidit Gaur*
The i-Clip-Riser® is a technological product developed by IFPEN, which enables rapid connection of drilling riser componentsa offshore.
As a result of the weight of the assembly and swell movements, it is constantly subjected to fatigue stresses. A detailed analysis of these, based on non-linear numerical simulations, has demonstrated their multiaxial cyclical nature, which the conventional fatigue criteria reported in the literature cannot express.
A very comprehensive series of experiments – dynamic multiaxial mechanical tests, SEM and TEM observationsb, X-ray diffraction – revealed different crack initiation mechanisms, depending on the stress conditions.
In addition, it made it possible to quantify the impact of several dimensioning parameters, such as:
- mean stress,
- the biaxiality ratio of stresses,
- or the presence of the saline environment(1).
A fatigue endurance limit criterion, including only two material parameters, was defined on the basis of this large-scale testing program and was validated in all the conditions tested(2). Based on the Gerber approachc, it enables characterization of the resistance of the connector under the multiaxial loads encountered during operation. Easily usable for structure calculations, this enables a more realistic validation of the behavior of the i-Clip-Riser® in response to fatigue.
It is planned to extend this research to predict the life estimation, and also to perform validation tests on a prototype (scale 1:1).
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the various tests performed (symbol) in the effective
stress plane, as a function of the maximum principal
stress.
a- A set of tubes, including one main pipeline, connecting the drilling platform to the subsea wellhead.
b- Scanning and transmission electron microscopy.
c- Approach that expresses the effect of mean stress on the endurance limit under uniaxial tension.
* Thesis entitled: Fatigue and corrosion-fatigue in Cr-Mo steel in biaxal tension.
(1) V. Gaur, V. Doquet, E. Persent, C. Mareau, E. Roguet, J. Kittel, Surface versus internal fatigue crack initiation in steel: Influence of mean stress, International Journal of Fatigue, 2016, 82-3, 437-448.
>> DOI: 10.1016/j.ijfatigue.2015.08.028
(2) V. Gaur, V. Doquet, E. Persent, E. Roguet, Effect of biaxial cyclic tension on the fatigue life and damage mechanisms of Cr–Mo steel, Int. Journal of Fatigue, 2016, 87, 124-131.
>> DOI: 10.1016/j.ijfatigue.2016.01.021
Scientific contact: eleonore.roguet@ifpen.fr