Fatigue Damage Assessment to a Rigid Planar Jumper on Model Scale

A rigid jumper is an important part of a subsea production system, and it may experience significant vortex-induced vibrations (VIVs) if subjected to current. It has normally a non-straight geometry shape in three-dimensional space. Consequently, the response of a rigid jumper under VIVs is much more complicated compared with straight pipeline structures. Currently, there are very limited studies and design guidelines including methods on how to assess the fatigue damage of rigid jumpers under VIVs. The methodology used for straight pipelines is often applied by ignoring the non-straight geometry characteristics and the multiaxial stress states. However, both experimental and numerical results show that the torsional stress does exist besides the flexural stress for rigid jumpers under VIVs. The objective of this study is to do a fatigue assessment practice based on the state-of-the-art calculation methods to a rigid jumper on model scale. The VIV response is obtained from experimental tests and numerical calculations by either force or response model methods. The influence of torsional stress on fatigue assessment is studied. Two approaches have been investigated. In the first approach, the flexural and torsional stresses are evaluated separately. The second approach uses the first principal stress to calculate the fatigue damage; thus, the flexural and torsional stresses are evaluated together. It appears that the use of the first principal stress gives higher fatigue damage if the torsional stress contribution is significant. Furthermore, the principal stress method is also less time-consuming in processing the results.