A Comparative Study of Fatigue Damage Assessment Methods to a Rigid Planar Jumper

Abstract

A rigid jumper is an important part of the subsea production system, it may experience significant vortex induced vibrations (VIV) if subjected to current. It has normally non-straight geometry shape in three-dimensional space. Consequently, the response of a rigid jumper under VIV is much more complicated compared to 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 VIV. The methodology used for straight pipelines is often applied by ignoring the non-straight geometry characteristics and the multi-axial stress states (coexisting of flexural and torsional stress). However, both experimental and numerical results show that the torsional stress does exist besides the flexural stress for rigid jumpers under VIV. On the other side, the response of the rigid jumper under VIV is also challenging. The objective of this study is to do a fatigue assessment practice based on state-of-the-art calculation methods to a rigid jumper on model scale. The VIV response is inherited from experimental tests and numerical calculations by either force or response model methods. The influence of torsional stress on fatigue assessment is demonstrated. Two approaches have been investigated. In the first method, the flexural and torsional stresses are evaluated separately. The second method uses the 1st principle stress to calculate the fatigue damage, thus the flexural and torsional stresses are evaluated together. It is shown that the use of the 1st principle stress gives higher fatigue damage if the torsional stress contribution is significant. Further, the principle stress method is also less time-consuming on processing the results. Detailed discussions based on results have been performed, which could be also applied to general real scale rigid jumpers.