Steel Catenary Riser Fatigue Due to Vortex Induced Spar Motions
Authors: Bai Y. , Tang A. , O&apos;Sullivan E. , Uppu K.C. , Ramakrishnan S. 
Published: October 2008

Spars are considered to have excellent motion characteristics in wave environments. However, they are susceptible to vortex-induced motions (VIM), where they can exhibit significant in-line and transverse motions under current conditions. Such motions have implications for the fatigue life of risers attached to the Spars. Due to the potential impact of VIM on riser fatigue life, it is imperative that this issue be properly addressed in riser design and analysis. Furthermore, the higher degree of uncertainty inherent in the prediction of Spar VIM (compared to similar predictions for typical motions of floating systems) complicates the problem. However, recognizing VIM uncertainties and careful selection of VIM suppression devices may achieve a reliable riser system [1].

Currently, the impact of vortex induced Spar motions on riser fatigue life does not receive full consideration in the design of deep-water risers such as steel catenary risers (SCRs). However, when performing a comprehensive fatigue analysis, all fatigue damages contributions should be combined at the same point to predict the total fatigue life of any riser system. In certain cases, it is seen that VIM induced fatigue is almost as important as the other aspects such as VIV fatigue, wave induced fatigue and installation fatigue, in the fatigue design of the riser.

In this paper we shall address the impact of Vortex Induced Motions (VIM) of a Spar on riser design. A 12" gas export SCR and a 12" oil export SCR under environmental conditions representative of the Gulf of Mexico region are treated in detail, in order to address the critical issues.

In addition, a sensitivity analysis is carried out to evaluate the influence of the soil stiffness, flex-joint stiffness, design pressure and hang-off angle.

In one of the examples of fatigue analyses presented in this paper, while combining fatigue damages from VIV, wave induced fatigue and installation fatigue, the riser system met the required criterion of 200 years. However, under additional consideration of VIM fatigue damage, the system failed to do the same. This indicates the importance of considering Spar VIM in SCR fatigue design.

It is also concluded that currents applied out-of-plane to riser generate higher fatigue damage than currents applied in plane to the SCR. Moreover, VIM fatigue life is observed to be sensitive to the hang-off angles.

The paper concludes with a summary of the sensitivity analysis results.

INTRODUCTION

VIM of Spars originates when fluid passing a bluff body causes low-pressure eddies (or vortices) to form down stream of the body. These vortices are shed periodically at frequencies that are fluid velocity dependent. Primarily, vortex shedding induces loading on the body normal to the direction of current flow. If the frequency of excitation of the vortices is close to the natural frequency of the Spar, resonance will occur. Consequently, large and damaging amplitudes of oscillation may be induced when interaction between the flow (usually current) and structure motion causes lock-in. Spar motions induce cyclic ranges of tensions and curvatures in the riser causing fatigue damages. Hence it is important to investigate the effects of VIM on riser fatigue. In the absence of preventive measures, fatigue failure of the risers may occur.