A Sensitivity Study on Fatigue Damage of a Drilling Riser Caused by Vortex-Induced Vibrations
Authors: Roveri F.E. 
Published: October 2008

Abstract
This paper focuses on the evaluation of the fatigue damage caused by current induced vibrations on a drilling riser in 1900 meters water depth. Shear71,2 is the well known computer program most utilized by the offshore industry for assessment of fatigue damage on slender structures, such as risers, caused by vortex induced vibrations (VIV) in the cross-flow direction, originated from sheared currents. However it has some limitations, such as, the planar velocities assumption. It has to be assumed for utilization of the program that the current profile is unidirectional and perpendicular to the plane of riser vibration, therefore one can define two vertical planes, the current direction (in-plane) and the perpendicular (out-ofplane). In Campos Basin current profiles are multi-directional, sometimes rotating 90 degrees through the water column. There are basically three approaches for dealing with such 3-D currents:to project the current velocities onto both inplane and out-of-plane, and add both fatigue damages;to apply all the current full velocities in-plane and all the current full velocities out-of-plane and choose the most damaging case andto define angular sectors and wherever along the depth the current direction aligns within a specific sector, consider full velocity, whilst if the current direction is outside that sector null velocity is considered.
The analysis is firstly performed for a set of variables defined as the Base Case. Fatigue damage is then assessed considering the three methodologies and the resulting fatigue lives are compared. A parametric study is also conducted for the determination of the influence of main parameters on the response. This study is warranted by the uncertainties about input parameters, such as current velocities and the several user defined Shear7 data. The aim is to establish the relative weight of the various factors on the resulting fatigue damage due to VIV. If the resulting damage exceeds the limits, vortex suppression equipment such as strakes or fairings may be necessary. However such devices are costly and time consuming for installation.

Introduction
Considering the multi-directional characteristics of the current profiles at Campos Basin, some simplification of the problem has to be made in order to allow the assessment of the fatigue damage due to VIV. Rotation of the velocity direction of 90 degrees or more through the water column is not uncommon. Therefore the hypothesis of planar velocities along water depth may no longer be valid and the industry has considered different approaches to deal with the problem. Such approaches will be described hereinafter.
A drilling riser system in 1900 meters water depth is utilized for investigating the results of the different approaches for dealing with non-planar current profiles in assessing the fatigue damage due to VIV. The main characteristics of the riser system are shown in Table 1. Generally there are two components in the riser that need to be checked for fatigue: the weld between the pipe and connector and the connector. It is assumed, based on previous calculations, that for this system the connector is the critical component for fatigue.