A Comparative Study of the Performance of Top-Tensioned Composite and Steel Risers under Vortex-induced Loading
Authors: Omar A.F. , Karayaka M. , Murray J.J. 
Published: October 2008

ABSTRACT
Current efforts on the development of composite riser joints have been mainly focused on low-cost manufacturing and failure strength evaluation of tube body and joint connection. The effect of material properties, such as structural damping and structural stiffness, on the riser performance under hydrodynamic loading, e.g., the Vortex-Induced Vibration (VIV) has not been addressed to the same level as other issues. In this paper, two top-tensioned steel and ccmposite risers identical in operational capacity are used to study the effect of structural mass and damping on the characteristics of VIV. The study considered tension capacity requirements. VIV-induced local stress, VIV amplitudes, and damage rates as the aspects of the comparison between steel and composite riser configurations. The comparisons revealed that composite risers have superior dynamic response characteristics compared to steel risers. For example, the maximum VIV stresses induced in the composite riser is about 50% of that induced in the steel riser. This indicates that the composite risers would have considerably longer fatigue lives compared to steel risers.

INTRODUCTION
As the offshore industry moves aggressively to pursue deeper water developments, composite materials are finding a wide range of new applications for both topside and subsea tructures. While most of the current applications are secondary structures in the topside facilities, several major U.S. and international initiatives are underway to develop primary load-bearing system components [1-7].
Composite production riser (CPR) joints are being seriously considered in the development of deep water floating platforms. This is because of their inherent lightweight, superior fatigue, corrosion resistance, and controllable strength and stiffness properties. Current efforts on the development of CPR joints have been mainly focused on low-cost manufacturing and failure strength evaluation of CPR tube body and CPR joint connections. To the authors knowledge, the important issues of system (i.e., the floating platform that containing multiple CPR strings) hydrodynamics and VIV have not been addressed.
VIV is of primary concern in the oil and gas industry for almost all type of drilling and production risers, especially with the developing activities heading towards deeper water and harsher environments. It is well known that long slender cylindrical structures, such as risers end cables, often exhibit a harmonic flow induced vibration response known as "lock-in". In this case the cylinder experiences a large increase in the steady in-line current induced drag forces. Under different flow conditions, uniform flow (e.g., Sarpkaya [8]), shear flow (e.g., Chung [9]) and non-uniform oscillatory flow (e.g., Omar [10 and [11]), lock-in has been extensively studied. The most obvious consequences of VIV are increased fatigue on the riser and wellhead, and increased stresses at hang-off point.
When lock-in occurs, the dynamic response due to VIV is mainly governed by two parameters; 1] the reduced velocity, Vr= V / fn D, and 2) the reduced damping, ks = 2 m ?s/(?D2), where, V is the current velocity, D the riser diameter, fn is riser nth modal frequency, m is the riser mass per unit length (including content and added mass), ? is mass density of sea water and ?s is the logarithmic decrement of the riser structural damping. It is the effect of material structural mass and damping which is the focus of this paper.