This paper describes the methodology used to analyze the Auger TLP production and steel catenary export risers for vortex induced vibrations. Presented are:
a discussion of the development of testing facilities for deepwater riser vortex induced vibration modeling;
an overview of the tests used to calibrate a response prediction program;
calibration of the response prediction program,'
the analysis procedure for both riser types;
the analyses' results; and
a summary of the vortex suppression technology used to choose triple-start helical strakes for the export risers.
It is well known that long, slender, cylindrical structures exposed to currents can fail by fatigue due to vortex-induced vibrations (VIV). The has been well documented for currents in which the flow velocity is uniform in magnitude and direction along the cylinder span; Marris, I Sarpkaya,2 Bearman,3 and Blevins4 have all published insightful reviews of VIV tests conducted on cylindrical structures exposed to uniform flows. Prediction of VIV response, in uniform flows, can usually be accomplished with reasonable accuracy. Ignoring the possibility of VIV, or inaccurately assessing VIV, during the design process has sometimes proved disastrous. It is also well known that the largest vibrations tend to be in the direction perpendicular to the current, called the out-of-plane, normal, cross flow, or transverse direction. However, substantial accelerations and bending stresses can also occur in the in-line direction.
That non-uniform flows can also excite a cylindrical structure into substantial VIV is not as nearly well established. Some initial research into VIV of cylinders exposed to non-uniform, or sheared flows has been conducted by severai investigators.5-18 Since about mid-1988, the author has been leading a research program on sheared flow VIV of deepwater tubulars such as risers, tendons, etc., exposed to the Gulf of Mexico (GOM) Loop Current and its associated eddies. This research program has included: development of suitable research facilities and the procurement of relevant VIV data for long tubulars in uniform and sheared flows; field measurements of VIV for a full buoyancy drilling riser in a 3-knot eddy; extensive modification and calibration of the response prediction program called SHEAR;13 based upon the current tank tests' results, comprehensive current tank tests to qualify various VIV suppression devices; and tests to size the helical strakes for the Auger TLP steel catenary export risers. Some details of this research program, including its impact on the analysis and suppression of the Auger risers, are presented in the following sections.
The computer program SHEAR was originally developed by Prof. J. Kim Vandiver of the Massachusetts Institute of Technology Department of Ocean Engineering and his thesis student T. Y. Chung. 19 The program's original version used: a Green's function solution technique to solve, in the frequency domain, the governing structural equations for a constant tension cable; a co-spectrum to model the lift forces from VIV; and a linear model for estimating the hydrodynamic damping. Subsequent versions have included some modeling of non-linear damping and the ability to model cables and beams with linearly varying tension.