Drag amplification and fatigue damage in vortex-induced vibrations

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.Includes bibliographical references (p. 181-184).Fatigue damage and drag force amplification due to Vortex-Induced-Vibrations (VIV) continue to cause significant problems in the design of structures which operate in ocean current environments. These problems are magnified by the uncertainty in VIV prediction, particularly with regard to fatigue damage. Although the last fifteen years has seen significant advancement in VIV prediction, important fatigue and drag related questions remain unanswered. This research addresses two important problems. The first is the difficulty in measuring local drag coefficients on long flexible cylinders, excited by VIV. At best engineers are forced to use spatially averaged drag coefficients. This is especially inaccurate when the pipe and flow properties change, either due to partial coverage with VIV mitigation devices, such as strakes or fairings, or shear in the incident current profile. The second problem is the lack of design procedures that account for the effect on fatigue damage due to the higher harmonics in the VIV strain response. To address these problems, two experiments were performed to collect data, the first in October of 2004 and the second in October of 2006. Both of these experiments were designed specifically to collect strain measurements from a densely instrumented pipe undergoing VIV at high mode numbers when subjected to current profiles with varying amounts of shear. Data from these experiments was used to develop a method to extract local drag forces from the measured mean strain. This method, when applied to a partially faired pipe undergoing VIV, successfully and accurately distinguished the dissimilar local drag coefficient between the bare pipe region and the region with fairings. In bare pipes, for the first time the method allowed for the measurement of the variation of local drag coefficient along the length of a flexible pipe undergoing VIV in sheared current.(cont.) Further by using filtering techniques, the higher harmonics were isolated and analyzed, particularly for their magnitude and phase response characteristics. Interesting features about the phase relationships between the first, second and third harmonics were observed when the primary VIV response was in the form of a traveling wave. Finally, data revealed some inaccuracies in the fatigue estimation techniques currently being used by the oil and gas industry. Two methods are suggested to incorporate the higher harmonics in VIV related fatigue design while correcting the observed inaccuracies in the current methods. The results revealed limitations in the commonly used, vibration-amplitude based methods of calculating local drag coefficients and may lead to modifications to correct these limitations. These findings also provide tools for researchers to include the higher harmonics in VIV related fatigue damage calculations and remove some of the uncertainty involved in VIV fatigue estimation and could lead to smaller safety factors in VIV fatigue design.by Vikas Gopal Jhingran.Ph.D

VIV Excitation Competition Between Bare and Buoyant Segments of Flexible Cylinders

This paper addresses a practical problem: “Under which coverage of buoyancy modules, would the Vortex Induced Vibration (VIV) excitation on buoyant segments dominate the response?” This paper explores the excitation competition between bare and buoyant segments of a 38 meter long model riser. The source of data is a recent model test, conducted by SHELL Exploration and Production at the MARINTEK Ocean Basin in Trondheim Norway. A pipe model with five buoyancy configurations was tested. The results of these tests show that (1) the excitation on the bare and buoyant regions could be identified by frequency, because the bare and buoyant regions are associated with two different frequencies due to the different diameters; (2) a new phenomenon was observed; A third frequency in the spectrum is found not to be a multiple of the frequency associated with either bare or buoyancy regions, but the sum of the frequency associated with bare region and twice of the frequency associated with buoyancy region; (3) the contribution of the response at this third frequency to the total amplitude is small; (4) the power dissipated by damping at each excitation frequency is the metric used to determine the winner of excitation competition. For most buoyancy configurations, the excitation on buoyancy regions dominates the VIV response; (5) a formula is proposed to predict the winner of the excitation competition between bare and buoyant segments for a given buoyancy coverage.DeepStar (Consortium)SHEAR7 JI

The Effect of Exposure Length on Vortex Induced Vibration of Flexible Cylinders

This paper addresses a practical problem: “What portion of fairing or strake coverage may be lost or damaged, before the operator must take corrective measures?” This paper explores the effect of lost fairings (the exposure length) on Vortex-Induced Vibration (VIV) of flexible cylinders. The source of data is a recent model test, conducted by SHELL Exploration and Production. A 38m long pipe model with varying amounts of fairings was tested. Response as a function of percent exposure length is reported. Unexpected results are also reported: (i) the flexible ribbon fairings used in the experiment did not suppress VIV at speeds above 1 m/s; (ii) Above 1 m/s, a competition was observed between VIV excited in the faired and bare regions of the cylinder, (iii) Unusual traveling wave behavior was documented—waves generated in the bare region periodically changed direction, and exhibited variation in VIV response frequency. The results of these tests showed that (1) the excitation on the bare and faired regions could be identified by frequency, because the faired region exhibited a much lower Strouhal number; (2) as expected, the response to VIV on the bare region increased with exposure length; (3) the response to VIV on the faired region decreased with exposure length.DeepStar (Consortium)SHEAR7 JI

Reynolds Number Effects on the Vortex-Induced Vibration of Flexible Marine Risers

This paper explores the Reynolds number dependence of the Vortex-Induced Vibration (VIV) of flexible marine risers. Emphasis is placed on revealing the trends that exist between the Strouhal number and the Reynolds number and between the dimensionless amplitude (A/D) and Reynolds number. Data is drawn from recent towing tank experiments which used flexible cylinders of three different diameters. The 38m long pipes were exposed to uniform and sheared currents. The Reynolds number range extended from approximately 5,000 to 220,000 — well into the critical regime — with the larger diameter pipes responding in up to the 13th mode and the smaller diameter pipe responding well above the 20th mode. The results and trends from this set of experiments are compared to previous results from laboratory and field experiments.SHEAR7 JI

Comprehensive Riser VIV Model Tests in Uniform and Sheared Flow

Despite of considerable research activity during the last decades considerable uncertainties still remain in prediction of Vortex Induced Vibrations (VIV) of risers. Model tests of risers subjected to current have been shown to be a useful method for investigation of the VIV behavior of risers with and without suppression devices. In order to get further insight on VIV of risers, an extensive hydrodynamic test program of riser models subjected to vortex-induced vibrations was undertaken during the winter 2010 by Shell Oil Company. The VIV-model test campaign was performed in the MARINTEK Offshore Basin Laboratory. A new test rig was constructed and showed to give good test conditions. Three different 38m long riser models were towed horizontally at different speeds, simulating uniform and linearly varying sheared current. Measurements were made In-Line (IL) and Cross-Flow (CF) of micro bending strains and accelerations along the risers. The test program compromised about 400 tests, which give a rich test material for further studies. In the present paper the test set-up and program are presented and selected results are reported

Eddy formation and propagation in the eastern tropical Pacific

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references: p. 28.Issued also on microfiche from Lange Micrographics.Observations of eddies in the eastern tropical Pacific from TOPEX altimetry data show that there are seasonal and interannual variations in eddy activity. Comparisons between time of eddy formation and corresponding wind data show that not all eddies are caused by winds blowing offshore from the coast of Central America. Plots of eddy tracks from TOPEX data show that some of these eddies last for over 6 months and travel more than 250 of longitude toward the west. Others go more towards the equator and dissipate quickly. A General Circulation Model is used to study the formation and propagation aspects of these eddies. Results from experiments exploring the formation mechanism show that high frequency wind bursts are sufficient but not necessary for eddy formation in the eastern tropical Pacific. Eddy activity remains almost the same if only the annual harmonic of the wind field is used to force the model. Forcing the model with only the high frequency wind component produces almost no eddies. The formation of eddies during periods of weak offshore winds suggests other possible mechanisms, such as unstable mean flows, for the formation of the eddies. Experiments done to study the propagation of the eddies show that the eddies are greatly affected by the structure of the background flow. Eddies formed in September or October encounter a strong westward flowing current and do not dissipate rapidly. These eddies do not travel south beyond the region of shear between the currents. They last for more than 6 months and travel westward for more than 250 of longitude. Eddies formed in March and April encounter a strong eastward flow dissipate quickly and propagate towards the equator where they disappear. These eddies last for less than four months and cover less than 150 of longitude. Eddies generated in January show properties between these two extreme cases