The Effectiveness of Two- versus Three-start Helical Strakes in Suppressing VIV of Flexible Cylinder

Floating spar platforms are widely used in the Gulf of Mexico for oil production. The spar is a bluff, vertical cylinder which is subject to Vortex Induced Motions (VIM) when current velocities exceed a few knots. All spars to date have been constructed with helical strakes to mitigate VIM in order to reduce the loads on the risers and moorings. Model tests have indicated that the effectiveness of these strakes is influenced greatly by details of their design, by appurtenances placed on the outside of the hull and by current direction. At this time there is limited full scale data to validate the model test results and little understanding of the mechanisms at work in strake performance. The authors have been investigating the use of CFD as a means for predicting full scale VIM performance and for facilitating the design of spars for reduced VIM. This paper reports on the results of a study to benchmark the CFD results for a truss spar with a set of model experiments carried out in a towing tank. The focus is on the effect of current direction, reduced velocity and strake pitch on the VIM response. The tests were carried out on a 1:40 scale model of an actual truss spar design, and all computations were carried out at model scale. Future study will consider the effect of external appurtenances on the hull and scale-up to full scale Reynolds’ numbers on the results.

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Experimental Investigation of Vortex-Induced Vibrations on Yawed and Inclined Flexible Cylinders

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.4051650 ◽

2021 ◽

pp. 1-16

Author(s):

Daniel P. Vieira ◽

Guilherme R. Franzini ◽

Fredi Cenci ◽

Andre Fujarra

Keyword(s):

A Priori ◽

Flexible Structures ◽

Resonance Region ◽

Offshore Structures ◽

Natural Mode ◽

Flexible Cylinder ◽

Decomposition Scheme ◽

Independence Principle ◽

Modal Amplitude ◽

Time Histories

Abstract An experimental setup was built to investigate the Vortex-Induced Vibration (VIV) phenomenon on yawed and inclined flexible cylinders, in which five yaw angles θ = 0°, 10°, 20°, 30° and 45° and five azimuth angles ß = 0°, 45°, 90°, 135°, and 180° were combined. The experiments were carried out in a towing tank facility at Reynolds numbers from 1800 to 18000, comprising vibrations up to the eighth natural mode. Time histories of displacements were recorded using a submerged optical system that tracks 17 reflective targets. A modal decomposition scheme based on Galerkin's method was applied, aiming multimodal behavior investigations. Such an approach allowed the analysis of the modal amplitude throughout time, revealing interesting results for such a class of VIV tests. The flexible cylinder total response is generally a combination of two or more modes. Only for azimuths 0°, 90°, and 180°, a unimodal response was observed for the two first lock-in regimes. The frequency response showed that, when the response was multimodal, non-dominant modes can follow the vibration frequency of the dominant one. Assuming a priori the Independence Principle (IP) valid to define the reduced velocities (Vr), it was observed that the resonance region was restricted to 3 <= Vr <= 8 for the tested cases, indicating that the IP can be at least partially applied for flexible structures. As the literature scarcely explores the simultaneous yawed and inclined configurations, the present work may contribute to further code validation and improvements regarding the design of slender offshore structures.

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Simulations on Hydrodynamic Coefficients of Stationary Cactus-Shaped Cylinders at a Low Reynolds Number

Volume 2: CFD and FSI ◽

10.1115/omae2018-78472 ◽

2018 ◽

Author(s):

J. Wang ◽

C. Shi ◽

Y. Liu ◽

X. Bao

Keyword(s):

Reynolds Number ◽

Low Reynolds Number ◽

Bending Moment ◽

Hydrodynamic Coefficients ◽

Rotational Angle ◽

Desert Region ◽

Helical Strakes ◽

Wind Velocities ◽

Low Reynolds ◽

Very High

Flexible cylinders, such as marine risers, often experience sustained vortex-induced vibration (VIV). Both helical strakes and fairings are demonstrated to be effective in suppressing VIV, while, helical strakes result in large drag, which increases the rotational angle and bending moment at the riser hang-off location and, fairings are cumbersome in term of storage, installation and maintenance. This study was inspired by the giant Saguaro Cacti which grow in desert region. Saguaro Cacti have shallow root system, but can grow up to fifty feet in height and can withstand very high wind velocities. In this study, numerical simulations of flow past a stationary cactus-shaped cylinder are performed in two-dimensional field at a low Reynolds number of 200. The hydrodynamic coefficients and the vortex-shedding patterns of a cactus-shaped cylinder are compared with those of a circular cylinder. In addition, a set of two cactus-shaped cylinders of tandem arrangement are also studied to investigate the effects of wake. Results showed that a cactus-shaped cylinder can reduce the drag, lift, and Strouhal number, which suggests its potential as an alternative technology to suppress VIV of a riser.

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Influence From Helical Strakes on Vortex Induced Vibrations and Static Deflection of Drilling Risers

24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 1, Parts A and B ◽

10.1115/omae2005-67192 ◽

2005 ◽

Author(s):

Carl M. Larsen ◽

Gro Sagli Baarholm ◽

Halvor Lie

Keyword(s):

Dynamic Response ◽

Test Data ◽

Oscillation Amplitude ◽

Cross Flow ◽

Current Profile ◽

Dynamic Stresses ◽

Drag Forces ◽

Vortex Induced Vibrations ◽

Bending Stresses ◽

Helical Strakes

Helical strakes are known to reduce and even eliminate the oscillation amplitude of vortex induced vibrations (VIV). This reduction will increase fatigue life, and also reduce drag magnification from cross-flow vibrations. But sections with strakes will also have a larger drag coefficient than the bare riser. Hence, the extension of a section with strakes along a riser should be large enough to reduce oscillations, but not too long in order to limit drag forces from current and waves. The optimum length and position for a given riser will therefore vary with current profile. Dynamic response from waves should also be taken into account. The purpose of the present paper is to illustrate the influence from strakes on VIV, as well as on static and dynamic response for a drilling riser. Hydrodynamic coefficients for a cylinder with helical strakes are found from experiments and applied in an empirical model for the analysis of VIV. The result from the VIV analysis is used for a second calculation of drag forces that are applied in an updated static analysis. Dynamic stresses from regular waves are also presented, but VIV are not considered for these cases. A simple study of length and position of the section with strakes is carried out for some standard current profiles. Results are presented in terms of oscillation amplitudes, fatigue damage, bending stresses and riser angles at ends. The study is based on test data for one particular strake geometry, but the analysis method as such is general, and the computer programs used in the study can easily apply other test data.

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Experimental Study on Flexible Bare Pipe Arrays Undergoing Vortex-Induced Vibrations

Volume 1: Offshore Technology ◽

10.1115/omae2018-78471 ◽

2018 ◽

Author(s):

Y. Liu ◽

C. Shi ◽

Z. Liu ◽

J. Wang ◽

X. Bao

Keyword(s):

Fatigue Damage ◽

Dynamic Responses ◽

Array Configuration ◽

Damage Rate ◽

Flexible Pipes ◽

Vortex Induced Vibrations ◽

Helical Strakes ◽

Preliminary Study ◽

Single Pipe ◽

Center Distance

Vortex-induced vibration (VIV) excited by current is a major contributor to the fatigue accumulation of marine risers. For deepwater operations, several risers are often arranged together in an array configuration. In this study, a set of four identical flexible pipes of a rectangular arrangement were tested in a water tunnel. By comparing the dynamic responses of a pipe in an array with that of a single isolated pipe, the effects of the current speed and the center-to-center distance between the up-stream and downstream pipes on their dynamic responses were investigated. Fatigue damages accumulated on each pipe in an array was calculated and a factor, termed “fatigue damage amplification factor”, was defined as a ratio between the fatigue damage rate of pipe in an array and the fatigue damage rate of a single pipe at a same current condition. The results showed that for bare pipes (i.e., without helical strakes), the downstream pipes in an array configuration may have larger dynamic responses and fatigue damage rates than those of a single pipe; and, it is not always conservative to assume that the fatigue damage rate estimated for a single pipe can be used to represent the fatigue damage rates of pipes in an array. This preliminary study provided some meaningful results for the design, analysis and operation of marine riser arrays.

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