Zhibiao Rao

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.Copyright

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.Copyright

Effects of Strake Coverage and Marine Growth on Flexible Cylinder Vortex-Induced Vibrations

In this paper we present some results from the recent SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 220,000. The main objective is to present the experimental results describing the effectiveness of different amounts of strake coverage and to explore the influence of simulated marine growth. The data is presented in terms of CF response amplitudes and rainflow-counted damage rates due to the combined CF and IL bending stresses. All results are compared with the bare cylinder cases which will be used as a reference to determine how effective the strakes are in suppressing VIV and how this effectiveness can be affected by marine growth. The results show that even small bare sections (missing strakes) can lead to significant VIV response. We also observe that moderate amounts of marine growth can quickly negate any suppression coming from the strakes. INTRODUCTION As oil and gas exploration and production moves into deeper and deeper waters, the fatigue damage accumulated due to Vortex-Induced Vibrations (VIV) is quickly becoming one of the most critical aspects of deep-water riser design. Helical strakes and other suppression devices are commonly used to try and minimize the effects of VIV. They work by ensuring that the flow separation points are not aligned along the structure or by moving the reattachment points further downstream of the cylinder, in any case they interrupt the coherence of the shed vortex sheet thus minimizing the power into the structure. Over the past years several strake and fairing designs have been proven to be extremely effective in disrupting VIV on rigid or fully covered flexible cylinders. The question that remains to be answered is how much strake coverage is actually necessary to minimize VIV on a flexible cylinder or conversely; how many strakes can one lose before VIV becomes a serious source of fatigue damage on the now partially straked riser. The other pressing issue that needs to be addressed once a strake design and coverage has been chosen is to determine how the suppression characteristics of the chosen device will be affected by the inevitable presence of marine growth. This might be done in order to determine a cleaning schedule or to simply account for the accumulated damage so that one has more confidence in the design or future operation of the riser. This work makes use of test data gathered on behalf of SHELL International Exploration and Production Co. by MARINTEK. The tests involved towing three different 38m long flexible pipes/cylinders in uniform and sheared currents with different suppressions devices, amounts of strake coverage and simulated marine growth. In the first part of this paper we propose a framework/methodology for experimentally determining the worst possible combination of Cross-Flow (CF) and In-Line (IL) damage, i.e. we calculate the position around the circumference of the cylinder’s cross-section that will experience the most severe loading due to the simultaneous motion in both CF and IL directions. The analysis shows that the damage due to the IL motion is comparable to that of the CF motion, and their combined effect results in a damage rate of the same order of magnitude as the pure CF signal and is usually situated at a position around the circumference of the cross-section that is very close to or coincides with the CF direction. The second part of this paper focuses on how effectively four different strake coverage amounts suppress VIV (always using the same strake design). The data is presented in terms of response amplitudes and damage rates, and it is demonstrated that the much larger reduction in damage rate is Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering OMAE2014 June 8-13, 2014, San Francisco, California, USA

Effects of Strake Coverage and Marine Growth on Flexible Cylinder VIV

In this paper we present some results from the recent SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 220,000. The main objective is to present the experimental results describing the effectiveness of different amounts of strake coverage and to explore the influence of simulated marine growth.The data is presented in terms of CF response amplitudes and rainflow-counted damage rates due to the combined CF and IL bending stresses. All results are compared with the bare cylinder cases which will be used as a reference to determine how effective the strakes are in suppressing VIV and how this effectiveness can be affected by marine growth. The results show that even small bare sections (missing strakes) can lead to significant VIV response. We also observe that moderate amounts of marine growth can quickly negate any suppression coming from the strakes.Copyright

Estimation of the Damping Parameter Governing the VIV of Long Flexible Cylinders

Much effort in the past half century has been made to explain the role of damping in the prediction of VIV. Scruton (1965), Griffin et al. (1975), Klamo, et al. (2005) and Govardhan & Williamson (2006) all made significant contributions. None fully characterized the role of damping in governing the response over the full range reduced velocities, which encompass the wake synchronized region. In 2012 Vandiver devised a way to do that with a new damping parameter c*. His results were verified using 2D spring-mounted cylinders in uniform flow.The primary objective of the research described in this paper is to find a c* -like quantity for flexible cylinders, which is capable of organizing response data for flexible cylinders, which may have many modes, be exposed to sheared flows and possess spatially varying properties, such as the coverage of strakes and fairings. Data from a recent high mode VIV model test campaign conducted by SHELL Exploration and Production Company are used to illustrate the application of c* to flexible cylinders. It is shown that, if one accounts for Reynolds number, the response of flexible cylinders with varying strake coverage in the SHELL Tests collapse onto a single curve.Copyright