This paper presents results from two field experiments using long flexible cylinders, suspended vertically from surface vessels. The experiments were designed to investigate vortex-induced vibration (VIV) at higher than tenth mode in uniform and sheared flows. The results of both experiments revealed significant vibration energy at the expected Strouhal frequency (referred to in this paper as the fundamental frequency) and also at two and three times the Strouhal frequency. Although higher harmonics have been reported before, this was the first time that the contribution to fatigue damage, resulting from the third harmonic, could be estimated with some certainty. This was enabled by the direct measurement of closely spaced strain gauges in one of the experiments. In some circumstances the largest RMS stress and fatigue damage due to VIV are caused by these higher harmonics. The total fatigue damage rate including the third harmonic is shown to be up to forty times greater than the damage rate due to the vibration at the fundamental vortex-shedding frequency alone. This dramatic increase in damage rate due to the third harmonic appears to be associated with a narrow range of reduced velocities in regions of the pipe associated with significant flow-induced excitation.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractTriple helical strakes can play an important role in the suppression of VIV on offshore platforms. This paper will present results of two field experiments, one conducted at Lake Seneca in upstate New York and the second in the Gulf Stream near Miami, Florida. Three different distributions of triple helical strakes were tested. These experiments were designed to explore the effects of VIV on both bare pipes and pipes with strakes at mode numbers greater than the tenth mode in uniform and sheared currents. At Lake Seneca, bare pipe and full strake coverage pipes were tested in uniform currents. In the Gulf Stream two different configurations of strakes were tested and compared to the VIV response of a bare cylinder. The two configurations are referred to as the 40% coverage case and the 70% staggered coverage configuration. The results of these tests showed a reduction in the amplitude of the vibration and also the frequency content of the vibrations. In particular, a large third harmonic component, which contributes significantly to the fatigue damage rate, was suppressed by the configurations with strakes. Together these reductions will greatly increase the fatigue life of the pipe.

Electrochemistry is primarily taught in first-year undergraduate courses through batteries; this lab focuses instead on corrosion to apply electrochemical concepts of electrolytes, standard reduction potentials, galvanic cells, and other chemistry concepts including Le Chatelier's Principle and Henry's Law. Students investigate galvanic corrosion under neutral and acidic conditions quantitatively by measuring voltage, amperage, and mass loss and qualitatively using phenolphthalein. They also investigate protection against corrosion through the use of sacrificial anodes and impressed current cathodic protection.

The response amplitude and the non-dimensional frequency of flexible cylinder vortex-induced vibrations from laboratory and field experiments show significant trends with increasing Reynolds number from 103 to 2 * 105. The analysis uses complex data from experiments with wide variations in the physical parameters of the system, including length-to-diameter ratios from 82 to 4236, tension dominated natural frequencies and bending stiffness dominated natural frequencies, sub-critical and critical Reynolds numbers, different damping coefficients, standing wave and traveling wave vibrations, mode numbers from 1 – 25th, and different mass ratios.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents the initial results from VIV (Vortex-Induced Vibration) field testing of a long, flexible, model riser at high mode number. The experiments were designed to better understand the dynamic behavior of a long riser in uniform flow responding at mode numbers from 10 to 25 in cross-flow vibration. The 1.31 inch, (0.0333 m), diameter riser model was made from fiberglass line pipe, manufactured by Fiberspar Corp. Two model configurations with length of 201 feet (61.26m) and 401 feet (122.23m) were towed behind a vessel in a deep lake in upstate New York. Motion was recorded with twenty four evenly spaced internal tri-axial accelerometers. Observed reduced velocity and RMS displacement response levels are reported. Mean drag coefficients and hydrodynamic damping derived from measured data are compared to calculated values from formulas commonly used in engineering design of offshore systems.

The primary objective of this research is to locate the source of the vortex-induced vibrations (VIV) for long flexible cylinders at high-mode number and to help determine the source region for future predictions. The two Gulf Stream tests were conducted to collect data on a scale-model pipe that was excited at high-mode numbers. The high density of the sensors on the pipe allowed for analysis that had not previously been done. Two methodologies are presented to locate the area of the region that is the source of the vibration. In VIV, the current which causes the vibrations is important, because the speed of the current will determine the frequency of the vibration. Therefore, one important question is which section of the pipe will be the source of the vibrations for a known current profile. This source region is known as the power-in region. Regions on the pipe that are not a source of power instead damp the structural vibrations. Once the region where the vibration originated has been found, the different phenomena that effect the location of the power-in region that were discovered are shown. Four different factors are presented that effect the locations of the power-in region: the angle of the pipe with respect to the vertical, the gradient of the current direction, the current profile, and the end effects at high mode number. A dimensionless parameter is presented which help in the prediction of VIV given a current profile. The power-in factor predicts the region where the source of the vibration occurs using a combination of the current velocity and the source region length.

When using the theory of Time Sharing of frequencies for predicting damage and fatigue from Vortex-Induced Vibrations (VIV), the percent of time that a riser is vibrating with high amplitude VIV will significantly influence the fatigue rate of a riser. To accurately predict the fatigue of a riser using time sharing or another stochastic prediction method an accurate model is needed for the percent of time spent at these high amplitude vibrations. Using multiple laboratory tests with both sheared and uniform flow, the percent of time that each section of a pipe vibrated at high amplitude was determined. The larger issue was determining the factors that control the percentage of time at high amplitude vibration. Energy in the system was determined to be the largest influence on the percentage of time. At both low levels and high levels of energy the system had a low percentage of high amplitude VIV. A middle set of energy is the condition that produced high amplitude VIV for the largest percentage of time.

Most empirical codes for prediction of vortex-induced vibrations (VIV) has so far been limited to cross-flow response. The reason for this is that cross-flow amplitudes are normally larger that in-line amplitudes. Additionally the in-line response is considered to be driven by the cross-flow vibrations. However since the in-line frequency is twice the cross-flow frequency, fatigue damage from in-line vibrations may become as important and even exceed the damage from cross-flow vibrations. A way to predict in-line vibrations is to apply traditional methods that are used for cross-flow VIV and establish an empirical relationship between the cross-flow and in-line response. Previous work suggests that the ratio between the in-line and cross-flow amplitudes depends on the cross-flow mode number, Baarhom et al. (2004), but the empirical basis for this hypothesis is not strong. The motivation for the present work has been to verify or modify this hypothesis by extensive analysis of observed response. The present analysis uses complex data from experiments with wide variations in the physical parameters of the system, including length-to-diameter ratios from 82 to 4236, tension dominated natural frequencies and bending stiffness dominated natural frequencies, sub-critical and critical Reynolds numbers, different damping coefficients, uniform and sheared flows, standing wave and traveling wave vibrations, mode numbers from 1–25th, and different mass ratios. The conclusion from this work is that the cross-flow mode number is not the important parameter, but whether the frequency of vibration in the cross-flow direction is dominated by bending stiffness of tension.