Abstract:
During November and December 96, a drilling riser in the Schiehallion field (West of Shetlands in 360m of water) was instrumented with accelerometers to record its Vortex-Induced Vibration (VIV) response in current. A paper has already been presented on the data gathering campaign (Hassanein, 1998). The present paper concentrates on the analysis of selected samples of the recording. This emphasis of the paper is twofold. The first is the investigation of the relationship between the shear of the velocity profile and the probability of single mode dominated response. The second is the determination of the reduced velocity bandwidth, _Vr, associated with the fluid excitation of the riser. Little is known about these quantities in the high Reynolds regimes found offshore. The first direct application of this work will be to better calibrate and further develop VIV prediction programs. The results presented in this paper come from 83 samples of 28 minute long records. The raw accelerations were interpreted in terms of modal displacements (the normal modes were found from FE analysis). The current profile data are from acoustic Doppler current profile (ADCP) records that covered all but the top 60m of the water column.

Abstract:
In a previous study, the authors have shown that the expected duration of visit by the windspeed to the critical velocity interval of a structural member is an important time scale in determining the ultimate fatigue damage of the member due to vortex-induced vibration in naturally time varying winds. In this paper, a Gaussian windspeed assumption is introduced in which the expected duration of visit can be explicitly in terms of simple wind statistics. This assumption is verified with high sampling rate maritime wind data. The wind statistics necessary for calculation of the expected duration of visit are extracted from the raw wind data.

Abstract:
A cable strumming experiment was conducted at Castine, Maine in July and August of 1981. 75-foot long section of a 1.25-inch diameter cable, and a 1.631-inch diameter pipe were subjected to vortex induced vibration. Seven biaxial pairsof accelerometers were placed at different locations along the cable, and the pipe. Acceleration at these seven positions as well tension, tidal current velocity and drag force were simultaneously recorded. Current velocities ranged from 0 to 2.4 feet/second. A numerical double integration technique is presented in detail and used to obtain the transverse and in-line displacements. Modal identification is used to evaluate the motion in terms of the individual natural modal coordinates of the cable. Lockin and non-lockin examples are presented. Cross flow amplitudes are typically twice that of the in-line vibration. In-line response frequencies are typically twice that of the cross flow.

Abstract:
As a part of the Norwegian Deepwater Program (NDP) three drilling risers have been instrumented with accelerometers and rotation-rate meters for measurement of vortex-induced vibrations (VIV). In addition, current was measured at number of depths. The paper describes how the riser displacements were derived from the measurements and compared with the current. A major task has been to rid the acceleration measurements of the influence of gravity due to the riser’s rotations out of the vertical and include the measurements of angular motion in a consistent way. This has been done using modal decomposition and a least-squares method to estimate the modal weights. The main purpose of the work was to provide data for calibration of computer programs for prediction of VIV. Examples of results are given.

Abstract:
The vortex induced vibration response of long cables subjected to vertically sheared flow was investigated in two field experiments. In typical experiment, a weight was hung over the side of the research vessel by a cable which was instrumented with accelerometers. A typical experiment measured the acceleration response of the cable, the current profile, the tension and angle of inclination at the top of the cable. The total drag force was computed from the tension and angle measurements. Two braided Kevlar cables were tested at various lengths from 100 feet to 9050 feet. As a result of these experiments, several important conclusions can be drawn: (1) The wave propagation along the length of the cable was damped, and therefore, under most conditions the cable behaved like an infinite string; (2) Response spectra were quite broad-band, with centre frequencies determined by the flow speed in the region of the accelerometer; (3) Single mode lockin was not observed for long cables in sheared current profile; (4) The average drag coefficient of long cables subjected to sheared flow was considerably lower than observed on short cables in uniform flow; (5) the r.m.s. response was higher in regions of higher current speed. A new dimensionless parameter is proposed which incorporates the properties of the cable as well as the sheared flow. This parameter is useful in establishing the likelihood that locin may occur, as well as in estimating the number of modes likely to respond.

Abstract:
This paper presents the results of a research study to develop an empirical basis for modeling hydrodynamic vortex-induced vibrations in marine risers, tethers and other slender marine structures. Published model tests were reviewed, evaluated, including more than 150 model tests compiled to provide extensive insight and understanding of the hydrodynamic VIV parameters. The data could provide values for the VIV parameters used in a VIV analysis model. The most important VIV parameters are: the lift coefficient, the shedding frequency (Strouhal number), the correlation length, and the shedding frequency bandwidth. The empirical data are based on steady flow model tests that are applicable to long, flexible cylinders in simulating marine risers undergoing large amplitude vibrations of the order of up to one riser diameter. The empirical data account for the lock-in phenomenon that is the most important consideration to predict accurately VIV extreme response. Conclusions and recommendations are included to develop am empirical methodology that captures the hydrodynamic VIV phenomena. These recommendations provide the basis for the development of a VIV prediction model that is a significant extension from previous models in literature, because it can predict lock-in behavior of a flexible cylinder in shear flow.

Abstract:
This paper focuses on the evaluation of the fatigue damage caused by current induced vibrations on the SlenderEx system. SlenderEx is a low cost drilling system being evaluated by Petrobras for fast appraisal of exploratory wells. SHEAR7 is the well-known computer program developed by MIT for assessment of fatigue damage on slender offshore structures (such as cables and risers) caused by vortes induced vibrations (VIV) originated from sheared currents. The SlenderEx system consists of a 13-3/8 inch 72 lb/ft casing connecting the mud line to a semi-submersible platform and is planned for drilling exploratory wells in water depths up to 1800 meters. At the lower end the casing penetrates some 400 meters into the soil and the upper termination is a stress joint located under the surface BOP which is guided by conventional tensioning system. Fatigue damage is firstly evaluated for a set of variables defined as the Base Case. After that a parametric study is conducted for the determination of the influence of main parameters on response. This study is warranted by the uncertainties about input parameters. The investigation consists of a parametric variation of current velocity, riser tension, lock-in bandwidth, single/multi-mode response, and stress concentration factors. A total of 88 current profiles and the corresponding probabilities of occurrence are considered in the analysis. Some of the current profiles are originally non-planar. The current velocities have then to be projected onto a plane for being analyzed with SHEAR7. The differences arising from this limitation are also evaluated.

Abstract:
Results are presented from a flow-induced vibration experiment conducted in a sheared current. The site was an historic 1848 mill canal with a width of 58 feet. Variable headgates allowed a controllable horizontal shear to be produced. An instrumented cable 1.125 inches in diameter was deployed horizontally in flow. Response was measured for a variety of cable tensions and sheared flow profiles. A method of predicting the response of cables in sheared flow is introduced, which includes the effect of correlation length, hydrodynamic damping, lift coefficient with higher order harmonics and turbulence. Comparisons are made between predicted and measured response. The response is shown to contain significant amount of vibration up to fifth harmonic of the vortex shedding frequency. Hydrodynamic modal damping is shown to have a dramatic effect on the response, and to decrease with increasing frequency of vibration.

Abstract:
Cable strumming experiments were conducted at Castine, Maine in 1981 and from an icebreaker in 1983. the purpose was to study the vibration characteristics of long flexible cylinders subjected to vortex-induced oscillation. Particular emphasis was placed on the investigation of the relationship between in-line and cross flow vibration. Under non-lock-in, random vibration conditions, linear spectral analysis indicated that in-line and cross-flow response were linearly independent of each other, while the results of modal analysis showed that the moving average vibration energies in these two directions were strongly related. A higher order spectral analysis was performed to demonstrate a non-linear correlation between in-line and cross-flow vibration of flexible cylinders excited by vortex shedding in both uniform and sheared flow conditions. The results of bispectral analysis demonstrated the existence of a quadratic relationship between in-line and cross-flow motion under both lock-in and non-lock-in conditions. The well-known frequency doubling phenomenon in in-line response was proven to be a result of such quadratic correlation.

Abstract:
A variety of cables were studied under controlled conditions in the ocean. Sections of cable, 76.5 ft in length, were exposed to a spatially uniform, time-varying current. Vibration response, current velocity, and tension were simultaneously recorded. Care was taken to minimize complicating influences, such as lumped masses and nonuniform tensions. The data revealed strong interaction between the vortex shedding process and the natural frequencies of each cable. Under lock-in conditions the spanwise vibration mode shape corresponded correctly with the “lock-in” natural frequency. A cable with an antistrumming fairing was tested simultaneously with an unfaired length of the same cable; the vibration frequencies and amplitudes are compared.

Abstract:
Case studies drawn from 17 years of field and laboratory experiments are used to demonstrate and explain why the flow-induced vibration of long cylinders in ocean current varies from single mode lock-in to broad-band random vibration. It is shown that, with a careful consideration pf a few dimensionless parameters, the range of observed behavior is predictable. New interpretations are given to the significance of familiar parameters such as mass ratio and reduced damping. In addition, the fractional variations in flow velocity over the length of the cylinder and the number of natural frequencies within the bandwidth of vortex-shedding frequencies are shown to be of considerable importance. When consideration of the above parameters reveals that multiple mode response without lock-in is likely to occur, then hydrodynamic damping is revealed to have a powerful influence on dynamic response, and the simple product of the total damping ratio and the mode number slows one to anticipate the whole range of response behavior, from the wave propagation properties of infinite length cables to the standing wave features of short cylinders.

Abstract:
The practice of assuming full, steady state, vortex-induced-vibration response to wind is excessively conservative. This paper presents a practical method for estimating the reduction of fatigue damage rate due to the natural unsteady fluctuations in wind speed. Two dimensionless parameters are shown to be particularly important. The first is the ratio of the lock-in bandwidth to the turbulence intensity level of the wind. The second is the ratio of the length of time the wind remains in the lock-in interval to the rise time of the member to steady state response levels. The paper also shows that one must account for the discrete nature of wind aped and direction scatter diagrams in estimating the probability of occurrence of the critical wind speed.

Abstract:
Experimental results are presented which show that two dimensionless parameters may be used to predict the likelihood of lock-in under sheared flow conditions. One parameter is Ns, the number of potentially excited modes within the vortex-shedding frequency bandwidth generated by sheared flow. The second, ?V/Vavr, called the shear parameter in this paper, is the ratio of change in velocity of the flow over the length of the cylinder to the spatially averaged flow velocity over the length of the cylinder. Unexpected results are shown which reveal that lock-in may occur under highly sheared conditions. The probable cause of the lock-in is that, under highly sheared conditions, the power available to one particular mode may dominate all other modes. This condition is less likely to develop in flows with lower velocity gradients.

Abstract:
In the summer of 1981 a field experiment was conducted investigating the vibration response of long flexible cylinders to vortex shedding in a steady, uniform current. Two basic cylinder types were tested, both 75 feet in length. One cylinder was a cable 1.25 inches in diameter with seven pairs of internal biaxial accelerometers. The second cylinder was a steel pipe, 1.631 inches in diameter, carrying a cable inside. Drag force, current speed, tension and biaxial acceleration at seven locations were measured. Drag coefficients in excess of 3.0 were measured for both the pipe and the cable under lockin conditions.

Abstract:
The kurtosis statistic is introduced as a sensitive tool for efficient preliminary analysis of flow induced vibration response data. The kurtosis of sequential blocks of time series data allows one to distinguish single mode lock-in events. The problem of predicting lock-in is then discussed. Instructional examples are introduced to illustrate the relative importance of flow-speed, power-in length and diameter, when attempting to predict single frequency dominance. When VIV from two different flow speed regions compete, it is shown that the ratio of the flow speeds cubed is an important indicator, as is the ratio of the square of the length of the power-in regions. Two example cases from measured response on a drilling riser in the North Sea are presented. SHEAR7 predictions are compared to measured results.

Abstract:
Current research in the prediction of vortex-induced vibration of marine risers is driven by problems in several areas. These include understanding the physics of the fluid-structure interaction, developing adequate structural dynamic modeling tools, acquiring high quality full-scale response data, finding appropriate techniques for analysis of response data, calibrating response prediction programs, and inventing cost effective suppression and protection methodologies. Problems in each of these areas are defined and key limiting issues are described.

Abstract:
A method is proposed for the prediction of the flow induced vibration response of flexible cylinders such as cables, pipes and risers in a sheared flow. The significance of material damping is discussed. The reduced damping or response parameter plays a key role in response prediction. However, the dependence of the response parameter and therefore the response amplitude on the ratio of cylinder mass per unit length to the displaced fluid mass per unit length is shown to be widely misunderstood. Under lockin conditions, damping is important in determining the response amplitude, but cylinder mass per unit length is not.

Abstract:
One of the key problems in VIV response prediction in sheared flow has been the prediction of single-mode dominance in sheared flows in spite of many competing modes. This challenge has stimulated a new look into the meaning of the dimensionless parameter, which is known variously as the “response parameter”, the “reduced damping” or the Scruton number” It is known to be a rough predictor of response amplitude A/D. The various forms have small differences and there has been some debate over the years as to which is correct. Most of the controversy centers on the definitions of mass ratio and damping ratio. Our recent studies have shed some new light on this.

Abstract:
This paper presents results from experiments with an elastically supported cylinder subjected to support motions and fluid flow. We varied the support motion amplitude, frequency and flow velocity. We show how this external disturbance influences the vortex shedding frequency, cylinder oscillation amplitude at the vortex shedding frequency and at the support oscillation frequency. Energy dissipation from the cylinder into the fluid is given in terms of the transverse drag coefficient Cd. Results show that the external disturbance has limited influence on the vortex shedding frequency, except for the case where the vortex shedding frequency coincide with the support frequency. Response from support motion tends to be more damped in fluid flow than in still water. Response at the vortex shedding frequency is reduced by the external disturbance, if the disturbance frequency is different from the vortex shedding response frequency.

Abstract:
Vortex-induced vibration (VIV) of a long riser in sheared current is often considered as an energy balance problem: Excitation forces in the power-in region add an equal amount of energy to the system as is dissipated by damping forces outside this region and structural damping. A riser may have different excitation and damping regions depending on the actual oscillation frequency, cross-section properties and local flow velocity. A damping model must hence be able to handle higher and lower flow velocities than the excitation velocity range. In this paper the fluid damping models proposed by Venugopal are compared with the experiments conducted by Gopalkrishnan and Vikestad. The results show that the models are conservative at high and low reduced velocities.

Abstract:
The accuracy of the current modelling is critical when considering deepwater riser fatigue damage caused by vortex induced vibrations (VIV). In the present study the use of empirical orthogonal functions (EOF) to simplify huge amounts of current measurements has been assessed. The amplitudes of the time varying principal components (PC) have been organized into bins in scatter diagram. The accuracy of this scatter diagram approach with different numbers of EOF modes involved has been evaluated in terms of riser VIV fatigue damage.

Abstract:
The present paper will discuss various models for calculation of vortex induced vibrations (VIV) of free span pipelines, and present a new strategy for such analyses. Applications of traditional models are presented and their limitations discussed. The new approach is based on the combination of an empirical linear frequency domain model, and a non-linear time domain structural model. The first step is to carry out the VIV analysis according to linear response theory, and next introduce the calculated hydrodynamic forces to the non-linear structural model. The benefit from using the non-linear model is to describe stresses at the shoulders more accurately, which is important since fatigue damage in many cases will be largest in this area. The conclusion is that the interaction between pipe and seafloor is crucial for accurate stress prediction, and that a non-linear time domain model will give the most accurate result.

Abstract:
Vortex induced vibration (VIV) of free spanning pipelines in current is considered. In standard VIV estimation, one mode of oscillation is considered only. Increasing the length of the span, several modal shapes may be excited. Further, due to the sag effect of a long free span, the dynamic properties in vertical and horizontal direction of the span are different. This causes a much more complex VIV response pattern for long free spans than for short spans. The observed VIV response of long free spans during model testing is discussed. Hypotheses that may explain the observed behaviour are presented. Also a format of new design principles for long free spans is outlined.

Abstract:
A simple equation is presented which provides the maximum achievable mode number for a flexible cylinder, towed by the top end with a weight at the bottom end. The maximum achievable mode number, while towing in still water, is shown to depend primarily on mass ratio, length to diameter ratio and the maximum allowable angle of departure from vertical at the top end. Modal overlap in lock-in regions is shown to depend strongly on mass ratio in uniform flow, but not in sheared flow. The reduced velocity bandwidth parameter is introduced to quantify the extent of lock-in regions in sheared flow. Two shear parameters are shown to be useful in characterizing low and very high mode number response. Finite length lock-in regions are described for cylinders with infinite length dynamic properties.

Abstract:
A dimensionless parameter, Su, is proposed which enables a clear presentation of vortex-induced vibration response data in sheared and uniform flows. The ability of the parameter to account for the effects of damping, flow velocity and power-in length is demonstrated with response data from an unusual set of experiments conducted by SHELL Development Co. The historical relationship of the new parameter to the earlier work of Owen Griffin is also presented. Key words: Vortex-induced vibration, reduced damping.

Abstract:
Vortex-induced vibration (VIV) results in fatigue damage of offshore oil exploration and production structures. In recent years, the offshore industry has begun to employ curved slender structures such as steel catenary risers in deep-water offshore oil systems. The top-end vessel motion causes the slender riser to oscillate, creating an unsteady and non-uniform flow profile along the riser. The purpose of this research is to develop a VIV prediction model for slender structures under top-end periodic motions. The key approach to this problem requires identifying the dimensionless parameters important to the unsteady VIV. A set of data from a large-scale model test for highly compliant risers conducted by industry is available. The spectral analysis of the data showed a periodic pattern of the response frequencies. A constant Strouhal (St) number model was proposed such that shedding frequencies change with local inline velocities. The Keulegan-Carpenter number (KC) controls the number of vortex pairs shed per cycle. A KC threshold larger than 40 was found to have significant response for a long structure with finite length excitation region. An approximate solution to the response of an infinite beam with a finite excitation length was obtained; this solution provided an explanation for the high KC threshold. A model for an equivalent reduced damping Sg under a non-uniform, unsteady flow was proposed. This equivalent reduced damping Sg was used to establish a prediction model for the VIV under top-end periodic motions. A time domain simulation of unsteady VIV was demonstrated by using Green’s functions. The turning point problem wave propagation was solved for a pipe resting on a linearly varying stiffness foundation. Simple rules were established for conservative estimation of TDP fatigue damage with soil interactions. Guidelines for model test experiment design were provided based on dimensional analysis and scaling rules.