Halvor Lie

Reynolds Number Dependence of Flexible Cylinder VIV Response Data

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.Copyright

Norwegian Deepwater Program: Analysis of Vortex-Induced Vibrations of Marine Risers Based on Full-Scale Measurements

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. Introduction To design marine risers for use in deep waters it is important to be able to predict the character and amount of vortex-induced vibration the riser may have. In general VIV will be important with respect to fatigue as well as drag forces on the riser. The VIV response of a marine riser is a complicated pro??cess involving both the hydrodynamical and the structural properties of the riser. Model testing has given valuable insight in VIV. Different types of experimenting have been done, e.g. forced motion with rigid cylinders in a uniform flow (Ref. 1), spring-supported rigid cylinders in uniform flow (Ref 2) and scaled riser models in uniform and sheared flows (Ref. 3). Ref. 4 gives a comprehensive introduction to the phenomenon of VIV in general, while Ref. 5 is an account of the state of art when it comes to VIV of marine risers. Still, full-scale data are needed to verify VIV models at realistic Reynolds numbers and in realistic currents that vary with depth. The task of obtaining data by instrumentation and processing of the measurements is not trivial and, if done incorrectly, may lead to results of questionable value. In order to provide full-scale data for VIV verification, three drilling risers at fields offshore North Norway had been instrumented for measurement of riser response. Also the sea current was measured at a number of depths. The three fields were Nyk High, Vema and Helland-Hansen. The water depths at these sites are 1270 m (Nyk High), 1220 m (Vema) and 685 m (Helland-Hansen). The instrumentation at these sites differed mainly in details, e.g. the number of motion and current sensors. To use full-scale measurements in verification it is very important that the measurements are processed correctly before use. Such processing will in general be necessary in order to convert the data to a form that is suitable for the verification. The typical device for measurement of riser dynamics is the accelerometer. Apart from general filtering to remove unwanted noise and bias, it is very important that the signal from the accelerometer is corrected for the time-varying disturbance by the gravitational acceleration. Neglecting this and integrating the accelerometer outputs twice to obtain (hopefully) displacement may lead to significant error. The paper describes the method for processing and analysis that were applied to a large number of records of riser motion and current. The purpose of the analysis was to derive quantities that give meaningful information about the VIV phenomenon and could be used further for calibration of VIVprediction tools such as SHEAR7 (Ref. 7). A central task has been developing a method for estimating true lateral displacements from the gravity-contaminated measurements. The method of analysis is exemplified by a set of data from Helland-Hansen (which had the best current measurements). For a broader description of the VIV part of the Norwegian Deepwater Program, see Ref. 8. The measurements The instrument system to measure riser motion on HellandHansen consisted of six instrument containers attached to the riser in positions shown on Fig. 1. Each instrument unit con2 KAASEN, LIE, SOLAAS, VANDIVER OTC 11997 tained motion sensors, data acquisition hardware and batteries. The units were completely autonomous. The main sensors were accelerometers for measurement of horizontal acceleration in two orthogonal axes, X and Y, as shown in Fig. 2. In three of the instrument containers the accelerometers were supplemented with sensors for measuring angular velocity about the X and Y-axes. A seventh unit was installed in the drilling vessel. This unit was not used in the analysis. The duration of the measuring period was about twelve weeks. All measuring units were in operation for about a month. Then intrument unit 1 failed, followed by unit 5 a few days later. The instrument system and the collected data are described in detail in Ref. 6. Current was measured by a number of acoustic doppler current profilers (ADCP) mounted in a vertical mooring in the neighbourhood of the drilling vessel. In addition to the ADCPs a rotor-type current meter measured current near the seafloor. The raw data consisted of readings every ten minutes, representing the average values of the ten-minute interval. The current data used in the analysis had been obtained by smoothing and interpolation (to fill “holes” in the data) in time and space and gave the speed and direction of the current at a number of depths. For the top 100 metres of the water column reliable current data could not be given due to disturbance from the drilling vessel’s thrusters. After about eight weeks of operation the bottom meter failed. Estimation of Riser Lateral Displacements Accelerometer Signals. An accelerometer can measure true acceleration along its sensitive axis as long as the orientation of this axis is kept constant in space. For the accelerometers on the riser this will not be the case as the motion of the riser at any point will be a combination of sideways motion and rotation. That is, the centre line of the riser will deviate from the vertical causing the orientation of the accelerometer to deviate from the horizontal, thus exposing the sensing axis to gravity force. Assuming a small angle of rotation, it can be decomposed vectorally in rotations and about the horizontal axes X and Y, respectively. Letting and denote the components of true horizontal acceleration, the signals from the X and Y accelerometers will be x y

Suppressing Full-Scale Riser VIV With the VT Suppressor

The paper reports on recent full scale experimental tests with The Ventilated Trouser (VT), a novel device for VIV suppression of cylindrical structures exposed to external fluid flow. The VT suppressor is a loose fitting sleeve in the form of a light flexible net with integral bobbins in a special arrangement (Fig 1). It is omni-directional, rugged, and made from materials compatible with the offshore environment.The tests reported here, originated in an invitation from Statoil to test the VT on a slick riser section. They were undertaken at Marintek, Norway, with a 0.53m diameter riser in current velocities up to 2.3m/s, equivalent to post-Critical Reynolds Numbers of up to 1.2 × 106.The VT suppressed the maximum VIV amplitude of the slick joint by over 90%. This was consistent with the suppression performance of the VT from previous tests with model risers varying in size from 0.1m diameter to 0.3m diameter.The test results suggest the VT is a candidate suppressor fully capable of competing with conventional suppression devices.Copyright

VIV Force Identification Using Classical Optimal Control Algorithm

Due to the difficulty of direct force measurements in vortex induced vibration (VIV) experiments with long elastic cylinders, accelerometer and bending strain measurement are available. Still, obtaining information on the force is of great interest to researchers. The work presented in this paper follows the same principle as Maincon (2004), who estimated external forces acting on a riser subjected to VIV from measured response by using a classical optimal tracking algorithm. The objective of this study is to first present a method for extracting VIV forces from measured data with long elastic riser models subjected to current. The second objective is to extract first order (primary) cross-flow force coefficients by a combined use of modal filtering. The algorithm minimizes the sum of the squares of the discrepancies between measured and predicted response plus a constant times the sum of squares of the external forces, while satisfying the system’s dynamic equilibrium equation. FEM discretization of the riser with Euler beam elements leads to a stiffness and mass matrix. The dimension of these matrixes is reduced by eliminating the rotation degree of freedom using master-slave condensation, which greatly facilitates the matrix iteration. Displacement is used in this study as input to the algorithm to identify forces. The method is verified against synthetic measurement data. The results showed the algorithm’s capability to accurately estimate the input forces from noisy measurement data. The method is applied to the data from a rotating rig test to identify hydrodynamic forces in primary cross-flow vortex shedding frequency range. The emphasis is on extracting force coefficient database. One important finding is that the high mode component of the force contributed little to the response, while it resulted in complication of the coefficient data base. Therefore, they are neglected by filtering the measurement with modal analysis before the use of inverse force estimation. The excitation and added mass coefficients are calculated and their contour plots are generated. Comparisons with existing data are investigated.Copyright

Frequency Domain Model for Prediction of Stochastic Vortex Induced Vibrations for Deep Water Risers

This paper describes a new model for prediction of fatigue damage from VIV in risers. The method will overcome some of the shortcomings of previous methods. A fully 3D model is proposed, “cross-flow” and “in-line” response are predicted, response at higher order harmonic components will be added, and the stochastic nature of the response is accounted for by introducing a time varying envelope function combined with “time sharing” between dominating response frequencies. A model that reflects this behaviour is considered to be more realistic and is more likely to predict lower fatigue damage than the traditional discrete-frequency models. The model will predict a response that will appear as a combination of standing and travelling waves depending on boundary conditions, damping and load distribution. Fatigue damage will therefore become more evenly distributed along the riser, and less concentrated at anti-nodes for (dominating modes) than seen from traditional discrete frequency models. The proposed model needs empirical coefficients for simultaneous IL and CF response. In principle this requires a data base of added mass, excitation and damping coefficients for varying flow conditions and response frequencies, combinations of response amplitudes in both directions, varying phase between the two response components and even the presence of higher order motion components. Such data do not exist. We have therefore proposed to use the limited information we have on this matter at present. Future improvement of the model might therefore be possible if more data becomes available. The new model will be implemented in the VIVANA program and the enhancement of the code is in progress. The paper will present the background of the model, the basic assumption of the new model and a comparison between preliminary results obtained from a preliminary code and model test results. The cases include both 2D uniform current conditions and 3D (non-uniform) current conditions.Copyright

Mooring line damping estimation by a simplified dynamic model

When predicting slowly varying resonant vessel motions, a realistic estimate of the motion damping is crucial. Mooring line damping, which is mainly induced by the drag force on line, can dominate the total damping of catenary moored systems and methods for predicting mooring line damping are therefore required. Based on a simplified dynamic model of mooring line tension, an approach to estimate the corresponding damping is presented in this paper. Short-term time domain simulations of dynamic line tension are carried out to verify the accuracy of the simplified frequency domain approach. Compared with the simulation results, the practical simplified method proposed herein gives a maximum 30% lower prediction of the damping coefficient of each mooring line and an about 20% smaller estimate of the total line damping and therefore yields conservative estimates of the low frequency vessel motions.Copyright

Interaction and Clashing Between Bare or Straked Risers: Analyses of Experimental Data

Hydrodynamic interaction and clashing of two long flexible cylinders in uniform steady current have been studied. Model tests have been performed in the towing tank at Marintek in Trondheim. The riser models were free to move in vortex induced vibrations (VIV) and wake induced vibrations (WIO). Model tests were performed both for straked and bare risers, and for risers with and without bumper elements attached. The model tests where presented in an earlier paper. This paper will briefly present the tests while the primary objective of the present paper is to describe advanced analysis and results. The emphasis in the analysis of the recorded data has been to assess the spatial distribution of clashing, the relative impact velocities, WIO and VIV.Copyright

Experimental Investigation of Dual Riser Interaction

Hydrodynamic interaction and clashing of two long flexible cylinders in uniform steady current have been studied. Model tests have been performed in the towing tank at Marintek in Trondheim. The riser models were free to move in vortex induced vibrations (VIV) and wake induced vibrations (WIO). The main objectives of the experimental campaign were to acquire high quality data that can be used to enhance the understanding of the mechanisms that drives riser clashing and to provide benchmark data for riser clashing code validation. Model tests were performed both for straked and naked risers, and for risers with and without bumper elements attached. Riser spacing, inflow angle, riser pretension and current velocity were systematically varied. The analysis of the recorded data has emphasized on assessing the spatial distribution of clashing, the relative impact velocities, WIO and VIV.Copyright

Simplified Model for Evaluation of Fatigue From Vortex Induced Vibrations of Marine Risers

This paper presents a novel approach for approximate calculation of the fatigue damage from vortex-induced vibrations (VIV) of marine risers. The method is based on experience from a large number of laboratory tests with models of full-length risers, large-scale tests and also full-scale measurements. The method is intended to provide a conservative result and be used for screening purposes at the early design stage. The model is in particular aimed at predicting fatigue for risers that respond at very high mode orders (above 10), but may as well yield valid results for lower mode numbers. The model will, however, not be adequate for free span pipelines or other structures that normally will respond at first and second mode. The riser will be defined in terms of some key parameters like length, weight, tension, hydrodynamic diameter and stress diameter. A current profile perpendicular to the riser in one plane must be known. The program will apply a simple model for calculation of eigenfrequencies and mode shapes, and these are sorted into in-line (IL) and cross-flow (CF) bins. An effective current velocity and excitation length can be defined from the profile and will be applied to identify the dominating cross-flow response frequency and the total displacement rms value. The dominating in-line response frequency is taken as twice the cross-flow frequency, and inline response rms is taken as a given portion of the cross-flow rms value. A set of contributing modes is defined from an assumed frequency bandwidth that reflects observed bandwidths, but also modal composition for cases with discrete frequency response. A simple mode superposition technique is then used to find the set of modes that gives the identified rms values. Bending stresses will be found directly from the curvature of the mode shapes. Fatigue damage will be found from stress rms values, user defined stress concentration factor and given SN curves. The model has been implemented in a simple computer program and verified by comparing results to measurements. The ambition has not been to obtain an exact match between computed results and observations, but to verify that the model gives reasonable but conservative results in almost all cases. However, an unrealistic over prediction of the fatigue damage is not desired. The results are promising, but the need for more information from measurements and response analyses with programs like VIVANA and SHEAR7 is still obvious.Copyright

Frequency Components of Vortex Induced Vibrations in Sheared Current

Vortex induced vibrations (VIV) of long flexible risers subjected to ocean currents are of vital interest to the offshore industry. Although significant efforts have been seen during the last decades, reliable prediction of this complicated fluid structure interaction phenomenon is still a challenge.The primary objective of this paper is to characterize the frequency components of VIV measured in flexible beams subjected to sheared current, and try to establish a general model for frequency participation for use in semi-empirical models for calculation of fatigue damage from VIV. Experimental data from the well known Hanoytangen tests and the Norwegian Deep-water Programme (NDP) high mode experiments have been used in this study. The present paper is mainly based on results from Ziguang Zhao (2011).Wavelet analyses are applied to reveal the frequency components in the measured signals. These analyses give information on the time-varying intensity of each active frequency at a specific position on the beam. The dominating frequency and range of other active frequencies are two key parameters from the wavelet analyses that are further used in this work.By comparing synchronic measurements from various positions along the beam, we can see that neighbor locations often will display the same time-varying peak frequency. However, the response at two positions apart form each other may be dominated by different frequencies. Hence, the time sharing concept needs to be reformulated from describing a frequency variation valid for the total length of a riser, to consider different zones separately. Based on the observation above, a combined time sharing and space sharing model is proposed. The controlling parameters in this model are an energy based parameter that ranks the participating frequencies, a threshold for a frequency candidate to become active, and the length of the excitation zone for each frequency. All parameters can easily be found for cases of practical interest.Copyright