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