For analysis of measured VIV response within the Norwegian Deepwater Program (NDP) an estimator for riser displacements has been developed. The estimator uses information from of accelerometers and rotation rate sensors. Accelerometers mounted on a riser are sensitive to gravity due to the riser's angular oscillations. Neglecting this effect and doing double integration to obtain displacement may lead to results that have little similarity to reality, in particular for the low modes of vibration. The estimator overcomes this problem and is also capable of including measurements of rotation rate in a consistent manner. The principle of modal decomposition is used. A number of riser modeshapes are precalculated and the corresponding modal weights are found by leastsquares estimation in the frequency domain. The estimator's sensitivity to sensor noise is studied and it is established that it is the influence of gravity that enables the accelerometers to detect modes oscillating at very low frequencies. The performance of the estimator is illustrated by an example. The effect of neglecting the gravitational "disturbance" is demonstrated.
As a part of the Norwegian Deepwater Programme three drilling risers at sites outside the Norwegian coast were instrumented for measurement of vortex-induced vibrations (VIV). in addition to the measurements on the riser, the current was measured at a number of depths. The purpose of the instrumentation was to provide data for verification of theoretical models for VIV. This paper describes how the measurements of riser motion were processed to yield information that could be used in their verification. This information consisted of typically the lateral displacements of the riser along its length, the participating modes and their frequencies of oscillation. In principle, the same method of analysis was applied to the data from all the three locations, although the instrumentation differed somewhat.
