Figure 1　Locations of the six instrument containers on the Helland-Hansen drilling riser. A seventh container was installed in the drilling vessel, but was not utilized in the analysis (Figure by Robit Technology AS)

　

Figure 2　Modeshapes of riser displacement, calculated by FEM model. The circles show the locations of the instrument units. Units 3, 4 and 6 (from the bottom) contain rotation rate sensors in addition to the accelerometers.

　

Figure 3　Transfer function of riser displacement to accelerometer signal for a 1^st-mode motion, showing the effect of gravity. Solid line represents accelerometer mounted at height L/4 from the bottom. Dashed line is for unit mounted at height 3L/4. Dotted line is true acceleration.

　

Figure 4　Example of noise sensitivity: Sensitivity of estimate of first modal weight to measurement noise on accelerometers 1 and 3 and rotation rate sensor in instrument unit 3. Dashed vertical line indicates expected 1^st natural frequency.

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