Many times the folks on board a jack-up, or liftboat, are concerned about the swinging of the bubbles in the glass tubes of their heel and trim inclinometers. They seem to indicate that the rig is rocking. Invariably it is only swaying and the hull is staying level. However, the horizontal accelerations are causing the inclinometer bubbles to move.
The typical -5º to +5º glass inclinometer on a jack-up can be used to estimate the sway motion amplitudes of the jack-up hull.
The average period of the sway is determined (for example by measuring the time for 10 swings of the bubble and dividing by 10). Simultaneously the average amplitude of the bubble movement is noted.
A chart is consulted and the amplitude of the sway is found, based on the amplitude of the bubble travel in degrees and the average period of the sway in seconds.
Chart showing sway amplitude corresponding to inclinometer bubble amplitude at 3.5 seconds average sway period
The inclinometer response is not linear and usually a calibration of the inclinometer is required. The video shows an inclinometer being swung at constant period with a range of amplitudes. From the observed data a calibration curve is constructed. Semi-empirical theoretical formulae have also been developed by STA, as described elsewhere. Note that a series of charts is needed to cover a range of average sway periods (and operating leg lengths extended).
The inclinometer sway charts were first introduced on the Maleo Producer in 2007.