Dynamic positioning of marine vessels
Dynamic positioning of a vessel 10 connected to the seafloor 14 by a riser 12 utilizes a measurement of riser 12 bottom angle combined with a measurement of vessel 10 velocity, optionally obtained from a Doppler log 16 . These two signals are combined to produce a single position estimate using an algorithm such as a Kalman filter. Using riser bottom angle only would result in an unstable control system, since the bottom angle lags the vessel motion by a considerable amount and the relationship is non-linear. Using the velocity measurement alone would result in a slow drift of position. The combination of the two eliminates the disadvantages of the individual measurements.
1. A method of determining the position of a vessel on the surface of water relative to a reference point below the surface to which the vessel is connected by a riser, the method comprising:
making measurements of the velocity of the vessel to generate one or more velocity measurements;
making measurements of the angle of the riser adjacent to the reference point to generate one or more riser angle measurements;
providing to a filter the one or more velocity measurements and the one or more riser angle measurements as inputs to the filter; and
combining the velocity measurements and the riser angle measurements using the filter to derive an estimate of an actual position of the vessel as an output from the filter.
2. The method according to claim 1 , wherein the velocity measurements and the riser angle measurements are combined using a recursive algorithm that takes as inputs a velocity measurement, a riser angle measurement and an estimate of the position of the vessel at an earlier time.
3. The method according to claim 2 , further comprising subtracting the estimated position of the vessel at the earlier time from an independent measurement of the position of the vessel at the earlier time to derive an error measurement, wherein the error measurement provides a further input to the recursive algorithm.
4. The method according to claim 2 , wherein the recursive algorithm is a Kalman filter.
5. The method according to claim 1 , wherein the velocity measurements are measurements of the velocity of the vessel relative to the water surrounding the vessel.
6. The method according to claim 1 , wherein the velocity measurements are made using a Doppler log.
7. A method of dynamically positioning a vessel on the surface of water relative to a reference point below the surface to which the vessel is connected by a riser, the method comprising:
making measurements of the velocity of the vessel to generate one or more velocity measurements;
making measurements of the angle of the riser adjacent to the reference point to generate one or more riser angle measurements;
providing the one or more velocity measurements and the one or more riser angle measurements to a filter as inputs to the filter;
combining the velocity measurements and the riser angle measurements at the filter to derive an estimate of an actual position of the vessel as an output from the filter; and
controlling one or more thrusters using the output from the filter to dynamically control the positioning of the vessel.
8. The method of dynamically positioning a vessel according to claim 7 , wherein the control signals sent to the thrusters or measurements of the outputs of the thrusters are combined with the velocity measurements and the riser angle measurements to derive the estimate of the position of the vessel.
9. An apparatus for determining the position of a vessel on the surface of water relative to a reference point below the surface to which the vessel is connected by a riser, the apparatus comprising:
a first measurement system for measuring the velocity of the vessel;
second measurement system for measuring the angle of the riser adjacent to the reference point; and
at least one filter for combining the velocity measurements and the riser angle measurements to derive an estimate of an actual position of the vessel.
10. The apparatus according to claim 9 , wherein the first measurements system includes a Doppler log.
11. The apparatus according to claim 10 , wherein the Doppler log is in water lock.
12. The method of dynamically positioning a vessel according to claim 7 , further including:
taking measurements to determine a heading of the vessel; and
controlling one or more thrusters to alter the heading of the vessel based on the output from the filter and the heading.