Dynamic positioning of marine vessels

Abstract

Dynamic positioning of a vessel 10 connected to the seafloor 14 by a riser 12 utilises 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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of the six axes of motion of a vessel;

FIG. 2 shows schematically the measurement of riser angle used in the invention; and

FIG. 3 is a block diagram of a dynamic positioning control scheme for a single axis of motion of a vessel, in accordance with the invention.

Claims

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;making measurements of the angle of the riser adjacent to the reference point; andcombining the velocity measurements and the riser angle measurements to derive an estimate of the position of the vessel.

2. A 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. A 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. A method according to claim 2, wherein the recursive algorithm is a Kalman filter.

5. A 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. A 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: using a method according to claim 6 to determine the position of the vessel;taking measurements to determine the heading of the vessel; andcontrolling thrusters to alter the position or heading of the vessel.

8. A 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. 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: means for measuring the velocity of the vessel;means for measuring the angle of the riser adjacent to the reference point; andmeans for combining the velocity measurements and the riser angle measurements to derive an estimate of the position of the vessel.

10. Apparatus according to claim 9, wherein the means for measuring the velocity of the vessel is a Doppler log.

11. Apparatus according to claim 10, wherein the Doppler log is in water lock.