Trials of the Dynamic Positioning of a Drill Ship

Abstract

Introduction Let us go rapidly over file sub-assemblies which compose the dynamic positioning system of a drill ship like the Pelican. There are essentially four sub-assemblies: a. a displacement detector of the acoustic position measurement system deducts the horizontal displacement of the ship in relation to its assigned point, from a series of measurements of the length of time of ultrasonic wave transmission between the bottom and surface. b. dual digital computers having the following functions: - pace the start of the subsea acoustic system; - process the data from the acoustic position measurement system to deduct the position displacement; - work out thruster commands to pass on to the propulsion system; - check the performance of the parts of the dynamic positioning system and start off the alarms or switches in case of failure. c. a back-up analog processing unit, using the position displacement system data supplied by a taut-wire inclinometer in order to figure the thrusts to pass on to the propulsion system. d. a unit for auxiliary measurements, allowing the following data to be gathered: - measurements directly involved for calculating, like wind direction and velocity, which are used in the control loop for a wind pre-correction; - corrections of pitch and roll enabling the transmission time measurements to be adjusted to take into account the slope of the plane (in relation to the horizontal plane)where the hydrophones, installed on board, are located to receive the ultrasonic beams for displacement detection.