Dynamic Positionning systems allow a vessel to remain at a fixed point independent
of the influences of the environment. This is achieved by a system of reference points,
sensors and propulsive power units.
The reference points determine the vessels position relative to a datum, they do not necessarily determine the vessels exact location on the earths surface- only DGPS attempts to do this.
Sensors monitor the environmental effects on the vessel, such as the effects of swell and wind and use this to error compensate the reference systems.
Propulsive power units take the form of the main propulsion unit, thrusters and steering gear system ( in conjunction with propulsive unit). These units are mounted for and aft and may be either fixed or steerable.
The number of reference systems, sensors and propulsion units is determined by the required duties of the vessel
Types of reference systems
This is a non exhaustive list of the types of reference systems available;
The wire passes through a Gimball head which is free to move in X and Y axis up to the mechanical limitations of the assembly. The angle the head is at relative to the vertical for the X axis and horizontal for the Y axis is measured by potentiometers and sent to the DP computer. The wire length is measured by a line out counter and sent to the DP computer. With the weight on the bottom contant tension is placed on the wire keeping it taut, this is achieved by having a constant rpm electric motor coupled to the wire drum via an electric clutch. The field current on the clutch determines the degree of coupling and thereby the degree of torque ( note that this torque if far to small to lift the weight and a seperate hydraulic motor is provided).
As the vessel moves the angle between the head and weight as well as the wire length will alter. A calculation is made by the DP computer which gives the vessels relative position to the weight. Vessel movement through wave action is measured by the accelerometers and factored in.
Shown is a system which is deployed from the ships side, a limitation is placed on the vessels movement towards the weight when the vessel is 'walked' ( say moving into position or following an ROV). A second system would be mounted on the opposite side. Alternately a single system may be fitted operating through a moonpool in the centre of the vessel.
The size of the weight ( and corresponding wire diameter) is determined by the depth that the vessel operates and the operating conditions the vessel works in. Typically they would be not less than 350Kg
Described is a vertical taut wire system. Also available are horizontal taut wires which give the same degree of reliability without the need for the clump weight. The disadvantage is that they must be manually tethered.
This is an auto tracking system whereby a scanning head fires a laser beam through one lens and receives a reflecion of it through a second
The targets must be equiped with reflectors, reflective white tpae may be used but the range is very limited. Greater distance is achieved prisms. 360' coverage is achieved by mounting these prisms on a tube.
Position reference is by the laser ( this is a class one low power unit similar to that found in CD players) scanning for the target. The bearing and range of the target is found and by calculation using error correction from the vertical reference units the movement between the two points is known.
The GPS signal from a satellite does not give the degree of accuracy required for a vessel positioning system. To improve accuracy differential GPS is used
A satellite signal is received by both the vessel and by an installation whose position is precisely known. Any error in the signal from the satellite is converted into and error correction signal which is then broadcast. This signal is received by the vessel, a calculation is made and a more precise position now known.
This system is inherently risky for vessels working in critical positions and it is often required that GPS be not used as both of the minimum two sources required. The gps signal is sometimes intermittent as is the error compensation signal, they are effected by reflection from nearby installations and by ionospheric variation. Multiple stations are used to improve accuracy
This consists of two stations. One unit mounted on a fixed installation, a second
unit mounted on the vessel. These units consist of a rotateing antenna. When initialised
both units rotate untill they point at each other. Manual control is available to
speed this up. Once they have acquired each other the antenna horns point continuously
at each other
The fixed unit now knows the relative postion of the vessel to itself by measurement of the angle of the 'antenna relative to north. The microwave connection gives the relative distance of the vessel. This information is given to the vessel. As the vessel moves there will be alterations in the distance and the bearing between the units. An error correction is made for changes in the ships heading measured by the GYRO.
Hydro Acoustic (HPR)
This system consists of an acoustic transponder and a sensor mounted on the vessel. The transponder may be fixed to the sea bed or installation required , be lowered to the sea bed as required from the vessel or be attached to a moving unit such as a diver or ROV to allow the vessel to 'watch' the know their location.
The sensor may be fixed so that it looks in one direction only, or tracking where it can move to locate several targets. To prevent interference from the vessels hull the sensors are normally mounted on a long pole which may be lowered through an isolation valve . The installation allows access to the sensor head for maintenance
The system is subject to variations in water temperature and salinity which effects the sound velocity. At water stratification layers a degree of refraction occurs to the wave.
Wind- This measures both the wind speed and wind direction. A calculation based on parameters such as vessel topside area is made on the effect of the vessel. This signal is fed forward to the DP Computer so that action may be taken before the vessel moves off station
Gyro-Measures the vessels heading giving error correction for such reference systems as artemis.
Vertical reference Unit-used to measure the vessels pitch and roll and used as error corrections on such reference systems as taut wire, artemis etc.
Draught-Used on vessels such as heavy lift where ships operations may significantly effect the draught of the vessel
Draghead Force-Used on dredges where the vessels forward speed is governed by the loading of the draghead
There is generally no current measurement, instead the current is calculated by the DP computer which looks for a permanent off set in the thrust required to keep the vessel in position. Depending on the class of vessels all sensors may be duplicated