DYNAMIC POSITIONING SYSTEM NIDHIL MOHAMED A R HAREESH R

Dynamic positioning system is a computer controlled system which is used to maintain the position and heading of a vessel by using her own propellers and thrusters. DPS allows operation at sea where anchoring or mooring is not possible due to water depths,congestion on the sea bottom or other problems.

Dynamic positioning is much used in the offshore oil industry,for example in the North sea, Persian Gulf, Gulf of Mexico, West Africa and Brazil. Now a days there are more than 1000 DP ships

APPLICATIONS DPS is used in Aids to navigation. Cable layer Crane vessels Cruise vessels Diving support vessels Dredgers Drillships

FPSOs Flotels Landing platform docks Mine sweepers Pipe laying ships Platform supply vessels Sea launch Sea based X band Radar

SCOPE The Dynamic positioning system is concerned only about the motion of the ship in the horizontal plane. So the DPS controls only 3 dofs of the system ie i) surge. ii) sway. iii) yaw.

A ship that is to be used for DP requires: To maintain position and heading, first of all the position and heading need to be known. A control computer to calculate the required control actions to maintain position and correct for position errors. Thrust elements to apply forces to the ship as demanded by the control system.

REDUNDANCY Redundancy is the ability to cope with a single failure without loss of position. A single failure can be Thruster failure Generator failure Powerbus failure (when generators are combined on one powerbus) Control computer failure Position reference system failure Reference system failure

CLASSES The Classification Societies have issued rules for Dynamic Positioned Ships described as Class 1, Class 2 and Class 3. Equipment Class 1 has no redundancy. Loss of position may occur in the event of a single fault.

Equipment Class 2 has redundancy so that no single fault in an active system will cause the system to fail. Loss of position should not occur from a single fault of an active component or system such as generators, thruster, switchboards, remote controlled valves etc. But may occur after failure of a static component such as cables, pipes, manual valves etc.

Equipment Class 3 which also has to withstand fire or flood in any one compartment without the system failing. Loss of position should not occur from any single failure including a completely burnt fire sub division or flooded watertight compartment.

AHT HALUL OFFSHORE SPECIFICATION The vessel is fitted with a DPS that can hold the vessel in position in any of the loading conditions and following environmental conditions. Water Depth meters Wind Velocity - 30 Knots. Significant wave height -3.0 m (Frequency of 6 -8 seconds equivalent to a force 6 gale on Beaufort Scale) Current velocity m/sec

SYSTEM REQUIREMENTS The system to have interfacing to a) Gyro compass x 2 nos. b) Wind sensor x 2 nos. c) Motion Reference unit x 2 nos. d) DGPS x 2 nos. e) Laser type (Radius) reference system – x 1 no.

Following operational modes shall be included: Standby mode Joystick Mode Manual/Joystick mode Mixed manual/Auto mode Auto Heading Mode Auto Position Mode Auto pilot Mode

Following interfaces shall be provided. All thrusters. Main Switch board Joystick Power Management system Two Gyro compasses Two Wind sensors Two Motion reference units Two DGPS One Laser reference unit.

D YNAMIC P OSITIONING  Position Reference Systems  Heading Reference Systems  Other Inputs ______________________________________________________________________________

P OSITION R EFERENCE S YSTEMS  Traditional methods used for navigation are not accurate enough for dynamic positioning  Hence the requirement for state of the art Position Measuring Equipment / Position Reference Systems arises ______________________________________________________________________________

S OME EXAMPLES Differential GPS Acoustic systems Riser angle monitoring Light taught wire Laser based PRS Radar based systems Differential absolute relative positioning system Inertial navigation systems ______________________________________________________________________________

D IFFERENTIAL GPS ( DGPS ) An enhancement to Global Positioning System that uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. DGPS is used in aircraft navigation as well  Wide Area Augmentation System  European Geostationary Navigation Overlay Service  Japan's Multi-Functional Satellite Augmentation System  Canada's CDGPS  Commercial systems like VERIPOS, StarFire, OmniSTAR ______________________________________________________________________________

A COUSTIC S YSTEMS Consists of one or more transponders placed on the seabed and a transducer placed in the ship's hull The transducer sends an acoustic signal to the transponder, which is triggered to reply. As the velocity of sound through water is known (preferably a soundprofile is taken regularly), the distance is known. Because there are many elements on the transducer, the direction of the signal from the transponder can be determined. Now the position of the ship relative to the transponder can be calculated. Disadvantages are the vulnerability to noise by thrusters or other acoustic systems. Furthermore, the use is limited in shallow waters because of ray bending that occurs when sound travels through water horizontally. ______________________________________________________________________________

Three types of HPR systems are commonly used  USBL (Ultra Short Base Line) Because the angle to the transponder is measured, a correction needs to be made for the ship's roll and pitch. These are determined by Motion Reference Units. Because of the nature of angle measurement, the accuracy deteriorates with increasing water depth.  LBL (Long Base Line) This consists of an array of at least three transponders. The initial position of the transponders is determined by USBL and/ or by measuring the baselines between the transponders. Once that is done, only the ranges to the transponders need to be measured to determine a relative position. The position should theoretically be located at the intersection of imaginary spheres, one around each transponder, with a radius equal to the time between transmission and reception multiplied by the speed of sound through water. Because angle measurement is not necessary, the accuracy in large water depths is better than USBL.  SBL (Short Base Line) This works with an array of transducers in the ship's hull. These determine their position to a transponder, so a solution is found in the same way as with LBL. As the array is located on the ship, it needs to be corrected for roll and pitch. ______________________________________________________________________________

R ISER A NGLE M ONITORING On drillships, riser angle monitoring can be fed into the DP system It may be an electrical inclinometer or based on USBL (ultra short base line), where a riser angle monitoring transponder is fitted to the riser and a remote inclinometer unit is installed on the Blow Out Preventer (BOP) and interrogated through the ship’s HPR.

L IGHT T AUGHT W IRE The oldest position reference system used for DP is still very accurate in relative shallow water. Horizontal LTW’s are also used when operating close to a structure. ______________________________________________________________________________ A clumpweight is lowered to the seabed. By measuring the amount of wire paid out and the angle of the wire by a gimbal head, the relative position can be calculated. Care should be taken not to let the wire angle become too large to avoid dragging. For deeper water the system is less favorable, as currents will curve the wire. There are however systems that counteract this with a gimbal head on the clumpweight.

L ASER B ASED S YSTEMS A small prism needs to be installed on a nearby structure or ship. Lasers can be used for very accurate positioning Risks are the system locking on other reflecting objects and blocking of the signal. Range depends on the weather, but is typically more than 500 meters Examples – Fanbeam, CyScan ______________________________________________________________________________

R ADAR B ASED S YSTEMS A unit is placed on a nearby structure and aimed at the unit on board the ship The range is several kilometers Advantage is the reliable, all-weather performance Disadvantage is that the unit is rather heavy Examples – Artemis, RADius, RadaScan ______________________________________________________________________________

D IFFERENTIAL A BSOLUTE R ELATIVE P OSITIONING S YSTEM ( DARPS ) Commonly used on shuttle tankers while loading from a FPSO Both will have a GPS receiver As the errors are the same for the both of them, the signal does not need to be corrected The position from the FPSO is transmitted to the shuttle tanker, so a range and bearing can be calculated and fed into the DP system ______________________________________________________________________________

I NERTIAL N AVIGATION S YSTEM An inertial navigation system (INS) is a navigation aid that uses a computer, motion sensors (accelerometers) and rotation sensors (gyroscopes) to continuously calculate via dead reckoning the position, orientation, and velocity (direction and speed of movement) of a moving object without the need for external references. It is used on vehicles such as ships, aircraft, submarines, guided missiles, and spacecraft. Inertial navigation is very useful in combination with GPS (Seapath) and Hydroacoustics (HAIN). ______________________________________________________________________________

H EADING R EFERENCE S YSTEMS Gyrocompasses are normally used to determine heading More advanced methods are: - Ring-Laser gyroscopes - Fibre optic gyroscopes - Seapath, a combination of GPS and inertial sensors. ______________________________________________________________________________

G YROCOMPASS  A gyrocompass is similar to a gyroscope. It is a compass that finds true north by using an (electrically powered) fast-spinning wheel and friction forces in order to exploit the rotation of the Earth.  Gyrocompasses are widely used on ships.  They have two main advantages over magnetic compasses: - They find true north - They are far less susceptible to external magnetic fields Fun Fact  A gyrocompass is essentially a gyroscope, a spinning wheel mounted on gimbals so that the wheel's axis is free to orient itself in any way. Suppose it is spun up to speed with its axis pointing in some direction other than the celestial pole. Because of the law of conservation of angular momentum, such a wheel will maintain its original orientation. Since the Earth rotates, it appears to a stationary observer on Earth that a gyroscope's axis is rotating once every 24 hours. Such a rotating gyroscope cannot be used for navigation. The crucial additional ingredient needed for a gyrocompass is some mechanism that results in applied torque whenever the compass's axis is not pointing north. ______________________________________________________________________________

Cutaway of Anschütz gyrocompass

A Gyro Compass Repeater

O THER I NPUT S ENSORS  Motion Reference Unit (MRU)  Wind Sensor  Draught Sensor ______________________________________________________________________________

M OTION R EFERENCE U NIT Motion Reference Units, Vertical Reference Units, and Vertical Reference Sensors are IMUs An inertial measurement unit, or IMU, is an electronic device that measures and reports on a craft's velocity, orientation, and gravitational forces, using a combination of accelerometers and gyroscopes. IMUs are typically used to maneuver aircraft, including UAVs, among many others, and spacecraft, including shuttles, satellites and landers. The IMU is the main component of inertial guidance systems used in aircraft, spacecraft, and watercraft, including guided missiles. In this capacity, the data collected from the IMU's sensors allows a computer to track a craft's position, using a method known as dead reckoning. ______________________________________________________________________________

W IND S ENSOR Measures Wind Speed Wind Direction Using Anemometer Wind Vane ______________________________________________________________________________

C ONTROL S YSTEMS ______________________________________________________________________________

P OWER & P ROPULSION For maintaining position Azimuth thrusters (L-drive or Z-drive) Azipods Bow thrusters Stern thrusters Water jets Rudders Propellers DP ships are usually at least partially diesel-electric, as this allows a more flexible set-up and is better able to handle the large changes in power demand, typical for DP operations. ______________________________________________________________________________

The End