Dynamic Positioning (DP) Systems PDF

Literatura: Jan Babicz - Encyclopedia Of Ship Technology IMO - Guidelines For Vessels And Units With Dynamic Positioning (DP) Systems IMCA - Guidelines for The Design and Operation of Dynamically Positioned Vessels Encyclopedia Of Ship Technology Dynamic positioning is a capability of a vessel to...

Literatura: Jan Babicz - Encyclopedia Of Ship Technology IMO - Guidelines For Vessels And Units With Dynamic Positioning (DP) Systems IMCA - Guidelines for The Design and Operation of Dynamically Positioned Vessels Encyclopedia Of Ship Technology Dynamic positioning is a capability of a vessel to maintain its position automatically using its propulsion system. Dynamic positioning system – A hydrodynamic system which controls or maintains the position and heading of the vessel by centralized manual control or by automatic response to the variations of the environmental conditions within the specified limits. MSC.1/Circ. 1580 Guidelines for Vessels with Dynamic Positioning Systems Dynamically positioned vessel (DP vessel) means a unit or a vessel which automatically maintains its position and/or heading (fixed location, relative location or predetermined track) by means of thruster force. History The development of DP systems is strongly linked to the development of the oil industry in sea areas. Beginning of the 19th century - the Caspian Sea - the first well in the seabed about 30m from the shore using a wooden pier Further drillings from from piers with lengths up to 400m 1925 - Caspian Sea - the first self-elevating drilling and production platforms of the "Jack up" type. These platforms could carry out drilling and mining operations on waters with depths up to approx. 60 m. Historia Vessels, stabilizing their position by appropriate selection of tension forces on anchor ropes (chains) attached to anchors, which were previously appropriately arranged and dropped. This solution enabled drilling and exploitation of wells at depths up to around 600m.  History 1960 – California USA – CUSS (Continental, Union, Shell & Superior oil consortium) Core drilling Goal 180m Water depth up to 3500m Equipment: Four azimuth thrusters Manual control Visual observations Sonar tracking Historia 1961 – USA – launching the first drilling vessel with a fully functional (automatic) dynamic positioning system. EUREKA ship built for Shell. Drilling at depths up to 1300m (wave up to 6m wind up to 21 m / s) Historia 1963 – France - launching two DP ships (Salvor and Terebel) intended for laying pipelines on the seabed and securing underwater works. History 1964 – USA – launching of DP vessel Caldrill (Caldrill Offshore Company). Drills up to 2000m. History 1971 – United Kingdom – first DP systems constructed by British GEC Electrical Projects Ltd. 1974 – United Kingdom - conversion of the commercial ship Wimpey Sealab into a drilling vessel for exploring hard coal deposits 1977 – United Kingdom - launching the first semi- submersible drilling rig Uncle John Historia 1975 – Norway – first research on DP systems at the request of Stolt Nielsen by Kongsberg Vapenfabrikk (KV) 1977 – Norway – launching (Seaway Eagle) of the first vessel with the Norwegian DP system. The Kongsberg systems were highly appreciated, which made the company one of the largest producers of DP systems. According to an advertising folder from 2012, it covered over 75% of the world market of DP systems Applications of DP systems Diving Support Vessels Applications of DP systems Pipelay Vessels Applications of DP systems ROV Support Vessels ROV - remotely operated underwater vehicle Applications of DP systems Crane Vessels Applications of DP systems Float-over Vessels Applications of DP systems Accommodation Vessels Applications of DP systems Drilling Vessels Applications of DP systems FPSO Vessels floating production storage and offloading Applications of DP systems Shuttle Tankers Applications of DP systems Trenching Vessels Applications of DP systems Cable Lay/Repair Vessels Applications of DP systems Jack-up Vessels Applications of DP systems Offshore Supply Vessels Applications of DP systems Anchor Handling Vessels/Tug Applications of DP systems Well Stimulation Vessels Applications of DP systems Rock Placement Vessels Applications of DP systems Dredging Vessels Applications of DP systems Other applications Classes of DP systems A DP system consists of components and systems acting together to achieve sufficiently reliable position keeping capability. The necessary redundancy level for components and systems is determined by the consequence of a loss of position and/or heading keeping capability. To achieve this philosophy the requirements have been grouped into three equipment classes. For each equipment class, the associated worst-case failure should be defined as in below. The equipment class of the vessel required for a particular operation should be agreed between the company and the customer based on a risk analysis of the consequence of a loss of position and/or heading. Otherwise, the Administration or coastal State may decide the equipment class for the particular operation. For equipment class 1, a loss of position and/or heading may occur in the event of a single fault. For equipment class 2, a loss of position and/or heading will not occur in the event of a single fault in any active component or system. Common static components may be accepted in systems which will not immediately affect position keeping capabilities upon failure (e.g. ventilation and seawater systems not directly cooling running machinery). Normally such static components will not be considered to fail where adequate protection from damage is demonstrated to the satisfaction of the Administration. Single failure criteria include, but are not limited to: any active component or system (generators, thrusters, switchboards, communication networks, remote-controlled valves, etc.); and any normally static component (cables, pipes, manual valves, etc.) that may immediately affect position keeping capabilities upon failure or is not properly documented with respect to protection. For equipment class 3, a loss of position and/or heading will not occur in the event of a single fault or failure. A single failure includes: items listed above for class 2, and any normally static component assumed to fail; all components in any one watertight compartment, from fire or flooding; and all components in any one fire sub-division, from fire or flooding For equipment classes 2 and 3, a single inadvertent act should be considered as a single fault if such an act is reasonably probable. Degrees of freedom An example of six degree of freedom movement is the motion of a ship at sea. It is described as: Translations: Moving forward and backward on the X-axis. (Surging) Moving left and right on the Y-axis. (Swaying) Moving up and down on the Z-axis. (Heaving) Rotations: Tilting side to side on the X-axis. (Rolling) Tilting forward and backward on the Y-axis. (Pitching) Turning left and right on the Z-axis. (Yawing) Degrees of freedom Main Principles of Operation A seagoing vessel is subjected to forces from wind, waves and currents as well as from forces generated by the propulsion system. The vessel's response to these forces, i.e. its changes in position, heading and speed, is measured by the position- reference systems, the gyrocompass and the vertical reference sensors. Wind speed and direction are measured by the wind sensors. The system calculates the deviation between the measured (actual) position of the vessel and the required position, and then calculates the forces that the thrusters must produce in order to make the deviation as small as possible. In addition, the system calculates the forces of wind, wave and water current which act upon the vessel and the thrust required to counteract them. Normally the system controls the vessel's motion in three horizontal degrees of freedom - surge, sway and yaw Main Principles of Operation Traditional DP systems are based on the mathematical model which requaries following data: wind speed and direction thruster/propeller pitch/rpm and direction sea current and waves Main Principles of Operation The model is a mathematical description of how the vessel reacts or moves as a function of the forces acting upon it. The model is a hydrodynamic description, i.e. it involves the vessel's characteristics such as mass and drag. The design criterion for the model is an as accurate as possible description of the vessel's motions and reaction to any external forces. The mathematical model is affected by the same forces as the vessel itself. Wind forces are calculated as a function of measured wind speed and direction, while thruster forces are calculated as a function of thruster/propeller pitch/rpm and direction. The system incorporates algorithms for the estimation of sea current and waves, and the forces caused by these. Main Principles of Operation The main outputs from the mathematical model are filtered estimates of the vessel's heading, position and speed in each of the three degrees of freedom - surge, sway and yaw. The mathematical model itself is never a 100% accurate representation of the real vessel. However, by using the Kalman filtering technique, the model can be continuously corrected. The vessel's heading and position are measured using the gyrocompasses and position-reference systems, and are used as the input data to the DP system. This data is compared to the predicted or estimated data produced by the mathematical model, and the differences are calculated. These differences are then used to update the mathematical model to the actual situation. Main Principles of Operation initial position, heading and speed wanted position, heading and speed initial position, heading and speed predicted position, measured position, heading and speed heading and speed initial position, measured position, heading and speed heading and speed predicted position, heading and speed measured position, heading and speed initial position, heading and speed predicted position, heading and speed DP construction The following components can be distinguished in the DP system : Power supply system Thruster system Reference systems Sensors Control system Steering console System operator DP construction - Power supply system All subsystems of the DP system use this system. This system must therefore be characterized by high flexibility of work, due to the possibility of sudden changes in the power demand caused by, for example, irregular operation of thruster system. The control system, control console, displays and alarm systems and reference systems of the DP system, even in the event of failure of the main power system, must be able to operate for at least 30 minutes. DP construction - Thruster system Propeller – one or more, in most cases Controllable Pitch Propeller (CPP) DP construction - Thruster system Rudder – usually modified steering gear, eg Becker rudder, Schilling rudder, Kort nozzle DP construction - Thruster system Manoeuvring thrusters – transversal propulsion device built into, or mounted to, either the bow or stern, of a ship DP construction - Thruster system Azimuth thrusters DP construction - Thruster system Azipod thrusters DP construction - Thruster system Voit-Schneider thrusters DP construction - Reference systems The reference systems included in the DP system must be characterized by high accuracy, high reliability and continuity of work. The number and type of reference systems used depends on: The level of risk associated with the operation being performed Required level of redundancy Availability of reference systems The effects of losing one or more reference systems DP construction - Reference systems Traditional navigation systems (GPS, Glonass) do not meet the requirements for reference systems used in DP systems The most commonly used systems are: Mechanical taut wire Microwave Artemis, RADius, RadaScan, Miniranger, Trispondeur Laser Fanbeam, CyScan differential satellite hydroacoustic DP construction – Sensors A system of sensors and other measuring devices used to determine the ship's motion parameters, its course, estimation of external disturbances and measurement of other parameters or factors required in the dynamic positioning process. DP construction – Sensors Heading sensors - gyrocompasses Depending on the required level of redundancy, two, three or more gyrocompasses are the main source of heading DP construction – Sensors Heading sensors – satellite compasses Satellite compasses can be used as a additional source of heading DP construction – Sensors Heading sensors – satellite compasses Satellite compasses can be used as a additional source of heading DP construction – Sensors Heading sensors – satellite compasses DP construction - Sensors Attitude sensors Movements in the vertical plane (roll, pitch and heave) are not compensated, knowledge of their current values is necessary in the stabilization proces. Possible types of motion sensors: Pendulum devices Fluid Stabilized Devices VRU VRU / GPS Aided Inertial sensors DP construction – Attitude sensors Pendulum devices, or inclinometers, are normally applied to DP systems as a solid state unit with two sensors mounted fore/aft and port/starboard. By measuring the component of gravity in each of these axes we can derive roll and pitch. Although solid state the effect under marine dynamics can be similar to that experienced by a mechanical pendulum, i.e. follow-up errors, low accuracy and inability to cope with short term accelerations. Advantages: Low cost Good performance in static conditions Disadvantages: Low Performance Poor performance in Dynamics Low update rate and Latency DP construction – Attitude sensors Fluid Stabilized Devices The next stage is to put the pendulum type of device into a dampened environment to counter the vessel dynamics. Such units use a pick up coil that floats in a oil bath to sense rotation about primary coils that are fixed in the roll and pitch axes. Although fairly accurate and reliable, such units have disadvantages in terms of size and handling restrictions and cost of routine maintenance. Advantages: Relatively accurate and reliable Tried and trusted Disadvantages: Size and Handling Life Cycle Costs Installation difficulties Latency DP construction – Attitude sensors VRU - Vertical Reference Units Advances in and the availability of solid state inertial sensors heralded the development of strapdown motion sensors. Such sensors use an orthogonal array of 3 accelerometers and 3 angular rate sensors ( gyros ) and deploy a vertical reference algorithm to compute Roll and Pitch. Advantages: Good accuracy for GPS and Acoustic Stabilization Range of performance / price sensors available Relatively low cost Disadvantages: No heading information DP construction – Attitude sensors VRU / GPS Advantages: Heading and Position Information Good Accuracy Disadvantages: Relatively High Cost Heading and Position GPS dependent DP construction – Sensors Anemometers: Mechanical anemometers (cup, vane) Ultrasonic DP construction – Sensors Anemometers: W I ND W I ND W I NDS EN SO R EXP O SE D F OR CEOF WI ND E XA GGER A TE D A TWI NDS EN SOR V ESS ELH U LL AN D S T RU CT U RE P L AT FO RM S U PER STR U CT UR E I NWI NDS HA D OW W IN DS EN SO RE XP OS E D T OFU L LF OR CE OF WI ND V E SS EL INL EE OF P L AT FO RM E V ER YGU ST CA U SE S VE SS E LT ODRI VE T OW AR DPL A TFORM A N DLO S EHE AD ING VE SS E L E X CU RS I ON DP construction – Sensors Anemometers: D OWN DR AF TF ROMHE L IC OP TER R OTO RB L AD ESMAYC A USE W I N D S EN SO RT O I NPU TF AL SE H I GHV AL UE ST HUSDE S TAB I LI SIN G T HED PC AP AB I LI TY DP construction – Sensors Anemometers: DP construction – Sensors Anemometers: W I NDS E NS OR L OC AT I ON WI NDS ENS ORSL OC ATE D I NPOS IT I ONS1A ND2 WI LLG IV EIN PUT 4 DI STO RT EDBY 3 T U RB U LEN CE F R OM ST RUC TUR E L O CA T I ONS3AN D4 AREB ET TER BUT I NPUT M D A O Y W N E N D E D U T E O T O BE S A L T C I T A U L E D D E 2 EXAG GE RAT I NGT HE 1 WI NDST REN GTH DP construction – Sensors Waves sensors - Hydrometeorological buoys DP construction – Sensors Current sensors – logs DP construction – Sensors Draft sensors Ships’ speed sensors – main source - reference (positioning) systems, additional supporting sensors (logs) Depth sensors Additional sensors (loading) DP construction - Control system The first DP systems used analogue techniques, digital techniques have been used since 1968 and microprocessors since 1980.  Praxis DP construction Steering console: BridgeMate Kongsberg DP construction Reference systems Operator Heading sensors Steering console Power system Attitude sensors Control system External conditions Thruster system sensors