Glossary of terms relating to Dynamic Positioning

This Glossary is a reference to the terms that may have been used in the examination system. The intention of this document is to align Training Centres and guide them to use the same definitions that may be used in the online examination system.

Age of Data (AoD)

Age of Data. The latency of the corrections within the DGPS system. The lower the data update-rate the greater the AoD. For Position-reference data to be reliable and relevant it must have a low AoD.

Artemis

A microwave position reference system using one shore-based and one shipboard transceiver, giving range and bearing information.

Astern operation (c.p. propeller)

A c.p. (controllable-pitch) propeller is less efficient when operating in the astern mode. Figures quoted by propeller manufacturers state that thrust output is typically between 50% and 70% of corresponding ahead values.

Auto heading select

A DP function in which the system determines the heading of the vessel. Sometimes referred to as “Weathervane” or “Minimum Power Heading” or “Optimum Heading” this function calculates the heading at which the vessel may be kept in position with zero Sway force.

Auto Position

The primary operational function of Dynamic Positioning. In “Auto Position” mode the vessel is enabled to maintain position and heading automatically.

Available power

In a diesel-electric power plant, “Available Power” is the amount of reserve power immediately available without starting additional generators. Sometimes referred to as “Spinning Reserve”.

Azimuth barring

Otherwise referred to as “Prohibited Azimuth Zone”, this is a function which prevents an azimuth thruster from assuming an azimuth configuration in which exhaust water stream is discharged directly into other thrusters. Other functions of this facility include the prevention of the discharged water stream from affecting operational elements such as a diving bell or ROV. This function may be Fixed, or Operator-selectable; if the latter the perator may have the capability of setting the barred azimuth limits.

Azimuth thrusters

Any type of thruster unit that has the capability of delivering thrust in any operator- or system-selected azimuth. In DP mode azimuth thrusters will normally have a full 360° azimuth capability. Certain modes will restrict this capability or cause thrusters azimuth to be fixed.

Azipod unit / thruster

A type of azimuth thruster in which the drive motor is contained within the pod unit, with normally a fixed-pitch propeller usually, but not invariably operating in the “pulling” mode.

Blackout prevention

An important function of a Power Management system. When Available Power is dwindling, the Power Management system will automatically take steps to prevent total blackout and total loss of control of the vessel.

Bus Tie breaker

A breaker allowing the connection or isolation of individual sections of switchboard.

Capability plot / Capability diagram

A Capability diagram is an approximate indication of the positional and heading capability of the vessel under a variety of stated conditions. A more realistic indication is given by the online capability plot, in which the data is recalculated at intervals, taking into account the conditions obtaining at the time of calculation.

Catastrophic Failure

A system failure which results in the loss of the ability to maintain position and/or heading.

Center of Rotation

The reference spot within the vessel about which the vessel will rotate when in full automatic positioning mode. A vessel DP system may be configured with more than one Centre of Rotation, operator-selectable. Vessel positional deviations or errors are given in terms of distance of the Centre of Rotation from the Set Point, or desired, position.

Checklist

A pre-prepared list of tasks and checks to be completed prior to commencing an operation or an individual phase of an operation.

Common Mode Failure

A situation in which multple elements within a redundant system fail simultaneously due to a common cause or factor. The term is also used to identify the possibility of this event.

Consequence Analysis

A DP software function required in vessels of Equipment Classes 2 and 3 which continually verifies that the vessel will remain in position subsequent to the event identified as “worst-case failure”.

Controllable pitch (c.p.) propeller

Any propeller in which the magnitude of thrust is wholly or partly effected by adjustment of the pitch angle of the blades.

Cyscan

A laser-based position reference system manufactured by Guidance Navigation.

DARPS

Differential Absolute and Relative Positioning System, a DGPS-related PRS used by shuttle tankers and FPSO

Dead Reckoning mode

A mode of positioning in which the DP system continues to function after loss of all position and / or heading reference, modelling the vessel’s position. The system will default to this if such reference is lost. Quality of positioning will deteriorate with time.

DGPS

Differential GPS (see GPS). A position reference system based upon the Global Positioning System with enhanced accuracy provided from reference stations located at fixed locations ashore or on platform installations. Most differential corrections are provided by means of subscription services.

Diesel-electric

A configuration of vessel power and propulsion in which diesel prime movers drive electrical alternators. Power generated is used to power most vessel functions including thrusters and propulsion units.

Diesel-electric hybrid

A configuration of vessel power and propulsion in which diesel prime movers are used to drive alternators. Power generated is used to drive thrusters, while other thrusters and propulsion units are driven by direct-coupled (or geared) diesel engines.

DP console

Sometimes referred to as the HMI (Human-Machine Interface) also the “DP Desk” or “DP Operator Station”. The collection of panels and screens allowing the DPO full control of all DP functions, and displaying all information as to the status of the vessel and system.

DP Control system

A collective term including all processors/computers within the DP system, also including the DP console and any remotely-located or backup units.

DP Current

A term loosely and erroneously used to describe the sea current derived from the mathematical model. It actually consists of a vector which represents sea current and all other unknown forces.

DP desk

See DP Console

DP Operator

The designated watchkeeping officer responsible for managing the Dynamic Positioning of the vessel.

DP Operator station

See DP Console

DP System

The DP system consists of 7 hardware elements plus the DP Operator. The 7 elements are: The DP Console, DP computer / processors, Position Reference systems, Heading reference systems, Environment reference systems, Power system and Propulsion systems (including thrusters).

DQI

DGPS Quality Indicator.

Draught sensor

Any device which remotely monitors the draught of the vessel and transmits this information to a remote location. DP vessels which undergo large changes in draught and mass whilst undertaking DP operations may have feedback of draught information enabled into the DP system.

Dual frequency

A GPS facility which uses both transmitted channels, L1 and the encrypted L2. A dual-frequency system has increased reliability at times of high Iononspheric interference resulting from sunspot activity.

Duplex

A term often used to describe a DP system having two computers to provide redundancy.

Dynamic Positioning

The ability of a vessel to maintain her position and heading automatically, exclusively by means of active thrust. The shipboard system which enables this ability.

Emergency Exit Strategy

See “Escape Route”. A pre-planned route away from hazards within any working area or worksite.

Environmental reference systems

Sensors which provide feedback data on wind speed and direction, also vessel attitude and motions.

Equipment Class

A measure of the level of overall redundancy within the DP capability of the vessel. Described in IMO MSC Circ 645, three Equipment Classes are described, Classes 1, 2 and 3; the higher the number the greater the level of redundancy.

Escape route

An essential component of any DP operation. An identified and defined route away from any hazards or nearby structures.

Exclusion zone (platform)

A defined area around an offshore structure or complex within which vessel and other operations are within the jurisdiction of the Offshore Installation Manager (OIM). Commonly this exclusion zone is set at 500m but may vary.

External force

Any force, measured or unknown, which affects the positioning or heading control of the vessel. Typical external forces include those caused by wind, waves, current, hawser tension, cable tension.

False eastings

Within the UTM (Universal Transverse Mercator) projection and co-ordinate system, an artificial value of 500,000m added to easting values to allow for all eastings within a zone to have positive values.

Fanbeam

An optical Laser radar position reference system.

Feedback

Any data measured by sensor which is monitored by the DP system and acted upon within its computations.

Fixed Azimuth mode

A function relating to Azimuth thrusters or Azipod units. In Fixed Azimuth mode these units individually or wholly take up a pre-determined azimuth configuration.

Fixed pitch (f.p.) propeller

A propeller in which the pitch is non-controllable, the magnitude and direction of thrust from a f.p. propeller is controlled purely by varying the rpm and rotational direction.

FMCW

Frequency-modulated continuous wave. A technique used in short-range radar systems.

FMEA

Failure Mode Effects Analysis. A document containing a detailed description of the vessels DP and associated systems and the results of failures within those systems. The FMEA document is provided subsequent to a thorough audit, and is often a requirement of the vessel’s classification society notation for DP Equipment Class. The FMEA document must be revalidated at regular intervals.

Follow-Sub/Follow Target

A function often installed within the DP system enabling a vessel to maintain position relative to a moving target, usually an underwater vehicle.

Gain

A factor within the DP system which determines the magnitude of power applied in response to position or heading deviation. Normally an operator-selectable function, Low Gain allows positioning using less power and thrust, economising in fuel consumption, while a High gain setting will use greater power and thrust values which may result in more accurate positioning.

Gallileo

The European satellite navigation system.

Glonass

The Russian global navigation satellite system, the equivalent to the American GPS.

GNSS

Global Navigation Satellite System. Any satellite-based navigation system having global coverage. Examples of GNSS include the American GPS, the Russian Glonass, and the European Galileo system. These systems may be used in combination, and this combination may also be referred to as GNSS.

Guidelines for vessels with Dynamic Positioning Systems (IMO MSC C 645)

A document published by the International Maritime Organisation which details the requirements for the provision of DP and associated systems, and details the levels of redundancy within Equipmnt Class.

Gyro compass

A mechanical device providing directional and heading data independent of magnetic field.

Hawser tension feedback

A facility installed in vessels such as cable-lay ships, and offtake tankers in which hawser or cable tension is monitored and provides an external force feedback to the DP and other systems.

HDOP

Horizontal Dilution of Precision, a measure of accuracy in the GPS and DGPS systems.

Heading priority

A facility within the positioning algorithm of a vessel DP system allowing heading deviation to be given greater weighting than vessel position. Thus, if position and heading excursions occur simultaneously, the system will prioritise the reduction of heading deviation. In some systems “heading priority” is an operator-selectable facility.

Heave

Vertical bodily movement of the vessel. One of the six freedoms of movement of a vessel.

HPR

Hydroacoustic Position Reference.

Hub

Within the DGPS, the master reference station within a network of reference stations.

IMO

The International Maritime Organisation.

IMO MSC Circ 645

A document entitled “Guidelines for Vessels with Dynamic Positioning Systems”, published by the Maritime Safety Committee of the International Maritime Organisation. The document outlines standards for dynamic positioning systems and levels of redundancy.

Independent Joystick control

A control facility in a DP-capable vessel fully independent of the DP system, allowing multiple thrusters to be controlled manually from a single joystick.

INS

Inertial navigation system.

Ionospheric delay

Additional satellite signal time delay resulting from refraction at ionospheric altitudes. One of the factors reducing the accuracy of GPS data.

Joystick mode

A facility within the DP system whereby the vessel may be controlled in position and heading through a single joystick. Sometimes referred to as “Manual” DP mode.

Kalman filter

An algorithm which, in a DP system is utilised to generate an optimum estimate of position from incoming data signals, filtering noisy and intermittent data, and applying.

Latency

See “Age of Data”

LBL

Long Baseline Hydroacoustic Position Reference. A system incorporating a single hull-mounted interrogating transducer obtaining range data from a calibrated array of sea-floor transponders.

LUSBL

The Sonardyne Long and Ultrashort Baseline HPR system.

Manual DP

See “Joystick mode”

Mathematical model

A software algorithm containing all vessel positioning characteristics.

Median Test

A quality test continually applied to incoming positional data when three or more position reference systems are being monitored. The median test enables the rejection of a drifting PRS by means of “voting”.

Minimum Power Heading

See “Auto Heading select”.

Mobile transponder

An acoustic device which might be located on an ROV or any other moving vehicle. Normally used to track the vehicle, but may have an input to such DP functions as Target Follow, or Follow-Sub.

Model Control

Sometimes referred to as the “DR mode”, or “Dead Reckoning” mode, the ability of a DP system to continue to position the vessel subsequent to the loss of all position reference. The quality of the positioning will deteriorate with time subsequent to PRS loss. A similar facility applies to heading control in respect to loss of heading data feedback.

Motion Reference Unit

Motion reference unit or MRU is a device providing data on vessel attitude, motions and accelerations.

Multipath

A term related to position-reference systems, where the signal path is deflected or reflected by structure, resulting in erroneous data determination . Reception of Multipath data is particularly problematic in systems using a signal-ranging technique such as GPS, or USBL underwater acoustics.

Operational radius

Sometimes referred to as “Reaction Radius”, a factor within a “Follow Target” or “Follow Sub” mode.

Pipe tension feedback

A facility often installed in pipelay vessels.

Pitch

Vessel movement, a rotation about an athwartships axis. One of the six freedoms of movement; one which is monitored but not controlled.

Position Reference systems

Any of the numerous navigational systems providing positional feedback for use within the DP system.

Power Management

A computerised system designed to ensure continuous reliable power supplies to essential users. Typically a Power Management System (PMS) will monitor loads and available power, providing load shedding function, and blackout prevention.

Pre-DP checklist

A checklist intended to be completed immediately prior to transferring the vessel from conventional navigation to DP control. There may be separate Pre-DP checklists for bridge, and Machinery Control Room.

Pre-operational checklist

A checklist intended to be completed once the vessel is established under DP control, before commencing her operational tasks.

Prohibited azimuth zone (thruster)

See “Azimuth barring”

Propeller

Any propulsional device (other than a jet thruster) intended to provide thrust. Many types of propeller are in use, some incorporated into thruster units.

Push-pull mode

A propulsional DP mode relating to a twin-screw vessel. One propeller runs continually ahead, the other astern. The required surge force is obtained by varying the rpm and/or pitch values of the two units.

Radascan

A short-range position reference system using FMCW (Frequency modulated continuous wave) radar techniques to obtain position information.

RADius

A short-range position reference system using FMCW (Frequency modulated continuous wave) radar techniques to obtain position information. The active unit is an Interrogator located aboard the vessel, communicating with transponders typically located on platform structures.

Reaction radius/Operational radius

A term used by some DP system manufacturers to describe the radius of a circle defining the distance a mobile target may move without triggering a vessel repositioning. Used in the “Follow-Sub” or “Follow Target” modes of operation.

Redundancy

In a DP system, the term “Redundancy” means the ability to withstand the loss or failure of any component within the DP system without loss of position and/or heading control.

Reference station

Within the DGPS position reference system, reference stations located at known locations ashore provide differential correction data to user vessels within range.

Responder

A form of transponder where the interrogating signal is conducted by hard-wire rather than through water.

Riser Angle mode

The mode by which DP control is exercised in deep water drilling applications.

Riser Angle sensor

A sensor in use in deep water drillships. Installed at the wellhead it monitors the directional differential angle between the wellhead structure and the adjacent lower section of the riser. This data is of vital importance to the positioning of the drillship.

Roll

Vessel angular rotation about a longitudinal axis. One of the six freedoms of movement of a vessel. Not controlled by DP but monitored.

ROV

Remotely Operated Vehicle, usually an unmanned submersible.

SBL

Short Baseline HPR.

Sea current

In reality, sea current is the horizontal movement of water (set, or direction, and rate) experienced by the vessel. Since this value cannot be measured directly from the vessel, the DP system will calculate a value from mathematical modelling. The displayed value of “Sea Current” may contain errors and cannot be relied upon. Some DP system manufacturers do not refer to this residual value as “Sea Current”.

Sea force

The term adopted by some DP system manufacturers to describe the residual vector sometimes referred to as “Sea Current”.

Set-point

The Desired Value of any controlled variable. i.e. Set Point heading, and Set Point position.

Settling time

The amount of time necessary for a DP system to obtain optimum Position and Heading keeping performance. This amount of time is related to the speed at which the mathematical model builds.

Simplex

A non-redundant DP system, single computer.

Single point failure

Any component within a DP system, the failure of which would cause a loss of position and/or heading keeping capability. Essentially, a Single Point Failure is a failure of the system redundancy.

SSBL

See USBL.

STCW

An IMO (International Maritime Organisation) protocol on the Standards of Training, Competence and Watchkeeping for those taking charge of ships.

Surge

Vessel movement in the fore-and-aft direction. One of the six freedoms of movement, one which is controlled by the DP system.

Sway

Vessel movement in the transverse direction. One of the six freedoms of movement, one which is controlled by the DP system.

Taut Wire

A position reference system consisting of sensors detecting the length and angles of a tensioned wire leading to a weight on the sea bed.

Thrust allocation/Thruster Allocation

A term used to describe the algorithm which determines individual thruster and propeller outputs from an overall positioning and heading vector demand.

Thruster

A term used to describe any propulsion unit other than a vessel’s main propellers.

Thruster Bias

A function which, when selected by the DP Operator, enables two thrusters to work in opposition. This may be enabled to prevent excessive azimuth hunting in calm conditions, or to artificially increase power loads.

Thruster Exclusion Zone

A term used to describe an azimuth sector, or range of azimuth sectors in which thrust output from an azimuth thrusters is prohibited or limited. Various terms may be used to describe this function, e.g. “Azimuth Barring”, “Prohibited Azimuth Zone”. The limits of the sector(s) may be selected and / or adjusted by the DP Operator, or they may be permanently applied. Also, there may be a control function to enable/disable this function. See also “Azimuth Barring”.

Touchdown point

When laying a cable or pipe, the touchdown point is the point on the sea floor on which the pipe or cable contacts the sea floor.

Track Follow / Autotrack

A DP function allowing the vessel to follow a predetermined track via waypoints, details of which are input by the DP Operator.

Transducer

The acoustic probe on the vessel's bottom used as part of the HPR system.

Transponder

A sea floor or equipment-mounted beacon used with HPR. A transponder normally functions in an interrogation-reply mode, being triggered by acoustic interrogation through-water, and replying through-water.

Triplex

A term often used to describe a DP system having three computers providing redundancy through voting logic.

Tunnel thrusters

Propulsion units consisting of propellers located at the bow or stern of the vessel, able to thrust in an athwartships direction, and located in a tunnel structure within the hull.

Umbilical

The tether connecting a diver to the diving bell carrying air supply, hot water and communications. Also any similar tether used with an ROV or any underwater vehicle.

UPS

Uninterruptible Power Supply. A low-voltage system installed to ensure secure power supply to DP control elements and other vital functions. Normally features a battery back-up facility.

USBL

Ultra Short Baseline, a principle used with HPR. Synonymous with SSBL (Super-short baseline). An acoustic position reference technique involving two-way communication between a hull-mounted transducer and a transponder located on the sea floor or other underwater fixed location.

UTM

The Universal Transverse Mercator projection and co-ordinate system. Within a UTM zone, a vessel’s position is described by Northing and Easting co-ordinates in metres.

UTM zone

One of the 60 zones, each covering 6° of longitude used within the UTM co-ordinate system. Variations of the UTM system exist in some localities.

Vertical Reference Sensors

Vertical Reference Sensor, a device giving accurate values for vessel Roll and Pitch values.

Weathervane/Minimum power heading mode

See “Auto Heading Select”.

WGS84

The World Geodetic Spheroid upon which the GPS system is based.

Wind Feed Forward / wind compensation

A facility in which rapid changes in wind strength are processed immediately allowing for immediate thruster compensation in gusting conditions. The configuration of this function varies between different DP system manufacturers, and different function naming may be found.

Windsensor

Any device enabling the detection of wind strength and direction, transmitting data to remote displays and equipment.

Worst Case Failure

A system failure resulting in the greatest functional degradation within the system.

Yaw

Vessel rotation about the vertical axis; Change of heading.