DP PRINCIPLES

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2 IMO DEFINITIONS (REF. MSC/CIRC. 645. 6 JUNE 1994)

2.1 DEFINITIONS In addition to the definition in the MODU Code 1989 the following definitions are necessary for the guidelines:

2.1.1 Dynamically positioned vessel (DP-Vessel) means a unit or a vessel which automatically maintains its position (fixed location or predetermined track) exclusively by means of thruster force.

2.1.2 Dynamic positioning system (DP-system) means the complete installation necessary for dynamically positioning a vessel comprising the following sub-systems:

.1 Power system

.2 Thruster system, and

.3 DP-control system

2.1.3 Position keeping means maintaining a desired position within the normal excursions of the control system and the environmental conditions.

2.1.4 Power system means all components and systems necessary to supply the DPsystem with power. The power system includes:

.1 Prime movers with necessary auxiliary systems including piping,

.2 Generators.

.3 Switchboards, and

.4 Distributing system (cabling and cable routing)

2.1.5 Thruster system means all components and systems necessary to supply the DP-system with thrust force and direction. The thruster system includes:

.1 Thruster with drive units and necessary auxiliary systems including piping. .2 Main propellers and rudders if these are under the control of the DPsystem. .3 Thruster control electronics, .4 Manual thruster controls, and .5 Associated cabling and cable routing

2.1.6 DP-control system means all control components and systems, hardware and software necessary to dynamically position the vessel. The DP-control system consists of the following:

.1 Computer system/joystick system.

.2 Sensor system,

.3 Display system (operator panels),

.4 Position reference system, and

.5 Associated cabling and cable routing.

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2.1.7 Computer system means a system consisting of one or several computers including software and their interfaces.

2.1.8 Redundancy means ability of a component or system to maintain or restore its function, when a single failure has occurred. Redundancy can be achieved for instance by installation of multiple components, systems or alternative means of performing a function.

2.2 Other definitions

Flag State is the country in which the vessel is registered by the National Authority. The vessel shall carry the Flag State’s flag, and show the port of registry at the stern, but may have no other apparent connection to the Flag State.

Coastal State is the country where the vessel is located when it is not in international waters, i.e. when in harbour or in some country’s territorial water. The Coastal State may apply restrictions that exceed those of the Flag State.

Main Class is the set of rules that are compulsory for any vessel classified by the specific Classification Society. Each part or section of the Rules that are within Main Class will be identified as such. Typically, for DNV, Part 4 Machinery and Systems is marked Main Class. This Part includes both Electrical Installations and Instrumentation and Automation Chapters.

Additional Class is sets of rules that are optional to a vessel to document a specific type of vessel or specific capabilities of the vessel. The DP rules is a typical case of Additional Class rules, which the vessel owner has chosen to apply for his vessel.

Class Notation is another name used for Additional Class. Thus, DYNPOS-AUTR is a Class Notation, and it will be stated in the vessel’s Class Certificate.

Segregation means that two or more units of equipment are located physically separated, without stating the degree of separation. For instance, A60 is a case of segregation where there are specific requirements to the degree of protection regarding propagation of fire.

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Independence, in relation to technical systems, means that one unit or system will operate without assistance from another system such that the one system will remain in operation after the other has failed. Mutual independence signifies that this will be true both ways.

Redundancy signifies that there are at least two units or systems that are able to serve a specific function, so that this function will remain operable if the one unit or system fails. See definition under 2.1.8. Thus, there can hardly be redundancy without independence. But redundancy can be achieved with a minimum of segregation, e.g. redundant components in a common cabinet.

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Basic Forces and Motions

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Basic Forces and Motions A seagoing vessel is subjected to forces from wind, waves and current 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. Reference systems readings are corrected for roll and pitch using readings from the vertical reference sensors. Wind speed and direction are measured by the wind sensors.

The K-Pos control system calculates the forces that the thrusters must produce in order to control the vessel’s motion in three degrees of freedom - surge, sway and yaw - in the horizontal plane.

Pitch(+ = bow up)

Roll(+ = stbd down)

Heave(+ = down)

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DP SYSTEM PRINCIPLES A simplified overview of the DP control system is illustrated in the following figure:

DragForces

Manual ForceDemand

THRUSTER

COMMAND

SYSTEM

THRUSTER

FEEDBACK

SYSTEMTHRUSTERS

POWER

OVERLOAD

CONTROL

THRUSTER

MODEL

THRUSTER

ALLOCATION

JOYSTICK

GAIN

CONTROLLER

GAIN

CONTROL

DAMPING

CONTROL

DRAG

COMPUTATION

KALMAN

FILTER

ERROR

COMPUTATION

WIND

MODEL

VESSEL MODEL

CARROT

COMPUTATION

MeasuredPosition & Heading

Position &HeadingDifference

ErrorCompensation

Force

Measured WindSpeed & Direction

WindForce

PredictedPosition &Heading

VesselModelUpdate External

Forces

EstimatedPosition &HeadingEstimated

Speed

ThrusterForce

Position &HeadingDeviation

SelectedGain

Feed Forward

Resulting ForceDemandJoystick

Gain & Linearity

Force Demandfrom Joystick

ThrusterSetpoints

ThrusterSetpoints

Generator StatusBus Switch StatusPower Consumption

ThrusterAllocation Mode

ThrusterFeedback

CarrotPosition & Heading

Setpoint

Position & Heading Setpoint

Speed Settings

NEW SETPOINT

PRESENT

Draught Input

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The Vessel Model The DP system is based on a Vessel Model which contains a hydrodynamic description of the vessel, including characteristics such as drag coefficients and mass data. This model describes how the vessel reacts or moves as a function of the forces acting upon it. Note ! Text in capitals in the following description refer to items in figure on page 19. The Vessel Model is provided with information describing the forces that are acting on the vessel: o A Wind Model uses a set of wind coefficients for various

angles of attack to calculate the WIND FORCE as a function of the wind speed and direction.

o A Thruster Model uses force/pitch/rpm characteristics to

calculate the THRUSTER FORCE according to the feedback signals from the thrusters/propellers.

Using the vessel characteristics and the applied forces, the Vessel Model calculates the ESTIMATED SPEED and the ESTIMATED POSITION AND HEADING in each of the three horizontal degrees of freedom - surge, sway and yaw. During sea trials, the Vessel Model is tuned to optimise the description of the vessel characteristics.

Vessel Model Update The Vessel Model can never be a completely accurate representation of the real vessel. However, by using a technique known as Kalman Filtering, the model estimates of position and heading are continuously updated with measured position information from position-reference systems and gyrocompasses. The PREDICTED POSITION AND HEADING from the Vessel Model are compared with the MEASURED POSITION AND HEADING to produce a POSITION AND HEADING DIFFERENCE. Since these differences may be caused by no ise in the measured values, they are filtered before being used to update the Vessel Model. Together, the Vessel Model and the Kalman filtering technique provide effective noise filtering of the heading and position

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measurements and optimum combination of data from the different reference systems. If the reference system measurements are completely lost (position or heading dropout), there is no immediate effect on the positioning capability of the system. The Vessel Model will continue to generate position estimates even though there are no further model updates. This "dead reckoning" positioning will initially be very accurate but will gradually deteriorate with time.

Error Compensation Force Even if appropriate thruster/propeller forces are applied to counteract the effect of the measured forces on the vessel, the vessel would still tend to move out of position due to forces that are not measured directly, such as waves and sea current (together with any errors in the modelled forces). These additional forces acting on the vessel are calculated over a period of time according to the filtered POSITION AND HEADING DIFFERENCE to produce an ERROR COMPENSATION FORCE, which is added to the modelled forces to represent the total EXTERNAL FORCES.

The error compensation force is presented to the operator as being entirely due to sea current since this is the main component.

Force Demand The force demand that is required to keep the vessel at the required position is composed of the following parts:

o The Force Demand for axes that are under automatic control o The Force Demand for axes that are under manual control o The Feed Forward

Force Demand for Axes under automatic Control This consists of two parts:

o a force demand that is proportional to the deviation between

the estimated position and heading and the position and heading setpoints

o a force demand that is proportional to the deviation between

the wanted and actual speed

The position and heading setpoints, specified by the operator, are compared with the estimated position and heading from the Vessel Model. The differences are multiplied by gain factors that

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are calculated and adjusted to optimize the station keeping capability with minimum power consumption.

The wanted speed is compared with the estimated speed from the Vessel Model. If the vessel is to maintain a stationary position, the wanted speed will be zero. This part of the force demand therefore acts as a damping factor in order to reduce the vessel's speed to zero.

Force demand for axes under manual control When any of the axes are not under automatic control, you can use the joystick to manually control the force exerted by the thrusters/propellers in those axes.

Feed forward In order to counteract changes in the external forces as soon as they are detected, rather than first allowing the vessel to drift away from the required position, the calculated EXTERNAL FORCES are fed forward as an additional force demand.

Thruster Allocation The force demand in the surge and sway axes (the directional force demand), and in the yaw axis (the rotational moment demand), are distributed as pitch and/or rpm setpoint signals to each thruster/propeller.

The demand is distributed in such a way as to obtain the directional force and rotational moment required for position and heading control, while also ensuring optimum thruster/propeller use with minimum power consumption and minimum wear and tear on the propulsion equipment.

If it is not possible to maintain both the rotational moment and the directional force demand due to insufficient available thrust, priority is normally set to obtain the rotational moment demand (heading). If required, you can request that the priority is changed to maintain position rather than heading.

If a thruster/propeller is out of service or deselected, the "lost" thrust is automatically redistributed to the remaining thrusters/propellers.

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RotationalMomentDemand

DirectionalForceDemand

= Rotation centre

= Tunnel thruster

= Wake from thruster/propeller

= Propeller/rudder

= Direction of thrust

Allocationof demand

(MD0001)

Power Overload Control The load on the main bus or on isolated bus sections is monitored, and power is reduced on the connected thrusters/propellers by reducing the pitch/rpm demand if the estimated load exceeds the nominal limit. The reduction value is shared between the connected thrusters/propellers in such a way that the effect on the position and heading control is minimised. This function acts as an addition to the vessel’s own Power Management System (PMS). The power reduction criteria are set at lower overload levels than the load reduction initiated by the PMS system.

Reference System and Sensor Data Processing Measurements of a vessel's position and heading at any point in time are essential for dynamic positioning.

Several different position-reference systems are normally used with the DP system. The first position-reference system selected and accepted for use with the DP system becomes the reference origin (the origin of the internal coordinate system).