A Savelyev DYNAMIC POSITIONING SYSTEM

Dynamic Positioning System Chapter 1 PRINCIPLES OF DYNAMIC POSITIONING SYSTEM

Track Follow - precisely controls the track of the vessel.

Auto Slowdown – provides tension control.

Auto Pilot and Auto Sail – expedites arrival on location.

ROV Follow – locate cable for retrieval or repair.

DP Mode – used during repairs.

CRANE BARGES AND CONSTRUCTION VESSELS

Objective: Used in construction and de-commissioning operations in the oilfield. These vessels are also used in wreck recovery or salvage work.

DP Advantage:

Since these vessels have to operate relatively close to object(s) being lifted, they have to maintain position and heading to prevent collision. DP systems in these vessels are often more feasible than anchoring.

DP Modes Used:

Standard DP Modes including special position measurement (DARPS, Artemis) for accuracy in measuring distances between structure and vessel.

CRUISE AND PASSENGER VESSELS

Objective: Provide transportation within the travel industry.

DP Advantage:

Increase maneuverability. Vessels are built very large to fulfill the needs of an expanding industry. However, the controlling depth of most channels remains the same. Therefore, DP systems now allow and use vessels to enter ports and harbors.

DP Modes Used:

Standard DP Modes.

DIVING AND UNDERWATER SUPPORT VESSELS

Objective: Deploy and recover divers, safely. Dive support vessels deploy divers for a variety of underwater operations:

Inspection

Installation Configuration Monitoring Recovery Survey, and more.

DP Advantage:

It may be possible to change the position and/or heading of the vessel to allow closer approach to the worksite, or to effectively locate the thrusters further away from the worksite. It may be possible to stop the nearest thruster to the diving location.

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Dynamic Positioning System Chapter 1 PRINCIPLES OF DYNAMIC POSITIONING SYSTEM

Due to hazards associated with dive operations, DP Diving Support Vessels have several arrangements in place to protect divers:

The length of the diver’s umbilical isrestricted to prevent him/her from being sucked into a rotating propeller.

A standby diver is used to tend to the diver’s umbilical or to assist in an emergency. For dive operations in water deeper than 300 meters, a diver must wear an atmospheric diving suit (ADS) or use a remote operated vehicle (ROV).

DP Modes Used:

Major DP system are duplicated or triplicated to ensure that divers are recovered, regardless of possible failure modes.

ROV Support – used in conjunction with the diving.

DRILLSHIPS

Objective: Involved with oil or gas exploration.

DP Advantage:

Dynamic Positioning, usually class III system, is used to keep the drillship as directly above the well as possible. If the drillship wanders off position to the extent that the connection to the well is severed, uncontrolled release of hydrocarbons could pollute and damage the environment. In addition, reconnecting to the well can be costly and time consuming.

The lower main riser angle is constantly monitored to ensure that the vessel remains within a predefined angle. A riser angle changes beyond the predefined angle that the drillship is drifting off location.. Some DP systems on drillships have “riser angle mode” function to ensure that the drillship is automatically maneuvered to reduce the riser angle. Allows for quick arrival and drilling at the specified location.

DP Modes Used:

Riser Angle Mode – maintain the drill string riser angle.

ROV Support – used in conjunction with the drilling.

PIPE LAY VESSELS

Objective: Precisely lay pipe and provide steady tension on the pipeline.

DP Advantage:

Pipeline tension data is automatically transmitted to the DP system.

System provides the necessary thrusters commands to enable the vessel to maintain tension. Enables these vessels to follow the exact track required to lay the pipeline.

Enables the vessel to work in areas where the use of an extensive anchor spread is not feasible.

Enables the vessel to work in awkward positions, including close to surface or sub sea structures, and without the need for other anchored vessels.

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Dynamic Positioning System Chapter 1 PRINCIPLES OF DYNAMIC POSITIONING SYSTEM

Minimizes the amount of time spent in close-in situations; Manoeuvre speed is not restricted by the need to reset anchors. Positions can be located quickly. Since pipe lay operations are expensive, delays for any reason are costly.

Reduces costly delays in operations.

DP Modes Used:

Track Follow Mode – controls track of vessel.

ROCK DUMPING AND DREDGING VESSELS

Objective: Clearing channels and harbors to recover rock and aggregates. Protect from damage and maintain control of the track and speed. Protect seabeds and remedy erosion problems by dumping rock.

DP Advantage:

Provide accurate track and speed control over the pipeline to be covered. Ensure accuracy in dredging a defined area which prevents increased expenses.

DP Modes Used:

Autotrack Mode - allows vessels to spread rocks and dredge more evenly and economically. Autoslow Down Mode – supports Autotrack Mode and monitors forces.

Track Follow Mode – precisely controls the track of the vessel.

FLOATING PRODUCTION, STORAGE, AND OFFTAKE UNITS (FPSO) VESSELS

Objective: A turret moored tanker which must prevent hydrocarbon release.

DP Advantage: Maintains position with increased anchor effectiveness.

DP Modes Used:

Weathervaning Mode - to maintain heading into the weather.

Anchor Mode – to increase anchor effectiveness.

SHUTTLE TANKER

Objective: FPSOs use shutter tankers to transport the oil. As the FPSO changes position, the shutter tanker has to maintain a relative position to prevent breaking the hose.

DP Advantage:

Relative positioning of the FPSO to avoid emergency disconnect.

Can approach safely.

DP Modes Used:

Weathervaning Mode – for close proximity movment.

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Dynamic Positioning System Chapter 1 PRINCIPLES OF DYNAMIC POSITIONING SYSTEM

SUPPLY AND STANDBY VESSELS

Objective: Replenishment operations for oil industry.

DP Advantage:

Allows close proximity to oil rigs, platforms, barges, or other vessels. Maintains location and distance from obstructions, therefore, reducing hazards.

DP Modes Used:

ROV Support.

Standard DP Modes.

Track Follow Mode – precisely controls the track of the vessel.

Weathervaning Mode – for close proximity movment.

OTHER VESSEL TYPES

Firefighting requires special utility vessels that can perform in inclement weather.

Military operation sometimes use vessels with DP systems installed to facilitate critical operations, for example, underway replenishment, mine countermeasures, etc.

SIX FREEDOMS OF MOVEMENT

A vessel at sea is subject to three planes with six freedoms or axis of movement.

Dynamic Positioning is concerned with the horizontal plane. However, it is necessary to sense vessel motion in other planes, monitor the wind, and make corrections to Position Measurement Equipment (PME) and sensor readings.

The six axis of movement, three rotations and three translations, are the traditional names for a vessel’s motion:

Roll - the rotational motion of a vessel about its longitudinal axis.

Pitch - the rotational motion of a vessel about its transverse axis.

Heave - the upward and downward motion of a vessel.

Surge - the forward and backward motion of a vessel.

Sway - the sideward motion of a vessel.

Yaw - the directional motion of a vessel about its vertical axis.

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Dynamic Positioning System Chapter 1 PRINCIPLES OF DYNAMIC POSITIONING SYSTEM

SIX FREEDOMS OF MOVEMENT

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Dynamic Positioning System Chapter 1 PRINCIPLES OF DYNAMIC POSITIONING SYSTEM

MOVEMENTS CONTROLLED AND MONITORED BY DP

DP is concerned with the individual and automatic control of surge, sway, and yaw, i.e. position and heading. DP maintains and controls a vessel’s position by controlling surge (X axis) and sway (Y axis). The position movement is measured by reference to high accuracy PMEs.

Yaw (N axis) is critical in controlling the vessels heading. The heading motion is measured through gyrocompass. Both are maintained by thruster action.

The remaining freedoms of movement, roll, pitch, and heave are monitored by DP.

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Dynamic Positioning System Chapter 2 ELEMENTS OF A DYNAMIC POSITION SYSTEM

SUBSYSTEMS

DP system is a complex combination of subsystems interacting to automatically maintain a vessel’s position and heading with active thrust. It can be divided into five subsystems:

Propulsion and Thrusters. Control Elements. Power Generation.

Position Measurement Equipment. Sensors.

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Dynamic Positioning System Chapter 2 ELEMENTS OF A DYNAMIC POSITION SYSTEM

ELEMENTS OF A DYNAMIC POSITION SYSTEM

PROPULSION AND THRUSTERS

DP system is a complex combination of subsystems interacting to automatically maintain a vessel’s position and heading with active thrust. Thrusters are positioned to provide optimum Dynamic Positioning capability and movement, with minimum interference with other thrusters and sensors, using minimum fuel consumption, and, in the case of tunnel thrusters, as far below the water line as possible. It is also important that thrusters are positioned to control the vessel with minimal fuel consumption and to reduce the wear on the thruster. Three basic types of thrusters are

strategically fitted to DP vessels to optimize control of heading, sway, and surge: Main Propulsion.

Tunnel Thrusters.

Azimuth Thrusters.

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Dynamic Positioning System Chapter 2 ELEMENTS OF A DYNAMIC POSITION SYSTEM

MAIN PROPULSION

Main Propulsion, consisting of a single or twin screw propellers, is positioned at or near the stern of the vessel.One of two types of propellers is used in main propulsion:

Variable or Controllable-Pitch Propeller (CPP) – The shaft turns at a constant speed while the propeller pitch is adjusted to influence the vessel’s speed and directionforward or aft.

Advantages:

Vessel can change speed and direction by adjusting only pitch.

Relative fast respond time. Limitations:

High maintenance cost and fuel.

Fixed-Pitch propeller – The speed and direction of the shaft are adjusted to control the speed and direction of the vessel.

Advantages:

Relatively low maintenance cost.

Limitations:

Propeller shaft has to be adjusted in order to change vessel speed and direction. Relative slow response time.

NOTE: Rudder(s) is used in conjunction with controllable-pitch propeller(s) or fixed-pitch propellers(s) for rotational (yaw) control.

TUNNEL THRUSTER

The propeller is encased in a transverse tunnel. Advantages:

Very effective.

Relatively low maintenance cost (fixed-pitch). A screen may be used at tunnel openings to keep debris out.

Limitations:

Full propeller immersion is necessary for maximum effectiveness.

Vessel speed over two knots, ahead or astern, incrementally reduces thruster effectiveness. A long tunnel reduces thruster efficiency.

Thruster may be more effective in one direction, port or starboard, than the other.

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Dynamic Positioning System Chapter 2 ELEMENTS OF A DYNAMIC POSITION SYSTEM

AZIMUTH THRUSTER

Azimuth thruster is a propeller designed to operate through 360 degrees.

Advantages:

Propeller can be fixed-pitch or controllable-pitch.

Thruster is effective in any direction.

Rudder is not required for rotational co

Thruster unit may be fixed or retractable.

Limitations:

Relatively high maintenance cost.

The type of vessel, DP operation, environment, level of redundancy, and many other factors go into determining

propeller/thruster configuration. However, there are three minimum configurations installed on DP vessel:

Minimum thrusters and a propeller.

A tunnel thruster and an azimuthing thruster.

Bow thruster conventional propeller.

Each of these configurations will control the vessel in X, Y, and N.

NOTE: Most DP vessels have propulsion/thruster configurations

beyond the minimum for redundancy. Thrusters are positioned to provide optimum movement, minimum interference from other thrusters, and minimum fuel consumption.

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