Dynamic positioning

Dynamic positioning is a computer-controlled system to automatically maintain a vessel's position and heading by using its own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors, and gyrocompasses, provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position. Examples of vessel types that employ DP include ships and semi-submersible mobile offshore drilling units, oceanographic research vessels, cable layer ships, and cruise ships.
The computer program contains a mathematical model of the vessel that includes information pertaining to the wind and current drag of the vessel and the location of the thrusters. This knowledge, combined with the sensor information, allows the computer to calculate the required steering angle and thruster output for each thruster. This allows operations at sea where mooring or anchoring is not feasible due to deep water, congestion on the sea bottom, or other problems.
Dynamic positioning may either be absolute in that the position is locked to a fixed point over the bottom, or relative to a moving object like another ship or an underwater vehicle. One may also position the ship at a favorable angle towards wind, waves, and current, called weathervaning.
Dynamic positioning is used by much of the offshore oil industry, for example, in the North Sea, Persian Gulf, Gulf of Mexico, West Africa, and off the coast of Brazil. There are currently more than 1800 DP ships.

History

Dynamic positioning began in the 1960s for offshore drilling. With drilling moving into ever deeper waters, Jack-up barges could not be used anymore, and anchoring in deep water was not economical.
As part of Project Mohole, in 1961, the drillship Cuss 1 was fitted with four steerable propellers. The Mohole project was attempting to drill to the Moho, which required a solution for deep water drilling. It was possible to keep the ship in position above a well off La Jolla, California, at a depth of 948 meters.
After this, off the coast of Guadalupe, Mexico, five holes were drilled, the deepest at below the sea floor in of water, while maintaining a position within a radius of. The ship's position was determined by radar ranging to buoys and sonar ranging from subsea beacons.
Whereas the Cuss 1 was kept in position manually, later in the same year, Shell launched the drilling ship Eureka that had an analogue control system interfaced with a taut wire, making it the first true DP ship.
While the first DP ships had analogue controllers and lacked redundancy, vast improvements have been made since then. DP nowadays is not only used in the oil industry but also on various other types of ships. In addition, DP is not limited to maintaining a fixed position anymore. One of the possibilities is sailing an exact track, useful for cablelay, pipelay, survey, and other tasks.

Comparison between position-keeping options

Other methods of position-keeping are the use of an anchor spread and the use of a jack-up barge. All have their own advantages and disadvantages.

	Jack-up barge	Anchoring	Dynamic positioning
Advantages	No complex systems with thrusters, extra generators and controllers. No chance of running off position by system failures or blackouts. No underwater hazards from thrusters.	No complex systems with thrusters, extra generators and controllers. No chance of running off position by system failures or blackouts. No underwater hazards from thrusters.	Maneuverability is excellent; it is easy to change position. No anchor handling tugs are required. Not dependent on water depth. Quick set-up. Not limited by obstructed seabed.
Disadvantages	No maneuverability once positioned. Limited to water depths of 175 meters.	Limited maneuverability once anchored. Anchor handling tugs are required. Less suitable in deep water. Time to anchor out varies from several hours to several days. Limited by obstructed seabed.	Complex systems with thrusters, extra generators and controllers. High initial costs of installation. High fuel costs. Chance of running off position in case of strong currents or winds, or due to system failures or blackouts. Underwater hazards from thrusters for divers and ROVs. Higher maintenance of the mechanical systems.

Although all methods have their own advantages, dynamic positioning has made many operations possible that were not feasible before.
The costs are falling due to newer and cheaper technologies, and the advantages are becoming more compelling as offshore work enters ever deeper water and the environment is given more respect. With container operations, crowded ports can be made more efficient by quicker and more accurate berthing techniques. Cruise ship operations benefit from faster berthing and non-anchored "moorings" off beaches or inaccessible ports.

Applications

Important applications include:

Servicing aids to navigation
Cable-laying
Crane vessels
Cruise ships
Deep sea mining
Diving support vessels
Dredging
Drillships
Floating production storage and offloading units
Flotels
Landing platform docks
Maritime research
Mine sweepers
Pipe-laying ship
Platform supply vessels
Rock dumping
Sea Launch
Sea-based X-band radar
Shuttle tankers
Survey ships
Scope

A ship's motion can be described in terms of six degrees of freedom, consisting of translation and rotation along three perpendicular axes.
The three translational motions are:

Surge: movement along the longitudinal axis
Sway: movement along the lateral axis
Heave: movement along the vertical axis

The three rotational motions are:

Roll: rotation about the longitudinal axis
Pitch: rotation about the lateral axis
Yaw: rotation about the vertical axis

Dynamic positioning is primarily concerned with controlling a vessel in the horizontal plane, namely translation along the two horizontal axes and rotation about the vertical axis.

Requirements

For a vessel to operate with dynamic positioning, several essential components are required:

Position and heading information: Accurate knowledge of the ship's location and orientation is the foundation of DP.
Control system: A dedicated computer continuously calculates the corrective actions needed to maintain position and heading.
Thrust elements: Propellers and thrusters apply the forces demanded by the control system to counteract environmental influences.

When designing a DP vessel, careful consideration must be given to both the position reference systems and the available thrust. In particular, maintaining control in adverse weather requires sufficient thrust capability in all three axes of movement.
Maintaining a fixed position is especially challenging in polar conditions, where ice forces can change rapidly. Current ship-borne ice detection and mitigation technologies are not yet advanced enough to predict these forces reliably, though they may offer advantages over sensors deployed by helicopter.

Positioning systems

There are several means to determine a ship's position at sea. Most traditional methods used for ships navigation are not accurate enough for some modern requirements. For that reason, several positioning systems have been developed during the past decades. Producers of DP systems are: Marine Technologies LLC, Kongsberg Maritime, Navis Engineering Oy, GE, SIREHNA, Wärtsilä, MT-div. Chouest, Rolls-Royce plc, Praxis Automation Technology, Brunvoll AS. The term digital anchor has been used to describe such dynamic positioning systems. The applications and availability depends on the type of work and water depth. The most common position reference systems and position measuring systems are:

DGPS, Differential GPS. The position obtained by GPS is not accurate enough for use by DP. The position is improved by use of a fixed ground-based reference station that compares the GPS position to the known position of the station. The correction is sent to the DGPS receiver by long wave radio frequency. For use in DP an even higher accuracy and reliability is needed. Companies such as Veripos, Fugro or C-Nav supply differential signals via satellite, enabling the combination of several differential stations. The advantage of DGPS is that it is almost always available. Disadvantages include degradation of the signal by ionospheric or atmospheric disturbances, blockage of satellites by cranes or structures and deterioration of the signal at high altitudes. There are also systems installed on vessels that use various augmentation systems, as well as combining GPS position with GLONASS.
Acoustics. This system 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, 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. 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:
*Ultra- or super- short base line, USBL or SSBL. This works as described above. 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.
*Long base line, LBL. 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.
*Short baseline, SBL. 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.
Riser Angle Monitoring. On drillships, riser angle monitoring can be fed into the DP system. It may be an electrical inclinometer or based on USBL, where a riser angle monitoring transponder is fitted to the riser and a remote inclinometer unit is installed on the Blow Out Preventer and interrogated through the ship's HPR.
Light taut wire, LTW or LWTW. The oldest position reference system used for DP is still very accurate in relatively shallow water. 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 favourable, as current will curve the wire. There are however systems that counteract this with a gimbal head on the clumpweight. Horizontal LTW's are also used when operating close to a structure. Objects falling on the wire are a risk here.
Fanbeam and CyScan. These are laser based position reference systems. They are very straightforward system, as only a prism cluster or tape target needs to be installed on a nearby structure or ship. Risks are the system locking on other reflecting objects and blocking of the signal. However, the Cyscan Absolute Signature which was released in 2017 was launched to address this issue. It is able to engage in an active lock with the Absolute Signature prism which reduces the chance of a wrong target being tracked. Range depends on the weather, but is typically more than 500 meters. New advancement, borrowed from the autonomous vehicle industry, are leading to the introduction of target-less PRS using fast laser scanners then Simultaneously Locating And Mapping one scan to the next to work out the relative movement of the vessel. Guidance Marine introduced the SceneScan, a 2D scanner, in 2018. Furlong Sensing introduced the Unity, a 3D scanner in 2025. As these target-less technologies mature it may be expected that solutions requiring installed targets will be retired. The offshore wind farm industry is leading the way as adding targets to many wind towers is a cost that can now be avoided.
Artemis. A radar-based system. A unit is placed on a fixed station and the unit on board the mobile station locks on to it to report the range and bearing. The operational range is in excess of 4 kilometers. Advantage is the reliable, all-weather performance. Disadvantage is that the unit is rather heavy and costly. Current version is the Artemis Mk6.
DARPS, Differential, Absolute and Relative Positioning System. 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.
RADius and RadaScan. These are radar based systems; while the RADius has no moving parts, the RadaScan has a rotating antenna under the dome. Guidance Marine has improved the miniRadaScan with the RadaScan View which has an added advantage of radar back-scatter. This enhanced the DPO's situational awareness. These systems usually have responders which are active targets that send the signal back to the sensor to report the range and bearing. The range is typically up to 600 meters.
Inertial navigation is used in combination with any of the above reference systems, but typically with gnss and Hydroacoustics.