Dynamic positioning (DP) 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 the computer with information about the vessel's position and the magnitude and direction of the environmental forces affecting it. Examples of vessel types that employ DP include ships and semi-submersible mobile offshore drilling units (MODU), oceanographic research vessels, cable layer ships, and cruise ships.
The computer program contains a mathematical model of the vessel, including information such as its current drag and thrusters' locations. This knowledge, combined with sensor data (e.g. wind), allows the computer to calculate the required steering angle and the output for each thruster to support operations at sea where mooring or anchoring is not feasible due to deep water, seabed congestion (pipelines, templates), or other constraints.
Dynamic positioning may be either absolute, in that the position is locked to a fixed point on the bottom, or relative to a moving object, such as another ship or an underwater vehicle. One may also position the ship at a favourable angle to the wind, waves, and current, a practice 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.
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 aimed 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, Shell later in the same year launched the drilling ship Eureka, which had an analog control system interfaced with a taut wire, making it the first true DP ship.
While the first DP ships had analog controllers and lacked redundancy, vast improvements have been made since then. DP is now used not only in the oil industry but also on various other types of ships. In addition, DP is no longer limited to maintaining a fixed position. One use case is sailing an exact track, which is useful for cablelay, pipelay, survey, and other tasks.
Comparison between position-keeping options Other methods of position-keeping include using an anchor spread and a jack-up barge. All have their own advantages and disadvantages. + Comparison of position-keeping options Jack-up bargeAnchoringDynamic positioningAdvantagesNo complex systems with thrusters, extra generators, and controllers. No chance of running off position due to system failures or blackouts. No underwater hazards from thrusters.No complex systems with thrusters, extra generators, and controllers. No chance of running off position due to 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 setup. Not limited by an obstructed seabed.DisadvantagesNo 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 (pipelines, 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 enabled many operations that were previously infeasible.
The costs are falling due to newer, cheaper technologies, and the advantages are becoming more compelling as offshore work moves into ever-deeper water and the underwater environment (coral) is given greater respect. In container operations, crowded ports can be made more efficient through faster, 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 (ATON) Cable-laying Crane vessels Cruise ships Deep sea mining Diving support vessels Dredging Drillships Floating production storage and offloading units (FPSOs) 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 (forward/astern) Sway: movement along the lateral axis (starboard/port) Heave: movement along the vertical axis (up/down)
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 (surge and sway) and rotation about the vertical axis (yaw).
Requirements For a vessel to operate using dynamic positioning (DP), several essential components are required:
Position and heading information: Accurate information about the vessel's position and heading is fundamental to DP operation. Control system: A dedicated computer system continuously calculates the corrective actions required to maintain the vessel's position and heading. Thrust elements: Propellers and thrusters generate the forces required by the control system to counteract environmental forces.
When designing a DP vessel, careful consideration must be given to both the position reference systems and the available thrust. In particular, maintaining position and heading in adverse weather requires sufficient thrust capacity in all three controlled degrees of freedom: surge, sway, and yaw.
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 methods for determining a ship's position at sea. Most traditional methods of ship navigation are not accurate enough to meet 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 (ex L-3), Edison Chouest Offshore, 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 (PRS) and position measuring systems (PME) are:
DGPS, Differential GPS. The GPS position is not accurate enough for DP use. The position is improved by using a fixed ground-based reference station (differential station) that compares the GPS position to the station's known position. The correction is sent to the DGPS receiver by long-wave radio frequency. For use in DP, an even higher accuracy and reliability are 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 signal degradation due to ionospheric or atmospheric disturbances, satellite blockage from cranes or other structures, and signal degradation 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 (by means of piezoelectric elements) to the transponder, which is triggered to reply. As the velocity of sound through water is known (preferably, a sound profile is taken regularly), 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 ship's position relative to the transponder can be calculated. A disadvantage is vulnerability to noise from thrusters or other acoustic systems. Use is limited in shallow waters because of refraction that occurs when sound travels horizontally through water. 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 must be applied for the ship's roll and pitch. Motion Reference Units determine these. Because of the nature of angle measurement, accuracy deteriorates as water depth increases. Long baseline (LBL): This system consists of an array of at least three transponders. Their initial positions are determined using USBL and/or by measuring the baselines between them. Once these positions are established, only the ranges to the transponders need to be measured to determine the vessel's relative position. In theory, the position lies at the intersection of imaginary spheres centred on each transponder, with each sphere's radius determined by the signal travel time multiplied by the speed of sound in water. Because angular measurements are not required, LBL generally provides better accuracy than USBL in deep water. Short baseline (SBL): This system uses an array of transducers mounted in the vessel's hull. The transducers determine their position relative to a transponder, allowing the vessel's position to be calculated in a manner similar to LBL. Because the array is mounted on the vessel, the measurements must 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 a USBL-based system, in which a riser-angle monitoring transponder is fitted to the riser, and a remote inclinometer unit is installed on the Blow Out Preventer (BOP) and interrogated via the ship's HPR.
Light taut wire (LTW or LWTW): One of the oldest position reference systems used for DP, the light taut wire remains accurate in relatively shallow water. A clump weight is lowered to the seabed, and the vessel's relative position is calculated by measuring the length of wire paid out and its angle using a gimbal-mounted sensor. The wire angle must be kept within appropriate limits to prevent the clump weight from being dragged along the seabed. In deeper water, currents can cause the wire to curve, reducing accuracy, although some systems compensate for this by using a gimbal mechanism at the clump weight. Horizontal LTW systems may also be used when operating close to a structure, where falling objects can pose a risk to the wire.