Dynamic positioning (DP) is a computer controlled system to automatically maintain a ship’s position and heading by using her own propellers and thrusters. A ship /ʃɪp/ is a large vessel that floats on water Ships are generally distinguished from Boats based on size Position reference sensors, combined with wind sensors, motion sensors and gyro compasses, provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position. A gyrocompass is similar to a Gyroscope. It is a Compass that finds True north by using an (electrically powered fast-spinning wheel and friction
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 (pipelines, templates) 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 favourable angle towards wind, waves and current, called weathervaning.
Dynamic positioning is much used in the offshore oil industry, for example in the North Sea, Persian Gulf, Gulf of Mexico, West Africa and off Brazil. The North Sea is a marginal, Epeiric sea of the Atlantic Ocean on the European Continental shelf. The Persian Gulf, in the Southwest Asian region is an extension of the The Gulf of Mexico ( Spanish: Golfo de México) is the ninth largest Body of water in the world West Africa or Western Africa is the Westernmost Region of the African Continent. |utc_offset = -2 to -4 |time_zone_DST = BRST |utc_offset_DST = -2 to -5 |cctld Nowadays there are more than 1000 DP ships.
Dynamic positioning started in the 1960’s for offshore drilling. With drilling moving into ever deeper waters, Jack-up barges could not be used any more and anchoring became less economical. A jack-up rig is a type of mobile offshore oil and gas drilling platform that is able to stand still on the sea floor, resting on a number of supporting legs
In 1961 the drillship Cuss 1 was fitted with four steerable propellers, in an attempt to drill the first Moho well. The Mohorovičić discontinuity, usually referred to as the Moho, is the boundary between the Earth 's crust and the mantle. It was possible to keep the ship in position above the well off La Jolla, California, at a depth of 948 meter.
After this, off the coast of Guadalupe, Mexico, five holes were drilled, the deepest at 183 m (601 ft) below the sea floor in 3,500 m (11,700 ft) of water, while maintaining a position within a radius of 180 meter. The ship's position was determined by radar ranging to buoys and sonar ranging from subsea beacons. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships
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. Shell Oil Company is the United States -based affiliate of Royal Dutch Shell, a multinational oil company (" Oil major " of Anglo
While the first DP ships had analogue controllers and lacked redundancy, since then vast improvements have been made. Besides that, DP nowadays is not only used in the oil industry any more, but on various other types of ships. In addition, DP is not limited to maintaining a fixed position any more. One of the possibilities is sailing an exact track, useful for cablelay, pipelay, survey and other tasks.
Other methods of position-keeping are the use of an anchor spread and the use of a jack-up barge. An anchor is an object often made out of metal that is used to attach a ship to the bottom of a body of water at a specific point All have their own advantages and disadvantages.
|Comparison position-keeping options|
|Jack-up Barge||Anchoring||Dynamic Positioning|
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 (coral) 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.
Important applications include:
A ship can be considered to have six degrees of freedom in its motion, i. Sea-Based X-Band Radar is a floating self-propelled mobile Radar station designed to operate in high winds and heavy seas navigational aid (also known as aid to navigation ATON or navaid) is any sort of marker which aids the traveler in navigation the term is most commonly navigational aid (also known as aid to navigation ATON or navaid) is any sort of marker which aids the traveler in navigation the term is most commonly A cable layer or cable ship is a deep-sea vessel designed and used to lay underwater cables for telecommunications electricity and such A crane vessel, crane ship or floating crane is a Ship that is specialized in lifting heavy loads A cruise ship or cruise liner is a Passenger ship used for pleasure voyages where the voyage itself and the ship's amenities are part of the experience A diving support vessel is a Ship that is used as a floating base for Professional diving projects Dredging is an Excavation activity or operation usually carried out at least partly underwater in shallow seas or Fresh water areas with the purpose of A drillship is a maritime vessel that has been fitted with drilling apparatus A Floating Production Storage and Offloading vessel ( FPSO; also called a "unit" and a "system" is a type of floating tank system used by the offshore Flotel, a Portmanteau of the terms '''flo'''ating ho'''tel''', refers to the installation of living quarters on top of Rafts An amphibious transport dock (also called a landing platform dock or LPD) is an Amphibious warfare ship, a warship that embarks transports and lands elements A minesweeper is a Naval Warship designed to counter the threat posed by Naval mines The dedicated purpose-built minesweeper first appeared during Sea-Based X-Band Radar is a floating self-propelled mobile Radar station designed to operate in high winds and heavy seas A shuttle tanker is a ship designed for oil transport from an off-shore Oil field. A research vessel (R/V is a Ship designed and equipped to carry out Research at Sea. In mechanics, degrees of freedom (DOF are the set of independent displacements and/or rotations that specify completely the displaced or deformed position and orientation e. it can move in any of six axes.
Three of these involve translation:
and the other three rotation:
Dynamic positioning is concerned primarily with control of the ship in the horizontal plane, i. In Physics, translation is movement that changes the position of an object as opposed to Rotation. A rotation is a movement of an object in a circular motion A two- Dimensional object rotates around a center (or point) of rotation e. the three axis surge, sway and yaw.
A ship that is to be used for DP requires:
For most applications, the position reference systems and thrust elements must be carefully considered when designing a DP ship. In particular, for good control of position in adverse weather, the thrust capability of the ship in three axes must be adequate. The main manufacturers of DP systems are Kongsberg Maritime, Converteam (formerly a part of Alstom), L-3 Communications (formerly Nautronix), Rolls-Royce Marine, Marine Technologies and Navis Engineering Oy. Kongsberg Maritime (KM is a Norwegian technology enterprise within the Kongsberg Gruppen (KOG Converteam is the power conversion company that formerly traded as Alstom Power Conversion Alstom ( is a large French multinational conglomerate which holds interests in the power generation and Transport markets Not to be confused with Level 3 Communications, an Internet carrier L-3 Communications Holdings Inc
There are several means to determine a ship's position at sea. Most traditional methods used for ships navigation are not accurate enough. For that reason, several systems have been developed during the past decades. The availability depends on the type of work and water depth. The most common Position reference/Measuring systems /Equipment (PRS/PME) are:
More advanced methods are:
Besides position and heading, other variables are fed into the DP system through sensors:
In the beginning PID controllers were used and today are still used in the simpler DP systems. A proportional–integral–derivative controller (PID controller is a generic Control loop Feedback mechanism widely used in industrial Control systems But modern controllers use a mathematical model of the ship that is based on a hydrodynamic and aerodynamic description concerning some of the ship's characteristics such as mass and drag. Note The term model has a different meaning in Model theory, a branch of Mathematical logic. Fluid dynamics is the sub-discipline of Fluid mechanics dealing with fluid flow: Fluids ( Liquids and Gases in motion Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a Of course, this model is not entirely correct. The ship's position and heading are fed into the system and compared with the prediction made by the model. This difference is used to update the model by using Kalman filtering technique. The Kalman filter is an efficient Recursive filter that estimates the state of a Dynamic system from a series of noisy measurements For this reason, the model also has input from the windsensors and feedback from the thrusters. This method even allows not having input from any PRS for some time, depending on the quality of the model and the weather.
The accuracy and precision of the different PRS’s is not the same. While a DGPS has a high accuracy and precision, a USBL can have a much lower precision. For this reason, the PRS’s are weighed. Based on variance a PRS receives a weight between 0 and 1. In Probability theory and Statistics, the variance of a Random variable, Probability distribution, or sample is one measure of
To maintain position azimuth thrusters (L-drive or Z-drive), azipods, bow thrusters, stern thrusters, water jets, rudders and propellors are used. An Azimuth thruster is a configuration of Ship propellers placed in pods that can be rotated in any horizontal direction making a Rudder unnecessary A L-drive is a type of Azimuth thruster in which the pod-mounted propellers are driven mechanically rather than electrically A Z-drive is a type of Azimuth thruster in which the pod-mounted propellers are driven mechanically rather than electrically Azipod is the registered Brand name of the ABB Group for their Azimuth thruster. A bow thruster, is a propulsion device built into or mounted to the bow of a Ship or Boat to enhance its maneuverability A pump-jet or water jet is a marine system that creates a jet of Water for propulsion. A rudder is a device used to steer a Ship, Boat, Submarine, Hovercraft, or other conveyance that move through a fluid (generally air or A propeller is essentially a type of fan which transmits power by converting Rotational motion into Thrust for propulsion of a vehicle such as an DP ships are usually at least partially diesel-electric, as this allows a more flexible set-up and is better able to handle the large changes in power demand, typical for DP operations. A number of vehicles use a diesel-electric Powertrain for providing locomotion.
The set-up depends on the DP class of the ship. A Class 1 can be relatively simple, whereas the system of a Class 3 ship is quite complex.
On Class 2 and 3 ships, all computers and reference systems should be powered through a UPS. An uninterruptible power supply ( UPS) also known as a continuous power supply ( CPS) or a battery backup is a device which maintains a continuous
Based on IMO (International Maritime Organization) publication 645 the Classification Societies have issued rules for Dynamic Positioned Ships described as Class 1, Class 2 and Class 3. The International Maritime Organization ( IMO) formerly known as the Inter-Governmental Maritime Consultative Organization ( IMCO) is a late 20th century In the Shipping industry classification societies are Non-governmental organizations or groups of professionals Ship surveyors and representatives of offices
Classification Societies have their own Class notations:
|Manual position control and automatic heading control under specified maximum environmental conditions||-||DP(CM)||DNV-T||-||DPS-0|
|Automatic and manual position and heading control under specified maximum environmental conditions||Class 1||DP(AM)||DNV-AUT DNV-AUTS||DP 1||DPS-1|
|Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault excluding loss of a compartment. The International Maritime Organization ( IMO) formerly known as the Inter-Governmental Maritime Consultative Organization ( IMCO) is a late 20th century The Lloyd's Register Group is a maritime Classification society and independent risk management organisation providing risk assessment and mitigation services Det Norske Veritas or DNV is a Norwegian company established in 1864 The Germanischer Lloyd AG is a Classification society based in the city of Hamburg Germany. The American Bureau of Shipping (ABS is a Classification society, with a mission to promote the security of life property and the natural environment primarily through (Two independent computer systems).||Class 2||DP(AA)||DNV-AUTR||DP 2||DPS-2|
|Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault including loss of a compartment due to fire or flood. (At least two independent computer systems with a separate backup system separated by A60 class division).||Class 3||DP(AAA)||DNV-AUTRO||DP 3||DPS-3|
Where IMO leaves the decision of which Class applies to what kind of operation to the operator of the DP ship and its client, the Norwegian Maritime Directorate (NMD) has specified what Class should be used in regard to the risk of an operation. In the NMD Guidelines and Notes No. 28, enclosure A four classes are defined:
Based on this the type of ship is specified for each operation:
Redundancy is the ability to cope with a single failure without loss of position. In Engineering, redundancy is the duplication of critical components of a system with the intention of increasing reliability of the System, usually A single failure can be, amongst others:
For certain operations redundancy is not required. For instance, if a survey ship loses its DP capability, there is normally no risk of damage or injuries. These operations will normally be done in Class 1.
For other operations, such as diving and heavy lifting, there is a risk of damage or injuries. Depending on the risk, the operation is done in Class 2 or 3. This means at least three Position reference systems should be selected. This allows the principle of voting logic, so the failing PRS can be found. For this reason, there are also three DP control computers, three gyrocompasses, three MRU’s and three wind sensors on Class 3 ships. If a single fault occurs that jeopardizes the redundancy, i. e. failing of a thruster, generator or a PRS, and this cannot be resolved immediately, the operation should be abandoned as quickly as possible.
To have enough redundancy, enough generators and thrusters should be on-line so the failure of one does not result in a loss of position. This is to the judgement of the DP operator. For Class 2 and Class 3 a Consequence Analyses should be incorporated in the system to assist the DPO in this process.
Disadvantage is that a generator can never operate at full load, resulting in less economy and fouling of the engines.
The DP operator judges whether there is enough redundancy available at any given moment of the operation. IMO issued MSC/Circ. 738 (Guidelines for dynamic positioning system (DP) operator training) on 24-06-1996. This refers to IMCA (International Marine Contractors Association) M 117 as acceptable standard.
To qualify as a DP operator the following path should be followed:
When the watchkeeping is done on a Class 1 DP ship, a limited certificate will be issued; otherwise a full certificate will be issued.
The DP Traning and Certification scheme is operated by The Nautical Institute (NI). The NI issue logbooks to trainees, they accredit training centres and control the issuance of certification.
The International Marine Contractors Association was formed in April 1995 from the amalgamation of AODC (originally the International Association of Offshore Diving Contractors), founded in 1972, and DPVOA (the Dynamic Positioning Vessel Owners Association), founded in 1990.  It represents offshore, marine and underwater engineering contractors. Acergy, Allseas, Heerema Marine Contractors, Helix Energy Solutions Group, Saipem, Subsea 7 and Technip have representation on IMCA's Council and provide the president. Previous presidents are:
While it started with the collection and analysis of DP Incidents, since then it has produced publications on different subjects to improve DP standards. It also works with IMO and other regulatory bodies.