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Automatic ship station- and heading-holding systems From Wikipedia, the free encyclopedia
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 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 (MODU), oceanographic research vessels, cable layer ships and cruise ships.
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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 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.[1]
Dynamic positioning began in the 1960s for offshore drilling. With drilling moving into ever deeper waters, Jack-up barges could not be used any more, 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 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 meters. 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.[2]
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, but also 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. All have their own advantages and disadvantages.
Jack-up barge | Anchoring | Dynamic positioning | |
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Disadvantages |
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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.e., it can translate and rotate on three perpendicular 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.e. the translation along the two horizontal axes (surge and sway) and rotation on the vertical axis (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.
Maintaining a fixed position is particularly difficult in polar conditions because ice forces can change rapidly. Ship-borne ice detection and mitigation is not sufficiently developed to predict these forces, but may be preferable to sensors placed by helicopter.[3]
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 (ex L-3), MT-div. Chouest,[check spelling] Rolls-Royce plc, Praxis Automation Technology, Brunvoll AS. The term digital anchor has been used to describe such dynamic positioning systems.[4] . 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:
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. 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. 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. For this reason, the model also has input from the wind sensors 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. This process is known as dead reckoning.
The accuracy and precision of the different PRSs 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 weighted. Based on variance a PRS receives a weight between 0 and 1.
To maintain position azimuth thrusters (electric, L-drive or Z-drive) bow thrusters, stern thrusters, water jets, rudders and propellers are used. 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. These fluctuations may be suitable for hybrid operation. An LNG-powered platform supply vessel started operation in 2016 with a 653 kWh/1600 kW battery acting as spinning reserve during DP2, saving 15-30% fuel.[11] The 154-meter North Sea Giant has combined 3 powerpacks, switchboards and 2 MWh batteries to operate in DP3 using only one engine,[12][13] keeping the engine load between 60% and 80%.[14]
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.
Based on IMO (International Maritime Organization) publication 645[15] the Classification Societies have issued rules for Dynamic Positioned Ships described as Class 1, Class 2 and Class 3.
Classification Societies have their own Class notations:
Description | IMO Equipment Class | LR Equipment Class | DNV Equipment Class | GL Equipment Class | ABS Equipment Class | NK Equipment Class | BV Equipment Class |
Manual position control and automatic heading control under specified maximum environmental conditions | - | DP(CM) | DYNPOS-AUTS | - | DPS-0 | - | |
Automatic and manual position and heading control under specified maximum environmental conditions | Class 1 | DP(AM) | DYNPOS-AUT & DPS1 | DP 1 | DPS-1 | DPS A | DYNAPOS AM/AT |
Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault excluding loss of a compartment. (Two independent computer systems). | Class 2 | DP(AA) | DYNPOS-AUTR & DPS2 | DP 2 | DPS-2 | DPS B | DYNAPOS AM/AT R |
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) | DYNPOS-AUTRO & DPS3 | DP 3 | DPS-3 | DPS C | DYNAPOS AM/AT RS |
DNV rules 2011 Pt6 Ch7 introduced "DPS" series of classification to compete with ABS "DPS" series.
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 Authority(NMA) has specified what Class should be used in regard to the risk of an operation. In the NMA Guidelines and Notes No. 28, enclosure A four classes are defined:
Based on this the type of ship is specified for each operation:
Loss of position, also known as runoff, can be a threat to safe operations and the environment, including possible loss of life, injury, damage to property or the environment, and loss of reputation and time. Incident records indicate that even vessels with redundant dynamic positioning systems are subject to occasional loss of position, which can be due to human error, procedural failure, dynamic positioning system failures, or bad design.[16]
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Dynamic positioning failure results in an inability to maintain position or heading control, and can be a drift off caused by insufficient thrust, or a drive off caused by inappropriate thrust.[16]
The basic response with a closed bell is similar to wet bell, but after stowing umbilicals, the hatch will be sealed so that internal pressure can be retained. The bell will be recovered as rapidly as possible in a red alert, and may be recovered if there is doubt that a yellow alert will be downgraded.[19]
Redundancy is the ability to withstand, while on DP mode, the loss of equipment which is online, without losing position or heading. 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 sufficient redundancy, enough generators and thrusters should be on-line so the failure of one does not result in a loss of position. This is left to the judgment of the DP operator. For Class 2 and Class 3 a Consequence Analysis should be incorporated in the system to assist the DPO in this process.
The redundancy of a DP ship should be judged by a failure mode and effects analysis (FMEA) study and proved by FMEA trials.[20] Besides that, annual trials are done and normally DP function tests are completed prior to each project.
The DP operator (DPO) 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[21] 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 training 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.
With ever more DP ships and with increasing manpower demands, the position of DPO is gaining increasing prominence. This shifting landscape led to the creation of The International Dynamic Positioning Operators Association (IDPOA) in 2009. www.dpoperators.org
IDPOA membership is made up of certified DPO's who qualify for fellowship (fDPO), while Members (mDPO) are those with DP experience or who may already be working within the DP certification scheme.
The International Marine Contractors Association was formed in April 1995 from the amalgamation of the Dynamic Positioning Vessel Owners Association, founded in 1990, and the International Association of Offshore Diving Contractors, founded in 1972.[22]
While it started with the collection and analysis of DP Incidents,[23] since then it has produced publications on different subjects to improve standards for DP systems. It also works with IMO and other regulatory bodies.
The Marine Technology Society Dynamic Positioning (DP) Committee's mission is to facilitate incident free DP operations through sharing of knowledge. This committee of dedicated volunteers delivers value to the DP community of vessel owners, operators, Marine Class Societies, engineers and regulators through an annual DP Conference, topical workshops and an extensive set of Guidance Documents covering DP Design Philosophy, DP Operations and Professional Development of DP Personnel. In addition, a growing set of unique documents called TECHOP's address specific topics of significant interest and impact. Conference papers are available for download by the public, providing the most comprehensive single source of DP industry technical papers available anywhere.
The DP Guidance documents published by the MTS DP Committee are designed to disseminate the knowledge, methods and unique tools to aid the DP community in achieving incident free DP operations. The documents are free to download from the Committee's website http://dynamic-positioning.com
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