Dynamic positioning

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Dynamic positioning (DP) is a system to automatically maintain a ship’s position and heading by using her own propellers and thrusters. 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. Nowadays there are more than 1000 DP ships.

History

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.

In 1961 the drillship Cuss 1 was fitted with four steerable propellers, in an attempt to drill the first Moho well. 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.

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, 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.

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.

Comparison position-keeping options
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.	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.	Advantages: Manoeuvring is excellent; it is easy to change position. No anchor handling tugs are required. Not dependent on waterdepth. Quick set-up. Not limited by obstructed seabed.
Disadvantages: No manoeuvrability once positioned. Limited to water depths of ~150 meters.	Disadvantages: Limited manoeuvrability once anchored. Anchor handling tugs are required. Less suitable in deep water. Time to anchor out varies between several hours to several days. Limited by obstructed seabed (pipelines, seabed).	Disadvantages: Complex systems with thrusters, extra generators and controllers. High initial costs of installation. High fuel costs. Chance of running off position by system failures or blackouts. Underwater hazards from thrusters for divers and ROVs. Higher maintenance of the mechanical systems.

Although all methods have there 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.

Applications

Important applications include:

• Cable-laying
• Crane vessels
• Cruise ships
• Diving support vessels
• Dredging
• Drillships
• FPSO's
• Flotels
• Landing Platform Docks
• Maritime research
• Mine sweepers
• Pipe-laying
• Platform supply vessels
• Rockdumping
• Sea Launch
• Sea-based X-band Radar
• Shuttle tankers
• Survey ships

Scope of dynamic positioning

A ship can be considered to have six degrees of freedom in its motion, i.e. it can move in any of six axes.

Three of these involve translation:

• surge (forward/astern)
• sway (starboard/port)
• heave (up/down)

and the other three rotation:

• roll (rotation about surge axis)
• pitch (rotation about sway axis)
• yaw (rotation about heave axis)

Dynamic positioning is concerned primarily with control of the ship in the horizontal plane, i.e. the three axes surge, sway and yaw.

Requirements for dynamic positioning

A ship that is to be used for DP requires:

• to maintain position and heading, first of all the position and heading need to be known.
• a control computer to calculate the required control actions to maintain position and correct for position errors.
• thrust elements to apply forces to the ship as demanded by the control system.

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 Simrad and Converteam (formerly a part of Alstom).

Reference systems

Position reference 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 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 systems (PRS) 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 (differential station) that compares the GPS position to the known position of the station. The correction is send to the DGPS receiver by long wave radio frequency. For use in DP an even higher accuracy and reliability is needed. Companies as Fugro supply differential signals via satellite, enabling the combination of several differential stations. The advantage of DGPS is that it is almost always available. Disadvantages are degrading of the signal because of sunspots or atmospheric disturbances, blockage of satellites by cranes or structures and deterioration of the signal at high altitudes.
• Hydroacoustic Position Reference, HPR. 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 soundprofile 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 position of the ship relative to the transponder can be calculated. Disadvantages are the vulnerability to noise by thrusters or other acoustic systems. Furthermore, the use is limited in shallow waters because of ray bending that occurs when sound travels through water horizontally. Main manufacturers are Kongsberg Simrad, Sonardyne and Nautronix. 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 (BOP) and interrogated through the ship’s HPR.
• Light Taut Wire, LTW. The oldest position reference system used for DP is still very accurate in relative shallow water. A clumpweight is lower 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/ CyScan. Both are laser based position reference systems. A very straightforward system, as only a small prism needs to be installed on a nearby structure. Risks are the fanbeam locking on other reflecting objects and blocking of the signal. Range depends on the weather, but is typically more then 500 meter. CyScan has the added advantage of an Auto-Tilt mechanism which compensates for waves motion by the use of actuators and gyro's.
• Artemis. A radar based system. A unit is placed on a nearby structure and aimed at the unit on board the ship. The range is several kilometres. Disadvantage is that the unit is rather heavy.
• 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. A radar based system, but no moving parts as Artemis. Another advantage is that the transponders are much smaller then the Artemis unit. Disadvantage is the short range of 100-200 meter and a limited 90 degree coverage.
• RadaScan. A radar based system similar to RADius. Advantage is the target tracking distance up to 1000 meter and 360 degree coverage.
• Inertial navigation is used in combination with GPS (Seapath) and Hydroacoustics (HAIN).

Heading reference systems

• Gyrocompasses are normally used to determine heading.

More advanced methods are:

• Ring-Laser gyroscopes
• Fibre optic gyroscopes
• Seapath, a combination of GPS and inertial sensors.

Reference systems

Besides position and heading, other variables are fed into the DP system through sensors:

• Motion Reference Units, MRU’s, determine the ship's roll, pitch and heave.
• Wind sensors are fed into the DP system feed-forward, so the system can anticipate wind gusts before the ship is blown off position.
• Draught sensors, since a change of draught influences the effect of wind and current on the hull.
• Other sensors depend on the kind of ship. A pipelay ship may measure the force needed to pull on the pipe, large crane vessels will have sensors to determine the cranes position, as this changes the wind model, enabling the calculation of a more accurate model (see Control systems).

Control systems

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 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.

Power and propulsion systems

To maintain position azimuth thrusters, bow thrusters, stern thrusters, water jets, rudders and propellors 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.

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.

Class Requirements

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.

• Equipment Class 1 has no redundancy.
  Loss of position may occur in the event of a single fault.

• Equipment Class 2 has redundancy so that no single fault in an active system will cause the system to fail.
  Loss of position should not occur from a single fault of an active component or system such as generators, thruster, switchboards, remote controlled valves etc. But may occur after failure of a static component such as cables, pipes, manual valves etc.

• Equipment Class 3 which also has to withstand fire or flood in any one compartment without the system failing.
  Loss of position should not occur from any single failure including a completely burnt fire sub division or flooded watertight compartment.

Classification Societies have their own Class notations:

Description	IMO Equipment Class	LR Equipment Class	DnV Equipment Class	ABS Equipment Class
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	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. (Two independent computer systems).	Class 2	DP(AA)	DNV-AUTR	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	DPS-3

NMD

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:

• Class 0 Operations where loss of position keeping capability is not considered to endanger human lives, or cause damage.
• Class 1 Operations where loss of position keeping capability may cause damage or pollution of small consequence.
• Class 2 Operations where loss of position keeping capability may cause personnel injury, pollution, or damage with large economic consequences.
• Class 3 Operations where loss of position keeping capability may cause fatal accidents, or severe pollution or damage with major economic consequences.

Based on this the type of ship is specified for each operation:

• Class 1 DP units with equipment class 1 should be used during operations where loss of position is not considered to endanger human lives, cause significant damage or cause more than minimal pollution.
• Class 2 DP units with equipment class 2 should be used during operations where loss of position could cause personnel injury, pollution or damage with great economic consequences.
• Class 3 DP units with equipment class 3 should be used during operations where loss of position could cause fatal accidents, severe pollution or damage with major economic consequences.

Redundancy

Redundancy is the ability to cope with a single failure without loss of position. A single failure can be, amongst others:

• Thruster failure
• Generator failure
• Powerbus failure (when generators are combined on one powerbus)
• Control computer failure
• Position reference system failure
• Reference system failure

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 redundancy of a DP ship should be judged by a FMEA study and proved by FMEA trials. Besides that, annual trials are done and normally DP function tests are completed prior to each project.

DP Operator

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:

1. a DP Induction course
2. a minimum of 30 days seagoing DP familiarisation
3. a DP Advanced course
4. a minimum of 6 months watchkeeping on a DP ship
5. a statement of suitability by the master of a DP ship

When the watchkeeping is done on a Class 1 DP ship, a limited certificate will be issued; otherwise a full certificate will be issued.

IMCA

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:

• 1995-6 - Derek Leach, Coflexip Stena Offshore
• 1997-8 - Hein Mulder, Heerema Marine Contractors
• 1999/2000 - Donald Carmichael, Coflexip Stena Offshore
• 2001-2 - John Smith, Halliburton Subsea/Subsea 7
• 2003-4 - Steve Preston, - Heerema Marine Contractors
• 2005 - Frits Janmaat, Allseas Group

(2005 Vice-President - Knut Boe, Technip)

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.

References

External links

• [IMO, International Maritime Organization]
• [IMCA, International Marine Contractors Association]
• [IMCA DP Intro]
• [NMD, Norwegian Maritime Directorate]
• [OPL Oilfield Seamanship Series - Volume 9: Dynamic Positioning - 2nd Edition] by David Bray

 

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