Dynamic positioning

Factory

Dynamic positioning started in the 1960s 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 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.

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.

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.

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.

Applications

Important applications include:

Servicing Aids to Navigation (ATON)

Cable-laying

Crane vessels

Cruise ships

Diving support vessels

Dredging

Drillships

FPSOs

Flotels

Landing platform docks

Maritime research

Mine sweepers

Pipe-laying ship

Platform supply vessels

Rockdumping

Sea Launch

Sea-based X-band radar

Shuttle tankers

Survey ships

Scope

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

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.

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.

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 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.

Power and propulsion systems

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.

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:

DNV rules 2011 Pt6 Ch7 introduced "DPS" series of classification to compete with ABS "DPS" series.

NMA

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:

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 withstand, while on DP mode, the loss of equipment which is online, without losing position and/or heading. 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 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. Besides that, annual trials are done and normally DP function tests are completed prior to each project.

DP operator

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 as acceptable standard.

To qualify as a DP operator the following path should be followed:

1. a DP Induction course + On-line Examination
2. a minimum of 60 days seagoing DP familiarisation
3. a DP Advanced course + On-line Examination
4. a minimum of 60 days 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.

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.

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, J. Ray McDermott, 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 standards for DP systems. It also works with IMO and other regulatory bodies.

Marine Technology Society Dynamic Positioning Committee

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