Marine Dynamic Positioning
For most vessels, complicated manoeuvres are only made when berthing or unberthing but for vessels working in certain sectors – particularly the offshore sector – the ability to accurately hold station for long periods is essential. To accommodate this need, vessels are fitted with some form of dynamic positioning (DP).
DP systems evolved over time and did so outside of the IMO regulations. So as to bring some uniformity to the development of DP systems, the IMO issued guidelines in 1994 in MSC/Circ.645 and this has evolved into an established international standard for DP systems. The guidelines have successfully provided the framework on which national regulations and classification society rules are based. Since MSC/Circ.645 was first published, DP has evolved from being a tool primarily for mobile offshore drilling units (MODUs) maintaining position over offshore wells, to being employed for a wide range of position-keeping operations, with systems being fitted on much larger numbers of new vessels and on an increasingly diverse set of vessels, from offshore units to shuttle tankers and passenger vessels.
In March 2016 at IMO’s Sub-Committee on Ship Systems and Equipment (SSE 3), it was agreed that further revisions were needed to the guidelines. The new guidelines finally appeared as MSC.1/Circ.1580 released in June 2017. Although voluntary unless declared otherwise by flag states, the IMO has recommended that the guidelines should apply to all vessels completed after their adoption. The earlier guidelines can still apply to older vessels. The guidelines are readily available and, at just 21 pages, short enough to make an easy read. They describe the equipment needed for various levels of DP ability and levels of training required for operators.
A vessel fitted with DP has a computerised control system that automatically maintains a vessel’s position and heading by using the ship’s propellers and thrusters to offset wind and current forces. Position reference sensors, combined with wind sensors, motion sensors and gyrocompasses, provide information to the computer about to the vessel’s position and the magnitude and direction of environmental forces affecting its position.
Although DP is related to navigation, it is normal for the DP operation centre to be located away from the main bridge. DP operators spend most of their working time in front of computer screens and there would be no advantage for them to be housed on the bridge.
The computer program needs to be populated with 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 thrust output for each thruster. 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.
As dynamic positioning systems are so specialised the manufacturing sector is quite small. There are presently three different classes of dynamic positioning systems recognised within the IMO guidelines with increasing degrees of sophistication made possible by technological developments that have evolved since the first DP system was devised.
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Equipment Class 1 has no redundancy. Loss of position may occur in the event of a single fault.
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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, thrusters, switchboards and remote-controlled valves but may occur after failure of a static component such as a cable, pipe or manual valve.
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Equipment Class 3 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 subdivision or flooded watertight compartment.
For ships to remain in one position requires highly accurate position referencing. This can be supplied either by satellite positioning or from a fixed point on the seabed if the vessel involved is a drilling unit or similar. If satellite position fixing is used, it should be noted that the position obtained by GPS is not accurate enough for use by DP, so differential GPS (DGPS) is required. DGPS makes use of a fixed ground-based reference station (the differential station) that compares its position as reported by GPS with the known position of the station. The correction is sent to the DGPS receiver by long wave radiofrequency.
For use in DP, an even higher accuracy and reliability is desirable. Companies such as Veripos, Fugro or C&C Technologies supply differential signals via satellite, enabling the combination of several differential stations. There are also systems installed on vessels that use various augmentation systems, as well as combining GPS position with GLONASS.
The choice of DP class for particular tasks is a matter for the shipowner and the client although in territorial waters the state involved may impose rules. In Norwegian waters for example, the Norwegian Maritime Authority (NMA) has specified what class should be used in regard to the risk of an operation.
Unlike the IMO guidelines, in the NMA guidelines four classes are defined :
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Class 0 Operations, where loss of position-keeping capability is not considered to endanger human lives, or cause damage.
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Class 1 Operations, where loss of position-keeping capability may cause damage or pollution of small consequence.
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Class 2 Operations, where loss of position-keeping capability may cause personnel injury, pollution, or damage with large economic consequences.
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Class 3 Operations where loss of position-keeping capability may cause fatal accidents or severe pollution or damage with major economic consequences.
Although DP systems employ computers to make the rapid adjustments to thrusters needed to maintain position, the overall operation must be monitored and controlled by trained DP Operator (DPOs). The main role of the DPO is to determine whether there is enough redundancy available at any given moment of the operation and to take appropriate action in the event of any equipment failure. In 1996, the IMO issued MSC/Circ.738 (Guidelines for dynamic positioning system (DP) operator training).
Although the IMO has issued the guidelines for DO Operator training it is a fact that in many of the main offshore oil and gas fields, rules other than SOLAS and MARPOL take precedence within a set distance from offshore facilities. These rules are drawn up by national health and safety authorities and there are differences depending on where ships are operating. In addition, the charterers of offshore ships often have their own requirements for DP operator experience and qualifications.
The IMO guidelines were in fact the work of the International Marine Contractors’ Association (IMCA) and in June 2017, the IMO approved Rev.2 of MSC.1/Circ.738, which provides a reference to guidelines issued by IMCA.
Source : https://shipinsight.com/guides/dynamic-positioning
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