DYNAMIC POSITIONING

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A Dynamic Positioning Ship is very helpful in monitoring the natural occurrences that take place offshore and aids in ships to maintain its position in the deep sea by pinpointing about the wind and the wave data which would otherwise make a ship lose control and veer off its course. Through Dynamic Positioning, a ship does not require the usage of anchors to maintain its course in the deep waters and thus can carry out its main purpose well. Ships with dynamic positioning system are known as dynamic positioning ships. 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 gyro compasses, 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, but are not limited to, ships and semi-submersible mobile offshore drilling units (MODU), oceanographic research vessels and cruise ships.
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.

How Dynamic Positioning Ships Work?
The working of a Dynamic Positioning Ship is quite simple. There is a control panel which notes the wind and the wave fluctuation and accordingly sends appropriate signals to the propellers so as to enable the ship to steady and maintain its course. There are, however three different levels of Dynamic Positioning that can be used and it depends on the type of the ship on which Dynamic Positioning has to be enabled.
Level I Dynamic Positioning System
Ships whose off-course drifting will not have any impact on the life of the crew or on any marine creature are generally enabled with a Level I Dynamic Positioning System. This is the most basic Dynamic Positioning system and it does not have any advanced tools that ships with the other two Dynamic Positioning systems require.
Level II Dynamic Positioning System
A Level II Dynamic Positioning system in built in a ship whose off-course veering will tend to cause serious problems. A Dynamic Positioning Ship enabled with a Level II Dynamic Positioning system contains high-end computer applications and diving watercrafts in case the ship encounters any major problem in the deep sea.
Level III Dynamic Positioning System
A Dynamic Positioning Ship with a Level III Dynamic Positioning contains similar equipments like a Level II Dynamic Positioning system but with a back-up Dynamic Positioning system at some other location. The aspect of back-up is important because they will act as emergencies in case the main Dynamic Positioning system gets destroyed due to any water penetration or occurrence of fire or short circuit or any other inadvertent casualty. Generally oil tankers which drill in the deep parts of the ocean are equipped with a Level III Dynamic Positioning system.
At present, only a few elite ships and naval vessels incorporate the usage of Dynamic Positioning. But since conserving and preserving the environment and the eco-system has become the need of the hour, it is only logical that Dynamic Positioning becomes the future of marine vessels in order to preserve the marine ecology with every bit of technological knowledge man has in his power and disposal.
Dynamic Positioning is generally used in research ships and drilling vessels which have to venture into the deepest parts of the ocean and sea where winds and waves tend to be perpetually altering. In situations like this, it could prove very tedious for a ship’s crew to lay the anchors. A ship enabled with Dynamic Positioning can get to know about the changes in the wind and the waves and thus alter its course suitably without having to compromise on its main purpose.

Ships enabled with Dynamic Positioning are independent of anchors and other support system in the sense that a Dynamic Positioning Ship enables the use of pushers and propellers to make the ship stay on course and steady rather than get carried away by the fluctuating winds and waves. This is perhaps the most advantageous feature of the system of Dynamic Positioning. In the earlier days, when ships used to enter the deeper parts of the ocean and the seas, there always used to be a threat of ships colliding with another ship because of natural movement of the wind and the waves or ships veering off course and thus getting lost and never to be found. But since the development of Dynamic Positioning which incorporates the usage of SONAR, Radar and other comprehensive detection, ships have started finding it easy to maintain their pace and steady their being in the deeper parts of the oceanic and sea waters.

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
2. a minimum of 30 days seagoing DP familiarisation
3. a DP Advanced course
4. a minimum of 180 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

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.

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 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 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 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