DP Class

There are 3 accepted classes of dynamic positioning setups, each relating to the level of redundancy that system provides.

Class 1 offers no redundancy. A single failure in the system will result in a loss of station keeping ability. A class 1 vessel will be equipped with a single control computer & associated systems.

Class 2 is set up so that any single equipment fault will not cause the system to fail. This is a fully redundant system. Class 2 vessels have dual control computers & enough thrusters that the vessel will still retain position even if you lose half of them, provided the operator ensures Redundancy is maintained.

Class 3 systems take class 2 a step further by adding a 3rd control station, so that even a completely burnt or flooded compartment will not cause a failure. Drill ships are a good example of class 3.  A control room equipped with an additional control station is stationed separate from the main bridge. This way in the event of a flooding or fire that affects the main bridge, control can be transferred to the auxiliary station & position keeping should not be compromised.

In basic terms, equipment Class 1 refers to non-redundant vessels, Class 2 relates to vessels with full redundancy of systems and equipment, while vessels built or fitted to equipment Class 3 are able to withstand the loss of all systems in any one compartment from the effects of fire or flooding.

References

3 seperate Position Reference Systems are required for Class 2 & 3 operations.  Some examples are:

• DPGS
• Tautwire
• Hydro Acoustic Beacons
• Fanbeam
• Artemis

It should be noted that while many vessels run multiple GPS systems, when it comes to DP operations they must be classed as a single reference, even if they receive their differential corrections from different sources.  This is because the main source of GPS error is Ionospheric Delay, which effects all GPS systems in the area equally.

The Norwegian Maritime Directorate

The NMD additionally defines 4 operational levels of risk & specifies what class of dynamic positioning vessel is to be used for those operations.

• 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 dynamic positioning 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.

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

Consequence Analysis

One of the requirements of the IMO Class 2 and 3 guidelines, is a system of Online Consequence Analysis to be incorporated in the DP system. This function continually performs an analysis of the vessel’s ability to maintain its position and heading after a predefined, worst case failure during operation. Possible consequences are based on the actual weather conditions, enabled thrusters and power plant status. Typical worst-case single failures are:

• failure in the most critical thruster
• failure in one thruster group
• failure in one power bus section

The typical redundant DP vessel is based on two almost identical half systems for power & thruster configuration & is controlled by a dual control system.  When setup properly each half shall be able to continue operation after full failure of the other half.  Both halves will continue normally even after the failure of one control system.