With reference to dynamic positioning system what do you understand by the term redundancy?

With the aid of a simple block diagram explain the difference between [dynamic positioning] DP 1, DP 2, And DP3. Enumerating their relative advantages and disadvantages.

“Ability of a component or system to maintain or restore its function immediately or in an acceptable time for the task of the ship, when a single failure has occurred. Redundancy can be achieved for instance by installation of multiple components, multiple systems or alternative means of performing a required function.

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.

Disadvantage is that a generator can never operate at full load, resulting in less economy and fouling of the engines.

To avoid the engines to work under low loads, there's an allocation mode to the thrusters called "biasing", in which the thrusters are set in pairs counteracting each other, with a precise control of such counteraction, in order to adjust the generators/engines working load. When an engine works under a low load for some time it accumulates carbon deposits on the upper part of the cylinder, which can damage the pistons and all other moving equipment related to the piston movement.

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.

Automatic and manual position and heading control under specified maximum environmental conditions	DP 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).	DP 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).	DP 3

[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]