2.1 A DP system consists of components and systems acting together to
achieve sufficiently reliable position keeping capability. The necessary redundancy
level for components and systems is determined by the consequence of a loss of
position and/or heading keeping capability. To achieve this philosophy the
requirements have been grouped into three equipment classes. For each equipment
class, the associated worst-case failure should be defined as in paragraph 2.2
below. The equipment class of the vessel required for a particular operation should
be agreed between the company and the customer based on a risk analysis of the
consequence of a loss of position and/or heading. Otherwise, the Administration or
coastal State may decide the equipment class for the particular operation.
2.2 The equipment classes are defined by their worst-case failure modes as
follows:
-
.1 For equipment class 1, a loss of position and/or heading may occur in the
event of a single fault.
-
.2 For equipment class 2, a loss of position and/or heading will not occur in
the event of a single fault in any active component or system. Common static
components may be accepted in systems which will not immediately affect
position keeping capabilities upon failure (e.g. ventilation and seawater
systems not directly cooling running machinery). Normally such static
components will not be considered to fail where adequate protection from
damage is demonstrated to the satisfaction of the Administration. Single
failure criteria include, but are not limited to:
-
.1 any active component or system (generators, thrusters,
switchboards, communication networks, remote-controlled valves,
etc.); and
-
.2 any normally static component (cables, pipes, manual valves, etc.)
that may immediately affect position keeping capabilities upon
failure or is not properly documented with respect to
protection.
-
.3 For equipment class 3, a loss of position and/or heading will not occur in
the event of a single fault or failure. A single failure includes:
-
.1 items listed above for class 2, and any normally static component
assumed to fail;
-
.2 all components in any one watertight compartment, from fire or
flooding; and
-
.3 all components in any one fire sub-division, from fire or flooding
(for cables, see also paragraph 3.5.1).
2.3 For equipment classes 2 and 3, a single inadvertent act should be considered as a
single fault if such an act is reasonably probable.
2.4 Based on the single failure criteria in paragraph 2.2, the worst-case failure
should be determined and used as the criterion for the consequence analysis (see
paragraph 3.4.2.4).
2.5 The Administration should assign the relevant equipment class to a DP vessel
based on the criteria in paragraph 2.2 and state it in the DPVAD (see paragraph
5.2).
2.6 When a DP vessel is assigned an equipment class this means that the DP vessel is
suitable for DP operations within the assigned and lower equipment classes.
2.7 It is a provision of the Guidelines that the DP vessel is operated in such a way
that the worst-case failure, as determined in paragraph 2.2, can occur at any time
without causing a breach of acceptable excursion criteria set for loss of position
and/or heading for equipment classes 2 and 3.