Section
13 Thruster-assisted positional mooring
13.1 General
13.1.1 Thrusters which provide safety functions are subject to the requirements of
classification.
13.1.2 Thrusters used for positioning of the unit into the weather during normal operation
to minimise motions of the unit and optimise production are not normally considered
to be providing a safety function and therefore not subject to classification.
13.1.3 Thrusters used for heading control of the unit during side to side offloading
operations may be subject to the requirements of classification depending on the
operational intent during offloading. An appropriate risk assessment using a
methodology acceptable to LR shall be carried out to determine whether the thrusters
are providing a safety function and classed for such offloading duty or not.
13.1.4 The risk assessment shall consider whether the thrusters are providing any safety
functions before, during or at the end of off-loading.
13.1.5 The risk assessment shall consider the boundary of the system to be assessed. This
boundary condition should include items such as offloading arms, ship to ship
coupling and decoupling systems that are structurally attached to the vessel.
13.1.6 The risk assessment should be carried out in accordance with a recognised National or
International Standard, such as those detailed in IEC/ ISO 31010 Risk Management
– Risk Assessment techniques; see also
Pt 3, Ch 15, 1.1 General for the use of a risk
assessment to identify safety critical systems.
13.1.7 Where a Thruster-assisted positioning system is classed, it shall be for a clearly
defined operational duty. Should the operational duty change, the system shall be
subject to a new risk assessment.
13.2 Class Notations
13.2.1 Where the positional mooring system is assisted by thrusters, as defined
in Pt 3, Ch 10, 4 Design aspects, units complying with the requirements of
this Section together with the requirements in will be eligible
for one of the following class notations as specified in 1.2:
13.3 Thrust units
13.3.1 Thruster installations are to be designed to minimise potential
interference with other thrusters, sensors, hull or other surfaces which could be
encountered in the service for which the unit is intended.
13.3.2 Thruster intakes are to be located at sufficient depth to reduce the
possibility of ingesting floating debris and vortex formation.
13.3.3 Steerable thrusters and thrusters having variable pitch propellers are to
be provided with two independent supplies of motive power to the pitch and direction
actuating mechanisms.
13.3.4 Each thruster unit is to be provided with a high power alarm. The
setting of this alarm is to be adjustable and below the maximum thruster output.
13.3.5 The response and repeatability of thrusters to changes in propeller pitch
or propeller speed/direction of rotation are to be suitable for maintaining the area
of operation and the heading deviation specified.
13.3.6 The thrust unit housing is to be tested at a hydraulic pressure of not
less than 1,5 times the service immersion head of water or 1,5 bar (1,5
kgf/cm2), whichever is the greater.
13.4 Electrical equipment
13.4.4 Where the electrical power requirements are supplied by one generator
set, on loss of power there is to be provision for automatic starting and connection
to the switchboard of a standby set and automatic restarting of essential auxiliary
services. For other requirements relevant to particular thruster-assisted class
notations, see
Pt 3, Ch 10, 14 Thruster-assist class notation requirements.
13.4.5 An alarm is to be initiated at the thruster-assisted positioning control
station(s) when the total electrical load of all operating thruster units exceeds a
preset percentage of the running generator(s) capacity. This alarm is to be
adjustable between 50 and 100 per cent of the full load capacity, having regard to
the number of electrical generators in service.
13.4.6 The number and ratings of power transformers are to be sufficient to
ensure full load operation of the thruster-assisted positioning system even when one
transformer is out of service. This does not require duplication of a transformer
provided as part of a transformer/silicon controlled rectifier (SCR) drive unit.
13.4.7 Thruster auxiliaries, control computers, reference systems and
environmental sensors are to be served by individual circuits. Services that are
duplicated are to be separated throughout their length as widely as is practical and
without the use of common feeders, transformers, converters, protective devices or
control circuits.
13.4.8 Where the auxiliary services and positioning mooring thrusters are
supplied from a common source, the following requirements are to be complied
with:
- The voltage regulation and current-sharing requirements defined
in Pt 6, Ch 2, 8.1 Hazard identification, calculations and testing are to be maintained over
the full range of power factors that may occur in service.
- Where SCR converters are used to feed the thruster motors, and
the instantaneous value of the line-to-line voltage wave-form on the a.c.
auxiliary system busbars deviates by more than 10 per cent of
times the r.m.s. voltage from the instantaneous value of
the fundamental harmonic, the essential auxiliary services are to be capable
of withstanding the additional temperature rise due to the harmonic
distortion. Control, alarm and safety equipment is to operate satisfactorily
with the maximum supply system wave-form distortion, or be provided with
suitably filtered/converted supplies.
- When the control system incorporates volatile memory it is to
be supplied via uninterruptible power supplies provision for automatic
starting and connection to the (UPS), see also
Pt 6, Ch 1, 2.9 Programmable electronic systems – General requirements.
13.5 Control engineering systems – Additional
requirements
13.5.2 Indication of the following is to be provided at each station from which
it is possible to control the thruster-assisted positioning system, as
applicable:
- The heading and location of the vessel relative to the desired
reference point or course.
- Vectorial thrust output, individual and total.
- Operational status of position reference systems and
environmental sensors.
- Environmental conditions, e.g., wind speed and direction.
- Availability status of standby thruster units.
13.5.3 Alarms are to be provided for the following fault conditions where
applicable:
- When the unit deviates from its predetermined area of
operation.
- When the unit deviates from its predetermined heading
limits.
- Position reference system fault (for each reference
system).
- Gyrocompass fault.
- Vertical reference sensor fault.
- Wind sensor fault.
- Taut wire excursion limit.
- Automatic changeover to a standby position reference system or
environmental sensor.
- Control computer system fault.
- Automatic changeover to a standby control computer system,
see
Pt 3, Ch 10, 14.3 Notation TA(3) 14.3.3
13.5.4 Suitable processing and comparative techniques are to be provided to
validate the control system inputs from position and other sensors, to ensure the
optimum performance of the thruster-assisted mooring system.
13.5.6 The control system for thruster-assisted positioning operation is to be
stable throughout its operational range and is to meet the specified performance and
accuracy criteria.
13.5.7 Automatic controls are to be provided to maintain the desired heading of
the unit.
13.5.8 The deviation from the desired heading is to be adjustable, but is not
to exceed the specified limits. Arrangements are to be provided to fix and identify
the set point for the desired heading.
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