Section 13 Thruster-assisted positional mooring

13.1 General

13.1.1 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 Pt 3, Ch 10, 13 Thruster-assisted positional mooring will be eligible for one of the following class notations as specified in 1.2:

TA(1) See 13.1.
TA(2) See 13.2.
TA(3) See 13.3.

13.1.2 Machinery items are to be constructed, installed and tested in accordance with the relevant requirements of Pt 5 MAIN AND AUXILIARY MACHINERY, together with the requirements of 13.2 and Section 14.

13.2 Thrust units

13.2.1 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.

13.2.2 Thruster intakes are to be located at sufficient depth to reduce the possibility of ingesting floating debris and vortex formation.

13.2.3 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.

13.2.4 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.

13.2.5 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.

13.2.6 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.

13.3 Electrical equipment

13.3.1 The electrical installation is to be designed, constructed and installed in accordance with the relevant requirements ofPt 6, Ch 2 Electrical Engineering together with the requirements of 13.3.3 to 13.3.8, and the relevant requirements of Pt 3, Ch 10, 14 Thruster-assist class notation requirements.

13.3.2 Where the thruster units are electrically driven, the relevant requirements, including surveys, defined in Pt 6, Ch 2, 15 Navigation and manoeuvring systems are to be complied with.

13.3.3 The total generating capacity is to be in accordance withPt 3, Ch 10, 14.1 Notation TA(1), Pt 3, Ch 10, 14.2 Notation TA(2) and Pt 3, Ch 10, 14.3 Notation TA(3), as applicable.

13.3.4 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.

13.3.5 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.

13.3.6 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.

13.3.7 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.

13.3.8 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 inPt 6, Ch 2, 8 Protection from electric arc hazards within electrical equipmentare 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.

13.4 Control engineering systems – Additional requirements

13.4.1 The control engineering systems are to be designed in accordance with the relevant requirements of Pt 3, Ch 10, 12 Electrical and control equipment together with the additional requirements of 12.4.2 to 12.4.3 and the relevant requirements of Pt 3, Ch 10, 14 Thruster-assist class notation requirements.

13.4.2 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.

13.4.3 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)

13.4.4 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.

13.4.5 Abnormal signal errors revealed by the validity checks required by 13.4.4 are to operate alarms.

13.4.6 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.

13.4.7 Automatic controls are to be provided to maintain the desired heading of the unit.

13.4.8 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.

13.4.9 Sufficient instrumentation is to be fitted at the central control station to ensure effective control and indicate that the system is functioning correctly, see 13.4.2.