Q. Briefly discuss the following with respect to safety of navigation:
(a) Bridge Navigation Watchkeeping Alarm System (BNWAS) (6 marks)
(b) Long Range Identification and Tracking of ships (LRIT) (5 marks)
(c) Voyage Simplified voyage recorder (VDR/S-VDR) (5 marks)
BNWAS is a monitoring & Alarm system which informs other navigational officers or master of the ship if the officer on watch(OOW) does not acknowledge or he/she is incapable of carrying out the watch duties efficiently, which can lead to maritime accidents.
The function of the bridge navigational watch alarm system (BNWAS) is to monitor bridge activity & detect operator inability which could lead to marine accidents. The system monitors the awareness of the Officer of the Watch (OOW) & automatically alerts the Master or another qualified OOW if for any reason the OOW becomes not capable of carrying out the OOWs duties. This is achieved through a mix of alarms & indications which alert backup OOWs as well as the Master. BNWAS warnings are given in the case of inability of the watch keeping officer due to accident, sickness or in the event of a security breach, e.g. piracy &/or hijacking. Unless decided by the Master only, BNWAS shall remain operational at all times.
Why Ships Need BNWAS?
There have been many accidents in the past wherein ships have collided or grounded due to wrong decision or inability in taking a decision at the correct time. If during an emergency situation, a navigational officer is not competant of handling that situation, it can lead to devastating scenarios. To prevent this BNWAS is fitted on the bridge which acts similar to a dead man alarm in the engine room.
A series of alert & alarm is first sounded by the BNWAS in the Navigation Bridge to alert officer on watch. If there is no response to the series of alarms, then BNWAS will alert other Deck officers, which may include Master of ship, so that someone can come out on the bridge & handle the situation & tackle the problem.
Operational Requirements of BNWAS-The BNWAS primarily has 3 modes of operation:
2. Manual ON
3. Manual OFF
Alarms and Indications
- Once the BNWAS is started, the dormant period should be between 3 to 12 minutes. This dormant period is the time in which the BNWAS is working without giving any alarm & it only once the dormant period is over that the alarm is sounded & the alarm/indication is sounded & the reset function required to be activated.
- Once the dormant period ends, a visual indication(first stage; flashing indication) must be activated indicating/demanding that the officer rest it, if available & active
- If not acknowledged within 15 seconds of the visual indication, an audible alarm is activated(first stage)
- If at the first stage the audible alarm is not acknowledged, 15 seconds after the audible alarm, other audible alarm (second stage; sound should have its own characteristic tone or modulation intended to alert, but not to startle, the OOW) is sounded in the backup officer’s &/or Master’s cabin
- If at the second stage the audible alarm is not acknowledged, 90 seconds after it, another audible alarm is sounded (third stage; easily recognizable, indicates urgency, volume sufficient for it to be heard throughout the locations above & to wake sleeping persons) at the locations of further crew members competent of taking corrective actions
- Except for passenger vessels, the second & third stage alarm can be combined to sound at all locations. If this is applied, the third stage alarm may be omitted
- For the very large vessels, sufficient time of up to 3 minutes must be accounted for the backup officer or the Master to the reach the Bridge to tend to the situation.
The Reset Function
- Reset can only be done from physically located areas on the bridge providing proper look out.
- Reset can be only be performed with a single operator action (for those familiar with BNWAS, one stroke to the round reset switch) which starts the dormant period further, cancelling the alarms/indications
- Constant activation of the dormant period vis a vis the reset button should not extend the dormant period to more than that is set or bring about any suppression of the alarms/indications
- An “Emergency Call” function must be present to immediately skip to the second & third stage alarms. This is present for the OOW to ask immediate assistance
- Accuracy of the BNWAS should be within 5% or 5 seconds, whichever is lesser
- Operational Mode & the duration of the Dormant Period setting shall be restricted to the Master only
- To be powered from the main power supply. The malfunction indication, & all elements of the Emergency Call facility should be powered from a battery maintained supply
- Outputs should be available for the integration with other bridge equipment if required.
Regulations for BNWAS
SOLAS Chapter V Regulation 19 states:
- Cargo ships of 150 gross tonnage & more & passenger ships irrespective of size constructed on or after 1st July 2011
- Passenger ships irrespective of size constructed before 1st July 2011, not later than the first survey after 1st July 2012
- Cargo ships of 3,000 gross tonnage & more constructed before 1st July 2011, not later than the first survey after 1st July 2012
- Cargo ships of 500 gross tonnage & more but less than 3,000 gross tonnage constructed before 1st July 2011, not later than the first safety survey after 1st July 2013
- Cargo ships of 150 gross tonnage & more but less than 500 gross tonnage constructed before 1st July 2011, not later than the first survey after 1 July 2014
A BNWAS fitted prior to 1st July 2011 may subsequently be exempted from full compliance with the standards accepted by the organization, at the discretion of the Administration.
b. Long Range Identification and Tracking (LRIT) System
LRIT is an international identification & tracking system included by the IMO under its SOLAS convention to assure a thorough tracking system for the ships across the world.
It came into existence on the 19th May 2006 & was included formally starting from January 2008. Based on these lines, those ships which were built on or following 31st Dec. 2008 were required to have this system of vessel identification.
The vessel tracking system is a clear system that does not permit any confusion to creep in with respect to the existing UNCLOS. In a similar manner, the tracking of ships system does not interfere with the individual maritime operational laws of the countries where it is applicable.
As per the LRIT requirements, the ships that come under its purview are:
- All ships used in passenger transportation. Such a criteria includes even the faster & speedier ships
- All offshore rigs used for the purpose of drilling oil in the high seas
- All ships used for the purpose of cargo transportation. This criterion also includes speedier vessels as also ships with a weight of over 300 gross tons(GT).
The main requirements to the system’s application are the following:
- The transmitting device & gadget to send the information data
- Specific providers for this type of communication service. One such service provider is the Absolute Maritime Tracking Services, Inc. (AMTS) formed by the Panama Flag Registry. This service provider is the only service provider to those ships under the Panamanian registration
- Centers of data for the system
- Service providers for the overall application of the system
- A thorough distribution plan for the data collected from the system
- International data exchange for the LRIT application also forms an important part of the overall system requirements.
(C) Voyage Data Recorder/ Simplified-VDR (VDR/S-VDR)
A VDR or voyage data recorder is an instrument safely fitted on the ship to continuously record vital information linked to the operation of the vessel. It contains a voice recording system for a period of at least 12 hours. This recording is recovered & made use of for investigation in events of accidents.
The IMO defines the Voyage Data Recorder as a complete system, including any items needed to interface with the sources of input signals, their processing & encoding, the final recording medium, the playback equipment, the power supply & dedicated reserve power source.
Akin to the ‘Black Box’ on airplanes, a Voyage Data Recorder is an equipment installed onboard the ships that record the various data on the ship which can be used for reconstruction of the voyage details & vital information during an accident investigation.
Information is stored in a secure & retrievable form, associated to the position, movement, physical status, command & control of a ship over the period & following an incident. This information is used during any subsequent safety investigation to identify the reason of the accident. Aside from its usage in accident investigation, it can also be used for the preventive maintenance, performance efficiency monitoring, heavy weather damage analysis, accident avoidance & training purposes to enhance safety & reduce running costs.
Carriage requirements for VDR
As with all the navigational equipment carried onboard, the VDR also comes under the purview of the SOLAS Chapter V, Regulation 20 as well as Annex 10.
The VDR at least must record the following:
- Date and time (SVDR)
- Ship’s position (SVDR)
- Speed and heading (SVDR)
- Bridge audio (SVDR)
- Communication audio (radio) (SVDR)
- Radar data (SVDR)
- ECDIS data (SVDR)
- Echo sounder
- Main alarms
- Rudder order and response
- Hull opening (doors) status
- Watertight and fire door status
- Speed and acceleration
- Hull stresses
- Wind speed and direction
The SVDR is nothing but a simplified VDR that records information that is only absolutely essential & does not record information as extensive as the VDR. Generally, it is more cost effective & more in usage on board merchant ships. The concept of the SVDR can be best understood by comparing the data below with that of the VDR. Mandatory information to be recorded in an SVDR are marked next to the category above. The last two interfaces of the Radar & ECDIS may be recorded only if there are standard interfaces available.
Q. Explain three methods of over current protection for electrical circuit. (8 marks) June2019,Feb-2019
Ans- Overcurrent protection is protection against the excessive currents or current beyond the acceptable current rating of the equipment. It normally works instantly. Short circuit is a type of overcurrent. Magnetic circuit breakers, fuses & overcurrent relays are commonly used to provide overcurrent protection.
Magnetic circuit breaker- is a thermal bimetallic strip that responds to the prolonged low level electrical surges or overloads of the electrical currents. Excess electrical currents will heat the bimetallic strip enough to bend it towards a trip bar that turns the circuit off.
Fuses- In the field of electronics or electrical, a fuse is an essential device used in various electrical circuits which gives the protection from the overcurrent. It consists of a strip or a metal wire that dissolves when the heavy flow of current supplies through it. Once this device has functioned, an open circuit, it ought to rewire or changed based on the type of fuse. A fuse is an automatic disconnection of supply.
Overcurrent relay- A relay that operates or picks up when its current exceeds a predetermined value (setting value) is called Overcurrent Relay. Overcurrent relays are used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors.
Q. Explain with aid of diagram the meaning of the term inverse current time characteristic( 8 mark). June 2019
Ans- Time-current curves are used to show the amount of time required for a circuit breaker to trip at a given overcurrent level.
Time-current curves are generally shown on a log-log plot. Figures along the horizontal axis of the curve shows the continuous current rating (In) for the circuit breaker. The figures along the vertical axis shows time in seconds.
To find how long a breaker will take to trip: find the current multiple of (In) at the bottom of the graph. Next, draw a vertical line to the point where it intersects the curve & then draw a horizontal line to the left side of the graph to get the trip time.
The total clearing time of the circuit breaker is the sum of the breaker’s sensing time, unlatching time, mechanical operating time & arcing time. Curves are formed using predefined specifications such as operation at an ambient temperature of 40°C, so keep in mind that the actual operating conditions of the circuit breaker can cause variations in its performance.
Most curves contain information box that will define which circuit breaker the curve applies to. This information box may also contain important notes from the manufacturer such as the allowable deviation from the trip times.
Inverse Time Relay
In this type of relays, the time of operation rely upon the magnitude of actuating quantity. If the magnitude of actuating quantity is very high, the relay operation is very fast. In other words, the relay operating time that is time delay in the relay is inversely proportional to the magnitude of the actuating quantity. The general characteristics of an inverse time relay is shown in the figure below.
In the graph it is clear that, when, actuating quantity is OA, the operating time of the relay is OA’, when actuating quantity is OB, the relay operating time is OB’ & when actuating quantity is OC, the relay operating quantity is OC.
In the graph above, it is also clear that, when actuating quantity is less than OA, the relay operating time becomes infinity, which means for actuating quantity less than(<) OA, the relay does not at all actuate. This minimum value of actuating quantity for which a relay initiates its operation is called as pick up value of actuating quantity. Here it is denoted as OA.
It is also clear from the graph that, when actuating quantity near to the infinity along x axis the operating time does not approach to zero(0). The curve approaches to an approximately constant operating time. This is approximately minimum time needed to operate the relay.
The inverse time relay, where the actuating quantity is current, is called as inverse current relay.
In this type of relay, the inverse time is obtained by attaching some mechanical accessories in the relay.
Inverse time delay is achieved in the induction disc relay by providing a permanent magnet in such a way, that, when disc rotates, it cuts the flux of the permanent magnet. Due to this, current is induced in the disc which slows down the movement of the disc. A solenoid relay can be converted to inverse time relay, by providing a piston & an oil dash-pot. A piston, attached to the moving iron plunger, is submerged in the oil in a dash-pot. When the solenoid relay is actuated, the piston moves upwards along with iron plunger.
Viscosity of oil slows the upward movement of the plunger. The speed of this upward movement against gravity also rely upon how strongly the solenoid attracts the iron plunger. This attraction force of the solenoid rely upon the magnitude of the actuating current. Hence, time of operation of the relay is inversely proportional to the actuating current.
Q. Briefly explain the principle of operation of induction motor.(4 mark ). June2019 (12.).
(a) What is slip for an induction motor? (4Mark). June2019
Slip- Slip can be defined as the difference between the flux speed (Ns) & the rotor speed (Nr). Speed of the rotor of an induction motor is always less than(<) its synchronous speed. It is generally expressed as a percentage of the synchronous speed (Ns) & represented by the symbol ‘S’.
% Slip= Ns – Nr/Ns X 100
Q. Draw a simple ladder logic diagram of star delta starting of an induction motor ( 8 mark).
Rung 1 Main contactor:
The main contactor depends upon the normally open input start push button (I1), normally closed stop button (I2) & normally closed overload relay.
It means that Main contactor will only be energized if start button is pressed, while stop is not pressed & overload relay is not activated. A normally open input named (Q1) is added in parallel to the start button I1.
By doing so, a push button is created which means that once motor is started, it will be kept started even if the start button is released
Rung 2 Star contactor:
Star contactor depends upon main contactor, normally close contacts of timer (T1), & normally close contacts of output delta contactor (Q3).
So star contactor will only be energized if main contactor is ON, time output is not activated & delta contactor is not energized.
Timer T1 measures the time after which the winding connection of star delta starter is to be changed. It will start counting time after main contactor is energized.
Rung 3 Delta contactor:
Delta contactor will be energized when the main contactor (Q1) is energized, timer T1 is activated & star contactor (Q2) is de-energized.