With reference to hull protection against corrosion and fouling, write brief explanation of each following
(a) Sacrificial anodes
(b) Impressed current system. Explain construction of impressed current cathodic protection system.
(c) means of preventing marine growth and fouling
(d) Describe various methods that being use to minimize effect of corrosion
(e) How does this system protect hull and rudder?
(a) Sacrificial anode system
- Sacrificial anodes are metals or alloys attached to the hull
- Fit within the hull, fit in the ballast tanks
- When immersed in SW, anodic more potential than steel
- Current flow from one metal(cathode) to other (anode), result of potential difference between them.
- Current flow result in metal remove from anode, cathode protect.
- Anodes supply cathodic protection current, consumed
- Require replacement for the protection to be maintained
- In oil tanker, Magnesium anodes not used in cargo tanks due to spark hazard
- Magnesium anodes fall and strike tank structure, when gaseous conditions exit, an explosion occur
- Zinc anodes are mostly used for safety
(b) Principle of ICCP system
ICCP system basically consists of
(1) DC current source
(2) anodes
(3) reference cell
(4) converter and controller units
(5) high quality inert protective coating on hull around the anodes.
- In converter unit, AC current transformed and rectified to DC current before supplying it to anodes.
- Anodes made of lead or platinized titanium, fitted in special recess boxes on ship’s hull.
- Reference cells made of silver / silver chloride.
- Protective coating of epoxy resin is applied, on hull around anodes for a radius of 1 meter or more, to provide a dielectric shield.
- Controller unit detects voltage between hull and reference cell and amplifies micro range reference cell current.
- Compares with preset value and automatically regulates anode current to required value.
- Impressed current required varies with (1) underwater hull area, (2) ship speed, (3) salinity (4) hull paint condition
- Low current cause inadequate protection and a high current damage paint work.
- Propeller shaft and rudder bonding arrangements fitted with impressed current systems to ensure propeller and rudder protection.
- When alongside a jetty, system switched-off, not to protect jetty.
- For tanker, while loading or discharging, system switched off
(c) MGPS: Marine Growth Protection System
Marine growth grow in ship internal sea water piping systems. This causes;
1. Engine Overheat
2. Pipes and valves become partly choked or totally blocked.
- MGPS system is based on electrolytic principle. Anode and Cathode metals submerged in sea chest or special electrolysis tank.
- A small dc current allowed to flow through copper and aluminum anodes.
- Copper produces copper ion which prevents marine growth
- Aluminum produces aluminum hydroxide which prevents corrosion on internal surface of pipes.
- This system is effective, reliable and safe in operation.
(d) Types of Hull protection systems used on ships are:
(1) Sacrificial Anode System: anodes fitted on ship hull to protect hull metal wastages.
(2) ICCP: Impressed Current Cathodic Prevention System
(3) Paint: cover layer of hull metal.
(e) System protect hull and rudder
- When ship’s steel hull in sea water, small corrosion cells set up due to slight variation in electrical potential of metal’s surface
- Electrical currents flow between high and low potential points, metal corroded from point where current leaves metal(anode)
- At point where current re-enters metal (cathode), protect
- Cathodic protection operates by providing reverse current flow to metal of hull and rudder.
What are the certificates and documents required to be kept on board the ships for the international voyages.
Certificates and Documents required to be kept on board for International Voyages For all ships
1. International Tonnage Certificate (1969)
2. International Load Line Certificate
3. Intact stability booklet
4. Damage control booklets
5. Cargo Securing Manual
6. Minimum safe manning document
7. Certificates for masters, officers or ratings
8. International Oil Pollution Prevention Certificate (IOPP)
9. International Sewage Pollution Prevention Certificate (ISPP)
10. International Air Pollution Certificate ((IAPP)
11. Oil Record Book
12. Shipboard Oil Pollution Emergency Plan (SOPEP)
13. Garbage Management Plan
14. Garbage Record Book
15. Document of Compliance (copy of DOC)
16. Safety Management Certificate(SMC)
17. International Ship Security Certificate (ISSC)
Additional Certificates and Documents for a Passenger Ship
1. Passenger Ship Safety Certificate and a Record of Equipment
2. Exemption Certificate (if exemption has been granted to the ship),
3. Search and Rescue co-operation Plan for ships trading on fixed routes
Classification Certificates
1. Classification of Hull
2. Classification of Machinery
3. Classification of Automated Installations
4. Classification of Boilers
5. Classification of Refrigeration Installations
Commercial and other Documents
1. Ship Sanitation Certificate (WHO)
2. Maritime declaration of health
3. International Certificates of Vaccination
4. Seaman: books Deck log book Engine room log book
(a) What are destructive tests and non-destructive tests?
(b) Describe two methods of destructive test & one method of the non-destructive test.
(a) Destructive test
- Test on test pieces
- Damaged after test.
Determine mechanical properties of test piece under test.
(1) Tensile test
(2) Bend test
(3) Impact test
(4) Hardness test
(5) Fatigue test
(6) Creep test
Non-Destructive Test
- Test on components
- Not damaged after test
Determine flaws or imperfection during manufacture (or) service.
(1) Liquid penetrating
(2) Ultrasonic method
(3) Magnetic crack detection
(4) Electrical test method
(5) Radiographic inspection
Destructive test
(1) Tensile Test
- Tensile test is used to determine the behaviour of a material up to its breaking point.
- A special shape specimen of standard size is gripped in the jaws of a testing machine. A load is gradually applied to draw the ends of the specimen apart such that it is subject to tensile stress up to yield point.
- The highest value of stress is known as the ultimate tensile stress (UTS) of the material.
(2) Bend Test
Specimen is bent through an angle of 1800 with an internal radius of 1.5 times the thickness of the specimen without cracking at the edges.
(3) Impact Test
- The testing machine basically consists of a pendulum which is raised and allowed to fall, striking and rupturing the specimen.
- In swinging through its arc of travel past the specimen, the pendulum will assume a lower position at the end of its travel due to loss of energy when it strikes the specimen.
- Energy given up to the specimen is its impact strength.
(4) Hardness Test
- In Brinell hardness test, a hardened steel ball is forced into the surface of the specimen with standard load.
- The diameter of the steel ball is 10mm. The load is 3000 kg for steel and 500 kg for soft metals such as brasses and bronzes. The loading period is for 15 sec. The diameter impression indicates the hardness number.
(5) Fatigue Test
- ‘Fatigue’ is defined as the failure of a material due to repeatedly applied stress.
- The specimen is rotated under load in a testing machine. So it is subject to tension and compression stresses alternately.
- The number of cycles imposed before is recorded.
(6) Creep Test
- Creep test use to find safe working stress for material working at high temperature
- It is permanent deformation resulting from loading over long period of time
- Test piece mount vertically and constant tensile load under constant temperature.
- Temperature range between 600’C to 1000’C and test period is 1000,10000,100000 hours
Non-Destructive Test
(1) Liquid penetrate test
- Industrial method, indicate presence crack, lamination lap and surface porosity.
- Fluorescent dye method and Aerosol dye method.
Fluorescent dye method
- First, the surface is cleaned using a volatile cleaner and degreaser.
- Then a fluorescent dye is applied and a certain time allowed for it to enter any flaws under capillary action.
- Then the surface is wiped clean using the cleaning spray.
- An ultra violet light is shone on the surface, any flaws showing up as the dye fluorescent.
Aerosol dye method
- The more commonly used dye penetrant method is similar in application.
- The surface is cleaned and the low viscosity penetrant is sprayed on.
- After a set time, the surface is cleaned again.
- Then a developer is used which coats the surface in a fine white chalky dust.
- The dye seeps out and stains the developer typically a red colour.
(2) Ultrasonic Test
- The probe of the test equipment transmits high frequency sound waves about 0.5 MHz to 20 MHz which are reflected by any flaws in the object
- Reflected sound waves displayed on monitor screen of cathode ray oscilloscope.
- Suitable for detection, identification and size assessment of a wide variety of both surface and sub-surface defects in materials.
- Measured thickness of material or to detect internal or surface defects in welds, casting or forging either during manufacture or when in service.
(3) Radiographic Test
- Image produced on film.
- X rays and gamma rays are used for inspection of welds, castings, forging and pressure vessels etc.
- Exposure time for x-rays and gamma rays vary with type of material, thickness and the intensity of rays.
- Faults in the metal effect the intensity of rays which passes through the material
- Film exposed by the rays gives the shadow photograph
- Used on both metallic and nonmetallic material, both ferrous and non-ferrous metal.
(a) Sketch and describe how bulwarks are constructed and attached to the deck plates.
(b) Since these are in the highly-stressed region explain how transmission of stress to the sheer strake is prevented.
(a) Bulwarks
- Bulwarks at least 1 meter high on the exposed freeboard and superstructure decks, weather decks.
- Consists of a vertical plate stiffened at top by a strong rail section
- Supported by stays from the deck. On the forecastle of ships, stays are more closely spaced.
- Protection for personnel. High stresses in bulwarks avoided
- Not intended as a major structural feature
- Connections to adjacent structure
- Light scantlings. Scantlings – size of various parts of ship, upright in house
- Freeing ports are cut in bulwarks forming wells on decks, water quickly drain away
- Required area of freeing ports is due to Load Line Convention requirement.
(b)
- Bulwark is cut for any reason, the corners are well rounded and compensation provided.
- No openings permitted in bulwarks near end of superstructure.
- lower edge of bulwark not welded to sheerstrake, because the sheerstrak cracked due to stress.
- “Floating bulwark” used. The gap between it and the deck edge acts as freeing ports.
