Hull survey methods, are means and procedures to detect failure and damage at an early stage to avoid premature breakdown.
Hull survey methods are therefore not only comprehensive means of detecting deficiencies or monitoring structural condition, but also of defining schemes for inspection between the last overhaul and before the occurrence of failure.
Means of detection of defects and condition monitoring are inter alia:
- Visual inspections
- Non-destructive testing (NDT) and calibrating
- Examination of tightness, function and centre of gravity
- Measurements of thickness, vibration
Schemes of inspection are periodical survey requirements which by virtue of design and operational experience are envisaged to discover deficiencies completely and early enough before they may lead to breakdown.
1. MEANS OF HULL CONDITION ASSESSMENT
1.1 Visual Inspection
A major part of hull surveying work is carried out using visual skills to perform the examinations and to arrive at an opinion on the state of a vessel’s condition.
Such visual examinations can be carried out as:
- Over-all inspections, a general sighting of a vessel's hull condition, followed by
- Close-up examinations at
- locations where discontinuities, ruptures or deformations have been found and
- certain hull structures as stipulated by rules and/or requirements, for instance in way of cargo area of oil tanks.
- Examination of areas of suspected crack and corrosion concentration.
The methods of visual inspection procedures may be applied as follows:
1.1.1 Visual over-all inspection
Examination of external hull body
Visual attention is to be focused on the vessel’s shapes, lines and curves for the detection of
- unusual deformation,
- misalignment of structures along bottom plating, side shells, bilge keels, decks.
As a result, permanent deformations of misshaped sections can be caused by:
- scattered set out
- local deflections from the original structure.
For measuring purposes, a wire or a piano line may be stretched out from forward to aft and gauging derived from such a zero basis.
Inside inspections in holds, tanks, hull parts
Similar visual examinations can be carried out inside of compartments:
Attention should be concentrated on the lack of straightness of structures, along side lines from forward to aft and from port to starboard, with regard to:
- stringers and longitudinal frames,
- walls, longitudinal bulkheads and corrugations,
- platforms, transverse members and bulkheads,
- frames, brackets, deck beams,
- floors and attached stiffeners.
Lines and/or structures showing misalignment, deflection, buckling or other discontinuities, are an indication of existing defects requiring close-up inspection.
When a vessel is dry-docked, attention has to be focused on:
- discovery of deformations and/or discontinuities along keel plates, bottom, and side plates, bilge keels, and attachments,
- checks for leakages from inside to outside, if the ballast
- tanks have overflow prior to this inspection,
- removal of the drain plug at the rudder blade. If water leaks out this is an indication that the blade has suffered water ingress (which may otherwise have remained hidden);
- measurement of rudder bearing clearances by feelers can also be considered a visual approach to assess wear-down. Ditto calibration of anchor chain links by caliper slide,
- condition of rudder flange; bolts or nut(s) must be absolutely tight;
- condition of welding at seams and butts and in way of outlet openings.
1.1.2 Close-up examination
If indentations and/or deformations have been located, visual close-up examinations are necessary.
The area under scrutiny should be accessible for visual inspection within bodily reach.
Such inspections should be carried out with floodlight etc. A good torch and a test hammer should always be available, as well as a scraper to remove rust scale and debris to reveal the bare material underneath.
In case of deformation
Deformations that may have been produced as a result of external or internal forces should be carefully analyzed.
Without apparent extra loads along shell, deck, or bottom, likelihood of the following should be checked:
- internal movement of cargo, liquids etc.
- excessive flexibility of the structure.
- local stress concentrations (point loads excessive).
Further examinations for fractures and incipient cracks may be necessary.
Also other identical locations should be examined to see whether similar defects exist or are developing.
In case of cracks
- location of crack,
- configuration of the structure/element,
- starting point of the crack,
- length and direction,
- depth and width of the crack,
- possible cause(s):
= defective welding of assembled parts,
= discontinuation of joints,
= compression or tension of adjoining parts,
= twisting motion,
= reduced thickness,
= type of corrosion, etc.
should be checked not only in the respective area, but also in other identical locations, especially at the opposite side.
1.1.3 Areas of concern for cracks and corrosion
Locations of stress concentration and crack raisers
- Corners of hatches on weather decks,
- corners at deck connections to deckhouses and superstructures,
- deck plating between cargo hatches, especially where plate thickness changes,
- at bulwark stay deck connections.
- Cutouts at webframes where longitudinal pass,
- cutouts at bulkheads where longitudinal passages are closed,
- tips of bracing plates (knee brackets) at bulkhead connections,
- areas where longitudinal members meet vertical structures.
In machinery spaces:
As above in under-deck locations and especially
- at areas of induced vibration (around oscillating machinery),
- underneath of engine seats/along foundations,
- at thrust bearing seats.
Locations where accelerated corrosion is likely
- Generally where the coating is inadequate, defective, or poorly maintained,
- corners and dead ends where water is restricted from draining or flowing away (i.e. bottom connection at aft bulkheads),
- inside of scupper pipes, especially at the elbows where the scuppers are led into the shell,
- at bulwark and coamings stays in way of deck connection,
- along deck connections with coamings of hatches, venti1ation trunks, air pipes, etc.,
- on top or underneath of air and ventilation pipes/trunks, especially where galvanized parts are fitted to steel.
At hatch covers:
- between panel joints and especially along rain gutters, sealing bars, and rubber channels,
- along underside of panel side walls in contact to hatch coaming,
- in pockets of lashing points, etc.
At hatch coamings:
- along sealing bar,- along roller tracks.
Under deck (cargo holds/tanks):
- along aft transverse bulkheads in way of deck/tank deck connection where water or cargo rests are likely to stay,
- inside of bilge trunk,
- base of sounding pipes (where doublers should be fitted),
- base of suction pipe bell mouths,
- in way of pipe clamps and fittings,
- at the undersides of pipelines where condensate is dripping,
- in ballast tanks along the area of air between filling level and tank top,
- at pipes, especially along their outer rear side, fittings and outer undersides.
1.2 Non-destructive Testing Methods
The detection of cracks by visual methods is rather limited. Additionally internal welding seam imperfections or flaws in material parts cannot be discovered without suitable means of examination and instrumentation. To discover these suitable means of non-destructive testing (N.D.T.) are used, such as:
- Dye checks with liquid penetrants
- Magnetic particle checks
- Radiographic checks, or
- Ultrasonic measurements.
1.2.1 Liquid penetrant methods (dye checks)
One type of test uses a low viscosity liquid, containing a fluorescent dye. The area to be tested is sprayed or soaked to allow for penetration by capillary action, and after a time lapse is wiped dry. When viewed under ultra violet light, any faults will be shown by the glow of the penetrant in them.
Another test uses a penetrant containing a powerful dye. This is sprayed on the suspect area with an aerosol. After allowing time for penetration, the area is wiped clean and covered with a liquid which dries to leave chalky sediment (developer). The penetrant stains the developer along the line of the crack.
These methods are based on old chalk and paraffin tests but the penetrant can have a hydrocarbon or alcohol base. Some are emulsifiable for removal by water spray, others can be cleaned off with solvents to reduce possible fire risk.
1.2.2 Magnetic crack detection
This type of test is suitable only for materials which can be magnetized (cannot be used for austenitic steels or non-ferrous metals). After the test the component is normally de-magnetized.
A magnetic field is produced in the component by means of an electric current or permanent magnet and magnetic particles are spread on the surface. Cracks are revealed by a line of magnetic particles.
The powder used may be black iron oxide held in suspension in thin oil. It is poured onto the surface, the surplus being collected in a tray beneath. Colored magnetic inks in aerosols are also available and the dry method makes use of powder only and this is dusted on the surface. Powder tends to collect at a crack in the same way as iron filings will stick to the junction of two bar magnets, placed to end with opposite poles together.
1.2.3 Radiographic inspection
X-rays and gamma rays are used for inspection of welds, castings, forgings etc. Faults in the metal affect the intensity of rays passing -through the material. Film exposed by the rays gives a shadow photograph when developed.
There is a requirement for radiographic examination of many welds, particularly those in pressure vessels.
Defects such as porosity, slag inclusions, lack of fusion, poor penetration, cracks and undercutting are shown on the film.
Films of radiographic examination provide a permanent record of quality of welds etc. and must be identified by serial numbers or other location marks. Image quality indicators are placed on or adjacent to welds.
Radiographs are viewed by a radiologist on a uniformly illuminated diffusing screen. Training is necessary for the interpretation of film, both with regards to the faults in the part being examined and misleading marks that sometimes appear on film.
A skilled radiographer is required for the obtaining of photographs.
Exposure times for gamma rays vary with the type of material, its thickness and the intensity of the rays. X-ray machine voltage and exposure time are also varied to suit the material and its thickness. Distances between ray source, faults and film are important for image definition.
Rays are harmful either in a large dose or a series of small ones where the effect is cumulative.
Monitoring against overdose is necessary with film badges, medical examination and blood counts.
Direct exposure is avoided by the use of protective barriers but there is a danger that objects in the ray path will scatter radiation.
1.2.4 Ultrasonic testing
Internal flaw detection by ultrasonic means is in principle similar to radar. The probe emits high frequency sound waves which are reflected back by any flaws in the object. Reflect ions are also received back from the opposite surface. The probe is connected to a cathode ray oscilloscope which shows the results in a simple way.
A single probe can be used, which combines both transmitting and receiving functions. Alternatively separate devices for transmitting and receiving the sound signals are available.
Any flaw in the material being inspected will also produce a peak.
The following details of "US Testing of Hull Butt welds" from BUREAU VERITAS Weld testing principle:
Transverse waves are emitted from an angle probe moved on the plate surface on either side of the weld.
The probe displacement should be sufficient for scanning the whole weld over a single or a double traverse, as shown on Figure 8.
As far as possible, and taking into account the plate thickness, scan from both sides of the weld, especially for detecting longitudinal defects.
- The scanning operation depends on the type of plate edge preparation before welding and on the configuration of the weldment, i.e. on the difficulty of access for the probe.
- The expanded time-base sweep should be chosen so that a triple traverse is displayed on the screen. The sweep may, however, be modified according to the difficulty of access and to the welded joint.
- Scanning for longitudinal defects (aligned in the direction of the welded joint) is performed by a transverse displacement of the probe with respect to the axis of the weld. The lateral displacement of the probe, which depends on the dimensions of the transducer, should be such as to ensure the over-lapping of the scanned areas; see Figure 9.
- When an anomaly has been detected, the weld may be inspected further by moving the probe parallel to the weld and swinging it back and forth by la to 30°. Then the speed of time-base sweep may be set for displaying an ultrasonic path equal to a double traverse.
- For scanning flush welds one may place the probe on the centre line for the welded joint and direct the ultrasonic beam along the longitudinal axis of the weld.
1.3. Pressure and Tightness Tests
Pressure or tightness tests are required during ship construction and thereafter at periodical surveys or after repairs when the tightness of the respective section(s) has to be proved again.
For such tests the methods are different for either ship tanks and/or cargo tanks.
1.3.1 Basic requirements for tanks (except cargo tanks)
All ballast, trim, feed water, freshwater, and heeling tanks as well as oil tanks for fuel and lubricants, shall be pressure tested by water corresponding to a water column of 2.5 m above the upper tank level; under certain circumstances a pressure test with air followed by a later function test with the liquid is allowed.
Should the deep load line be higher than 2.5 m above the upper level of the tank, the tightness is to be tested with a water column corresponding to the deep load line.
In all cases the testing shall be carried out with a water column reaching to the uppermost level of the overflow or air pipe.
1.3.2 Pressure test of cargo tank
Pressure/tightness tests of cargo tanks of oil and chemical tankers, cargo tanks on dry cargo vessels, etc. are to be carried out as follows:
Prior to the vessel’s launching a tightness test should be carried out by water pressure in the cargo tanks and cofferdams. This test is to be carried out in such a way that the cargo tank bulkheads and the cofferdam bulkheads are tested at least from one side. The test shall be carried out prior to the application of the first protective coating.
Should the test with water not be possible during the vessel’s stay at the vessel 15 building place or dock, hydrostatic pressure test can also be carried out after launching.
For cargo tanks the test requires a water column corresponding to 2.5 m above the upper level of the tank. Any specific weight of the cargo above 1.025 t/m3 has to be taken into account.
For cofferdams a water level up to the upper edge of the access hatch is sufficient.
1.3.3 Tightness test of hatch cover
Weather deck hatch covers should be tested for “weathertightness”.
These tests should usually be carried out by hose testing using a fireline with a nozzle of 12.5 mm diameter at a pressure of at least 2.0 bar from a distance of 1.5 m.
1.4 Function Tests
Function tests or operation tests should prove by demonstration that the tested component
- fulfils its respective purpose under the conditions for which it is designed, and that
- all relevant aspects of safety are satisfied when the component is in operation, in open and/or closed position.
1.4.1 Basic requirements
Function tests shall be carried out with the Surveyor of the
Administration attending and the shipbuilder acting according to the following guidelines:
A definite testing procedure with details of all single tests is to be agreed upon, containing information on the duties and actions of all persons involved.
All relevant safety valves and/or pressure or temperature or flow control s should be readjusted and checked in the workshop before field installation and testing.
For reasons of safety the following should be considered and provided:
- means of escape,
- good lighting, including emergency lighting,
- shipboard electricity in function and backed up, including blackout back-up,
- means for fire fighting to be ready,
- the persons engaged in testing shall be limited to a minimum number,
the testing director shall be selected and nominated.
1.4.2 Items to be tested
The following should be considered for each function and/or operation test:
- Testing of all operational conditions under which the system should prove safe operationability (such situations may also be simulated ).
- Testing of all relevant means of built-in control s, indicators, valves, and fittings; tightness of respective piping, admissible motor load, etc.
- The minimum or maximum data expected; the relevant limits should be reached and demonstrated.
1.4.3 Hull function tests
Function tests forming part of hull surveys are inter alia:
- anchoring tests
- mooring winch tests
- hatch cover operation tests
- cargo gear load tests
- maneuvering tests
- bollard pull tests
- heeling tank tests
- accommodation ladder tests
- pilot lift tests
- cargo lift tests
- cargo ramp tests
- cargo door tests
1.5 Inclining Test
For each new building or after each modification of the vessel which influences stability an inclining test is to be carried out prior to sea trials for the vessel’s recommissioning into service. This test is to be carried out with the Surveyor of the Administration attending and under suitable conditions.
1.5.1 Condition for testing
- Tanks should be empty and the vessel more or less in a completed state in respect of installation work and the equipment installed. Unavoidable tank contents should be concentrated to a tank with vertical side walls.
- The additional weights on board shall not exceed 20% of the lightweight, provided no other stringent reasons request a higher percentage for additional weights.
- Tanks should be completely filled up to 100%. Should a tank be partly filled, the free surfaces must be such that they do not change considerably during testing.
- Vessels must be free of persons which are not actually carrying out testing and control measurements.
- The vessel should be unlimited in movements, i.e. mooring rape free and no contact to quay walls.
- Cooling water, fire fighting, sanitary, fuel, lube oil systems should be filled up to operational conditions.
Ditto boiler and cargo cooling or hydraulic systems.
- Wind and current should not affect vessel' s free movement during the test.
1.5.2 Testing procedure
- The inclination angles should be between 1.5º and 2.0º In any case limits of 1.00 and 2.500 have to be maintained. The inclinations to each side should be carried out twice. The zero points should be noted in the protocol.
- The inclination test is to be calculated by using the hull form data for the actual waterline (buoyancy with trim correction).
- Inclining tests can be omitted for sister vessels of the same type built by the same shipyard without deviation of building data which could influence stability, provided the test results of two previously built vessels produce comparable results. For this the written approval of the owners (and possibly of the Administration) is required, but a deadweight calculation is to be carried out in the presence of the Surveyor for the Administration.
- If applied for, the inclination test can be omitted with huge tankers and bulk carriers of a length of above 250 m provided again the ship owner (and possibly the Administration) approves this in writing and the deadweight calculation is carried out under the attendance of the Surveyor of the Administration.
- For vessels with built-in heeling moments, f.i. with cranes at one side only, also this moment is to be calculated in connection with the evaluation of the inclining test.
1.6 Thickness measurements
1.6.1 Anchor cables
Anchor chains are usually measured by using caliper slides.
Chain links in the vicinity of the chain ends should be measured in 2 cross sectional directions.
The locations for measurement must be chosen at the link ends where maximum wear and/or deformation is to be expected and/or visible.
1.6.2 Thickness measurements of hull scantlings
In general, thickness measurements are made by ultrasonic thickness gauges (see above 1.2.4).
If carried out professionally and in a representative way, measurements of the actual thickness of scantlings can generally reveal the actual overall condition of a vessel with respect to its structural strength.
The scope of the measurements required is determined by the rules of the classification society based on the type and age of a vessel under survey. The actual conditions of the structure, verified by visual observations, may request premature and/or additional measurements.
As a general rule, the smaller the thicknesses the more the extensive measurements have to be.
In areas of heavy corrosion testing is to be increased to show the extent of wear and to allow proper judgment if the area is to be renewed or otherwise repaired.
1.7 Vibration Measurements
Detailed vibration investigations should be made during the design period of a vessel to predict the vibration levels in accommodation and working spaces and to avoid damage by excessive accelerations to hull structures and machinery.
For vessels with slow-speed 2-stroke engines an overall vibration examination should be carried out for hull and superstructure.
Vibrations can be excited by periodical forces, such as the main engine (as a function of the firing frequency), the periodical propeller blade force s at blade frequency, and other free vibrating masses.
Tank sides and shell plating areas in way of the engine room and propeller area should be designed so that structural frequencies are higher than the respective exciting frequency.
For vessels with medium speed engines the possibility of propeller blade induced vibrations should be examined. This type of engine induces excitations with firing frequencies between 23 and 33 Hz. Calculations, of natural frequencies of local structures are therefore necessary.
Whether other systems as masts, rudder arrangements or shaft-lines are to be investigated, depends on the individual case.
Local structures should have' natural frequencies of about 20 - 25% above the highest main exciter frequency. Such calculations may be carried out by using simple formulas, or by the finite element (FE) methods.
FE models which are used for strength calculations may also be utilized for the vibration analysis.
Classification societies can greatly assist ship-owners or builders with such calculations which may avoid expensive modifications or structural alterations after unfavorable seatrials.
Vibration measurements are usually carried out in new-buildings during sea trials.
Occasionally these measurements are not sufficient and have to be repeated in a fully or partly loaded condition of the vessel and occasionally also under certain engine operation modes.
Measurements are then carried out by a special surveyor team, using vibration registration equipment positioned in specially selected locations to record simultaneously engine operation modes together with local structural excitation frequencies, amplitudes and acceleration in order to identify resonances.
2. INSPECTION SCHEMES
The recognized Classification Societies have developed systematic hull inspection programs which ensure that a vessel's structural parts, components and compartments are duly kept under control by periodical examinations and are subjected partially or totally to the above described visual inspections, testing and examination methods.
These survey programmes are:
- Periodical Class Renewal procedures after 4 years, with a possible extension to 5 years if satisfactorily subjected to a class extension survey;
- Continuous Survey procedures for Hull (CSH) with the renewal survey program divided into partial inspections of abt. 20% for each year) over a period of 5 years;
- Class Extension Surveys
- Dry-docking Surveys at intervals of at least 2.5 years.
All these scheduled inspection systems ensure that a vessels condition is regularly controlled and properly supervised within the respective survey system.
The respective inspection schemes are as follows:
2.1 Periodical Class Renewal Surveys (also called “Special Surveys”)
For the Renewal of the Class, the ship’s hull, machinery including electrical installations and the automatic/remote control systems are to be subjected to surveys at the fixed intervals.
A class renewal survey can, under special circumstances, be carried out in several steps. Here, the total survey period must not exceed 12 months.
A bottom survey within this period of time can likewise be recognized if the requirements for class renewal are fulfilled.
The examination of certain covered parts may be dispensed with at a Class Renewal Survey if the Surveyor is completely satisfied of their efficient condition, and if the Owner undertakes to have them exposed for examination within 12 months. A corresponding entry will be made in the Certificate of Classification.
Class Renewals Hull is to be effected in the sequence I, II, III, IV and subsequent to IV. The Class Renewal, No. IV and the following correspond to Class Renewal III.
2.2 Continuous Survey Hull – CSH
Instead of the Class Renewal procedure according to 2.1 the Owner may apply for Continuous Class Renewal for Hull and Machinery. The Class Renewal procedure can, however, also be adopted only for the hull or only for the machinery, including the electrical plant.
The required surveys under CSH extend over a period not exceeding
5 years. It has to be made sure that during the Continuous Surveys all parts of the ship’s hull and/or machinery, including the electrical plant, be surveyed at intervals not exceeding the periods normally required for the maintenance of class.
The Surveyor may re-inspect compartments or structures are deemed necessary.
At the end of the period of class the extent of survey of the hull depends on the scope of the respective class renewal due, I or II or III or IV.
Where both, a ship's hull and machinery, including the electrical plant, are surveyed in accordance with the continuous class renewal procedure, the 5 years' period of class is valid for both sectors. This is conditional upon the prescribed survey intervals and respective scope of survey required being observed.
Where either only the hull or the machinery, including the electrical plant, is subject to the continuous class renewal procedure, a 4 years' period of class is valid for both sectors. Class extension by 12 months is possible. Surveys according to the continuous class renewal procedure are performed al so during the period of class extension.
2.3 Class Extension Surveys
On Owners' request el ass can be extended by not more than 12 months after survey of the vessel - at least to the scope of the requirements for an Annual Survey afloat. Class may be extended only if hull and machinery, including the electrical plant, are in perfect condition and if, since the bottom was last surveyed, no incidents occurred resulting in damages expected to have been caused to the underwater body.
Ships having a character of classification different from 100 A 4 (highest GL class character) cannot have their class extended.
Dry-docking intervals are to be observed for class extensions.
At a Class Extension Survey the ship is to be inspected, if practicable, when it is not loaded, so that the hatches, the cargo holds, the tweendeck spaces, the watertight doors, etc. can be examined; if necessary, tanks will also be examined. In the case of oil tankers and ships carrying combined cargoes (e.g. OBO-ships) the ballast tanks located in the cargo area will be subjected to a general condition survey. An inspection of the machinery, including the electrical plant, is to be made to verify, in particular, satisfactory operation. Automatic/remote control systems are to be examined, taking into account records of operation.
2.4 Docking Surveys
Underwater hull inspections at regular intervals shall ensure that the outside and the steering facility of a ship remain in a satisfactory condition. Such inspections are also carried out for the control pf the propeller, the shaft-line bearings and seals. In addition inlet and outlet piping, valves, seachests and sea filters are examined.
A special type of underwater hull survey is the “in-water survey” which can be applied under special considerations.
For seagoing ships with character of class 100 A 4 an in-water survey may be recognized as a substitute for every second periodical bottom survey, provided
- the required special equipment is available, documents have been issued and trial requirements complied with and if the survey is carried out as required and with approved firms and satisfactory results,
- this survey is not part of a class renewal.
For ships of more than 10 years of age the intervals between dry-docking must not exceed 2.5 years.