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Non-Destructive Testing and Malta Drydocks J. Cremona - Malta Drydocks 'Malta Contact

The origins of Malta Drydocks (MDD) date back 1689, when the Knights of Malta, started building stores, residences or hotels for galley captains and three arches for slipping galleys. These were built on the same land that MDD now stands on. Several of these original stores survived the war and are still in use today. Malta Drydocks has seven docks that can accommodate ships up to 250000 tons. MDD has carried out lots of different work, ranging from shipbuilding, repairs, conversions etc. Work is carried out to different procedures, depending on client and Classification Societies' requirements for any particular job.

Malta Drydocks, situated in the centre of the Mediterranean Sea, had always used this effect as its leading edge. The number of employees is at an average of 2900 workers and most of these are skilled workers. It is also the largest single employer on this small island state. Unfortunately, like many other shipyards around the world, it requires government subsidies to stay in business. Shiprepair is a cutthroat business, where pricing and duration are the prime motivators.

MDD took a step forward when it was decided to split the organisation into separate entities or units. These are the two main divisions, namely Shiprepair and Enterprises. To supplement these two, three smaller units were also organised, these being a Common, Marketing and Finance units.

As the name implies, the Shiprepair Division still has shiprepair as its main area of activity and the core of its activities remain the same as before.

The Enterprises Division has been set up to acquire work other shiprepair. This work was never given the necessary importance. Now separate marketing is being carried out and some sort of progress has already been achieved. Several local and foreign contracts have already been won. These range from petroleum products pipelines' to pressure vessels. Contracts are also being taken together with several other partners. The NDT department is part of this division.

The Common Unit supplements the two main Divisions with labour and services that is required by both. These include plant maintenance, civil works, safety and QA/QC among others.

The NDT department provides services to both Shiprepair and local Industry. The complement is five persons. The methods that are practised are Ultrasonics, Radiography, Magnetic Particle and Liquid Penetrant. Although they do not form part of the NDT department, MDD also offers Leak testing and Visual Inspection (Welding). The QC department caters for these two methods. Personnel are certified to a variety of qualifications from ASNT Level I to ASNT Level III and PCN Level I and II. The department also carries out in situ Hardness testing using portable equipment and also tensile and bend testing.

The quality system at MDD ensures that all welders are qualified. The NDT department plays a major role as all welders are qualified by producing test pieces and these are then radiographed. A third party inspector, usually a Classification Society Surveyor, and one of our interpreters view the radiographs and accept or reject the work. Radiography, tensile, bending and etching also test welding procedures when required. During production work, random testing is carried out. Any defective areas are investigated and necessary repair action is taken. After production, additional testing is carried out, This would normally be in areas indicated by a third party or in the absence of such, random areas selected by our personnel. Work procedures are carried out to Classification Societies' rules, client specifications and in the absence of these according to Yard procedures. These normally are based on International standards such as ASME and EN.

Another task that the NDT department carries out is the calibration of several machines such as chain testing, wire testing, tensile machine and several tensometers. This is done by the use of proving rings. All calibration procedures are based on EN Standards.

The most important field of work for this department is that of Ultrasonic thickness gauging. We are approved by several leading Classification Societies to carry out enhanced thickness measurement on vessels. The approvals are from American Bureau of Shipping (ABS), Lloyds Register of Shipping (LR), Det Norske Veritas (DNV), Germanischer Lloyds (GL), Bureau Veritas (BV) and Nippon Kaiji Kyokai (NKK). These surveys are carried out to regulations issued by the International Association of Classification Societies (IACS). The following will show how a typical survey of a cargo tanker is carried out.

Upon the confirmation of a job from the commercial department, a meeting is held between MDD and the client's representative. Depending on which Classification Society the vessel is classed; the regulations of that particular class are discussed. Actually there are two surveys that are carried out simultaneously,

• Close up inspection by Class Surveyor,
• Thickness measurement survey by an approved company.

The most important factors that decide the extent of both surveys are the age and the general condition of the vessel. A typical layout of an oil tanker is shown below.

The following table shows the extent of close up inspection that the Class Surveyor must follow during the survey according to the vessel's age. All areas are to be visually inspected and any damage or corrosion is noted.

Special Periodical Survey Number 1 (Age 5 years)		Special Periodical Survey Number 2 (5 < Age 10 years)		Special Periodical Survey Number 3 (10 < Age 15 years)		Subsequent Special Periodical Surveys (Age >15 years)
1	One complete transverse web frame ring including adjacent structural members in a ballast wing tank, if any or a cargo wing tank used primarily for water ballast	1	All complete transverse web frame rings including adjacent structural members in a ballast wing tank, if any or a cargo wing tank used primarily for water ballast	1	All complete transverse web frame rings including adjacent structural members in all ballast tanks in a cargo wing tank	1	All complete transverse web frame rings including adjacent structural members in all ballast tanks in a cargo wing tank
2	One deck transverse including adjacent deck structural members in a cargo wing tank.	2	One deck transverse including adjacent deck structural members in each remaining ballast tanks, if any in a cargo wing tank. in two centre cargo tanks	2	One complete transverse web frame ring including adjacent structural members in each remaining cargo wing tank.	2	One complete transverse web frame ring including adjacent structural members in each remaining cargo wing tank.
3	Lower part of transverse bulkhead including girder system and adjacent structural members in one ballast tank in one cargo oil wing tank in one cargo oil centre tank.	3	Both transverse bulkheads including girder system and adjacent structural members in a wing ballast tank, if any, or a cargo wing tank used primarily for water ballast	3	One deck and bottom transverse including adjacent structural members in each cargo centre tank.	3	One deck and bottom transverse including adjacent structural members in each cargo centre tank.
		4	Lower part of transverse bulkhead including girder system and adjacent structural members in each remaining ballast tank in one cargo oil wing tank in two cargo oil centre tank.	4	All transverse bulkheads including girder and stiffener systems and adjacent members in all cargo and ballast tanks.	4	All transverse bulkheads including girder and stiffener systems and adjacent members in all cargo and ballast tanks.
				5	Additional complete transverse web frame rings as considered necessary by the Surveyor	5	Additional complete transverse web frame rings as considered necessary by the Surveyor
						6	Any additional tanks and structure as considered necessary by the Surveyor.
Table 1: REQUIREMENTS FOR CLOSE-UP SURVEY OF OIL TANKERS, ORE/OIL SHIPS, ETC.

The following table shows the extent of thickness measurements that the gauging company must follow during the survey.

Special Periodical Survey Number 1 (Age 5 years)		Special Periodical Survey Number 2 (5 < Age 10 years)		Special Periodical Survey Number 3 (10 < Age 15 years)		Subsequent Special Periodical Surveys (Age >15 years)
1	One transverse section of deck plating for the full beam of the ship within amidships 0.5L (in way of a ballast tank, if any, or a cargo tank used primarily for water ballast).	1	All main deck plating within the amidships 0.5L or cargo area, whichever is longer	1	All main deck plating within the amidships 0.5L or cargo area, whichever is longer	1	All main deck plating within the cargo area, all exposed main deck plating outside of cargo area, and all exposed 1st tier superstructure deck plating (poop, bridge and forecastle decks)
2	Measurement, for general assessment and recording of corrosion patterns, of structural members subject to Close-up Surveys.	2	One transverse section within the amidships 0.5L	2	Two transverses section within the amidships 0.5L	2	A minimum of three transverse sections within the amidships 0.5L
3	Suspect areas throughout the vessel	3	Plating in two wind and water strakes outside the amidships 0.5L.	3	All plating in two wind and water strakes, port and starboard, full length.	3	All plating in two wind and water strakes, port and starboard, full length.
		4	Plating and stiffeners or transverse bulkheads and internals in forepeak and afterpeak tanks.	4	Plating and stiffeners or transverse bulkheads and internals in forepeak and afterpeak tanks.	4	Plating and stiffeners or transverse bulkheads and internals in forepeak and afterpeak tanks.
		5	Measurement, for general assessment and recording of corrosion patterns, of structural members subject to Close-up Surveys.	5	Measurement, for general assessment and recording of corrosion patterns, of structural members subject to Close-up Surveys.	5	All keel and bottom plating full length.
		6	Suspect areas throughout the vessel	6	Suspect areas throughout the vessel	6	Measurement, for general assessment and recording of corrosion patterns, of structural members subject to Close-up Surveys.
						7	Suspect areas throughout the vessel
Table 2: Requirements for Thickness Measurements at Special Survey of Oil Tankers etc.

As can be seen the older the vessel, the more strenuous the inspections. It must be appreciated that Close- up Survey and subsequent thickness measurement require a lot of inspection in areas that are not easily reached. Therefore during the first meeting a survey plan will be devised so as to minimise the amount of time required. A complete set of drawings is normally made available so that preliminary planning and sketches are made.

From the gauging point of view, the last two items of the Thickness Measurements at Special Survey are the most time consuming. The extent usually depends upon the condition of the tanks. If the tanks are in very good condition, only a few gaugings are necessary to prove that what is being seen is actually true, (i.e. One could have a plate with 50% reduction that has been blasted and coated. If no gaugings are taken, it could be assumed that the plate is still very good) On the other hand if there are signs of corrosion, hundreds of gaugings are usually needed. Obviously the worse the state of the plates, the greater the number of gaugings.

It is normal practice that after the cleaning process of the tanks from sludge, the tanks are filled with water, according to a ballast plan. An inflatable boat is then lowered and inflated inside the tank. The Close-up Inspection and thickness measurement can then be carried out. De-ballasting or filling up the tanks reaches different inspection levels. The duration of this process depends upon the size of the vessel and its pumping capabilities. This can be carried out in port or during a voyage.

Alternatively, the owner may opt for staging inside the tanks, especially if extensive repairs are to be carried out. This will allow access for Close-up Inspection and thickness measurement. Obviously, this is very time consuming as the staging takes some time to be erected in so many areas. Another very irritating problem with this method is that we have to work during silent hours as workmen are usually working on the same staging, burning, fitting or welding.

As the measurements are taken, these are reported in a formalised manner. The readings taken are fed daily into a computer, which will work out the respective reduction from the original thickness for each measurement. Each specific area is allowed different percentage reduction from the original thickness. If this percentage exceeds what is allowed, more readings are necessary to determine if repairs to that area are necessary. But if this percentage is more than 75% of that allowable for that area, the area will be considered as an area of Substantial Corrosion, which will involve the taking of more readings. Rules specify the required measurement patterns for areas of Substantial Corrosion.

The daily reports are passed on to the Classification Society's Surveyor for record and checking purposes. Once the job is finished, a complete report is produced, signed by our technicians and passed on to Class for verification. The Class surveyor signs the report and then it is distributed to all concerned parties, i.e., Client, Class, MDD and any other third party that could be involved, such as Coast guard etc.

As can be seen, Classification rules try to survey as much as possible areas that are subject to corrosion and stress areas. Apart from these special surveys, intermediate and annual surveys are also carried out; targeting areas also specified in the rules. These are not as extensive as the special surveys.

From all of the above, everything effort is made to monitor the integrity of the vessel. To ensure this there must be full co-operation between the owners, Classification Societies and gauging companies. The end result, after any recommendations that are made are satisfied, should be a safe and sound vessel.

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