Requirement Thickness measurement in Cargo oil tank

1. ·Home
·Table of Contents
·Terrestrial and Marine Transportation
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

2. 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.

3. 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
5 Additional
complete

4. transverse web
frame rings as
considered
necessary by the
Surveyor
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 SURVEYOF 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. 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.

6. 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.
© AIPnD , created by NDT.net |Home| |Top|