VESSEL
TANKERSHIP
A tanker (or tank ship or tankship) is
a merchant vessel
designed to transport liquids
or gases in bulk.
Major types of tankship include the oil
tanker, the chemical tanker, and gas
carrier. In the United States Navy and Military Sealift Command, any type of tanker used to
refuel other ships is called an oiler.
u Tankers can range in size of
capacity from several hundred tons, which includes vessels for
servicing small harbours and coastal settlements, to several hundred thousand
tons, for long-range haulage. Besides ocean- or seagoing tankers there are also
specialized inland-waterway tankers which operate on rivers and canals with an
average cargo capacity up to some thousand
tons. A wide range of products are carried by tankers, including:
Different
products require different handling and transport, with specialised variants
such as "chemical tankers", "oil
tankers",
and "LNG carriers" developed to handle
dangerous chemicals, oil and oil-derived products, and liquefied natural gas respectively. These broad
variants may be further differentiated with respect to ability to carry only a
single product or simultaneously transport mixed cargoes such as several
different chemicals or refined petroleum products.[1]
Among oil tankers, supertankers are designed for transporting
oil around the Horn of Africa from the Middle
East. The
supertanker Seawise Giant, scrapped in 2010, was 458
meters (1,503 ft) in length and 69 meters (226 ft) wide. Supertankers
are one of the three preferred methods for transporting large quantities of
oil, along with pipeline transport and rail.
Tankers used for liquid fuels are
classified according to their capacity.
In 1954,
Shell Oil developed the average freight rate assessment (AFRA) system which classifies
tankers of different sizes. To make it an independent instrument, Shell
consulted the London Tanker Brokers’ Panel (LTBP). At first, they divided the
groups as General Purpose for tankers under 25,000 tons deadweight
(DWT); Medium Range for ships between 25,000 and 45,000 DWT
and Large Range for the then-enormous ships that were larger than
45,000 DWT. The ships became larger during
the 1970s, and the list was extended, where the tons are long
tons:[3]
Cargo Operation
Operations
aboard oil tankers are governed by an established body of best practices and a
large body of international law.[94]
Cargo can be moved on or off of an oil tanker in several ways. One method is
for the ship to moor alongside a pier, connect with cargo hoses or marine loading arms. Another method involves mooring
to offshore buoys, such as a single point mooring,
and making a cargo connection via underwater cargo hoses.[95]
A third method is by ship-to-ship transfer, also known as lightering. In this method, two ships come
alongside in open sea and oil is transferred manifold to manifold via flexible
hoses.[96] Lightering is sometimes used
where a loaded tanker is too large to enter a specific port.
Pre-transfer
preparation
Prior
to any transfer of cargo, the chief
officer must
develop a transfer plan detailing specifics of the operation such as how much
cargo will be moved, which tanks will be cleaned, and how the ship's ballasting
will change.[97] The next step before a transfer
is the pretransfer conference.[98]
The pretransfer conference covers issues such as what products will be moved,
the order of movement, names and titles of key people, particulars of shipboard
and shore equipment, critical states of the transfer, regulations in effect,
emergency and spill-containment procedures, watch and shift arrangements, and
shutdown procedures.[98]
After
the conference is complete, the person in charge on the ship and the person in
charge of the shore installation go over a final inspection checklist.[98]
In the United States, the checklist is called a Declaration of Inspection or
DOI.[98] Outside the U.S., the document
is called the
"Ship/Shore
Safety Checklist."[98] Items on the checklist include
proper signals and signs are displayed,[98]
secure mooring of the vessel,[98]
choice of language for communication,[99]
securing of all connections,[99]
that emergency equipment is in place,[99]
and that no repair work is taking place.[
Loading Cargo
Loading
an oil tanker consists primarily of pumping cargo into the ship's tanks.[99]
As oil enters the tank, the vapors inside the tank must be somehow expelled.[99]
Depending on local regulations, the vapors can be expelled into the atmosphere
or discharged back to the pumping station by way of a vapor recovery line.[99]
It is also common for the ship to move water ballast during the loading of
cargo to maintain proper trim.[99]
Loading
starts slowly at a low pressure to ensure that equipment is working correctly
and that connections are secure.[99]
Then a steady pressure is achieved and held until the "topping-off"
phase when the tanks are nearly full.[99]
Topping off is a very dangerous time in handling oil, and the procedure is
handled particularly carefully.[99]
Tank-gauging equipment is used to tell the person in charge how much space is
left in the tank, and all tankers have at least two independent methods for
tank-gauging.[99] As the tanker becomes full, crew
members open and close valves to direct the flow of product and maintain close
communication with the pumping facility to decrease and finally stop the flow
of liquid.
Unloading Cargo
The
process of moving oil off of a tanker is similar to loading, but has some key
differences. The first step in the operation is following the same pretransfer
procedures as used in loading. When the transfer begins, it is the ship's cargo
pumps that are used to move the product ashore.[101]
As in loading, the transfer starts at low pressure to ensure that equipment is
working correctly and that connections are secure.[101]
Then a steady pressure is achieved and held during the operation.[102]
While pumping, tank levels are carefully watched and key locations, such as the
connection at the cargo manifold and the ship's pumproom are constantly
monitored.[100] Under the direction of the
person in charge, crew members open and close valves to direct the flow of
product and maintain close communication with the receiving facility to
decrease and finally stop the flow of liquid
Tank Cleaning
a
special crude oil washing (COW) system is part of the
cleaning process.[103] The COW system circulates part
of the cargo through the fixed tank-cleaning system to remove wax and Tanks
must be cleaned from time to time for various reasons. One reason is to change
the type of product carried inside a tank.[103]
Also, when tanks are to be inspected or maintenance must be performed within a
tank, it must be not only cleaned, but made gas-free.[103]
On most
crude-oil tankersasphaltic deposits.[103]
Tanks that carry less viscous cargoes are washed with water. Fixed and portable
automated tank cleaning machines, which clean tanks with
high-pressure water jets, are widely used.[103]
Some systems use rotating high-pressure water jets to spray hot water on all
the internal surfaces of the tank.[103]
As the spraying takes place, the liquid is pumped out of the tank.[103]
After a
tank is cleaned, provided that it is going to be prepared for entry, it will be
purged. Purging is accomplished by pumping inert gas into the tank until
hydrocarbons have been sufficiently expelled. Next the tank is gas freed
which is usually accomplished by blowing fresh air into the space with portable
air powered or water powered air blowers. "Gas freeing" brings the
oxygen content of the tank up to 20.8% O2. The inert gas buffer between fuel
and oxygen atmospheres ensures they are never capable of ignition. Specially
trained personnel monitor the tank's atmosphere, often using hand-held gas
indicators which measure the percentage of hydrocarbons present.[104]
After a tank is gas-free, it may be further hand-cleaned in a manual process
known as mucking.[105] Mucking requires protocols for
entry into confined spaces, protective clothing, designated safety observers,
and possibly the use of airline
respirators.
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