Tankers are necessarily a technical subject and the CTX will sometimes need to use some tanker jargon. This should be done as little as possible. But, when you are forced to use technical terms, please put a link in the document to the explanations in this glossary, to make your contribution as accessible as possible.

A tanker's deadweight is a measure of ship's carrying capacity in tons. It includes both the weight of the cargo and the weight of the fuel the tanker carries for her own propulsion purposes. This non-cargo fuel is called bunkers. For big tankers, the weight of the bunkers is a small proportion of the deadweight, usually less than 5%. So tankermen sometimes mis-use the word deadweight, when they are really talking about the weight of the cargo.
Despite the name, a tanker's deadweight has nothing to do with the weight of the ship itself, that is, the weight of the ship when empty, which is confusingly called the lightweight. The sum of the deadweight (everything that the ship carries) and the lightweight (the weight of the ship) is know as the displacement because this is the weight of sea water that the ship will displace when floating in the ocean.
Tankers come in an enormous range of sizes, from a few hundred tons deadweight to over 500,000 tons deadweight. The workhorse big tanker is the prosaically named VLCC, which stands for Very Large Crude Carrier. So much for names redolent in the lore of the sea. A VLCC is a is a ship with a deadweight of from 200,000 to 315,000 tons. The standard VLCC is about 300,000 tons which will allow is to carry about 2 million barrels of oil. The lightweight of such a ship will be around 42,000 tons.
In the mid-late 70's, some 80 tankers were built which were considerably larger than the VLCC. For want of imagination, these ships were called ULCC's (Ultra Large Crude Carriers). These ships ranged in size from around 320,000 tons deadweight to 550,000 tons. There have only been four ULCC's built in the last 25 years.
In many situations, the tanker is too large to enter the load or discharge port. The most prominent example is the USA which has almost no ports which can accept drafts in excess of 13 meters. A fully loaded VLCC will have a draft in excess of 20 meters. A ULCC can have a draft of as much as 25 meters. When these ships bring oil to the United States, they almost always have to off-load their cargo to a smaller tankers at sea. The smaller tankers, called lighters, then actually deliver the oil to the refineries which in some cases are well in-shore. For example, the Exxon refinery at Baton Rouge is 200 miles up the Mississippi River.

This process is called lightering. Lightering normally takes place 20 to 50 miles offshore. The lighters typically have a cargo capacity of about 500,000 barrels. Thus, a VLCC will offload to about four lighters in a single discharge, and a ULCC to as many as six. The lighters have a deadweight of about 80,000 tons. Their loaded displacement is about the same as a nuclear aircaft carrier. They are bigger than the largest ship afloat in 1960. The lighters may be small compared to a VLCC or a ULCC, but they are big ships.

In order to lighter, the mooring master must bring this aircraft-carrier-sized tanker alongside the mothership with the delicacy of a watchmaker, while both ships are moving thru open water at 4 to 6 knots. It is one of the more thrilling and spectacular sights in the industrial world. It is also an inherently dangerous process. I have never tired of watching this show and I have never failed to give a little sigh of relief when the two ships were moored together.

As you read this, there are some half-dozen VLCC's being lightered off the Gulf Coast and another one or two off California.

There is one very important exception to lightering in the United States. And that is the Louisiana Offshore Oil Port or LOOP. LOOP consists of three large Single Buoy Moorings or SBM's about 25 miles off the Louisianna coast. A VLCC or ULCC ties up to one of these buoys. The buoys are fitted with hoses. The hoses are connected to the tanker, which discharges the cargo to the buoy and then to an undersea pipeline that runs ashore, and connects directly to the refineries by other pipelines. Similiar facilities exist thruout the world, but LOOP is the only SBM terminal in the United States.

The CTX believes that the SBM-based system is far safer than lightering and transporting 500,000 barrel cargos up rivers and bayous via tanker. It's also cheaper.

Oil transport is essentially a one-way trade. Tankers take oil from places that have it to places that don't. They are too specialized to carry anything back. (Like many statements in this glossary, this a slight over-simplification.) But they can't go back empty. The propeller would be out of the water, the bow subject to slamming, and the ship nearly unmaneuverable. Therefore, tankers use sea water as ballast. Ballast is both a noun and a verb, and can be used as an adjective. The process of taking on ballast is known as ballasting and pumping the ballast back into the sea at the load port as deballasting. The portion of the voyage in which the ship doesn't have cargo on-board is called the ballast leg . The revenue earning portion of the trip is the loaded leg
IMO stands for the International Maritime Organization. This is a UN body whose role is the regulation of marine transportation. Like all UN bodies, it has no real power itself, but is completely dependent on the Member States to ratify and then enforce any IMO regulations. In fact, the regulations are not even drafted by IMO bureaucrats but by an unwieldy system of ad hoc committees, few of whose members have practical tanker experience, some of whose members have little technical competence, but almost all of whom represent some special interest.
MARPOL and pre-Marpol tankers
Strictly speaking, MARPOL refers to the set of regulations adopted by the International Convention for the Prevention of Pollution from Ships which functions under the auspices of IMO. These regulations are being modified more or less continuously. The most recent Consolidated Version published by IMO is called Marpol 73/78, Consolidated Edition, 2002, ISBN 92-801-5125-8.

MARPOL can also be used as an adjective. Often the CTX will refer to a ship as a MARPOL tanker or a pre-MARPOL tanker. In the wake of the loss of the Amoco Cadiz in 1978, a set of new tanker regulations was eventually adopted which became known as the MARPOL rules. For present purposes, the key such rule was the imposition of protectively located, segregated ballast on (almost) all tankers delivered after 1979.

Prior to this regulation, almost all tankers used a system in which a portion of the cargo tanks was also used to carry sea water ballast on the ballast leg. This meant that some of the oil remaining in these tanks after the loaded leg was later discharged into the sea when the tanks were subsequently deballasted. A surprisingly effective system for limiting the amount of oil in this ballast was developed. But as long as tanks are used both for cargo and ballast, there is no way of eliminating this discharge entirely.

Although it had nothing to do with the Amoco Cadiz spill, the new MARPOL rules imposed segregated ballast on all new tankers. In this system, a tank is either a cargo tank or a ballast tank, but not both. The cargo piping and ballast piping are also completely separate. This eliminates the discharge of ballast contaminated with oil from the tank's cargo on the previous loaded leg. It has also had a number of unintended consequences including more spillage in groundings.

CTX use the term Marpol tanker to refer to the tankers that were built to the MARPOL rules. These were single hull, segregated ballast tankers, almost all built between 1980 and the very early 1990's. In the early 90's, the industry switched to double hulls which also use segregated ballast.

Tankers built before 1980, which CTX calls pre-Marpol tankers, were semi-godfathered under the MARPOL rules. These single hulled ships did not have to use segregated ballast but at age 25 they either had to switch to protectively located ballast or be subject to a requirement called Hydrostatically Balanced Loading (HBL).

Normally stability is not a big problem for tankers, especially sizable tankers. However, with the introduction of double hulls, some yards went to one-across ships for tankers up to 100,000 tons deadweight. These ship have only one column of cargo tanks which extend from the inner side of the double hull on one side all the way to the inner side of the double hull on the other side. These tanks were so wide relative to the size of the ship, that they became unstable unless the crew was very careful to keep almost all these tanks either completely full or completely empty. This instability was caused by the liquid in the tank flowing to the low side of the tank. This did not result in a capsize, but a sudden change in list to one side or the other. This change in list, called lolling was often 5 degrees or more, which caused havoc with loading arms, hoses, and mooring lines. In commercial practice with different cargo parcels and different load/discharge sequences, it was nearly impossible for the crews to comply with the completely full or completely empty requirements.
Inerting is the process of filling the empty portion of a tank with a low oxygen gas usually called inert gas or IG. For tankers, this inert gas is usually drawn from the boiler exhaust or stack gas. If a boiler is properly design and operated, the O2 content of this stack gas will be between 2% and 4%, well below the limit which will support combustion. (Normal air has an O2 content of about 21%.) Some motor ships (all should be) are also equipped with a separate inert gas generator for producing inert gas when the boilers are not operating.

Cargo tank inerting was introduced in the late 60's and early 70's when a series of tank explosions was traced to static electricity generated by tank washing. Proper inerting effectively eliminates these casualties. Now all tanker cargo tanks must be inerted by law.

Since tankers usually burn a high sulfur fuel, the boiler stack gas is highly acidic containing as much as 1500 ppm S02. To reduce this sulfur, tankers scrub this gas. The inert gas scrubber is a little more than an oversized shower which sprays seawater on the gas. Most of the sulfur combines with the seawater and is drained to sea.