VLCC DESIGN AND CONSTRUCTION VLCC design, like all types of engineering, balances conflicting requirements to produce an economic and safe structure. The capacity of a VLCC is more important than the speed at which it travels, accounting for the boxy, barge-like look of large tankers. It is the task of tanker designers and builders to produce vessels that are structurally sound, are capable of maneuvering predictably, have reliable machinery, and can be operated continuously at an average of 345 days each year, often in areas far removed from assistance.

Design: Computers Check and Recheck Structural integrity is equally dependent upon sound design and high quality construction. The basic hull, or outside shell, of a tanker afloat is supported by a balance of upward buoyant forces from water on the outside and downward weight forces of the ship, its machinery, and its cargo or ballast. The framing and the internal walls, or bulkheads, of the tanker are needed to keep its outer shell from distortion under shifting loads of cargo and the impact of waves.

The bulkheads must be strong enough to divide the cargo tanks and other interior spaces into independent compartments, ensuring survival of the ship should several compartments be flooded in an accident. International regulations require this safety precaution, which all VLCCS satisfy.

The journals of marine technical societies record the great detail of theoretical and applied research on ship structure accomplished since World War II-work related perhaps more to tankers than to any other type of ship, especially since the late 1950s.

A sampling of this work shows that:

> Basic sizes of hull structural plates and

frames once were determined by empirical, or "cut and try" formulas derived from experience. Computers now are used to make a stress analysis of the entire hull, in order to determine more accurate hull plate and frame dimensions.

> Equally important is accurate information on waves and their effects on a tanker's hull. In a long-term program carried out by the American Bureau of Shipping, six large tankers carried instruments for monitoring wave stress for several years. The results were employed in subsequent design and construction. Organizations in other countries also have conducted such programs, and there is an international exchange of information.

> An Exxon VLCC, the guinea pig in a 1970 program, was fitted with 1,400 stress monitors throughout her hull. During a period of intensive testing, loads on the hull were varied by shifting ballast water, and the structural response was measured. The resulting information was made available to the world's marine structural experts for verification of their stress analysis programs. Results showed that stress analyses previously made by Exxon and other organizations were quite accurate.

> Another major safety step was the movement to "bridge aft" designs in the 1960s. With the bridge moved from the middle of the ship to the rear, the crew works near the source of shipboard power and equipmentthe best position for handling any shipboard emergency. In case of fire or explosion, the ship's crew can take refuge in the bridge structure aft of the cargo, or abandon ship, in greater safety. Also, most ship masters have found "bridge aft" an advantage in ship handling because, with the full length of the ship in front of them, they can determine the ship's maneuvers with more precision.

> With the new techniques used in VLCC de

sign, there has been a shift away from individual, ship-by-ship designs to more economical and better engineered standard, or "class" designs. This allows ships to be built more quickly, and, at the same time ensures that thoroughly engineered and tested components are built into every ship of a particular class.

Shipbuilding: More Quality Control

Progress in tanker construction techniques owes much to the tremendously competitive nature of shipbuilding, particularly outside the United States. By the mid-1960s, welding had replaced riveting of ship structures. While this change got its start during World War II, the all-welded ship initially experienced some cracking attributed to poor design details at joints between framing members, and metallurgical deficiencies in hull plates at low temperatures. Extensive research on each of these, especially in Europe and Japan, has overcome earlier difficulties.

Tankers built today have no rivets, nor the leakage associated with them. A typical VLCCS welding, nearly all performed automatically, allows greater precision and quality control. Improved techniques also are used for inspection, ensuring that all key joints are welded properly at every stage of construction.

In modern VLCC shipyards, nearly all hull plates and internal framing members are cut automatically by computer-controlled machines working to a degree of precision impossible to attain in building older ships.

There are many differences in VLCC shipyards around the world, but all have in common a carefully controlled and efficiently organized system of construction, using large prefabricated sub-assemblies, often weighing 1,000 tons. These frequently are constructed indoors. The typical yard of 20 years ago built tankers in many smaller segments, almost entirely outdoors, diminishing the workers' efficiency, particularly in the major tanker yards working through northern winters.

VLCC Maneuverability

VLCCS moving at top speed travel farther than smaller ships before they can be brought to a full stop. However, a VLCC seldom, if ever, is required to come to a full stop from maximum speed on open sea, where it has ample room to maneuver and avoid any obstacle. In closer waters, or when nearing a mooring, a VLCC moves at slow speeds, often aided by tugboats, and the stopping distance required is greatly reduced.

Before Exxon's first VLCCs were delivered in 1968, the company conducted extensive model tests in order to be sure the maneuvering capabilities of these larger vessels were compatible with standards applied to all other vessels.

> Tests with large models and computer calculations indicated that in some respects VLCCS would be easier to handle than smaller tankers-a finding confirmed in actual performance of Exxon's VLCCS.

> Tests demonstrated that directional instability-the tendency of a ship to veer right or left is easier to control by rudder on larger ships. Directional instability was not unexpected, for it is common to smaller craft, also. In fact, it contributes to maneuverability.

> Tests showed that VLCCS, like smaller ships, become sluggish in shallow water. Allowances are made for this, and VLCCS can be maneuvered just as safely.

Attention of major tanker owners and builders to maneuverability has stimulated development of this aspect of ship design. Since 1965, some advances which have been made in this field are:

> The first aircraft-type simulators of a VLCC's bridge, both for training mariners and conducting research, were built in the Netherlands. Exxon was the first operator to use simulators on shore routinely in training of

senior deck officers. Exxon has conducted extensive full-scale maneuvering trials with both 190,000-ton and 250,000-ton VLCCS, providing valuable information to help verify industry-wide computer programs for these simulators.

> Harbor authorities in Europe have made models of entire harbor entrances for study of wave and current patterns, and to establish safe limits for the sizes and characteristics of ships that will use the harbors.

> To ensure the safe maneuverability of a new VLCC design of 400,000 tons, Exxon recently conducted studies with a 30-foot model, the results of which were the basis of computer simulations of how the full size vessel would maneuver. These were verified by the construction of a 100-foot model in Japan-all before final design of the new vessel was started.

Most masters have found VLCCS to be more predictable vessels to handle than earlier, smaller tankers. Safe maneuvering of VLCCS can be accomplished with proper navigational planning and action.

Training at Grenoble, Delft and Wageningen

In order to develop new training techniques to help captains and harbor pilots make the transition from smaller tankers to VLCCS, in 1966 Exxon created Port Revel in Grenoble, France. Working with a private French hydraulics firm, Sogreah, the company remodeled a secluded lake at the foothills of the Alps to form in exact miniature, to 1/25 scale, typical waterways and tanker terminals throughout the world, including a crucial portion of the Suez Canal. By operating model ships, captains within a few weeks gain years of experience in reactions of large tankers under varied operating conditions. This is the only facility of its kind in the world.

Trainees at Grenoble receive classroom instruction, and perform maneuvering exercises in the ship models, both VLCC and smaller. Exercises include harbor approaches, mooring at offshore terminals, ship-to-ship berthing, and putting a ship alongside a dock. In 1970, Sogreah took over the management of Port

Revel, but Exxon and other tanker operators continue to make use of the training facility.

Complementing the training of tanker masters at Port Revel, are centers at Delft and Wageningen in the Netherlands that offer simulator training, a method widely used in the aviation industry to train pilots. Ships' masters work in a mock-up of a tanker bridge that is programmed by computer to reflect the forces affecting a ship. Projected on a screen in front of the bridge are the horizon, the ship forward of the bridge and anything else that is appropriate to the particular training session-a harbor entrance, for example. The scene changes according to the ship's motion, giving a very realistic simulation of the actual view from a ship's bridge. Through a series of exercises, a captain is able to perform maneuvers and become familiar with the "feel" of a large tanker before he may ever be in command of one.