proach to their design. Many design improvements came about through trial and error. Vessels often operated well for a period of time after modifications were made to correct problems. Then similar problems cropped up again and further modifications had to be made.

As operations moved into deeper waters and more hazardous locations, larger, sturdier liftboats were built. Water depth capabilities increased with longer legs and larger hulls.

With a goal of achieving some consistency in design and analysis methods, the Coast Guard has sponsored research projects to enhance the industry's understanding of liftboat strength and survivability against the forces of nature. The Coast Guard also participates with industry associations concerned with liftboats.

Association efforts

Most liftboat operators are members of the Offshore Marine Services Association headquartered in New Orleans, Louisiana. This association has worked with the Coast Guard for several years to achieve a balanced set of regulations governing the design and operation of existing liftboats. Coast Guard regulations for new liftboats will be a part of 46 CFR subchapter L, which addresses offshore supply vessels.

The Society of Naval Architects and Marine Engineers is involved in technical areas of liftboat design and operations. It has a liftboat committee which intends to produce a technical and research bulletin to address liftboat design and operation. The Coast Guard, the American

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Bureau of Shipping and the National Transportation Safety Board are active participants of the liftboat committee.

Operating precautions

Liftboat operations are unique -- completely different from those of any other vessel in the Gulf of Mexico. Liftboats are not and cannot be designed to operate in heavy seas. Typically, they are restricted to wave heights of four to five feet when jacking up or down.

Operational instructions for liftboat captains leaving port usually require them to assess weather conditions at the offshore destination and en route. If seas are predicted to be above the maximum safe limit for the vessel, the captain will stay in port. If seas are encountered en route approaching the safe limit for jacking, the captain will either stop and jack up the vessel, or return to port.

If a liftboat is jacked up on location and is preparing to return to port, a similar weather evaluation is made. If conditions are predicted to be rougher than the vessel can tolerate afloat, the captain will keep it elevated. Because a liftboat when elevated offshore usually encounters more turbulent seas than those in which it can jack up or down in safety, it must be designed to be able to operate while jacked up in rougher weather conditions than if it were afloat.

A typical liftboat spends about 90 percent of its life in service elevated offshore. Although many liftboats have been toppled by hurricanes when elevated, there are also many cases where they have survived. Their survivability when elevated is directly related to the water depth at their elevated location. The Coast Guard requires that existing boats demonstrate their ability to survive in 100-knot winds in shallow water when elevated.

Existing liftboats must satisfy afloat stability standards similar to, but not quite as stringent as those for mobile offshore drilling rigs (MODUs). They must comply with the Coast Guard's 1.4 area ratio (often presented in terms of an allowable center of gravity location curve) for a 50-knot wind speed. Existing boats are required to have a range of positive stability of just 10 degrees.

Liftboats generally have much lower freeboards than MODUS (often less than three feet), and cannot safely travel in seas more than six feet high. In higher seas, there is danger of capsizing after taking large amounts of green water on their open forward decks.

The industry is now trying to determine maximum safe conditions for liftboat operations based upon vessel design parameters. The practical experience of many years of liftboat operation is being combined with modern naval architectural science to achieve this goal.

The future

The liftboat market is less affected by the current downturn in the United States offshore industry than the drilling rig and supply boat market. This is partly because liftboats are still the best vessels for maintaining many existing offshore production platforms.

However, liftboats are beginning to leave United States waters and go to overseas markets in the Middle East, Africa and the Mediterranean, decreasing the number of available boats in the Gulf of Mexico.

The recent introduction of inspection requirements also affects the market. The new regulations have resulted in water depth restrictions being imposed on vessels that were previously limited only by leg lengths.

These two new occurrences are both helping to keep the liftboat market from collapse. Several new vessels have been built on the Gulf Coast in the last three years. With more emphasis on analysis, clear design targets and modern computer design aids, these new vessels are setting higher standards for liftboat safety and efficiency. It is hoped that the market will continue to support new liftboat construction.

Mr. Bil Stewart is president of Stewart Technology Associates, an engineering consultancy located at 5011 Darnell, Houston, TX 77096.

Telephone: (713) 665-7294.

Photographs accompanying this article are courtesy of Stewart Technology Associates.

Between 1980 and 1987, 46 liftboats were involved in accidents resulting in 57 deaths and injuries, and more than $20 million in damages. Well over half of the accidents were caused by failure of the legs, pads or jacking equipment. Operator error and inadequate stability accounted for most of the remaining accidents.

This accident record prompted liftboats to become Coast Guard-inspected vessels in the late 1980s. They are inspected under Coast Guard regulations for cargo and miscellaneous vessels, except for liftboats under 100 gross tons, which may be inspected under small passenger vessel regulations.

Liftboat inspections for certification, focus on safety features, including lifesaving and firefighting equipment, propulsion and auxiliary machinery, the electrical system, pollution prevention, licensing and documentation, hull and closure condition, and fire hazards.

Because they can elevate their hulls out of the water, liftboats carry their own drydocks with them, and rarely need the services of an actual drydock.

Inspection of the legs, support structure and jacking mechanisms are an important part of their hull examinations. For example, the straightness of the legs must be checked, because even slight deflections can result in catastrophic accidents. In addition, the welded connections between pads and legs need to be closely inspected to ensure they are sound.

Stability

Because liftboats operate both in afloat and elevated modes, their stability calculations are more involved than for standard vessels and must be carefully reviewed.

Adequate stability is absolutely vital to control wind and wave forces. Without it, liftboats are in danger of being flipped upside down. When a liftboat is afloat, traveling between jobs, its legs are up, extending high above the deck. Large waves can wash across the main deck and momentarily push the vessel down in the water, damaging cargo or the vessel. Big waves can cause a liftboat to "rock and roll" severly, because of the high center of gravity. Continued on page 32

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To improve stability, some companies permit their operators to lower the legs several feet to reduce the center of gravity and smooth out the ride. This can be disastrous, however, in shallow water or near shoals where the lowered legs could contact the bottom and get damaged, or even cause the liftboat to capsize.

When a liftboat is elevated, winds are of primary concern. Picture a liftboat with its pads on dry land and its hull jacked up almost to the tops of its legs. Besides looking scary, it is obvious that the hull can act like a sail. Such is the case even when a liftboat is in 100 feet of water and only elevated 25 feet above it. It is very important that it be able to withstand high winds without toppling over.

Since large waves frequently accompany high winds, a liftboat must be high enough out of the water so as not to get hit by the waves. When the vessel is elevated, big waves can actually lift it up off the sea floor and slam it back down again. Obviously, this can damage the legs and jacking machinery, but if the vessel is slammed down unevenly, it can be toppled. Since the length of the legs prevent liftboats from being jacked up indefinitely, there are limits to the depth of water in which liftboats can operate.

The ability of liftboats to withstand wind and waves determines the operating restrictions placed on them by the Coast Guard.

Operational guides

Liftboat operation methods have contributed to numerous casualties over the years. (See Proceedings, April 1982, November/December 1986, September 1987 and July/August 1992 for liftboat casualty cases.) Consequently, formal operations manuals are required for each liftboat. These manuals contain simple instructions for routine and emergency operations, and for maintaining stability.

Probably the riskiest of all liftboat operations is jacking up and down. A vessel is most susceptible to damage when its pads have just made contact with or are near the sea floor. If the seas unexpectedly cause the vessel to pitch or roll, or the sea floor is uneven, the load on one or more of the legs can be greater than it or they are designed to handle. Consequently, wave heights and sea condition limits are contained in all operations manuals.

Guidance for emergency situations is absolutely necessary in liftboat operations, as illustrated by the sinking of the Auco V on July 31, 1989. This vessel was attempting to get into port in severe weather when it was overcome by the seas and capsized, killing ten people. One of the causes cited was the company's failure to recall the liftboat into port early enough to avoid the heavy weather. Also contributing to the accident was the master's decision to lower the liftboat into heavy seas from a relatively safe elevated position. Since this incident, operations manuals have had to provide guidance for such difficult decisions as when to leave the area and head for safe refuge, when to evacuate the vessel and when to jack it down.

Operational manuals provide additional guidance such as warnings not to submerge the main deck to attempt to free a leg that is stuck in the mud, and not to rock or twist the vessel to break legs free from mud. There also are instructions for liftboat personnel to wear life preservers during jacking operations. These lessons were all learned the hard way.

LT Timothy M. Close is a marine inspector at MSO Morgan City, 800 David Drive, Morgan City, LA 70380-1304.

Telephone: (504) 384-5278, Ext. 132.