A DESIGNER OF A SHIP has a problem in combining into a workable whole the many characteristics desired by the intended owner. Speed vs. sea-keeping, cargo capacity vs. limiting draft in harbors to be visited, cargo capacity vs. endurance, and cargo capacity vs. exemption from admeasured tonnage are some of the many conflicting aspects of a marine design. Compromises are forced upon the designer and the owner in order to obtain a ship satisfying both the needs of the owner and the expected service conditions.

Just as the designer must appreciate the owner's and operator's intended use of the vessel, so too must the owner and operator appreciate the limitations of the design.

This article discusses in nontechnical terms the stability of the small, broad, shallow-draft vessels which have evolved along the Gulf of Mexico as mobile support vehicles for the offshore drilling sites. It was originally prepared to give operators of these vessels a better appreciation of the limitations and peculiarities of that design. It is included in the PROCEEDINGS because the basic

principles discussed herein are of general application.

The principal features of a typical offshore supply vessel are: LOA 120' to 130' -Beam 32' Depth 10'

Load Draft 8'-4"

Freeboard at load draft 20"
Displacement at load draft 700
long tons

Lightship weight 235 long tons
Deadweight 465 long tons

The major portion of the deadweight is for cargo on deck and liquids in the "ballast tanks"-a very minor part is crew, supplies for the crew and the ship, and fuel: (about 7 to 10 tons). If peak tanks forward and aft are filled, this nonpaying deadweight would be about 45 to 50 tons. This boils down to about 400 long tons of paying cargo at 20"' freeboard.

Basically the ship type is a deck barge which has been fined up in the bow for streamlining and cut away at the stern for the propellers. Pilothouse and crew quarters are forward above the main deck. Machinery exhaust, intake and access are contained

in trunks projecting up through the deck aft in an outboard position so as not to clutter the cargo area.

Cargo is carried on the broad open deck area bounded with rails, pipe stanchions, or closed bulwarks at the sides. From the quarters forward to the machinery space aft there usually is a below deck tunnel, centerline, which is flanked by large "ballast" deeptanks that extend from the bottom shell to the cargo deck. These tanks are used to transport liquids to the offshore rig.

SOME BASIC DEFINITIONS

The details of a vessel's stability can be quite technical and involved. An effort has been made to keep this discussion simple. As a first step we should review some basic terms:

Draft: Distance from keel to waterline.

Freeboard: Distance from waterline to deck-how close we are to being underwater. Metacentric height: GM-It is a measure of initial stability. As it gets numerically smaller, stability is reduced.

Figure 1 represents coordinate axis upon which stability is commonly shown in graphical form.

Figure 2 represents a transverse section of a vessel showing the relationship of G, the center of gravity; B, the center of buoyancy; M, the metacenter, the point through which B acts.

Figure 3 is representative of a typical offshore supply vessel. We are interested in trends and not specific numbers at this point. Specific numbers are left to the naval architect. In figure three:

1. The loadline: Defines the maximum draft or more correctly the minimum freeboard. (How close we are to being under the water.)

2. The required GM curve (S-S): This defines the minimum stability for survival at sea, assuming the cargo is well secured.

3. The "shaded" area of the graph is the area to avoid. If we are interested in survival we stay in the unshaded area.

NOW FOR THE SHIP AND ITS LOADING (FIG. THREE)

1. Place the "bareboat" ship on the graph as A. This is the basic ship without ballast, crew, cargo, fuel, etc.

2. Put on crew, fuel and supplies and we move to point B.

3. Add a given tonnage of pipe cargo stowed 4 feet high on deck and we move to point C.

4. Add drilling water in two below deck tanks till they are each half full; we move to D.

5. Press up those two "ballast" tanks and move to E. So far so good. 6. Now let us go back to point B and assume that instead of pipe we are transporting drilling mud in containers 7' x 7' x 18' or 8' x 8' x 16' mounted on skids which raise them another 6 inches off the deck. From point B load an identical tonnage to what we had before but this time the tonnage is in full or nearly full mud tanks. Since the center of gravity of the cargo is higher than before, we move to point C1.

7. Add same amount of "ballast" as before and move to D1 and E1. As you see, we are now into the shaded area. Numbers have intentionally been omitted, the principle is the important thing and that is all that is being demonstrated. It is definitely possible to load these vessels dangerously.

TO EMPHASIZE THE ABOVE

1. In adding cargo on deck we have moved toward the shaded area of the graph. Both freeboard and stability have decreased.

2. In adding ballast in these vessels, contrary to what you might think, the same thing happens; both stability and freeboard decrease. Certainly the stability did not decrease as fast as it would have if we had

ABOUT THE AUTHOR

added more deck cargo, but it did decrease.

3. In a conventional cargo vessel when you add low ballast you get a gain in GM shown in figure 4.

4. Such is not the case when you consider an offshore supply vessel with a broad shallow-draft hull form. One nontechnical reason is because, in these vessels, adding ballast does not appreciably lower the center of gravity. In these vessels, tonnage exemption considerations plus a required capability of transporting large quantities of river water "ballast" to the drilling site (to be used as drilling water) do not provide a tankage arrangement which improves stability. The addition of liquids in the deep "ballast" tanks of these vessels never improves stability. The only thing you can say of merit is that stability decreases less than if an equal amount of deck cargo had been added. A more sophisticated reason is that, in this hull form, the displacement is increasing without significant change in the waterplane. These effects combine to reduce GM drastically.

HOW MUCH STABILITY IS NEEDED?

Our goal is to stay out of the shaded area. To do this we must know the boundaries of the shaded area-this is for the naval architect to determine. After the boundaries are known, we must:

1. Assign the responsibility for maintenance of stability. There must

Figure 2.

LCDR Bleakley was recently assigned to the Technical Staff of CCGD9(m). He graduated from the Coast Guard Academy in 1951 and completed post graduate training in Naval Construction and Marine Engineering at M.I.T. in 1957. On previous duty assignments in the Coast Guard he has served in the Merchant Marine Technical Division at USCGHQ, in deck and engineering billets on both Atlantic and Pacific Ocean station vessels, and as a Ship Superintendent at the CG Yard.

DRAFT

GM

Figure 4.

be a man who knows the condition of the vessel at all times, i.e., someone in charge on board the vessel. This has customarily been the Master.

2. Give the Master a set of simple rules so that he can keep the vessel's stability "out of the shaded area."

The required GM curve which the Coast Guard feels is applicable to this type vessel is based on the work of Rahola (a professor at Helsinki) with a slight modification to take into account more recent studies by J. R. Paulling (a professor of the University of California). Paulling's studies concern the decrease in stability experienced when a ship has a wave crest amidships. For these broad shallow draft hull forms the stability decrease associated with a wave crest amidships is much more significant than in conventional forms. An example of when this stability decrease would be experienced is when running before a sea, where the vessel will be on the crest of a wave for a relatively long time in relation to the roll period. RAHOLA CRITERIA AND DYNAMIC STABILITY

It has been mentioned earlier in this discussion that metacentric height (GM) is a measure-a yardstick if you will-of initial stability, which for convenience can be considered as stability with no heel on the vessel.

Rahola uses another yardstick, related to GM, but taking into account other factors, such as the behavior in the sea. Rahola's yardstick is called dynamic stability.

Graphically, dynamic stability, at any point, is the area under the static stability curve up to that point, and it usually looks something like figure 6.

Rahola says, essentially, that a ship needs a certain amount (15 foot degrees) of dynamic stability to survive in a seaway. He measures this dynamic stability up to a critical angle which is determined by:

(a) A point where the dynamic stability ceases to increase appreciably with more heel; in other words, where the tendency or desire of the vessel to return to the upright condition begins to decrease. Rahola used the point where the angle of maximum righting arm or 40 degrees (whichever is less) has been reached.

If the vessel starts to flood before the previously mentioned point is reached, then "signals are off." The reason is that the vessel does not get the chance to develop its full survival capabilities which it otherwise would have if all openings were closed and the vessel did not flood. So Rahola adds the following:

(b) Or to the angle at which openings admit flood water-the down flooding angle.

A required GM curve can be computed which meets the Rahola criteria. If all openings are closed at all times the general shape of the required GM curve for this type vessel is shown by curve ABC of figure 7. But, if flooding will occur through hull openings then the vessel's heel must be limited to avoid exposing those openings, and the curve would then look something like ABD.

From this we can see that two points about openings on deck should be stressed.

1. The Master must see to it that openings which could flood the ship are kept closed. This is essential in this class of vessel where a roll of 6-10° puts the deck edge under.

2. The designer should assume that if something could go wrong it will go wrong. People will leave doors open, especially in situations with poor ventilation. Doors and other openings should be located well inboard or high up where they have less chance of being awash.

VESSEL'S TRIM IN RELATION TO LOADLINE

Sometime in your childhood days, you may well have gone swimming with other children on a raft. You

noticed when the raft got crowded, that as long as you were all evenly distributed or were all near the center of the raft, you were still afloat and the raft was level. If, however, you all went to one end of the raft (mind you, the same number of people on the raft in both cases) you quickly found that the crowded end went under and maybe even scooted out from under you.

A similar trimming action happens with the typical offshore supply vessel, or with any vessel for that matter. For example, load a barge evenly with cargo. The vessel is level and just has its loadline at the surface of the water. Now move the cargo to the stern. What happens? The freeboard decreases at the stern, but the loadline mark is still above water. The same thing would have happened if you had initially placed the same cargo at the stern.

The consequences of this are numerous and more significant in a vessel without a poop deck. Obviously, any hatches, such as access openings to the steering gear room, are that much closer to being under water or being washed over by waves, and any water on deck tends to reduce stability.

CARGO SHIFT

A ship with properly secured cargo can operate in the unshaded area of the previous diagram and be safe. The required GM curve shown on the left of the diagram assumes that the cargo is secured. But if the cargo is not secured and large heavy objects are allowed to careen across the deck, you are in trouble, and following all the other rules in the book won't help once the cargo has shifted and the vessel capsized.

Again we are back to the raft story, except that instead of moving the load to the stern we have moved it to the side of the ship. The results are generally more severe and the response quicker since there is less resistance to heeling than to trimming.

FREE SURFACE

A liquid free surface acts to the detriment of stability in a manner similar to a cargo shift. It is, however, a hidden free roving stability thief out of sight in a tank or a bilge. Compare the trip from the kitchen sink to the refrigerator with an ice tray full of water, first without and then with the cube divider. This example of the effects of a free surface has a direct parallel to the conditions on a ship with a cross connection between the two wing tanks open or closed.

Two partially full tanks side by side in a ship with an open cross connection between them create a stability loss due to free surface which is four times that which would occur without a cross connection.

Liquid free surfaces do rob you of stability; they must be controlled by elimination of bilge water and control of tank contents.

BLOCKED FREEING PORTS:

Water on deck, held there by bulwarks, is just like tons of deck cargo except that it is also moving. This reduces both freeboard and stability. Therefore, an important design detail is the requirement for an adequate amount of freeing ports to allow the water to drain clear. The operator must insure that freeing ports are not blocked by cargo.

DOWNFLOODING

Water flooded into the internal portions of a ship is a load which reduces freeboard and also reduces stability due to sloshing back and forth or "free surface" effect.

CONCLUDING REMARKS

We have brushed over the high spots of stability, as it affects the offshore supply vessels, in an attempt to describe a rather technical subject in nontechnical terms.

From the stability viewpoint, we still have three basic things to do:

1. Determine the boundaries of the shaded area which we want to avoid. (This is the naval architect's job.)

2. Assign the responsibility for the maintenance of stability.

3. Provide the responsible person with some rules for assuring himself that he does have adequate stability at all times.

Some progress has been made on step 1. This beginning involves a series of calculations done at USCG HQ for one hull form similar to figure 2 which is representative of this type of vessel. This information is currently being used as a basis for approving stability calculations subImitted to the Coast Guard for inspected and certificated vessels of this type. Because the hull forms do vary, additional work has been scheduled to include hard chine forms with more

deadrise, less deadrise, a double chine form and a conventional gradual turn of the bilge form, all of which are currently in service.

The aim of these analyses is to shortstop the calculations presently necessary to determine a Rahola required GM curve by devising a family of curves from which an appropriate curve for a particular vessel may be selected without calculation. Ultimately it would be desirable to derive a simple formula using principal dimensions such as beam, draft, depth and midship section shape which would provide a curve equal to a Rahola required GM curve.

In addition to giving the owners and operators a better appreciation of the stability limitations and peculiarities of these vessels, it is hoped this article will generate additional interest and progress in the areas of steps 2 and 3 which are vital to assured, rather than chance, operation in the zone of adequate stability.

REFERENCES

1. "The Judging of Stability of Ships and the Determination of the Minimum Amount of Stability"Rahola-Helsinki 1939.

2. "The Transverse Stability of a Ship In a Longitudinal Seaway"J. R. Paulling-pp. 37-47-Journal of Ship Research-Vol. 4, No. 4, March 1961, published by the Society of Naval Architects and Marine Engineers, 74 Trinity Place, New York, N.Y.

3. "Transverse Stability on Tuna Clippers"-J. R. Paulling Jr.-pp. 489-495-Fishing Boats of the World: 2-Published by Fishing News (Books) Ltd., 110 Fleet Street, London, E.C. 4.