Three Dimensional Enlargement Of Great Lakes Bulk Carriers — A Sound Investment

Existing Great Lakes bulk carriers may be economically enlarged since the new Poe Lock at Sault Ste Marie has removed the previous size restraint. It is now possible for a vessel of 1,000-foot overall length and 105-foot beam to transit the new lock. Two new vessels in the building stage will take advantage of the new lock size. One vessel has an average trip capacity close to 41,000 tons and the second vessel has an average trip capacity of about 52,000 tons. The largest vessels presently in service have an average trip capacity of about 25,000 tons.

By using a three dimensional (3-D) enlargement of an existing vessel, this disparity can be partially eliminated. Three dimensional enlargement consists of enlarging an existing vessel by increasing its length, beam, and depth.

There are 24 U.S.-flag Great Lakes bulk carriers which are prime candidates for 3-D enlargement, all built since 1950. There are also many vessels built prior to 1950 which have suitable powerplants, or have been repowered, and which are also suitable for 3-D enlargement. There are 31 vessels in this category.

Three dimensional enlargement of any Great Lakes bulk carrier now in service will increase its average trip capacity by about 60 percent. Operating expenses increase slightly and are comprised primarily of increased insurance and fuel costs.

Several 3-D enlargement projects have been investigated in some detail. They have all proved to make sound investments.

Investigated in greater detail was an existing vessel with a length of 710 feet, a 75-foot beam and 37-foot 6-inch depth, with an 8,500-shp steam turbine powerplant.

Economics

Table 1 compares the investment in an existing vessel with those for various enlargements. It includes in addition the investment in a new vessel. An arbitrary 1969 market value of $8,000,000 has been assigned to the existing vessel.

The economic comparison is a simple one, since the intent is to compare investments, and not to make a detailed analysis of each. Straight line depreciation over 20 years has been used. The effects of income surtax and investment credit have been ignored, since each investment will be affected equally by considering them. Accelerated depreciation will improve each investment. The method of financing will be significant, and may vary with the size of investment. None of these refinements will affect the standings.

In Table 1 the average annual gross revenue (line 3) is based on annual carrying capacity (line 1) at a freight rate of $1.90 per ton, except that $2.05 per ton has been used for the new vessel, which is a self unloader, and average annual operating costs (line 4) is an average figure for the 20-year period. It is about 30 percent in excess of current annual operating costs.

Using capital recovery factor as an index it is shown that the 3-D enlargement makes the best investment. Circumstances could justify an owner choosing an investment other than the best, but Table 1 clearly indicates the sacrifices he would make.

Choosing the New Dimensions

Existing vessel dimensions, proportions, lines, powerplant, and construction determine the maximum size to which a vessel may be three dimensionally enlarged.

Cargo hatch arrangement, and vessel shape at the cut points, help determine the amount of lengthening and widening possible. The increase in length will also be dependent upon existing bottom shell thickness and other existing structure.

The only purpose of deepening a vessel with adequate existing draft is to permit the maximum possible lengthening. For instance the 710-foot vessel may be lengthened about 96 feet maximum if the depth is not changed; the addition of large amounts of steel in the form of straps or doublers might permit a greater increase than this, but the cost becomes prohibitive. The same vessel may be economically lengthened by 168 feet if the depth is suitably increased. Each existing vessel will have its own particular maximum increases for each of the three dimensions.

It is generally possible to provide sufficient longitudinal strength to suit the increased vessel dimensions. Widening the vessel by cutting just inboard of the gunwale angle is particularly attractive, since it permits a large increase in spar deck stringer plate width, which provides a large increase in longitudinal strength. If the longitudinal cut for widening is made near the centerline of the vessel, the hatches will increase in size with the extra width, but no increase in spar deck stringer plate width is obtained. This means that the extra steel for required longitudinal strength must be added as straps, doublers, etc. Cutting near the centerline creates another problem; because of the quickly changing beam, the hatch crane is unable to travel sufficiently far forward and aft to handle hatch covers at the ends.

If the widening is performed within the ballast tanks the total ballast capacity of the enlarged vessel is considerably increased. In fact, for the vessel considered, ballast capacity increases almost 100 percent. It should not be necessary to increase the ballast pumping rate exactly in proportion to the increase in ballast capacity, but it may be advantageous to make some increase to the pumping rate to minimize time spent at the loading docks.

A three dimensionally enlarged vessel requires an increase in rudder area to maintain satisfactory maneuverability. The additional area may be obtained by cutting the rudder a few feet forward of the trailing edge and fairing in new double plate construction to give added length.

Increased rudder area results in increased rudder torque which necessitates modifications to the upper rudder stock and steering gear. It may also be necessary to make changes to the lower pintle and to the rudder coupling bolts. Replacing mild steel with low alloy steel is the most economical means of achieving the increased strength.

An increase in steering gear torque may be achieved, in many cases, by an increase in hydraulic pressure and a resetting of the relief valves. For the vessel under consideration a 20 percent increase in torque may be achieved in this way.

Three dimensional enlargement increases the equipment tonnage of a vessel and may require heavier anchors. An increase of anchor chain strength may also be necessary. For the vessel considered, only the anchor weight requires an increase. A change in chain size would mean an expensive anchor windlass modification or renewal.

A 60 percent increase in displacement will have some effect on vessel speed. For most 3-D enlargements a decrease in speed of about 3/4 mph should be expected, at the same shaft horsepower.

Any change in propeller pitch, found necessary as a result of 3-D enlargement, may be readily accomplished on most Great Lakes vessels.

Enlargement Procedure

The 3-D enlargement of a vessel appears at first to be a formidable job. When the process is examined step by step, it proves to be much less fearsome. Precise burning, accurate alignment, careful welding and the provision, prior to cutting, of means to hold the vessel's shape, are the essentials for a successful 3-D enlargement. These are exactly the same requirements to successfully build a new hull.

There are a number of different methods for accomplishing each stage of the enlargement. The method shown in Figures 1 to 6 is one satisfactory method.

The widening may be accomplished by different means. It is possible, for instance, to horizontally jack the side shell section of the cargo hold outboard the distance required for widening.

Loading

A three dimensionally enlarged vessel will be able to load at a number of the existing pocket docks. Predictions indicate that by 1970 two-thirds of the iron ore shipped on the Great Lakes will be in the form of pellets. By 1980, the tonnage of pellets is expected to increase to about 90 percent of the total. At many loading points, belt conveyors will be used to load the vessel. Belt loaders will decrease the overall time at the loading port.

Unloading

A Hulett unloader may not be able to reach out to the outboard side of the cargo hold of a three dimensionally enlarged vessel. Cargo remaining in the outboard portion of the cargo hold must be bulldozed into the reach of the Hulett unloader, requiring some increase in total unloading time.

Conclusion

It is technically and constructionally feasible to enlarge an existing Great Lakes bulk carrier three dimensionally. Such a 3-D enlargement is an attractive investment where increased capacity is required. Even if additional tonnage is not required, a 3-D enlargement would permit the retirement of an older less economical vessel. A fleet owner who has vessels suitable for 3-D enlargement should thoroughly investigate the merits of such an enlargement before choosing any other means of providing additional capacity.

Figure 1—Vessel is placed in drydock. First the bow is cut free and midsection plus stern floated back so that the distance from the bow to the stern is the final length of the enlarged vessel. Then the drydock is emptied and the stern cut loose from the midsection. In another floating operation the midsection is floated to its predetermined spot in the dock and the dock again pumped dry.

Figure 2—The spar deck and side shell are cut longitudinally and transversely so that sections can be lifted free and put on the ground alongside the dock.

Figure 3—The main deck and bottom shell are cut longitudinally and girth cuts are made so that side shell sections can be moved outboard. Install new bottom shell, main deck sections, etc.

Figure 4—Cut longitudinal tunnel bulkhead above main deck, jack up the spar deck to required height and insert new longitudinal sections in tunnel bulkhead and shell.

Figure 5—Install new spar deck section and lift removed side shell and spar deck sections into place.

Figure 6—Deepen the bow and stern section by cutting, jacking up and installing new steel. Erect new structure between bow and midsection, and stern and midsection. This completes the structural part of the 3-D enlargement.

Table 1. Comparison Of Investments (All figures, except percentages, in 1,000 units) — columns: 3-D Enlarged Vessel / New Vessel / Lengthened & Widened Vessel / Lengthened & Deepened Vessel / Lengthened Vessel / Existing Vessel 1 Annual (L.T.) Capacity: 1,514 / 1,715 / 1,315 / 1,216 / 1,117 / 966 2 Investment: 14,200 / 19,000 / 12,800 / 11,400 / 9,500 / 8,000 3 Average Annual Gross Revenue for 20 Years: 2,877 / 3,516 / 2,499 / 2,310 / 2,122 / 1,835 4 Average Annual Operating Costs for 20 Years: 1,500 / 1,800 / 1,474 / 1,434 / 1,408 / 1,380 5 Average Annual Net Revenue for 20 Years: 1,377 / 1,716 / 1,025 / 876 / 714 / 455 6 Depreciation = Investment/20: 710 / 950 / 640 / 570 / 475 / 400 7 Taxable Income: 667 / 766 / 385 / 306 / 239 / 55 8 Tax 48%: 320 / 368 / 185 / 147 / 115 / 26 9 Cash Flow: 1,057 / 1,348 / 840 / 729 / 599 / 429 10 Capital Recovery Factor After Tax: 7.44% / 7.09% / 6.56% / 6.39% / 6.31% / 5.36%

*Mr. White, director of engineering, Fraser Shipyards, Inc., Superior, Wis., presented the paper condensed here before a recent meeting of Great Lakes and Great Rivers Section of The Society of Naval Architects and Marine Engineers.