New Developments in Tanker Design
By E. L. Stewart
A modern Sfandard Oil of California tanker passing Mile Rock Lichthouse, San Francisco Bay.
THE DEVELOPMENT of the oil tanker from its very immature beginning to the present highly specialized carrier of bulk petroleum
products was one of "necessity." Necessity — occasioned by the remoteness of the oil fields from home and foreign markets, the volume of the petroleum produced, and the need for a safe, practical and economical means of handling and transporting petroleum products in seagoing vessels.
At first the petroleum was transported in wooden barrels. Subsequently, to reduce the tare weight of the containers and permit of more economical stowage, rectangular prism-shaped metal containers were used. Due to the considerable leakage of petroleum from the containers — with attendant hasards of explosive gas formation, and also by reason of the relatively high cost of containers at a time when the production of petroleum was rapid — attention was directed to the possibility of handling and transporting petroleum in bulk in seagoing carriers. The structural arrangements devised and used to effect such bulk petroleum transportation were numerous and varied, until in 1886 the German - conceived and English-built tanker Gluckauf, prototype of the present-day bulk petroleum tanker, was built. Although there is a far reach from the Gluckauf, of 3020 tons deadweight carrying
capacity, to the large bulk oil carriers of present - day commerce, the basic principles of general design embodied in the Gluckauf and its contemporary tankers have not undergone any material change in present application.
It would entail a long treatise to describe the transition of the tanker from vessels with propulsion machinery aft to vessels with propulsion machinery amidships and back again to propulsion machinery aft; from single deck vessels to those of twodeck "poop -bridge -forecastle" type and three-deck "flush" or "shelter" deck types, and back again to singledeck vessels with poop-bridge-forecastle; from vessels with separate expansion trunks to those with continuous expansion trunks or just access hatchways on deck; from vessels with transverse framing to those with longitudinal framing or hybrid transverse/longitudinal framing systems; from vessels with tank center line longitudinal bulkhead and topside summer tanks to vessels with twin and triple longitudinal bulkheads in cargo oil spaces; from the various developments and uses of steam reciprocating, combined steam reciprocating and exhaust turbine, geared turbine, to diesel (of four cycle and two cycle, single acting and double acting, air injection and solid
injection; supercharging and nonsupercharging), turbine -electric and diesel -electric; and from Scotch to water tubular and exhaust gas boilers.
Although such a treatise would be of historical interest and perhaps be especially intriguing to persons directly interested in tanker construction and operation, it would prove of little value in a practical consideration of the present-day tanker. In entering into such a consideration from the viewpoint of "New Developments in Tanker Design Especially as They Affect Speed and Transportation
Costs," it is recognized that there can be many shades of opinion as to whether or not the present-day bulk oil carrier embodies those features or developments which should provide a safe, practical, economical and readily maintained tanker to fill the needs of all normal services. These shades of opinion, however, are generally associated with the requirements of special or "tailored" services peculiar to the operations of individual operators and companies.
A Maritime Commision T-2 tanker, Turbo-electric drive.
( Photo Marinship )
Analyzing the economic significance of some of the present developments in the design and operation of seagoing bulk oil carriers, there are certain influencing factors upon which an agreement of opinions should obtain. Amongst such influencing factors are those related to the gradual increases which have been made in the deadweight carrying capacity and sea service speed of tankers during the past two decades. The gradual improvement in the design of the hull structure, hull form and its appendages, propulsive equipment and hull engineering items, with resultant relatively large decreases in the weight thereof, has made it possible to obtain an attendant gradual increase in deadweight carrying capacity and in sea service speed.
The increase in speed occasioned very close study of hull form and propulsion, and the resultant "fining" of tanker lines has permitted the present-day tanker to maintain speed in heavy weather. It is obvious that an increase in the sea service speed of a large tanker has a greater influence on the average speed of transportation of petroleum products than a like increase in the speed of a smaller tanker in the same service.
Constant efforts are being made to minimize the cost per cargo deadweight ton per mile for the transportation of the petroleum products carried and to obtain a maximum return on the capital invested in the vessel.
Compatible with the port facilities and the shoreside oil handling facilities involved, the average size of tankers as to deadweight capacity and cargo volumetric capacity has been gradually increased over the average size of tankers built ten years ago.
In this connection it is of interest to note that the world average deadweight carrying capacity of tankers built in 1930, principally motorships, was about 12,300 tons and the average speed about 11 1/4 knots.
The present-day tanker construction embodied in the Maritime Commission Tanker Building Program indicates an average of slightly over 16,600 tons deadweight capacity per tanker and a speed of slightly over 14 1/2 knots.
As of September M), 1944, the program of Maritime Commission tanker construction included contract groups comprised of 446 tankers contracted for and about ill tankers delivered, of the types designated T2-SE-A1 and T2-SE-A2. Such types of tankers
are rationalized commercial vessels having the same hull characteristics — symbolized by T2; like form of single-screw steam turbine electric propulsion — symbolized by SE; but different power of propulsion — symbolized by A1 for installation with propulsion motor capable of developing about 6600 shp at 93 rpm; and A2 for installations capable of developing about 10,000 shp at 106 rpm. Of the two types the contract groups embody 406 vessels of the A1 symbolization and 40 vessels of the A2.
The low freeboard of tankers demands extreme precaution in designing deck fittings for water tightness and affords opportunity for fine sea action shots.
The Maritime Commission's construction program has also included contract groups comprised of 15 tankers contracted for and 14 tankers delivered, of the type designated T3-S-A1, which in general is a geared steam turbine propulsion powered contemporary of the steam turbine-electric propulsion T2-SE-A1 tankers. In view of the preponderance of tankers constructed and delivered since the beginning of 1942, being of the Marcom T2-SE-A1 type, and by reason of the same hull form characteristics having been embodied in two other differently powered designs of privately constructed contemporary tankers, it is thought that a comparison of such designs is especially pertinent to the subject.
The three designs embody the use of fluted bulkheads in cargo oil tanks, and are of the single deck poophridge-forecastle erections type, with twin longitudinal bulkheads in the cargo tank body, and bracketed lont;itudinal framing of shell and deck.
While the T2-SE-A1 tankers are entirely of welded construction, the other two designs employed riveting in the seams of the side shell plating and upper deck plating — the butts were welded. The T2-SE-A1 tankers have no sheer of upper deck but are of increased molded depth, resulting in practically the same summer freeboard draft as that which obtains for the other two designs having a parabolic sheer of upper deck. The details of other features such as tank body disposition and volumetric capacity, location of cargo oil pump room, semi - bulbous bow, cruiser stern, are quite comparable in all three designs.
The propulsion power installations are, however, entirely different in the three designs — the T2-SE-A1 type have a 6000 shp (normal) at 90 rpm steam turbine electric installation, while the other two designs each have a geared steam turbine installation, one design having 4000 shp (normal) at 85 rpm, and the other design having 9000 shp (normal) at 90 rpm. Under certain conditions of propulsion power operation, as presently authorized by the War Shipping Administration, the motor of T2-SE-A1 tankers may he operated at about 7600 shp and 97 rpm. The average sea service speed and the corresponding "all-purpose fuel oil consumption" per 24 hours, for each of these comparable designs, should be: for the geared steam turbine vessels, 13 knots — 203 barrels and 16.5 knots — 415 barrels; and for the steam turbine electric vessels, 14% knots — 311 barrels and 15.3 knots — 348 barrels.
Further comparison of these three designs, as to their relative deadweight capacity and ratio of deadweight/displacement, at International summer freeboard draft and with vessels devoid of any national defense features, is:
4,000-shp vessel..I6,725 tons and 0.767 ratio
6,000-shp vessel..l6,76S tons and 0.766 ratio
(Without tipper Deck; Under-deck Supplementary Girders)
6,000-shp vessel. .16,610 tons and 0.759 ratio
(Without Upper Deck; Under-deck Supplementary Girders an modified After Accomodations)
9.000-shp vessel-16.585 tons and 0.759 ratio
In the case of the geared steam turbine turbine propulsion designs, the increase in power from 4,000 shp to 9,000 shp resulted in an increase in machinery weight of about 12 1/2 per cent and an increase in cost of about 10 per cent of the construction cost of vessel.
The increase in speed of about 1/2 knot for the T2-SE-A1 tanker is the result of relaxation of design rating and minor changes in the steam turbine unit.
Quantitative analysis of the effect of speed changes in these designs depends upon the evaluation of all items to meet the operating and cost structure peculiar to each operator; It should, however, be possible to operate and maintain in operation such
vessels at their higher speeds and, in doing so, reduce the "cost per cargo deadweight ton per mile" in the order of several per cent.