I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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phase, between sulfur dioxide in solution and the dissociated coniponents of nitrosyl sulfuric acid (HNOSO,). He concluded that the same reaction mechanism takes place in the more conventional chamber. Berl(1) arrived at a similar conclusion, but assigned an important role to the so-called “violet acid” (HaS04.NO) in the series of reactions. Petersen placed no importance in the violet acid, as it is found only when the p r o w is improperly operated. Berl also showed the importance of the reaction rate of oxidation of nitric oxide to nitrogen peroxide, and it is the slow speed of this react,ion with its negative temperature coefficient that is the ratedetermining step in the p r o w . He investigated the effect of presmre on the reactions involved and showed that the over-all rate could be greatly accelerated by pressure. It doea not appear that this development has much possibility for practical application, or could offer any economic advantage.
Vol. 42. No. 11
LITERATURE CITED (1) Berl, Ernst, Tram. Am. Inat. C h a . Engrs., 31, 193 (1935). (2) Fairlie, A. M., “Sulfuric Acid,” A.C.S. Monograph 69, p. 191, New York, Reinhold Publishing Corp., 1938. (3) Ibid., p. 214. (4) Fairlie, A. M., TTans. Am. Inst. C h a . Engra., 33, 563 (1937). (5) Harney, T. R., C h a . Met. Eng., 36, 402 (1929). (6) Larison, E. L., Ibid.,26,830 (1922). (7) Molstad, M. C., Abbey, R. G., Thompson, A. R., and McKinney, J. F., TTans. Am. Inst. Chem. Engrs., 38, 387 (1942); 39, 605 (1943). (8) Petersen, Hugo, C h a . - Z l u . , 55,493 (1911). (9) Peteraen, Hugo, 16me. Congr. chim. id.(Brussels, 6eptember 1935), 1936,80-7. (10) Wells and Fogg, Bur. Mines Bull. I84 (1918). RECEIVED April 13,SB50.
OFFSHORE SULFUR PRODUCTION JAMES M. TODD Jefferson Lake Sulphur Company, New Orleans, Lis.
Sulfur mining in offshore operations in the Gulf of Mexico can be considered at the present time, only as research activities to extend known reserves. The cost of offshore construction makes sulfur mining uneconomical on the present competitive sulfur market. It is entirely feasible: in fact, overwater production of sulfur has elready been accomplished in protected areas. However,
the extension of this expeFience to gulf offshore operations. at the present time, would be costly. Pipe lines from offshore oil locations on the gulf ffoorto land-based tankage are in daily use, and the pumping of mxlfur through such facilitiee can be accomplished. The special problems to be encountered in offshore sulfur production are discussed in this article.
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If one assumes that permanent platform serving only a limited area, and hence a limited number of production wells, can be economical in Frasch process mining, it is definite that the area to be mined must be extremely rich in sulfur deposit to pay the construction costs of such platforms. Cappel (I) has pointed out that the considerations affecting the design of stationary-type platforms include depth of water, fetch and exposure, bottom characterktics, normal wave action, hurricane wave action, hurricane winds, littoral currents, load concentrations, possible and probable cumulation of forces, rigidity under drilling conditions, areas required for machinery and supply layout, and crew quarters. Experience shows that 30-pound-per-square-foot wind velocity loads are not sufficient design stresses for offshore permanently fixed structures, but that structures designed on a 50-pound-persquare-foot basis have withstood gulf hurricane winds. “The height of hurricane waves as well as the horizontal forces exerted by them is directly related to the depth of water and the fetch-that distance over which the wind has free play on the water.” (1). The generally accepted maximum wave height is two thirds of the depth of the water.
HE mining of sulfur by the Frasch procem in water which is relatively shallow and reasonably protected from high wave and tide action presents no particularly difficult problem. This lias been done succeasfully and competitively by two sulfur operat ing companies. Jefferson Lake Sulphur Company a t its Lake Piegneur operations in Louisiana, mined sulfur by using permanent wharves with drilling equipment mounted on submersible barges. When fivst approached, the problem of submersible barge-mounted rigs offered some interesting prospects. The e84e with which refloating and moving of the rigs to new locations waa accomplished, was not only surprising but contributed substantially to the economy of the whole operation. The Freeport Sulphur Company, in its operations a t Grande I?caille, La., uses wharves and land-fill rig locations. They have riot found the problem of overwater operation too difficult or too costly for competitive production. The offsbre prospect, of aourse, presents many new conoiderations which as yet have not been dealt with economically or competitively in sulfur production. Of course, as long as sulfur can be produced from land-based operations and the competitive selling price of the product is a t its present level, prospective operations for sulfur on the continental shelf would seem only a remote possibility. However, i t does have the advantage of outlining additions to the present rapidly diminishing reserves. PLATFORM CONSTRUCTION ON OFFSHORE LOCATIONS The oil companies have demonstrabd the feasibility and the
costa of platform construction on offshore locations in the Gulf of Mexico.
DATA ON FIRBT PLATFORM IN GULF AREA
The original platform in the gulf area was built in 1937. It was erected in 13 feet of water, 6000 feet offshore, and was constructed of creosoted piling and timber. The lost time during construction because of weather was roughly 75% and considerable equipment was lost. Today the permanent platforms are built of steel. Owing to improvement in technique, the lost time during construction because of weather, over a %year period, has been considerably less than 10%. The minimum cost of platforms or permanent structures of
November 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY
sufficient strength and size to handle a small Fraech operation has been estimated by experienced operators in‘the gulf area to be 660,000dollars. No records on the effect of subsidence on such platforms are available. Those persons who have experienced the troubles and trials of subsidence on land-based operations, think in terms of serious fractures and damage to the construction members. One case within the compass of the author’s experience with water mining showed that an area of considerable extent aubsided 32 feet. The submersible type of equipment, at least for shallow water areas, has enough merit to warrant consideration; however, the economy of thie type of equipment for deep water operations is questionable.
FRESH WATER SUPPLY FOR FRASCH PROCESS Owing to the scarcity of fresh water sands and the possibility of quick salt water intrusion from the gulf on those that might be fresh, sources other than underground water supply must be developed if Frasch process mining is to be accomplished on offshore prospects. The use of salt sea water for Frasch process mining in the same manner aa fresh water, without expensive research, is out of the question. Of course there are processes which make fresh water from salt sea water, but at the present time these processes are not economically competitive. This is particularly true when one considers the large quantities of water necessary for Frasch process operations-3,000,000 gallons per day and upward. Equipment, working with the original McVoy (3)patents, is now offered which is guaranteed to make pure fresh water from sea water at a fuel cost of 1/n of a cent per gallon. Some of the oil operators on offshore platforms have used a Diesel engine-powered equipment with vapor compressor, pumps, evaporators, cooler, exhaust boiler, tanks, and controls for fresh water in small quantities. This equipment averages 220 pounds of water per pound of fuel. The unit size of this equipment is rated at 300 gallons per hour. For the quantities of water required for Frasch process mining, a multiplicity of these units to get capacity hardly seems practical. It is entirely probable that salt water could be heated directly, and successfully, by submerged combustion. As yet no published data on costs are available, even though i t is definite that underwater combustion may be considered past the experimental stage. Without a detailed economic study it seems that either thc transportation of water by barge or the laying of sufficient pipe line for the present distances involved may be uneconomical. At the present competitive sales price of the product, this method of water supply must be discounted. It is true that Freeport Sulphur Company at Grande Ecaille, La., is actually transporting fresh water a considerable distance by elevated pipe line and finding i t economical; nevertheless, the distances involved in present, offshore prospects, if not the laying and maintenance cost of underwater piping, would seem uneconomical for Frasch process mining. UNDERWATER PIPE LINES The California Company in its Bay Marchand and North Pass operations, originally put in service &inch underwater lines of 14,400 feet and 16,200 feet, respectively, and as of March 1950 had on the gulf floor a total length of 47,900 feet of 4-iich pipe (3 runs) and 23,800 feet of Finch pipe (8 runs). They point out further that economy of installation can be secured and cite a 20% reduction in costs on the repeater 4-inch line installation; a 46% reduction in costs on the last Zinch line jobs. These costs, however, are approximately four times that of laying comparable lines in the gulf shore marshes. It seems that the major problem are more economy of installa-
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tion, proper anchoring or marking of the lines to prevent damage, corrosion control, repairing underwater damage, and wing the correct type of scrapers (2). AVAILIBILITY OF IPUEL FOR MINING SULFUR The availability of fuel may not offer a serious problem because of the presence of both oil and gas in the offshore oil operations. The economical storage, transportation, and use of oil fuel, over water, from tank0173 or barges is a proved fact. If fuel lines are laid from land fuel sources one is again faced with the relatively high cost of laying and maintaining imdorwater pipe lines. This can be done, but again one must consider the economic considerations of a competitive price of thc end product, measured against costs which one knows cannot compete with land-based operations. The handling of supplies to and from mine location will entail an item of cost which when added to those economic factors cited above would again show that at present product competitive prices, economic competition with land-bmed operations can hardly prove other than unwise, Product transportation offers no special obstacles, for in liquid Form sulfur has been barged for reasonable distances. It 13, lu)wever, not an especially cheap niethod of transportation COST OF PROSPECTING FOR SULFUR RESERVES In event one should dhcount or surmount the above-mentioned cvonomic difficulties, a proposd was placed before Congress to yeduce the depletion allowance on sulfur operations from the existing 23% to 15%. Those conipanieu engaged in the businem o f extracting minerals such t t ~sulfur must expend large sums af money in the discovery of new reserves, not only to perpetuate the individual operating coinpany but also for national security. The author viFlualizes the: operation of offshore mining of sulfur by the Frasch process-granting for the Rake of argument that the economic pitfalls may mine day be eliminated by price change of the pioduct-in the following manner: The power plant would be either on a permanetit platform, or mobile, much like a ship. It could be brought t u location and moved when necessary, either under its own power or by tug boat. This mobile plant or platform, as in the case of land-based operations, would house the usual complement of boilers, electric pnerators, pumps, air compressors, water treatment and water eating e uipment fuel storage, and, in addition operating crew housing. ?t could Le either in a sin le unit, or muitiple desi n, depending upon m y unsolved probfemu, not the least of wkch is mechanical pib- line connections. The field operation would also be of the platform type similar in many ways to, but less elabor:bte than, the oil company operations a t present in the offshore Gulf area. These platforms would serve for field stations, well equipment, and production areas only. The drilling operations, in view of the fact that sulfur drilling operations are not so deep as for oil, would be conducted from floating rigs or semisubmersible types capable of easy movement from one location to another. From the field stations the liquid sulfur would be pumped into properly insulated and equipped barges for transportation to land-based storage and shipping faoilitiee. HURRICANE DAMAGE The element of hurricane damage, and possible shutrclown periods due to storms, must not be overlooked in any economic discussion of Frasch process mining in gulf offshore locations. The oil companies’ experiences in these respects indicate that these are not insurmountable, even if they are costly. LITERATURE CITED (1) Qappel, C. Glen, Civil hg., 18, No.7, 447 (1948). (2) Denrler, Scott, and West, Petroleum Branch, Am. Lnst. hlining Met. Engrs., San Antonio, Tex., October 1949. (3) MaVoy, V. P., U.8.Patent 2,027,396 (1936). RECEIVED April 13. 1960.
