Click for next page ( 6


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 5
2 DESCRIPTION OF THE PROBLEM . Fixed offshore structures consist of three main components--the superstructure or deck, which provides work space; the jacket; and the piling (see Figure 1~. The jacket rests on the ocean floor and has open pipe columns, or legs, which extend above the water surface. Tubular bracing members interconnect the legs to make the jacket a single rigid structural unit or space frame. Pilings are driven through the legs of the jacket into the ocean floor. Some jackets are as large as tall buildings or the largest ships ~ see Figure 2) . The j acket serves as a guide during pile installation and as a structural unit to support the deck and resist horizontal loads from wind, waves, earthquakes, and currents. For deep water, or for soft foundation conditions, it is often necessary to splice the piling by welding to reach the required pene- tration. For shallow water, the jacket is completely fabricated upright, carried to location on a cargo barge, picked up and set on bottom by a floating crane or derrick barge, and the piling then driven. In deeper water, the jacket is usually fabricated on its s ide , carried to location on a special launch barge, and launched into the water on location where it floats in a horizontal attitude. It is then rotated into the vertical position and lowered to the bottom by a derrick barge or by controlled flooding. The superstructure, consisting of several units or deck sections, is built onshore in a fabrication yard, carried to location by barge, and lifted into position by a derrick barge. NUMBER OF PLATFORItS The committee assembled information on the number of U. S. offshore structures, their size, water depth, and year installed from a variety of sources. Figure 3 shows total platforms installed per year and the cumulative number in the U.S. Gulf of Mexico, both in state and OCS waters. Table 1 shows the water depth of structures installed in the Gulf of Mexico, off California, and off Alaska. These data show that in 1983 there were 4,094 fixed offshore oil and gas drilling or product ion structures located in the U. S . territorial sea or on the continental shelf of the United States. An additional 1,461 5

OCR for page 5
6 IWO DR ILLING RIGS n ~~ DECK SECT10 A. JACKET OR I L Ll NG CONDUCTORS - ~, LY 'it/ ~ MEAN SEA LEVEL .. |/ ~ T_ ~~ ' ~ 'I''' ''1 1 ~ : LEGS : (72"T084"Dia.) L: \: 3S OCR for page 5

OCR for page 5
8 30O1 1 1 ~ ~ or to _, 200 In it or ~ 100 V, o m ZINSTALLED ~: /' V _ / _ /CUMULAT IRE 1 960 1 9 70 Y E A R S FIGURE 3 Gulf of Mexico offshore structures. 6000 f , rY ~~ it_ ~\tJ/ 1000 _ O 1 980 1990 to 5000 ~ 4000 ~ O 3 000 ~ O Z 2000- J SOURCE : Minerals Management Service ~ total federal OCS lands ); Offshore Oil Scouts Assoc iation ( state offshore lands) . Number of structures after 1982 based on committee estimates . structures are projected for installation through 1990.~ This is the population of structures that may require disposal in the next 35 years, the time-frame of this study. PLATFORM LIFE The life of an offshore structure extends until it is no longer serviceable and must be replaced, or until the function that it performs is no longer needed. A well-designed steel structure has no defined life as long as it has not been overloaded and has been properly maintained to prevent corrosion. The useful life of an offshore platform depends on the duration of oil and gas production from that location, not necessarily on the strength of the structure . Offshore platforms are subject to repeated loads that can cause fatigue damage. Fortunately, fatigue has not been a problem of any significance in the Gulf of Mexico because the normal day-to-day wave environment is very mild. In addition, the early structures built in relatively shallow water are stiff and not particularly subject to XAbsent a completely reliable basis for predicting future platform requirements, the committ~e's projection is based on general industry opinion.

OCR for page 5
9 TABLE 1 Water Depth of Structures Installed in the Gulf of Mexico, off California, and off Alaska as of 1983 Water Depth Number of Structures (feet) Gulf of Mexico California Alaska Total 0-20 l~lS2 1,152 21-S0 1,414 1,414 51-100 650 7 14 671 101-150 329 329 151-200 240 10 250 201-300 206 206 301-400 52 6 58 401-500 5 5 501-900 4 1 5 >900 4 4 Total 4,056 24 14 4,094 aDepth categories for California data are <100', 100'-200', 200'-400', >400'. ball Cook Inlet, Alaska platforms are in water approximately 100' deep. SOURCES: See Figure 3 . Also, Alaska Department of Envl ronmental Conservation and California Divis ion of State Lands . fatigue damage. As structures are built in deeper water they are more slender, and, therefore' more flexible. Platform motions and the resulting fatigue problems become more significant. However, better procedures are available allowing design of structures to resist fatigue loading based on a prediction of the loading history QXpQC ted during the life of the structure. FEW, if any, structures have been removed because they were no longer structurally sound or serviceable (except for structures damaged by collision, fire, or storms. Presently there are numerous offshore structures in moderate and deeper water depths that are IS to 20 years old. Based on experience, it is possible, though not anticipated, that fatigue of existing fixed platforms will be of significance. Future platforms in very deep water and nonconventional structures may be more likely to suffer fatigue damage, though advanced analysis, design, and construction techniques will minimize the likelihood. Better assessments can be made when the detailed des igns are completed and structure response predicted. These discussions assume that platforms are properly maintained (e.~., painting system, cathodic protection system, and general maintenance and repair). If a platform is not so maintained, gradual deterioration and eventual Esilure will inevitably result.

OCR for page 5
10 REMOVAL SCHEDULE A survey of the experience of several offshore operators indicates that offshore production platforms are typically kept in service about 25 years; larger structures tend to be kept in service for longer periods. The reason is that, as operations in deeper water are more expensive, only the larger and more productive fields are developed, and these tend to produce longer. Presently the Minerals Management Service (MMS) requires that all structures be removed after production ceases. The removal schedule developed herein is based upon this requirement. Thus it is predicted that (1) the smaller structures, single-well protectors, and we11 caissons in very shallow water will be removed after 20 to 25 years; (2) somewhat larger structures with more wells, in shallow to intermediate water, are projected to have a useful life of 2S to 30 years; and (3) larger structures in deeper water should have a SQrYiCQ life of at least 30 years. The removal of structures to date tend to follow this pattern. Using data on the number and age of structures and assumptions of structure life as described above, a forecast of structures to be removed was developed ~ see Figure 4) . 300~ z ~ 250 o o w a: In lo o 200 150- 100 50 6000 _ , ,_~ __ ,// , - at,, _ _ _ ~ / ~ ~ _ / , ~ i' NUMBER REMOVED /YR. / ''// ~ CUMULATIVE 1970 1960 1~ 1 990 Y E A RS 5000 ~ . JO 1000 ~ _ o 2000 2010 2020 FIGURE 4 Structures removed and to be removed--Gulf of Mexico. SOURCE: Historical data from Minerals Management Service and industry sources; forecast developed by committee based on assumptions described in the text.

OCR for page 5
11 OPTIONS FOR DISPOSING OF OFFSHORE PLATFORMS Disregarding current regulations, there are two basic options concerning the ultimate disposition of offshore platforms. They can be left in place (presumably for some other use), or they can be removed. If a platform is to be removed, it can be removed completely or partially. The platform or pieces of it can either be taken to shore, toppled in place, or emplaced or disposed of elsewhere in the marine environment. These options are arrayed in Figure 5. The figure also shows where a number of proposed disposal options fit into the general logical framework. These include: (1) Existing Rules. Removal of a platform is stipulated in the OCS lease. OCS Order No. 3 requires the lease operator to clear the location of obstructions to at least S meters (16 feet) below the mud line prior to relinquishing the 1QaSQ. (2) Removal to Allow Safe Subsurface Navigation. In a 27 July 1984 letter to the American Petroleum Institute, the Secretary of Defense proposed draft removal standards with the objective of providing for safe subsurface navigation ~ see Chapter 7 and Appendix C) . The proposed Department of Defense ~ DOD) standards provide that "removal shall be to within 5 meters above the seafloor in waters less than 400 meters, to within 15 meters above the seafloor in waters less than 2~000 meters and greater than 400 meters " (3) Removal to Allow Safe Surface Navigation. Various oil industry pos i Lions have been developed calling for the removal of offshore structures to a depth sufficient for the safety of surface navigation. One such position, expressed by the Oil Industry International Exploration and Production Forum Position (May 1984)~ calls for complete removal of structures in TOSS than 40 meters (132 feet) of water when they are no longer needed. In water deeper than 40 meters ~ it calls for clearing navigational obstructions to a depth of 40 meters, and also for marking the position and SiZQ of remaining installations on nautical charts. This industry policy statement is included in Appendix C. Numbers are keyed to F i Sure S . The Oil Industry International Exploration and Production Forum Is an international technical organization of oil industry operators. It provides a focus for national oil industry organizations In Europe, the United States, and elsewhere to seek common approaches to international authorities and nongovernment safety and environmental groups, with special emphasis on operations in the North Sea. The Forum policy statement in Appendix C is supported by a technical report ~ E&P Forum, 1984 ~ .

OCR for page 5
12 ED - _ Z _ _ ~ Z '~ ~ / 3 ~ ~ ^ ~ _ . ~ C: Ir ~ Z / ~ ~ :r / 11 / cr: i oW ~ \ UJ o 111 / J / ~ ~ ~ O _ J cat _ J _ / ~ O / Z / ~ \ ~ /_ 3 / \ > ~ ~ Z / \ ~ ~ 3 o / LL ~ UJ ' / ~ ' uJ in / ~ ~ ~ TIC ~ ~ Ul O ~ _ A,, ~ / ~ ~ \ I O O / \ o ~ tI, O \ ~ ~ _ _' ~ ~ ~ ~ _ ~ C: O\ Hi ~E ~ O. Z ~ ~ 1~1 0 ~ C Z \ ~ ~ ~ ._ X \ LL _ E LU ~ U) Z \ \ ~ h `, O O Z

OCR for page 5
13 (4) Emplace Elsewhere. This implies locating an obsolete structure purposefully in the marine environment for another use, e.~., for use as a fishing reef. Options for reef development include leaving platforms in place, toppling them in place, or locating them elsewhere. (5) Remove to Shore. In this option, similar to option 1, structural elements are cut free, towed to shore, and offloaded on shore for ultimate disposal. (6)* Ocean_Dumping. Some have advocated towing obsolete platforms to deeper water for ultimate disposal under authority of federal ocean dumping regulations. The Environmental Protection Agency has granted permits for the ocean dumping of structures that have been severely damaged by storm or accident. REFERENCE E&P Forum. 1984. The Decommissioning of Offshore Installations--A Worldwide Survey of Timing, Technology and Anticipated Costs London, U.~.: The Oil Industry Exploration and Production Forum. Report No. 10.5/108. * Numbers are keyed to Figure 5 . .