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3 ENGINEERING AND COST OF PLATFORM REMOVAL REMOVAL PROCEDURES During the 38-year life of the offshore industry, about 350 struc- tures have been removed in the Gulf of Mexico. In simplest terms, the procedures for removing f ixed steel platforms are the reverse of the installation procedure. The primary procedure has been to cut the platform into sections and remove by lifting. The size of the com- ponent to be lifted is determined by the capacity of the lifting equipment. In some instances it has been possible to separate the structure into its original components of deck and jacket. In others, deck and j acket have had to be cut i nto smaller components because of the limited size of the lifting equipment employed. Occasionally, the procedure used has been to dismember the struc- ture, separating it into small components or individual members that can be picked up by a small floating crane. In a very few instances, auxiliary flotation has been used. The jacket has been lifted of f the bottom using temporary, clamped-on buoyancy tanks for it to go to deeper water for ocean dumping, or, for one structure, to enable it to be moved to another area to serve as a fishing reef. A few platforms that were to be reused have been lifted or winched back onto launch barges, then relaunched at another location. One platform, a struc- ture located near Bermuda in about 200 feet of water, was dismantled in place using explosives. Before a platform is to be removed, miscellaneous equipment, such as living quarters and generators, is returned to shore for scrap or reuse. The deck section can then be cut into sections, lifted from the platform, and placed on cargo barges for transportation. Removal of the piles and jacket then follows. The piles and the jacket are grouted together on many structures; skirt pi les are always grouted . On the majority of structures, the piles and the jacket are connected only at the top of the j acket by welding . By removing the weld, the piles and the jacket can be handled separately. A major difficulty has been cutting the piling below the mud] ine . The eas test procedure has been to wash the soil out from inside the piling and detonate a charge inside the pile to sever it. However, this is not always satisfactory. Even when shaped charges are used, this procedure tends to expand the pile where it was cut so ~4
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15 that it cannot be lifted out of the jacket leg. Thus, it becomes necessary to lift the piling and the jacket as a unit. when the weight of the jacket in combination with the piles is too heavy for the lifting equipment, other procedures to cut the piles are neces- sary. The piles can be cut from the inside using procedures that do not expand the pi les . The most frequent procedure has been to use divers inside the piling, making the cut-off with a carbon arc. Various mechanical cutters also have been used, such as a milling machine, a liquid sand blast, or a drill string with expanding casing cutter . A major portion of the removal procedure, is the actual disposal or scrapping of the components of offshore structures after they have been returned to shore . Few components are sui table for reuse . (Thi s will be discussed later in this study. ~ All items--eguipment, modules, deck units, jackets, and piles--of significant size and weight must be cut into components that are compatible with the off- shore removal procedures and the capacity of the derrick barge. These components must then be offloaded on shore. This can present a problem, since the lifting capacity of land-based cranes is usually much less than offshore derrick barges. It may be necessary for an offshore derrick barge to accompany the salvaged components to shore for offloading, or special skidding arrangements must be developed. Onshore, the small equipment items and components can be transEerrad directly to a commercial salvage yard. This is usually a no-cost item s ince the salvage value of the components almost equals the transfer cost . This is not the case, however, for large structural units such an jackets and deck sections. These must be cut into small sections ~ which can be handled by commercial salvage yards. This dismembering process is slow, expensive, and far exceeds the commercial value of the scrapped steel. Production equipment and piping that have con- tained natural gas or crude oil must be purged and flushed before they are safe to dismantle. They can then be processed by salvage yard procedures similar to those used for automobile engines. Thus, although not truly part of the marine removal, the onshore di sposal of an offshore structure is a very complex and expensive operation to be carefully considered. TECHNOLOGY ADVANCES AND NEEDS To date, removal of offshore structures has not been a major indus- try. The removal of structures has been occasional, which has not promoted the development of more economical procedures. When the removal of offshore structures grows into a significant market, the technical prof ic iency in pl atform removal will improve. The industry has shown continuing developments in two areas that will improve removal capabili t i es . One i s the development of larger, more weather-resistant crane barges. The other is the improved technology
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16 in working under water with remotely operated vehicles and with improved diving systems that allow deeper dives for a longer period of time. Certain other technical developments could assist platform removal capabilities. For example, pile cutters, which can sever the pile below the mudline without using divers or without expanding the pile diameter, can be improved. Also, the ability to cut jacket members, legs and braces within the structure using a remotely operated vehicle and cutter not requiring divers would be very advantageous. Also possibly of benefit would be the development of temporary buoyancy systems with a positive means of attaching to the~jacket legs to assist in lifting the larger sections by flotation. For the typical Gulf of Mexico structure, the development of removal procedures is not ~ normal part of the original design effort. For most of the structures designed to date, the removal procedure has been cons idered primarily a reverse of the installation procedure. If the structure had been designed for installation by lifting, then the same or larger equipment could remove it. If the structure had been designed to float before installation on bottom, the jacket could likely be refloated by capping the legs and blowing out the water. No detailed analysis of platform removal procedures is normally performed other than to ensure in the design of the structure that adequate buoyancy is available. However, for deeper water depth structures that are likely to be cut into several sections' a more detailed analysis of a removal procedure is sometimes performed to ensure that removal is possible and to obtain a rough estimate of the removal cost. Actually a detailed removal procedure cannot be developed until the condition and ultimate disposition of the structure are known. For example, a different procedure would be used if the jacket is to be cut off below the waterline and the upper sections floated with auxiliary buoyancy to deep water for ocean dumping. Unless the final disposition of the structure is known when the structure is designed, the development of a detailed removal procedure during the design and approval phase of a project is probably a waste of effort. Moreover, since any removal procedure is necessarily based on equipment available at the time, and since it is impossible to establish, say, in 1985 what capacity crane barges will be available in 2010, the effort spent would probably be wasted. Consider, for example, a removal prognos i s wri then in 1960, when 2SO-ton crane barges were the largest available That prognosis would be of little value today, 25 years later, when barges wi th 2, OOO-ton plus capac i ty are avai fable . There are some enhancements, however, that could be included in the orig inal des ign that might make removal eas ier no matter what pro- cedure i s used . For example, the des igner could ensure that adequate buoyancy is available for removal and allow for a certain loss of buoyancy because of leakage through the years, as well as take into account buoyancy that would be lost from normal installation flooding. Lugs could be welded on legs of structures to allow a
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17 positive connection for bolt-on clamps for auxiliary or temporary buoyancy. In all probability the very large structures will have to be cut into sections for removal. It is not likely that a definite weak link such as a bolted or other easy-to-remove joint on the jacket legs would ever be acceptable since the weak link could weaken the overall structure. However, it might be possible to include planned separation points that would allow cutting fewer structural members. In any case, special care must be exercised to ensure that in-place integrity is not compromised. The removal of offshore structures is not a standard construction procedure. Because of modest weight and size, removal of the majority of structures will be relatively straightforward. As the structures become heavier and more complex, removal requires more detailed and sophisticated engineering. The removal of the largest structures will require state-of-the-art engineering, planning, and execution. Only the most experienced marine contractors are likely to have the engineering, technical, and logistical resources necessary to execute the largest jobs safely. REUSE OF P=TFO=S The reuse of the platform is an ideal concept, but not often prac- tical (Lawlor, 1975). An important aspect in considering reuse is that the offshore industry has been in operation for less than 40 years and is relet ively young compared to most other types of con- struction. The design and construction of offshore platforms has been a rapidly advanc ing technology. Des igns have improved as a result of increased knowledge of the marine environment and consequent enhanced understanding of des ign loads . Platforms have become stronger and heavier, and able to withstand more substantial storm forces. Many older platforms that met the design criteria in effect at the time they were built do not measure up to current design criteria. The historical service experience does not suggest actual inadequacy of existing platforms. However, any significant departure from current design criteria would likely make operators and regulators reluctant to accept reuse of many older platforms. A major concern in removing offshore structures is the disposal of equipment and the structural sections of the platform that have been removed. Individual i tems of deck equipment, such as cranes, genera- tors, living quarters, buildings, and heliports can be refurbished and reused with little difficulty. The same is true for individual pro- duction skids . Production piping, built into deck sections, and purpose~built deck modules have less chance of being reused. The drilling rig itself is not a part of the platform--it is moved to the next platform once all wells are completed. The structural portions of the jacket and deck are not as reusable as the deck equipment. On relatively rare occasions, an almost new platform needs to be relocated. In this instance, reuse presents
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~8 1 i ttle di f f iculty, provided the new locat ion has approve imately the same water depth . Naturally, new pi les wi 11 be necessary s ince most of the original piles are left in the ground. Also, different soil conditions will require different foundation design. In other i ns lances, i t i s neces sary to return the j acket to shore, modi fy the lower portion of the jacket to accommodate different water depth, and return it offshore for installation. Since most platforms are not suitable for reuse as platforms, they must be scrapped on shore or at sea. If a structure i s to be emplaced elsewhere in the marine environment, for use as an artificial reef for example, then the smaller and lighter individual equipment items on deck probably have to be removed. In addition, all tanks, piping, and other vessels that have contained oil or gas have to be removed or completely decontaminated. If the structure is returned to shore, shorebased equipment can often be used to remove the material from the barge and cut i t alp for scrap. If the structure i s relocated to a marine site, the offloading has to be performed wi th higher cost marine equipment. The length of the tow to the ultimate destination is another consideration. While it is true that structures with sufficient reserve buoyancy or with buoyancy tanks attached can be towed to shore, to an ocean dump-site, or to an artificial reef site, there is a definite risk of loss of buoyancy during the tow, especially if the structure is old. Moreover, buoyancy tanks as well as the entire towing operation are expensive (see discussion of cost in next section). COST OF PLATFORM REIlOVAL With several thousand structures in existence, it is impractical for a study of this type to analyze the cost of removing each offshore structure. Rather, the structures were divided into five categories on the basis of size and type. A removal estimate was performed for each category and a total estimate was developed accordingly . These estimates are based on current techniques and 1985 dollars not adjusted for inflation. Allowances are made for techniques expected to be developed in both design and removal technology, however, it is not expected that these will substantially reduce costs. Category T includes smaller structures, single-well caissons, well protectors, and other items that can be removed using equipment with lifting capacity not over 100 tons (jacket weighing less than 100 tons). Generally, these structures are in water depths of 20 feet or less. However, some of the very old structures in deeper water (up to 50 feet) also fall into this category. Category II corers typical eight-pile structures in water depths up to 100 feet, with jackets weighing 500 to 700 tons . Until better techniques become standard, these structures will also be removed by lifting .
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100,000: so,ooo 10000 In ~ is a 1 5,000 - 3 00 a Or 500 . ^~ CaT. ,? _ ~ _ _^ · T~' CaT. , L/ m / CAT ~ ' / CAT. CaT rot cat. :~ 0 200 400 600 800 I~0 WATER DEPTH—FEET FIGURE 6 Comparison of jacket weight versus water depth. Category III includes structures with jackets weighing from 1,200 to 1,500 tons. This encompasses typical present-day structures in water depths of loo to 200 feet. Category IV covers structures located generally in 200 to 400 feet of water. The cost estimates are based on cutting the jacket into sections, lifting the sections onto cargo barges, and returning them to shore. Category V includes all structures installed beyond the 400-foot water depth. Generalizations about the most favored removal procedure are not practical for structures of this size; each requires custom development of removal procedures. Figure 6 shows a comparison of jacket weight versus water depth for typical Gulf of Mexico drilling-production structures. Figure 7 employs the platform population data and life expectancy estimates presented in the previous chapter to estimate the number of structures in each category to be removed each year. The number of structures to be removed will gradually increase from about 30 a year, at present, to well over 200 in the future. However, as is shown, the bulk of these will be the small structures of Category I and Category II. These are relatively inexpensive structures to remove. The real
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20 250 .,, 200 L) o m 150 _ 100 z _C; CAT. V OVA R 400 ' ,::::::::: ::: :: :::: ~AT. IV ~00' TO 400' ^~ /~ SAT . 17..'./ ~ 1~ ' ' - : ................. elk ~ I , :, .... . .:.::# '::::::::::::::::.:::::::::::::::::::::::::::::::: :::::::::::::::.:::::.:::.:.::::::: -AT. I I ~0 ' TO 100 ' I:: ~ I.::::::.::::: ::::::::::::::::::::::: . :: ::::::: _ ~ _ I:-, 2] ~ · - 2 2. : ..... . . I_: :,:: :::: ~ _ I. ::::::: ::i, `::::::::::::::::::::::::::::::::::::::::::::::::::::::::.::::: :::::::::::::::::: :::::::: ::::::: .~ :: ' 2 '.2 " 22,.~ ,.~ I''''''''' ::::::: '"2"""""""2'' " """2'' ~ I'::::.: :""? I- '- ...' : 2, 2 ' .. 2. ' 2 2.'.22 ' .2'2""" ' ' ' ' ' ' ' ' "I it" "" " ' "' ' ::' "' " ~.2 . 2 . 2.2. . 2 . ." : ~ :- -:~: - - :.," · o I i I I I ! ! 1985 1990 Is95 2000 2005 2010 2015 2020 YE A R S FIGURE 7 Estimated number of structures to be removed by category--Gulf of Mexico. Ire fool TO 200' CAT. r o, TO 20' problems will not begin until Category IV structures begin to be removed somewhere in the 1995 to 2000 time-frame. Removal of the deep-water Category V structures built in the past few years, an well as others being contemplated, is not anticipated until around 2005. With few exceptions, Category I and II structures will be com- pletely removed and returned to shore. These are not difficult to remove, and therefore can be removed cheaply when no longer useable. Even if operators were allowed to leave structures in place, liability considerations and maintenance costs would dictate the removal of the bulk of these structures. Since the water depth of these structures is also relatively shallow, they are not likely to be treated as structures to be cut off at some point below the waterline with the bottom section left in place. For purposes of preparing an overall estimate, typical removal procedures ware developed for a structure of this category. The normal removal cost of a Category T structure is estimated to be in the range of $50,000 to $400,000. Larger equipment and more time on location is required for Category TI. It is esti- mated that the average removal cos t of these s true Lures wi ll range from $600,000 to $1.3 million. For this study, it was assumed that structures in Categories lIr, IV, and V would also be removed completely and returned to shore. Considering the additional SiZQ and complexity of these structures, it is estimated that the removal of Category III structures by present techniques would cost from $1 million to $2~5 million. For Category IV structures, an average cost would be between $5 million and $15 million. Similar removal procedures would be used, except when the weight of the jacket requires cutting into sections for convenient lifting and transporting to shore on cargo barges. Onshore dismantl-
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21 ing and disposal costs represent about 20 percent of the total removal costs for Category I-IV structures. For Category V, the very deep-water structures, the cost of removal would begin at $15 million. An unpublished, detailed cost study, prepared by an owner of one of these structures several years ago, estimated the removal cost at over $70 million. At the present time, this estimate would probably range from $90 million to $100 million. Very special and specific procedures would be required for each struc- ture in this category in order to make ~ satisfactory estimate. Onshore disposal costs for Category V structures would range from S3 million to $6 million. The co~ittee's estimates of removal cost are comparable to estimates prepared by the E&P Forum (E&P Forum, 1984). Based on the number of structures shown in Figure 7, and using the cost of the categories described, the total cost of removing the platforms in the Gulf of Mexico has been projected, as shown in Table 2. Cumulative costs are shown in Figure 8. Assuming 1985 dollars, the committee estimates that by the year 2005 about $2 billion will be required to remove the structures; this cost will rise to about $7.5 billion by 2020. These estimates do not take into account structures in Alaska and California, some of which will be expensive to remove. The estimates do not address advanced platform concepts intended for deeper water. Only one of these structures, a guyed tower, is in place on the U.S. outer continental shelf. When Alaska and California platforms are included (see Tables 3, 4A, and 4B), these costs increase to an estimated $2.5 billion by 2005 and $8.5 billion by 2020. soo In I_, 300 J C) `r 200 IS too v iSS I 990 1995 20= Y E A R S ~ 1 7 ~ , COST/YE ~ =~' / / C UM ULAT IVE , ~ ~ . 200s 2 oto 2 1 15 FIGURE 8 Total cost of removing Gulf of Mexico structures. , 14000 _ 2 20 20 In 8000 o J 6000 ~ o W > 4000 At 2000 *The estimates of costs shown in the tables and figures are based on the committee's experience and judgment.
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22 TABLE 2 Number of Structures to be Removed and Estimated Removal Costs in the Gulf of Mexico Time of Category Cos t Removal I IT III TV V Total (millions ) 1985 27 5 32 9 1986 3S 5 40 11 1987 26 21 41 24 1988 34 22 56 26 1989 48 18 66 25 1990 68 24 92 34 1991 92 34 1 127 49 1992 85 41 5 131 63 1993 68 36 2 106 49 994 86 50 12 148 83 1995 74 60 9 143 84 1996 IS 57 4 136 73 1997 7 7 62 5 144 80 1998 61 40 18 5 124 129 1999 54 26 19 11 110 1 77 2000 59 30 26 8 123 163 2001 56 46 25 3 130 125 2002 59 36 29 18 142 2 73 2003 59 29 23 9 120 167 2004 37 15 12 9 73 131 2005 63 24 18 23 128 295 2006 60 30 18 28 136 3 50 2001 44 46 20 17 1 128 294 2008 65 53 36 29 3 186 532 2009 103 4 7 28 20 1 1 99 3 5 1 2010 89 56 35 19 2 201 398 2011 7 9 68 3 7 1 2 2 1 98 340 2012 101 63 28 1 2 209 35 5 2013 132 67 14 8 2 223 271 2014 105 S3 16 5 2 181 226 201 S 105 53 14 7 1 180 203 2016 105 63 19 10 2 199 290 2017 105 65 25 13 4 212 412 2018 105 65 30 14 4 218 431 2019 105 65 30 20 4 224 491 2020 105 65 30 20 4 224 491 Totals 2, 746 1, 786 588 328 34 5, 482 $7, 505 Previ ous ly removed 95 246 5 346
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23 TABLE 3 Number of Structures to be Removed Off Alaska and Estimated Removal Costs . Number Costa Costa Time of Removal Removed (millions) (millions) 1985-1990 1990-1995 2 50 36 1995-2000 6 150 108 2000-2005 6 150 108 2005-2010 2010-2015 2015-2020 3 75 S4 Total 17 $425 $306 NOTE: All structures located in about 100 feet of water. aThis column assumes mobilization/demobilization of marine equipment from California, and both deck and jacket removed, taken to shore, and cut apart. Onshore dismantling and disposal costs will be about $6 million per structure (included in above figures). bThis column assumes mobilization/demobilization of marine equipment from California, the deck taken to shore, and cut apart, but the jacket removed and sunk in deep water. TABLE 4A Number of Structures to be Removed Off California and Estimated Removal Costs by Water Depth Water Depth Removal Costs (Millions) <100' Sa 5b 100' - 200' 7a 7b 200' - 400' 21a 14b >400' 25a lab aThese costs assume complete removal and transportation to shore to be cut apart. Onshore dismantling and disposal costs represent about 20 percent of total removal costs. bThese costs assume complete removal and transportation of j acket to a deep-water s i te for di sposal wi th the deck taken to shore.
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24 TABLE 4B Number of Structures to be Removed and Estimated Costs by T ime of Removal Time of Water Depth Removal Cost 400 ' (mi 11 i ons ) 198S-1990 1 5 5 1990-1995 4 1 27 27 1995-2000 7 1 70 63 2000-2005 2005-2010 2 1 1 60 46 2010-201 S 1 4 89 61 2015-2020 10 10 460 320 Total 7 10 16 11 $71la $522b Previ ous ly removed 1 aThese costs assume complete removal and transportation to shore to be cut apart . Onshore di smantl ing and di sposal costs represent about 20 percent of total removal costs. bThese costs assume complete removal and transportation of jacket to a deep-water s i te for di sposal wi th the deck taken to shore . COMPARISON OF COST OF RETURN-TO-SHORE MID OCEAN DISPOSAL OPTIONS The discussion above of cost of removal has been based on existing requirements for complete removal with disposal ashore. As has been explained, to remove a Category III-V j acket completely and return i t to shore, it is usually necessary to cut it into sections to be lifted onto a barge for transportation. Then it must be offloaded and further cut into sect ions for salvage . For the larger platforms (Categories III-V), the cost of removal and disposition could be sub- stantially reduced if other removal options were employed. For example, a large jacket could be lifted off the ocean bottom with auxiliary buoyancy or with a derrick barge after the piling has been cut. The jacket could then be towed to deep water for dumping (equipment and piping that had contained petroleum would be removed to shore). A recent unpublished study of the removal of a Gulf of Mexico eight-pile structure in over 3~)0 feet of water estimated that marine operations would cost $3.9 million and an additional $1.3 million would be required for offloading, dismantling, and disposal, for a total cost of $5.2 million. It is estimated that the total cost
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25 TABLE 5 Comparative Costs for Several Removal Options Category III (millions) Category IV (millions) Category (millions) Option Aa 1.0 - 2.5 5 - 15 15 - 90 Option BE 0.8 - I.5 3 - S 6 - 12 Option cC 3 - 4 4 - 8 aJacket severed below mudline, everything taken to shore. bracket severed below mudline, lifted off bottom, and transported to nearby (~25-30 miles) deep-water s i te and dumped . Deck and equi pment returned to shore . CJacket in ~500-600 feet of water, severed at mudline, and toppled in place. In waters deeper than 500-600 feet, only the top 200 feet would be removed and set on bottom adjacent to portion of structure remaining. Deck and equipment taken to shore. estimate would be reduced to $3.2 million using a derrick barge to transport the partially buoyant j acket to deep water . This procedure may not be cost-effective for shallow water structures (Category I and II), but total savings on the disposal of deep-water structures (Category III-V) would be substantial. Ocean dumping of Category III-V structures would reduce the total estimated cost to remove all offshore structures through the year 2020 by about S2.5 billion, or one-third. (This estimate is necessarily very uncertain since each of the largest platforms will have to be treated as a special case. ~ Estimated differences in costs for several removal options are shown in Table 5. Tax implications to the owner or the government are not included in Table S.~ ENGINEERING AND COST OF REMOVAL OF OTHER PLATFORM TYPES As the industry moves into deeper water' other types of offshore platforms may be the technology of choice as a result of performance The extent of the government's financial interest in offshore development can possibly influence the government's choice among alternative platform dispositions. Where petroleum taxes are very high or where the government owns a major share in the offshore concessions, a least cost disposition could have a significant impact on the federal treasury.
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26 _ ~~~ ....~. _ .--.. . ,.: ':: /,':, , Am/ W ! ~ Y- hi.. .. . - . . : , - _ . .. . I.—. - -- PA I R LEADS - . . . —. . . a ..... . If . . A.... ~ . - ,. .. .. ~ \ A = ., ._ .. \ \ ~ ~~ \ - GUYLINES FIGURE 9 Guyed tower (water depth 1,100 feet). evaluations and cost estimates. These may include compliant structures such as guyed towers or tension-leg platforms (see Figures 9 and 10), or subsea well completion structures coupled with floating storage systems. One guyed tower is in place on the OCS of the United States. Large concrete or steel gravity-base structures are also likely to be used. A concrete and steel gravity-base structure is already in use in the Alaskan Beaufort Sea. One or more concrete gravity-base structures, similar to those in the North Sea, could possibly be chosen for use in the southern Bering Sea. Removal of a guyed tower will be similar to the removal of a fixed platform, possibly easier. The structure has fewer piles and substantially more buoyancy, which may make the structure easier to float on its own. The removal of other types of structures, however, will be totally different from a fixed platform. For example, the removal of the upper portion of a tension leg platform will be substantially easier, since it will only be necessary to slacken and remove the others and then tow the upper section away.
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27 ~ < ! ~ ~ ~ . . ~ . ~ ~ ~ ~ -a TENSION ~E~BE~ .. .. .. . . .. . . _ ~ '~ p~3 a_ FIGURE 10 Tension leg platform (aster depth 485 feet).
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28 Removal of the lower portion or foundation uni t of this structure on the ocean floor will be an entirely different and a more difficult operation, Thi ~ on-bottom template foundation may protrude 5-10 meters above the seafloor ( see Figure 10) . ~ Complete removal will involve procedures and techniques that are essentially a reverse of the initial installation operations, with several notable additional complications. Severing foundation piles below the mudline is similar to well abandonment operations and can readily be accomplished by floating deep-water drilling vessels. Rigging the structure for lifting to the surface is likely to include reestablishing structure buoyancy, ei ther by deballas t i ng or attach i ng buoyancy modules . Beyond saturat ion diver depths, presently about 1 ,150 feet, these operations will entail extensive use of remotely operated vehicles and purpo s e -bu i 1 t equ i pmen t . L i f t i ng the s t rue t ure th rough the water column, although straightforward in principle, will generally require the use of purpose-built hea~ry-lift systems operating from large surface support vessels outfitted to moor in deep water or dynamically positioned. At the surface, the structure will either be loaded on barges, most likely after disassembly into pieces of manageable size and weight, or made seaworthy for surface transport by the addition of buoyancy and towing appliances. Although each of these tasks is technically feasible, they promise to be expensive and burdened with engineering and logistical difficulties, raising serious questions about the cost-effectiveness of this approach. Alternatives to complete removal, beyond the obvious approach of leaving the structure intact on the seafloor, would include destruction in place by explosives, leaving structural debris scattered over a small area. The dumping of rock or other fill material to cover low profile bottom structures could also be considered. Since on-bottom template structures will probably be installed only in deep water (greater than 1,000 feet) they will not interfere with navigat ion . Leaving on-bottom template structures in place would simplify salvage substantially. If total removal of these on-bottom units is required, it will be very expensive, even with the use of aux i 1 i ary buoyancy and dumpi ng at sea . There are about 200 concrete structures, some gravi ty-base and some pile supported, located in state waters in the Gulf of Mexico. These shallow water structures will be comparatively easy to remove. Their removal, in principle, consists only of removing topside facilities and any ballast,breaking loose from the seafloor, and then towing away. Removal of a large, North Sea-type, concrete, gravity-base platform (Figure 11) would be much more difficult. Refloating the whole structure may be impractical because of the sheer weight (some are more than 500,000 tons), or much of the buoyancy available during installation is lost, or, on some, tons of grout injected into the Similar on-bottom templates may be used with subset production systems and clusters of subsea wells.
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29 FIGURE 11 Concrete gravi ty-base platform--North Sea (water depth 520 feet) .
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30 spaces between the base of the structure and the supporting soil will adhere to the underside of the structure. Dismantling a concrete structure in place also has major problems. The decks can be removed without too much difficulty and the columns can be severed above the base, probably by explosives. It is the base that presents the major removal problem, for reasons noted above. One option would be to use explosives to reduce the base to rubble. Off Norway where there are some IS huge concrete gravity-base platforms, the Norwegian Petroleum Directorate requires that for each such platform, a complete Manual for Removal be developed, fully engineered, and approved prior to issuance of a permit for construc- tion. In all recent platforms, complete piping systems, disconnect devices, etc., have been installed. Careful analyses have been made of soil adhesion to skirts and dowels, and detailed deballasting plans, stability analyses, etc., have been prepared. In these instances, the same degree of engineering has been incorporated in planning for removal as in planning for construction and installation. Removal of conventional deep-water structures is difficult, complex, and expensive. For other types of platforms, removal may require even more thorough engineering and execution. The operas i ons must be planned with the same degree of care as the original installation ~ to avoid serious consequences . REFERENCES Lawlor, Frank James, III. 1975. "A Preliminary Technology Assessment of Alternative Uses for Offshore Petroleum Platforms." Unpublished master's thesis. Texas A&N University, College Station, Texas. E&P Forum. 1984. The Decommissioning of Offshore Installations--A World-Wide Survey of Timing, Technology and Anticipated Costs. London, U.K.: The Oil Industry International Exploration and Production Forum. Report No. 10.S/108. Shaped or block charges are used routinely to sever piles under- water and for other demolition tasks. Demolition of a large concrete gravi ty-base structure wi th explosives is technically fees ible, but has never been tried. The use of explos ives underwater may ki 11 f i sh in the area. The s ize of the f i sh ki 11 depends on the amount of employ ives used and the f i sh populat i on in the area .
Representative terms from entire chapter: