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Patent Application Publication Aug. 7,2003 Sheet 1 of 6 US 2003/0147703Al

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Patent Application Publication Aug. 7, 2003 Sheet 3 of 6 US 2003/0147703Al


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Patent Application Publication Aug. 7, 2003 Sheet 5 of 6 US 2003/0147703 Al

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TENSION LEG PLATFORM HAVING MODIFIEDWAVE RESPONSE CHARACTERISTICSBACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention[0002] The present invention relates to exploration of,drilling of, and production from offshore subterranean res-ervoirs. In another aspect, the present invention relates toapparatus and methods for the exploration of, drilling of, andproduction from offshore subterranean reservoirs.[0003] 2. Description of the Related Art[0004] The exploration for, drilling for, and production ofhydrocarbons from offshore subterranean reservoirs presentquite a number of challenges. A number of different tech-niques have developed over the years to explore for, drill forand produce hydrocarbons from offshore reservoirs.[0005] Economics and technology have mostly deter-mined the water depths in which drilling and productioncould reasonably be conducted. Offshore production at firstwas conducted in shallow waters, and as technology devel-oped and economics became more favorable, was conductedin deeper waters.[0006] Initially, offshore well operations were conductedfrom fixed platforms in relatively shallow water. However,as offshore operations chased potential hydrocarbon reser-voirs from shallow to deeper water the cost of fixed plat-forms became very expensive. In attempts to reduce expenseand to conduct well operations in deep water, differentsystems have been proposed, such as providing a completelysubmerged production station at the sea floor, providing afloating platform production system in which semisubmers-ible platforms are utilized, and providing a tension legplatform production system.[0007] In general, offshore hydrocarbon production sys-tems generally include a plurality of wells extending toundersea deposits of oil, with trees located on or above thesea floor, wherein each tree includes a plurality of valves andpipe couplings. Risers extend up from the trees to apparatusfloating at/near the sea surface that has oil handling equip-ment.[0008] These floating production platforms normally fallin one of two categories, either Direct Vertical Access (DVA)or Non-DVA.[0009] A DVA systems is most suitable where the wellscan be drilled, completed, and re-entered from a derricklocated on the platform. They offer the convenience of easyintervention on wells when needed, and reduced drillingcosts. Almost all the DVA systems installed in the world indeepwater (waterdepth greater than 1,000 m) are eitherTension Leg Platforms (TLP) or spars.[0010] A Non-DVA system, such as a floating platformproduction system, is suitable for reservoirs that are spreadover a large area, in which the wellheads are drilled,completed, and re-entered from a separate drilling semi-submersible or drillship. Drawbacks include the high drill-ing cost because of the requirement for a separate drillingvessel, and the high mobilization cost for intervention onwells.

Aug. 7, 20031

[0011] Tension leg production system involves the use oftension leg platforms ("TLP"), such as described in U.S. Pat.Nos. 3,780,685, 3,648,638, and 3,154,039. Tension legplatforms are characterized by the absence of heave, roll orpitch in response to wave motion and thus provides oppor-tunity for improved production efficiency and simplificationof the riser design and tensioning thereof. In general, atension leg platform consists of a platform deck supportedby a buoyancy structure, a bottom anchoring structure andtension elements extending between the buoyancy structureand the bottom anchoring structure. The buoyancy of such aplatform exceeds its weight by a margin termed the excessbuoyancy, which maintains the tension elements in tension.[0012] A spar comprises a spar buoy or spar in the form ofa body having a height that is a plurality of times its averagewidth, and usually at least 5 times as tall as wide. Dependingupon the width of the spar, there can be a anywhere frommoderate to extensive drift in reaction to winds, currents,and waves, which may result in anywhere from moderate toexcessive bending of the risers and fluid-carrying pipestherein. To keep the spar upright, its upper portion is madehighly buoyant while its lower portion contains considerableballast to weight it and thereby lower its center of gravity.[0013] Spars are deep draft floaters (normally over 200 mdraft for a typical Gulf of Mexico application). The hull iscomposed of a single vertical column 40 to 60 m in diameter.The truss spar is an improvement over the conventional spar,where the lower section of the hull is truncated about 60 mbelow the surface, and replaced by a truss structure withhorizontal plates to reduce the platform motion. Both typesof spars have a large column piercing the water surface, andtherefore exposed to severe wave and current loads, andrisers are supported by buoyancy cans to accommodaterelative displacement between the hull and the riser top.[0014] Currently, in ultradeep water (water depths greaterthan 1,500 meters), the only viable system is the spar.However, the main drawbacks of spars include: the largedisplacement, and therefore cost of the hull; the large lateralloads that require a substantial mooring system; the buoy-ancy cans because of their cost, installation and operationalissues, and health, safety and environment concerns; and thetendency of the hull to experience vortex-induced vibrations(VIV) because of its circular shape.[0015] The complexity of production from ultradeepwater(water depths greater than 1,500 meters) is best understoodby examining the wave environment. First, a substantialamount of the wave energy is concentrated in the first 100meters of water depth. Second, the frequency of the waveenergy is concentrated between 6 and 15 seconds.[0016] With a TLP, the problem is one of anchoring theTLP to provide sufficient stiffness so that the TLP will havea heave resonance generally less than the 6 to 15 secondrange, preferably less than 4 seconds. For example, at 1200meters of water depth, a TLP will typically have 8 tendons.However, at 2400 meters (i.e., at double the depth), thenumber of tendons for a similar TLP ("similar" TLP but not"identical", because at increased depth some of the designfeatures of the TLP are different, and the TLP system weightbecome more dominated by the weight of the tendons) mayincrease by order of magnitude (i.e., 80 tendons, by roughmodel calculations). Or in other words, increasing the depthby 2 times, increases the tendons by about 10 times. Gen-


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erally, cost and space limitations make application of morethan about 20 tendons to a TLP impractical.[0017] In addition to the use of spars for ultradeep water,several other alternative DVAconcepts have been proposed.[0018] One of these is the buoyant leg structure (Interna-tional Design Engineering and Analysis Services, Inc, SanFrancisco, Calif.) which is a shorter spar with a tether.However, this concept cannot easily be extended to largewaterdepth because the tendon size increases significantlywith waterdepth, and is cost-prohibitive beyond 5,000 ft.[0019] Another of these is the TPG3300 (Technip-Cof-lexip, Paris, France), a deep draft semi-submersible. How-ever, because of stability requirements, this concept requiresa very large displacement and therefore its cost is veryhigh,comparable to a spar, and its motion characteristics are suchthat very large riser tensionners are required to preserveintegrity of the risers.[0020] Even another of these is the minDOC (minDOC,LLC, New Orleans, La.), a spar-like structure with smallermultiple columns. This concept has a better Vortex-InducedVibrations (VIV) performance than a spar, however, due tothe complexity of the hull, its cost is very high.[0021] Still another of these is the ABB spar (ABB Lum-mus Global, Inc, Houston, Tex.), which is a short spar witha very large diameter. This concept also suffers from VIV,and due to the large buoyancy near the water line, itexperiences very large current and drift forces, and therequirements on the mooring systems are excessive. It is nota better alternative than a spar.[0022] Thus, in spite of the advancements in the art, therestill exists a need in the art for apparatus and methods for theexploration of, drilling of, and production from offshoresubterranean reservoirs.[0023] There is another need in the art for apparatus andmethods for the exploration of, drilling of, and productionfrom offshore subterranean reservoirs, which do not sufferfrom the disadvantages of the prior art apparatus and meth-ods.[0024] There is even another need in the art for apparatusand methods for the exploration of, drilling of, and produc-tion from offshore subterranean reservoirs in greater than1500 meters of water depth.[0025] These and other needs in the art will becomeapparent to those of skill in the art upon review of thisspecification, including its drawings and claims.

SUMMARY OF THE INVENTION[0026] It is an object of the present invention to providefor apparatus and methods for the exploration of, drilling of,and production from offshore subterranean reservoirs.[0027] It is another object of the present invention toprovide for the exploration of, drilling of, and productionfrom offshore subterranean reservoirs, which do not sufferfrom the disadvantages of the prior art apparatus and meth-ods.[0028] It is even another object of the present invention toprovide for the exploration of, drilling of, and productionfrom offshore subterranean reservoirs in greater than 1500meters of water depth.

Aug. 7, 20032

[0029] These and other objects of the present inventionwill become apparent to those of skill in the art upon reviewof this specification, including its drawings and claims.[0030] According to one embodiment of the present inven-tion, there is provided an offshore platform. The platformincludes a deck for supporting hydrocarbon exploration,drilling or production equipment, a buoyant member, anopen support structure positioned between the deck andbuoyant member and connected to both, and a plurality oftendons connected to the buoyant member suitable foranchoring the platform. When the platform is positionedoffshore, the deck is supported above the waterline, theupper end of the open structure is positioned above the waterline, with the lower end positioned at least 100 feet belowthe waterline; and the heave resonance of the platform is atleast 6 seconds.[0031] According to another embodiment of the presentinvention, there is provided a method of exploring anoffshore target zone for hydrocarbons. The method includespositioning a platform, as described above, offshore near thetarget zone. The method further includes conducting explo-ration activities from the platform.[0032] According to even another embodiment of thepresent invention, there is provided a method of drilling foror production of hydrocarbonds from an offshore targetzone. The method includes positioning a platform, asdescribed above, offshore near the target zone. The methodthen includes conducting drilling or production activitiesfrom the platform.[0033] These and other embodiments of the present inven-tion will become apparent to those of skill in the art uponreview of this specification, including its drawings andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS[0034] FIG. 1 is an illustration of a typical traditional TLP200, showing deck 10, buoyancy section 203, and tendons204.[0035] FIG. 2 is an illustration of one embodiment of SoftTLP 300 of the present invention, showing deck 10, supportsection 301, buoyancy member 303, and tendons 305.[0036] FIG. 3 is an illustration of a typical traditional trussstep spar 100, showing deck 10, spar 101 having a buoyantupper section 102 and a ballast lower section 103, trusssection 105, and ballast section 109.[0037] FIG. 4 is a detailed illustration of buoyant section303 and columns 304.[0038] FIGS. 5-11 show a typical installation sequence forthe Soft TLP of the present invention.[0039] FIG. 12 is a table showing the payload weights ofthe various major components for the proposed Soft TLPand the Brutus TLP, as utilized in the computer model for thecomparison of Example 1.[0040] FIGS. 13 and 14 are a table and graph, respec-tively, showing the results of the computer model compari-son of Example 1 for the proposed Soft TLP of the presentinvention, and the commercial Brutus TLP.


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DETAILED DESCRIPTION OF IREINVENTION

[0041] The apparatus of present invention is a modifiedversion of the standard TLP, and is sometimes referred toherein as "Soft TLP."[0042] The apparatus of the present invention will first bediscussed in contrast to the typical TLP and to the typicaltruss step spar. Referring now to FIGS. 1, 2, and 3, there areshown, respectively, a typical TLP 200 (FIG. 1), the SoftTLP 300 of the present invention (FIG. 2), and the tradi-tional truss step spar 100 (FIG. 3).[0043] In FIGS. 1, 2 and 3, a conventional deck 10 isshown supported by, respectively, conventional TLP 200,Soft TLP 300 of the present invention, and spar 100.[0044] Conventional prior art TLP further includes a buoy-ancy structure 203, a bottom anchoring structure (notshown) on the sea floor, and a plurality of tension elements204 extending between the buoyancy structure and thebottom anchoring structure. Notice that with the conven-tional TLP, the buoyancy structure 203 provides a largecross-section at the water level, subjecting TLP 200 to strongwave forces.[0045] Conventional prior art spar 100 further includesspar buoy 101 having a highly buoyant upper section 102,and a lower portion 103 containing considerable ballast tolower the center of gravity of spar buoy 101. As spar 100 isa "truss spar," lower section of spar 101 is truncated, andreplaced by truss structure 105 with horizontal plates 107 toreduce the platform motion. Further ballast section 109 ispositioned at the far end of truss structure 105. Mooringlines 108 serve to anchor spar 100. Like conventional TLP200, spar 100 provides a large surface area at the water level,thus subjecting spar 100 to strong wave forces.[0046] Focusing now on the Soft TLP 300 of the presentinvention attention is directed to FIG. 1 and additionally toFIG. 4 a detailed illustration of buoyant section 303 andcolumns 304.[0047] In the practice of the present invention, deck 10may be any deck as is known in the art. Deck 10 may includeseparate or integrated modules, compartments or sectionsfor drilling, production, quarters, and/or utilities.[0048] This deck 10 rests upon and is supported bysupport structure 301, which includes a suitable number ofcolumns 304, and reinforcing members 302. In the practiceof the present invention, Soft TLP 300 will generallyincludes at least 1 column 304, preferably at least 2, morepreferably at least 3, and even more preferably at least 4columns 304. Any suitable number of reinforcing membersare utilized to provide the desired stability. Columns 304 andreinforcing members 302 are designed generally to reduce,more preferably to minimize the effect of the waves uponSoft TLP 300.[0049] Buoyant section 303 provides the necessary buoy-ancy to support Soft TLP 300. This buoyant section 303 ispositioned at the end of columns 304, with the length ofcolumns 304 selected to generally position buoyant sectionat least 100 feet below the mean waterline, more preferablyat least 150 feet below the mean waterline, and even morepreferably at least 200 feet below the mean waterline, an stilleven more preferably at least 250 feet below the meanwaterline.

Aug. 7, 20033

[0050] Optional extension members 307 extend from eachcorner of buoyant member 303, and provide increased pitchstiffness.[0051] As compared to a conventional TLP or spar system,Soft TLP 300 exposes a much smaller surface area to waveforces in the first 100 feet of water depth. Columns 304 andreinforcing members 302 are generally designed to mini-mize their surface area. Buoyant section 303 is generallypositioned below the first 100 feet of water depth.[0052] In the practice of the present invention, any suit-able tendon may be utilized as tendons 305, and suchtendons 305 are secured to buoyant section 303 and theocean bottom as is known in the art.[0053] In the practice of the present invention, the SoftTLP is anchored with sufficient stiffness so that the Soft TLPwill have a heave resonance generally near the lower end ofthe 6 to 15 second range. Preferably, heave resonance rangefor the Soft TLP will generally have an upper end of about12 seconds, preferably about 10 seconds, and more prefer-ably about 8 seconds, and still more preferably about 7seconds, and a lower end generally about 6 seconds, pref-erably greater than 6 seconds, and more preferably about 7seconds. The prefered heave resonance range for the soft TIPis about 6 to about 10 seconds, preferably about 7 to about9 seconds, and more preferably about 7 to about 8 seconds.[0054] Referring now to FIGS. 5-11, there is shown theinstallation sequence for the Soft TLP of the present inven-tion. Referring first to FIG. 5, the hull, comprising buoyantsection 303 and support section 301 is towed to the desiredlocation. Referring next to FIG. 6 the hull is then ballastedto below the ultimate desired target depth (which in FIG. 6is shown as 220 feet of water depth). Referring next to FIG.7, tendons 305 are assembled by a construction vessel as isknown in the art. Referring next to FIG. 8, tendons 305 arepassed to the hull and pre-connected. Referring next to FIG.9, all tendons 305 are connected and tensioned, and buoyantsection 303 is partly de-ballasted. Referring next to FIG. 10,deck 10 is assembled onto the hull. Referring next to FIG.11, deck 10 is complete, and buoyant member 303 is fullyde-ballasted.

EXAMPLES[0055] The following examples have been providedmerely to illustrate a few embodiments the invention, andare not intended to, and do not limit the scope of the claims.

Example 1Comparison of Soft TLP (Proposed Design) to an

Actual TLP[0056] In this Example, a computer model comparison ismade between a proposed Soft TLP design of the presentinvention and an actual TLP, the Shell Brutus TLP, locatedin Green Canyon Block 158 in 2,985 feet of water in theGulf of Mexico.[0057] The payload weights of the various major compo-nents for the proposed Soft TLP and the Brutus TLP, asutilized in the computer model are shown in FIG. 12.[0058] The platform response was computed using theShell Oil Company in-house numerical tool COSMOS (soft-ware to compute the response of floating offshore plat-forms).


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[0059] COSMOS is a fully-coupled finite-element pro-gram that predicts the response (platform motion, mooringline or tendon tension) of various floating systems andconsiders the interaction between mooring lines or tendonsdynamic tensions and the floating vessel. Diffraction-radia-tion theory is used to compute wave loads, and Morisonequation to model viscous effects. A dynamic wind model isused to predict wind loads. The response of the platform iscomputed in a 100 year hurricane, and cumulative tendondamage is determined using an annual wave-scatter dia-gram.[0060] Results are presented in FIGS. 13 and 14.[0061] While the illustrative embodiments of the inven-tion have been described with particularity, it will be under-stood that various other modifications will be apparent toand can be readily made by those skilled in the art withoutdeparting from the spirit and scope of the invention, Accord-ingly, it is not intended that the scope of the claims appendedhereto be limited to the examples and descriptions set forthherein but rather that the claims be construed as encompass-ing all the features of patentable novelty which reside in thepresent invention, including all features which would betreated as equivalents thereof by those skilled in the art towhich this invention pertains.We claim:

1.An offshore platform comprising:(A) a deck for supporting hydrocarbon exploration, drill-ing or production equipment;

(B) a buoyant member;(C) an open support structure positioned between the deckand buoyant member, comprising an upper end con-nected to the deck, and comprising a lower end con-nected to the buoyant member;

(D) a plurality of tendons connected to the buoyantmember suitable for anchoring the platform;

wherein when the platform is positioned offshore, thedeck is supported above the waterline, the upper end ofthe open structure is positioned above the water line,with the lower end positioned at least 100 feet belowthe waterline; and the heave resonance of the platformis at least 6 seconds.

2. The platform of claim 1, wherein the heave resonanceof the platform is in the range of about 6 to about 10 seconds.3. The platform of claim 1, wherein the heave resonance

of the platform is in the range of about 7 to about 9 seconds.4. The platform of claim 1, wherein when the platform is

positioned offshore, the lower end is positioned at least 150feet below the waterline.5. The platform of claim 4, wherein the heave resonance

of the platform is in the range of about 6 to about 10 seconds.6. The platform of claim 4, wherein the heave resonance

of the platform is in the range of about 7 to about 9 seconds.7. The platform of claim 1, wherein when the platform is

positioned offshore, the lower end is positioned at least 200feet below the waterline.8. The platform of claim 7, wherein the heave resonance

of the platform is in the range of about 6 to about 10 seconds.9. The platform of claim 7, wherein the heave resonance

of the platform is in the range of about 7 to about 9 seconds.

Aug. 7, 20034

10. A method of exploring an offshore target zone forhydrocarbons, the method comprising:(A) positioning a platform offshore near the target zone;(B) conducting exploration activities from the platform,wherein the platform comprises:(i) a deck for supporting hydrocarbon explorationequipment;

(ii) a buoyant member;(iii) an open support structure positioned between thedeck and buoyant member, comprising an upper endconnected to the deck, and comprising a lower endconnected to the buoyant member;

(iv) a plurality of tendons connected to the buoyantmember suitable for anchoring the platform;

wherein when the platform is positioned offshore, thedeck is supported above the waterline, the upper endof the open structure is positioned above the waterline, with the lower end positioned at least 100 feetbelow the waterline; and the heave resonance of theplatform is at least 6 seconds.

11. The method of claim 10, wherein the heave resonanceof the platform is in the range of about 6 to about 10 seconds.12. The method of claim 10, wherein the heave resonance

of the platform is in the range of about 7 to about 9 seconds.13. The method of claim 10, wherein when the platform

is positioned offshore, the lower end is positioned at least150 feet below the waterline.14. The method of claim 13, wherein the heave resonance

of the platform is in the range of about 6 to about 10 seconds.15. The method of claim 13, wherein the heave resonance

of the platform is in the range of about 7 to about 9 seconds.16. The method of claim 10, wherein when the platform



of the platform is in the range of about 7 to about 9 seconds.19. A method of drilling for or production of hydrocar-

bons from an offshore target zone, the method comprising:(A) positioning a platform offshore near the target zone;(B) conducting drilling or production activities from theplatform,

wherein the platform comprises:(i) a deck for supporting hydrocarbon explorationequipment;

(ii) a buoyant member;(iii) an open support structure positioned between thedeck and buoyant member, comprising an upper endconnected to the deck, and comprising a lower endconnected to the buoyant member;

(iv) a plurality of tendons connected to the buoyantmember suitable for anchoring the platform;

wherein when the platform is positioned offshore, thedeck is supported above the waterline, the upper endof the open structure is positioned above the water


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line, with the lower end positioned at least 100 feetbelow the waterline; and the heave resonance of theplatform is at least 6 seconds.

20. The method of claim 19, wherein the heave resonanceof the platform is in the range of about 6 to about 10 seconds.

21. The method of claim 19, wherein the heave resonanceof the platform is in the range of about 7 to about 9 seconds.22. The method of claim 19, wherein when the platform



Aug. 7, 2003524. The method of claim 22, wherein the heave resonance

of the platform is in the range of about 7 to about 9 seconds.25. The method of claim 19, wherein when the platformis positioned offshore, the lower end is positioned at least

200 feet below the waterline.26. The method of claim 25, wherein the heave resonance



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10 068 767 tension leg platform having m

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