In the ongoing search for hydrocarbons off-shore, a major challenge faced by operatorshas been to reduce field development costs.In deepwater and in marginal productionareas, this challenge has led to the evolution

of structural technologies.In the past, costs dictated that central drilling

and production facilities require either oil pricesexceeding $18/ bbl or significant reservoirslarger than 100 million bbl of recoverable oil. Itwas predicted that oil production rates of50,000-75,000 b/d would require 40 to 60 wells,as typical flow rates per well did not exceed1,000 b/d.

The compliant tower (CT), based on earlierdata, was deemed to be unsuitable for marginalfield applications in water depths in the 1,200-2,500 ft range. With data accumulated fromrecent installations and subsequent cost stud-ies, however, initial presumptions about wellcounts and flow rates in deepwater fields havebeen revised. Individual wells can generateflows exceeding 7,000 b/d, requiring singledrilling rigs and fewer well slots.

Radical improvements in compliant towerdesign have reduced tonnages, improved con-structability and enhanced operating capability,thereby reducing capex, opex, and schedules.These attributes make compliant towers aproven, viable alternative to floating systems foruse in deepwater fields.

Tower compliancyThe term, compliancy connotes flexibility. In

deep waters, bottom founded and floating struc-tures must be designed to be compliant inorder to mitigate the impact of hurricane forcesof wind, waves and currents.

Under hurricane conditions, the time lag(period) between larger waves is approximately13 seconds. In order to prevent excessive ampli-fication of the environmental forces of wind,waves and current, it is imperative that the nat-ural period of the primary modes of response besubstantially different than the dominant peri-ods of the hurricane seastate.

Achieving compliant response requires con-trolling the mass and stiffness characteristics tode-tune the natural frequencies of vibration, rel-ative to the frequencies of the periodic forces ofwind and waves, in combination with current.

Compliant towers, with the use of flex ele-ments such as flex legs or axial tubes, typicallyachieve sway periods of 30-33 seconds. As aresult, resonance is reduced and wave forces arede-amplified. By comparison, typical shallowwater platforms will have periods of 3-4 seconds.

De-amplification of hurricaneforces enhances efficiencylevels with respect to ton-nages and constructionrequirements, as the struc-ture can be configured toadapt to existing fabricationand installation equipmentand facilities.

Advances in design proce-dures have resulted in addedsavings with the third gener-ation towers. Design andanalysis software can accu-rately account for threedimensional random seas,including refinements forcurrent blockage and con-ductor shielding. Windforces can be accounted fordynamically, thereby reduc-ing the levels of excessiveconservatism which mayoccur when using conven-tional design procedures.

Finally, the use ofresponse-based design crite-ria, which involves the deter-mination of 100-year return

period responses rather than metocean events,can help to ensure that the selected designenvironmental events will result in designforce levels with acceptable, but not excessive,levels of risk.

Tower historyThe compliant tower concept has essentially

progressed through an evolution of three con-figurations. The first emerged in the early 1980s with the

installation of Exxons Lena platform, a guyedtower in a water depth of 1,018 ft and sup-ported by 20 weighted guy wires to achievecompliancy and stability.

A second generation of structures, compliantpiled towers, was introduced during the late

Compliant towers:the next generationWell counts, design altering deepwater scenario

Steve WillMustang Engineering, Inc.

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3,000

Tower weight versus water depth Gulf of Mexico

100,000

Second generation towersFull drilling and production

2 drill rigs12,500-ton payload

40 to 48 wells

Third generation towersFull drilling and production

1 drill rig7,500-ton payload

18 to 21 wells

90,000

80,000

70,000

60,000

50,000

40,000

30,000

20,000

10,000

0500 1,000 1,500 2,000 2,500

Tota

l jake

t & pi

le (to

ns)

Water depth (ft)

The third generation of thecompliant tower for the USGulf of Mexico (Art byMustang Engineering).

Reprinted with revisions, from the July 1999 edition of OFFSHORECopyright 2000 by PennWell Corporation

1980s which relied on the piles forits flexibility and stability.

The newest generation of compli-ant tower designs is representedby the 1998 installation of: (1)Amerada Hess Baldpate compli-ant tower at Garden Banks Block260 in 1,650-ft water depths; (2) Texacos Petronius tower,designed for installation at VioscaKnoll Block 786 offshoreLouisiana in 1,754-ft water depths. The Baldpate tower gained its

compliance by utilizing axial tubesaffixed to its legs and an articulationpoint approximately 500 ft above thesea floor. The Petronius structure,referred to as a flex-leg structure,relied on flexible legs for its stabilityand flexure.

Operating advantagesThe compliant tower (CT) offers

many of the same advantages of any bottom-founded structure pro-ducing in shallower water depths.Additionally, the CT approach allowscertain pluses when compared tofloating methods. Among them are:(1) Drilling and production operations: The

compliant towers topside structure enablesdrilling and production to be carried outsimultaneously without the need for atten-dant mobile drilling equipment that can bedifficult and expensive to contract. Forexample, the Baldpate platforms 9,800-tontotal topsides weight included a tri-level decksection with a 28-man quarters and facilitiessufficient to support an API 20,000 ft. drillingrig along with the processing equipmentnecessary to accommodate production fromthe 18 wells. Like the fixed platform, the CTcan also support workover or well servicingoperations without having to rely on externalfloating support equipment.

The compliant tower is very stable. Its dis-placement, even under 100-year hurricane con-ditions, might be only 25-30 ft, or 1.5-2.0% of thewater depth. In contrast, floating systems gen-erally have lateral movement of up to 10% ofwater depth. Spars, with riser constraints, havea maximum lateral displacement at the waterline of approximately 6% of water depth.

The stability of the CT means that its down-time is limited to only the most extreme events,similar to shallow water platforms. This stabilityalso reduces the complexity of operations.There are no specially-trained crews needed tooperate ballast/deballast tanks or to adjust risertensions. Because it is not a floating system, itdoes not require ABS classification.(2) Production riser and wellhead support:

With the compliant tower, all wells can havedry trees in lieu of subsea wet trees. CTscan also serve effectively as a centralproduction facility, supporting platformdrilled wells or satellite subsea tiebacks.Also, wells can be predrilled and temporar-ily abandoned and tied back followinginstallation of the tower. The surface com-pletions improve accessibility for controls,maintenance and future well servicing.

Compared with floating systems, such as ten-sion leg platforms (TLP), mini-TLPs, and Spars,the production risers are conventional and aresubjected to less structural demands and flex-ing, as they are afforded maximum support and

protection by the CT structure. Thisfactor is particularly important infields where high currents are preva-lent, such as in the Campos Basinoffshore Brazil.

The production risers use conven-tional well systems, with conductorsand casings that become structuralelements. Once again, the simplicityof the CTs operation reduces mater-ial costs. Production tubulars aregenerally made of carbon steel asopposed to special materials neededto support flexing. The weight of therisers extends directly into theseabed and, along with the wellheadsand BOP stack, necessitates onlyminimum support by the jacket.(3) Export riser support: The com-pliant tower has an advantage in thatit can effectively support large diame-ter steel export risers, including steelcatenary risers (SCR), J-tubes, or pre-installed risers. In the case of theBaldpate installation, the 16-in. oiland 12-in. gas SCRs were suspendedfrom clamps attached to the jacketlegs approximately 400 ft below thewaterline, with the pipeline extend-

ing from the platform to touch the seabed nearly650 ft from the structures base.

Comparisons, economicsInitial projections of tonnages for compliant

towers have been revised downward followingthe design and construction of the Baldpate andPetronius towers. Earlier data suggested thatthe Baldpate tonnages would approximate50,000 tons for the 1,900-ft structure operatingin 1,650 ft water depths.

In actuality, a total of 34,000 tons of structuralsteel were used to construct the tower and thepiles. This included the 351-ft base sectionweighing 8,700 tons and the 1,320-ft towerweighing 20,200 tons. By comparison, one con-ventional fixed platform, with higher wellcounts and payload and currently producing inapproximately 1,350 ft water, weighed almost60,000 tons.

Similarly, a second conventional fixed plat-form currently producing in 1,290 ft waterdepths weighed in at approximately 43,500tons. Extrapolations from in-house studies indi-cate that compliant tower tonnages wouldallow operation at a water depth of 2,800 ft witha tonnage comparable to that of the 1,350 ftfixed platform and at a depth of 2,300 ft with atonnage approximating the fixed platform in1,290 ft depths.

The compliant towers economic viability wasfurther validated by the Baldpate installation.The capital expended for the development ofthe anticipated 118 million barrel of oil equiva-lent (BOE) reserves for the two primaryBaldpate reservoirs is about $300 million,

(Base 400 ft X 400 ft)

Base footprints of tower and fixed platform

Conventional fixed platformin 1,290ft water depth

Compliant tower in1,750 ft water depth(Base 110 X 110 ft)

Fixed platform for1,290 ft water depth

Compliant tower for1,750 ft water depth

Elevations of platform and tower

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including development drilling and completionscosts and pipelines. This translates into anapproximate capital expenditure (capex) of$2.80 BOE. By comparison, it has recently beenreported that the monohull mini-TLP in similarwater depths had a capex of $3.50 BOE.

Improved constructabilityThe compliant tower is a more slender, less

complex structure than is a conventionaldeepwater fixed platform. As such, it presentsfewer fabrication constraints and more opportu-nities for economy. For example, when compar-ing footprints, existing deepwater structures fordepths of 1,290 ft and 1,350 ft have base widthsexceeding 400 ft. By comparison, the Baldpatestructure has a base width of 90 ft, with the baseexpanding to 140 ft at the mudline.

The Petronius flex-leg structure has baseand tower widths of 110 ft square. The dra-matic reduction of tonnages and fabricationheights allows yard fabrication and assemblywith a minimum of large capacity, heavy liftand extended reach cranes and specializedequipment. In addition, the reduced fabrication heights provide significant safetyenhancements.

Reduced design force levels have also led tocompactness. As a graphic example, the designfootprint of the Baldpate tower was sufficientlycompact to fit comfortably within the 140 ft by200 ft launchbox created for the 1,290 ft jacketdiscussed earlier. In the Baldpate assembly, thelargest members were the 144-in. diameter legsof 3 5/8-in. rolled plate. These material dimen-sions can be accommodated by multiple rollingmills and fabrication yards along theTexas/Louisiana Gulf Coast.

Construction complexity of compliant towersis minimal. The design of these structures issimplistic, with a square plan and repetitiveframing used throughout the entire length ofthe tower section. There are no high costmechanical system components required for

long-term performance by compliant towers.Rather, all structural systems are composed offield proven, low unit cost materials and compo-nents which have been fabricated numeroustimes in fabrication yards experienced with thefabrication of offshore structures.

This relatively simplistic structure containsno items which have more complex construc-tion requirements, such as buoyancy tanks,mooring systems, ballast/deballast systems,riser tensioners, or flexible risers. In short,compliant towers are simple to build and easy tomaintain.

InstallationThe installation procedures for the two-piece

compliant tower are proven and can be handledby suitable launch barges residing in the Gulf ofMexico. Following the installation of foundationleveling piles on which the base is to be set, andtwo docking piles to guide the setting of thebase, the base itself is launched and installed.

In the case of Baldpate, 12,400-ton skirt piles(three per base leg) were driven to a depth of430 ft. Following the setting of the base, thetower was towed similarly by launch barge andlaunched end-on to then upright itself and belowered by a derrick barge, having been bal-lasted to 900 tons.

The underside of each tower leg had a dock-ing pin that stabbed into the receiving cones ofthe structures base. Once positioned, it wasadditionally ballasted and connections grouted.Shortly thereafter, the deck was transported tothe site and installed by the derrick barge in asingle lift.

Subsequently, the main deck package includ-ing the quarters, was lifted and set on the deck.Following hookup of flow lines and facilities,first oil production was recorded within twomonths. Using Baldpate and Petronius as exam-ples, the conventional manner in which thecompliant tower can be installed, requiring nospecial equipment, adds to its attractiveness.

ConclusionThe improved efficiencies in field develop-

ment have been produced by a combination ofdesign advances, improved configurations andreduced wellcounts. Options now exist for cen-tralized drilling and production with the designof new generation compliant tower configura-tions that are lighter, easier to fabricate andmore economical to install.

These more efficient CT configurations pro-vide a means of improving field developmentcosts with these efficiencies translating intolower capital expenditures needed for the devel-opment of marginal deepwater sites. The dis-cussions herein apply to towers designed forwell counts of 18-20 and simultaneous drillingand production operations.

Mini-towers, designed for fewer wells (5-8)and reduced payloads (drilling or productiononly opereations) will offer added savings.Similarly, milder environments will also resultin lighter, more compact configurations andreduced capex.;

ReferencesWill, S.A.; Edel, J.C; Kallaby, J.; des Deserts, L.D;

Design of the Baldpate Compliant Tower, 1999OTC No. 10915.

Simon, J.V.; Edel, J.C.; Melancon, C.H.; An Overview ofthe Baldpate Project, 1999 OTC No. 10914.

AuthorSteve A. Will, P.E., is senior consulting

engineer at Mustang Engineering, Inc. Will has31 years experience in of fshore engineering andconstruction. His background includes projectmanagement responsibility for some of theworlds tallest bottom founded structures. He isa graduate of Purdue University and is aregistered professional engineer in Texas andLouisiana.

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