Use of the Dual-Activity Drillship as a Field Development Tool

Copyright 1999, Offshore Technology Conference

This paper was prepared for presentation at the 1999 Offshore Technology Conference heldin Houston, Texas, 36 May 1999.

This paper was selected for presentation by the OTC Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Offshore Technology Conference and are subjectto correction by the author(s). The material, as presented, does not necessarily reflect anyposition of the Offshore Technology Conference or its officers. Electronic reproduction,distribution, or storage of any part of this paper for commercial purposes without the writtenconsent of the Offshore Technology Conference is prohibited. Permission to reproduce in printis restricted to an abstract of not more than 300 words; illustrations may not be copied. Theabstract must contain conspicuous acknowledgment of where and by whom the paper waspresented.

AbstractThe startup of the dual-activity drilling vessel DiscovererEnterprise in early 1999 heralds a new age for deepwaterdevelopment. For the first time a drilling vessel exists thatcan perform many of the aspects of deepwater fielddevelopment cost effectively, including a variety of subseaproduction equipment installations and flowline/pipelinelaying. This capability minimizes the barges and equipmentrequired for field development and yields schedule flexibilityfor the installation tasks.

Aspects of the dual-activity drillship related to non-drillingactivities are described in the paper. The second rig on thedrillship is only required part of the time to support drillingoperations. The remainder of the time can be used for otherfield development activities.

The subsea tree is run with the auxiliary rig while drillingwith the BOP and riser with the primary rig. In addition,subsea manifolds, umbilical pods, flowline jumpers andsupporting piles can be placed and secured with the auxiliaryrig while drilling progresses with the primary rig. Conceptsfor subsea equipment that can be installed with the dual-activity drill rig are presented.

Some flowline installation work can also be performed whiledrilling is in progress. The flowline installation technique,designated discrete lay and/or reverse lay, results insectional flowline placements using an anchor handling vesseland an ROV boat (for inspection and monitoring). This

technique can also be used for the placement of longjumpers.

Flowlines, as well as export pipelines in deeper waters, canalso be installed by the dual-activity drillship in pipelay modebefore or after drilling is completed. Using both rigs, twolines can simultaneously be laid which is an advantage forflowline sets requiring pigging loops or parallel oil and gasexport lines. Burial of the lines is also achievable with thedual-activity drillship.

A conceptual timeline of how drilling and non-drillingoperations can be achieved by the dual-activity drillship isprovided.

IntroductionThe concept of a dual-activity drillship has been in existencesince the mid-1990s. A year-long study performed byTransocean Offshore Inc. concluded that deepwater drillingefficiencies could be enhanced by design and construction of asingle drillship with dual drilling capabilities. Construction ofthe first dual-activity drillship, the Discoverer Enterprise,began in 1997 and is scheduled for delivery in the spring of1999. Two additional drillships, the Discoverer Spirit and theDiscoverer Deep Sea, are currently under construction and areexpected to be delivered in the year 2000.

One advantage identified in association with the dual-activity drillship is that it has the potential to enhance offshoreoperations in unconventional modes. Because the auxiliary rigis out of the critical path, it can be used for operations whichwould not be economic if the rig spread cost were applied tothe operation. For example, the second rig is capable of beingemployed to drill a disposal well for flared gas or forproduced water, or the second rig can be used to pull inpipelines, run production jumpers, run control lines or othersubsea construction work. The extra rig is available for heavysubsea lifts and work as a 5,000 hp ROV. In addition thesecond rig can help eliminate shallow water flow problems orreduce external hydrates. After the BOP stack is set there is

OTC 10891

Use of the Dual-Activity Drillship as a Field Development Tool

Johnce E. Hall, BP Amoco; Robert P. Herrmann, Consultant; Charles A. Holt and William J. Straub, BP Amoco; Rickey G.Weber, R & D Technical

2 J.E. HALL, R.P. HERRMANN, C.A. HOLT, W.J. STRAUB, R.G. WEBER OTC 10891

available time at the auxiliary rotary table for making uppipeline segments for installation on the seabed.

The biggest advantage in using the rig for field developmentis to eliminate the need for an additional construction vesselsuch as a derrick barge or pipelay barge. The capabilities ofthe Discoverer Enterprise as a dedicated deepwater pipelayvessel actually exceeds that of many specialized lay barges.By using the rig for this function, more schedule flexibility isrealized. It is not necessary to coordinate the arrival of aseparate construction vessel with the other field activities suchas drilling or host installation. No additional mobilization ofoffshore vessels is required. The rig can even be used to drillanchor piles and install the mooring lines. The rig can laypipelines in a piecemeal manner when needed rather than inone operation. The rig can be used at anytime within thedevelopment schedule for lay purposes or other tasks. Thispermits a more rational infrastructure development for thefield rather than that found when the lines have to be laid all atonce in a single operation.

Finally, there is the capability to increase the overallefficiency of the operation by changing the basic culture on therig floor. Because of the second drilling center more focus andattention will be given to planning the subsequent phases ofthe operation. One complete rig exists to support the other,and just keeping that operation running will improveorganization and help eliminate the unidentified unproductivetime that is found in analysis of the well construction process.This potential savings due to better planning is real and can beseen in investigation of the improvements in efficiency inbatch-drill operations. A significant amount of the batch-drillimprovements cannot be tied to the operations eliminated bythis process alone. Much of these improvements are fromabolishing the need to plan between stages of the operation.The second rig causes better planning and coordinationbetween stages because its primary function is to carry out thework for the next phase.

The Discoverer EnterpriseThe Discoverer Enterprise is an extremely large drillshipincorporating a multi-activity derrick and drillfloor. The hull ismodeled on a standard 100,000 ton North Sea dynamicallypositioned (DP) shuttle tanker outfitted with six 7,000 hpazimuthing thrusters. Total stationkeeping thrust totals 42,000hp, and the variable deck load is 20,000 tons.

The large stable platform provided by this hull allows use of adual capacity derrick with an 80 ft x 80 ft drill floor and a 220 ftheight. The derrick is fully outfitted with two drilling packagesincluding topdrives and motion compensators. Figure 1 showsthe basic floor arrangement. The allowable simultaneous hookloads are 2,000 kips each and the two rigs are outfitted to rackcasing in triples and drillpipe in quadruples (125 ft stands). Thedual derrick permits parallel and shared operations and placesmany rotary table activities out of the critical path. Thecontinuous setback can feed either rotary table and allowscasing, bottom hole assemblies, and running tools to be made up

and stood back with one rig while drilling hole with the other.The dual activity also permits the auxiliary rig to drill and casethe top hole while running the BOP/Riser with the main rig. Thederrick is designed to transit with two full drillpipe setbacks.

The drillship is outfitted with two mud systems, quarters forup to 200 men, and storage for 20,000 ft of riser. There is spacefor staging and running subsea trees, and for the normalshorebase storage and office facilities. The vessel's sleek linesand high power enable economical moves between operatingareas, and the ship can carry variables for up to five wells.There is 100,000 bbl crude storage potential for well testing orcompletion.

The motion response characteristics of the drillship areextremely good and are in fact better than those for a semi-submersible for waves in the 9-14 second head seas.

The design is especially suitable for field development work.The stationkeeping particulars, the high hoisting capacity and theability to work subsea in parallel with the drilling operationlends the opportunity to economically carry out work that wouldnormally be done with a dedicated construction vessel.

Dual-Activity Drilling OperationsFor single exploratory wells there are three main areas wheredual activity can be carried out: running the BOP stack andtop-hole operations; make up and testing of bottom holeassemblies and downhole tools; and make up and running ofcasing. Up to the time the BOP stack is landed both rigs are100% engaged, thus halving the time required to drill thatsection. One rig runs the BOP and riser while the other drillsthe top hole and runs casing down to 20 in. One rig does alltophole operations, pilot hole, drilling, tripping, running pipeetc. while the other is running the BOP and riser. Once thewellhead running tool is released the rig moves 20 ft and thestack is landed.

At the end of the well the auxiliary rig can be used toremove the wellhead or to run abandonment gear whilesimultaneously pulling the BOP stack.

After the stack is landed the auxiliary rig is devoted topreparing bottom hole assemblies and down hole tools for themain rig. In addition to the time saved simply from movingthese activities out of the critical path there is the benefit ofreduction of downtime from testing faulty tools. This can bedone with the auxiliary rig and allows time to troubleshootproblems that would have normally been charged against thespread costs. This is especially important in deepwater or inapplications requiring special tools. It is possible to buildstands outside the main rotary table with a special mouse holebut it is difficult to test running tools in this area while the drillstring is rotating or while tripping pipe.

In addition to servicing downhole tools the auxiliary rigmakes up the casing string in 125 ft stands and racks back forrunning. Time is saved in making up the shoe joints and floatcollars, and half of the casing joints are made-up out of thecritical path. Tubing strings, gravel pack and other specialstrings are made up by the auxiliary rig as well as drill pipe for

OTC 10891 USE OF THE DUAL-ACTIVITY DRILLSHIP AS A FIELD DEVELOPMENT TOOL 3

the main rig or for inspection.When used for field development the dual-activity rig

becomes even more efficient. After landing the BOP stack theship can be moved so that adjacent wells can be drilled by theauxiliary rig. The offset required to reach a new well is wellwithin the riser angle limit. Up to nine additional top holescan be drilled and cased through the auxiliary rotary tablewhile the main rig is drilling out under the 17-1/2 in. top holein the first well. The BOP can then be moved to the next wellwithout pulling to the surface, and the pressure section of thewells can be drilled batch style.

While drilling through the BOP the second rig can be usedto rig up and run the production trees. The well is secured,the BOP released and the rig positioned to land the tree. Thestack can be moved to another well or set atop a horizontaltree for completion. In any case the tree is safely run withouthaving to trip the BOP. In the event the tree does not test, it isnot necessary to double trip the stack to retrieve the tree.

Construction Vessel CapabilitiesAs a construction vessel the Discoverer Enterprise comparesfavorably to typical dedicated deepwater construction barges.The capacities of the derrick (used as a J-lay tower forpipeline installation), stationkeeping, and sea bed surveyequipment on the Discoverer Enterprise surpass those foundon the current fleet of deepwater construction vessels.Moreover, the dual derrick allows two pipelines to be laidsimultaneously without a significant increase in spread costs.Table 1 sets out a comparison between the DiscovererEnterprise and a typical deepwater J-lay barge. Figure 2illustrates the difference in size between the same two vessels.

Tower.Hook Load. Large lifting capacities are required to J-lay

pipe in deepwater. Flow assurance often demands heavy pipe-in-pipe insulation systems, and project economics may notallow export line wall thicknesses that can withstand thecollapse pressure acting on an evacuated line. Wet pipeweights can be as high as 180 lb/ft and, coupled with thetension requirements and dynamic loading, can result in towerloads up to 1,500 kips. According to Albaugh (Ref. 1) typicalcapacities of existing J-lay barges run between 150 kips and792 kips. This compares to the twin hookload capacity of2,000 kips for the Discoverer Enterprise. There are twohoisting drawworks in the dual derrick. Each is fully outfittedand rated for 2 million lb hookload. The normal rig landingstring is 6-5/8 in. 27.7 lb S drill pipe rated at 1 million lb. Forlarger loads the riser can be used. The 2 million lb hookload issimultaneous and can be achieved by both drawworks at thesame time. This is useful for emergency recovery or foroperation in 10,000 ft of water.

Motion Compensation. The drilling rig has two 1,000 kipmotion compensators and can in fact provide a constanttension at levels higher than that of the ultimate liftingcapacity of typical lay barge towers. The two compensators

can be put to good use for delicate hingeover operations or foremergency laydown or recovery operations.

Dynamic Derrick. The Dual Derrick on the DiscovererEnterprise is rated at 2 million lb each hook (simultaneous)while suffering 40 kt winds and 18 ft significant sea acting 30off the bow. A hook load of 1,800 kips on one rigsimultaneous to 1,400 kips on the other rig can be maintainedwhile in 60 kt winds and a 25 ft significant sea acting 30 offthe bow. The dynamic capacity of the derrick allowsemergency operations in rough weather. Typical lay bargetowers have reduced sea state and subsequent motionresistance capacity and are thus required to cease operationsmore often.

Dual Pipelay. Both rigs are rated to 2 million lb and can beused to simultaneously lay two lines. The rotary tables are 40ft apart and provide good separation between lines. Asillustrated in Figure 3 the rig can change heading 45 aboutthe neutral point and still achieve adequate separation betweenpipelines. When 45 from square the pipelines are only 11 ftcloser together which is insignificant in deep water. Theability to make a 45 heading change coupled with the verygood motion characteristics permit the dual lay operation evenin winter.

Stationkeeping Power. The Discoverer Enterprise is outfittedwith six 7,000 hp thrusters and designed to stay on locationduring a 50 year Gulf of Mexico hurricane. The thrusters cankeep station with a 60 kt wind directly on the beam and whileonly running at 80% capacity. The azimuthing thrusters candevelop 1,200 kips thrust. This compares to up to 800 kips ofthrust potential from the most powerful J-lay barges. Thedrillship power generation is 52,000 hp continuous andcapable of 70,000 hp instantaneous. This increasedstationkeeping allows the drillship to work year round and addto the enhancement of schedule flexibility.

Pipehandling Machines. The Discoverer Enterprise isequipped with two vertical pipehandling machines. Each unitis capable of working either rotary table or any setback andprovides backup. The pipe handlers are designed to handle138 ft stands of 3-1/2 in. - 20 in. pipe and can pick up a 125 ftstand of 9-1/2 in. drill collars (27,000 lb). The machines aredesigned to set and align threaded pipe for makeup and arecomputer controlled. The pipehandlers can be commanded tocenter the pipe in the rotary and align to vertical. The rig isoutfitted with all manner of power slips and casing elevatorsto run the pipe.

Special Operations.Subsea Power. There are four 2,200 hp high pressure mud

pumps on the Discoverer Enterprise which can deliver 8,000hydraulic hp @ 7,500 psi to seabed machinery. This enablesthe rig to power seabed winches, trenching machinery or otherequipment that heretofore were limited by the hydraulic power


available from the ROV. One such application is the burial ofpipelines. No equipment of such capacity is normallyprovided on pipelay barges.

Lift and Pull. Utilizing both drawworks allows one to lift apipeline and the other to pull in the pipeline. This helps inexecuting pipeline deflection and connection operations.

Torque. The rig can apply and hold torque to the pipelinewhen landing the second end. This enables the rig to steer thepipeline to a desired lay direction or to land the pipeline basein the correct vertical hub orientation.

Ability to Drill or Pile Pipeline Bases. The drillship is wellsuited to drill in piles for supporting pipeline bases ormanifolds. By offsetting the rig and allowing the riser to takea small angle the second rig can drill piles while the primaryrig is drilling through the BOP. In 7,000 ft of water these pilescan be as far away as 250 ft from the BOP stack. By drillingoff-line, only the material cost of the pile adds cost to theproject.

ROV and Survey. The drillship is equipped with two fullROV spreads including crews. Positioning reference systemsinclude twin DGPS, short baseline, long baseline, andcombined ultra-short/long baseline acoustic systems.

Timing with Drilling OperationsThe Discoverer Enterprise is actually a triple-activity rig forfloor operations. There are two fully functional rotary tablesand drawworks rigged up in the dual derrick, plus there is amechanized mousehole (smart mousehole) that, while usingthe aft pipe handler, can make up and break down bottom holeassemblies and make up and rack casing and completionstrings. A more accurate description of the drillship is a dualactivity unit for below water operations and triple-activity unitfor drillfloor operations. As such the second rotary isavailable whenever there is no need for dual in-wateroperations. This is a substantial amount of time and includesall time after landing the BOP stack up until the tree is movedto the center of the auxiliary rotary table for rigging up, andincludes the time from when the tree running string is retrievedup until abandonment. This is roughly 80% of the entire timeon location. The mousehole operation does however, requireuse of one of the pipehandling machines and will interfere withthe auxiliary rig operations when the second pipehandler isneeded. To be conservative it is assumed that the mouseholetime interferes 100% with non-drilling construction activities.This brings the available time for these operations to 65% ofthe location time.

Figure 4 depicts a detailed schedule for a theoreticaldeepwater well with no trouble time, a perfect well. Thedrilling and completion time is 37 days. This is a challengingschedule for planning concurrent field construction activities.In reality the perfect well gives the most pessimistic templatefor structuring the simultaneous operations. The auxiliaryrotary table operations are a fixed set of tasks and are notsubject to changes owing to drilling problems. Thus, if theprogram is delayed, then the extra drilling time can be

considered as more time available to the auxiliary rotary tablefor construction activities. Figure 5 is the perfect wellschedule filtered for all tasks involving the auxiliary rotarytable and the mouse hole. The schedule has been furtherenhanced by making two tasks to start as late as possible. Thisenables the available auxiliary rotary table time to be arrangedin two continuous blocks and yet not affect the overall drillingand completion time. The schedule also shows the slack timewhere neither the auxiliary rotary table nor the mouse hole isbeing used. The first large block occurs between the 17-1/2 in.section and running 9-5/8 in. pipe. There are 7.15 days shownwhere no auxiliary rotary table or mouse hole activity isscheduled. The second period is after landing the subsea treeand comprises almost 14 days. Thus, without reference to thewell operation, there are 21 days available out of the 37 in twolarge blocks. There are smaller segments of availableauxiliary rotary table time earlier on which make up the twodays needed for the 65% claim.

There are two types of field development work identified ascandidates for installation with the drillship while drilling.Installing sea bed equipment such as trees, manifold bases,jumpers, and support piles comprise one group. These tasksare normal drill rig activities and are done at the auxiliaryrotary table without interference to the normal drillingoperations at the main rotary table. The second group includespipeline construction and involves welding on the rig floor.This type of work should be undertaken during the first of thetime blocks, while drilling the well, rather than during the wellcompletion phase when the well is live.

Drillfloor Welding. The drillfloor is classified as a Zone IIhazardous area. As such welding is allowed if contained in anarea positively ventilated with air from outside the hazardouszone. This is the setup which allows use of direct current (DC)drilling motors and the other rig floor electric equipment onthe drillfloor. Additionally it is desired to keep hot weldingmaterial from falling into the moonpool and to isolate thewelding flash from the adjacent drilling crew. The rotarytables are 40 ft apart. Had this separation distance been 50 ftthen welding could be undertaken without the need for thepositive ventilation. To positively ventilate and to keep theflash off the drill crew a small house will enclose the pipewhile welding. This house is equipped with supply fans thatare ducted to take air from a safe area. The roof and fore andaft walls of the house open up to allow the pipe to be pickedup and set in position as seen in Figure 6. While aligning thepipe the house is closed and the ventilation is started. Thehouse can be as large as 10 ft x 15 ft, and will have a floor tokeep sparks and molten metal from falling into the moonpool.

Subsea Equipment Installation with Dual-ActivityDrillshipThe subsea installation most evident for the offline rotary tableis installation of the subsea tree. While the BOP is down anddrilling operations are being conducted with the main rig, the


tree is brought to the secondary rotary table and rigged up forrunning. Horizontal trees are run to the sea bed on drillpipeand landed without having to pull the BOP to the rig floor.Vertical trees can likewise be run by the auxiliary rig with aseparate riser system. The second rig also provides theopportunity to carry out activities not normally consideredpossible due to the cost of the operation spread. The rig canbe used to run pipeline bases and to pull in the pipeline.Heavy gear can be moved subsea, and the mud pumps canprovide hydraulics for seabed work such as trenching orpipeline pull-in. The offline rig can run tree jumpers, pipelinejumpers and manifolds all while the main rig is drillingthrough the BOP.

Subsea Tree. Normally the subsea tree is run after securingthe well and pulling the BOP stack. The stack or high pressureriser is then rerun and landed on the tree. Deepwater trippingof the BOP and riser is time-consuming and comprises asignificant percent of the well completion costs. Often most ofthe tree preparation work commences only after rigging downfrom pulling the BOP and the moonpool area under the rotarytable becomes free. To pull the stack, rig up and run the tree,and then rerun and test the BOP takes a minimum of 10 daysin deepwater. This does not account for the risk of failure ofthe BOP or high pressure lines to test once the riser system isre-deployed.

The dual-activity rig runs the tree on drillpipe while theBOP stack is on the wellhead. Care and planning arenecessary to execute this maneuver without problems causedby interference with the drilling riser. First, it is required thatthe tree, tree running tool or other bottom string equipment beprovided with fenders or rails to keep that part of the treesusceptible to hang up off of the marine riser. The fendersthemselves need to be smooth and snag free. Second, thedrillship has to be oriented so that the second string is pushedaway from the marine riser. This is accomplished by firstacquiring a current profile and then calculating the trajectoryof the second string as it runs to the bottom. If thedisplacement for all positions and all times is less than 40 ft,the rotary table separation, then the heading for best vesselmotion can be maintained. If the displacement is more thanthe rotary table separation and would lead to interference withthe marine riser, then it is necessary to orient the relativeposition between the running string and the riser by changingheading so that the displacement, whatever it is, runs awayfrom the riser. In the rare event where the displacement ismore than 40 ft and cuts through an arc greater than 180(when viewed from above), then it is necessary to also offsetthe vessel, increase the riser angle at the BOP, and increase theseparation between the two strings. It is also possible that aseries of heading changes are necessary as the trajectory shiftsdue to increasing hanging weight and alterations in current.Finally the position of the bottom of the string is continuouslymonitored while lowering. A bottom string transponder givesthe absolute position of the tree and is used to check against

that calculated from the trajectory analysis. An ROV followsthe tree to give both a visual and acoustic picture of thesituation.

Other Subsea Equipment. Besides the tree there is othersubsea equipment that can be deployed with the offline rotarytable and help reduce the field development costs. Figure 7illustrates a typical deepwater subsea setup.

Pipeline Manifold Base. Often a base is deployed as ananchor for pipeline termination or as a tie-in point for the treejumper and the main production flowline or simply to functionas a subsea manifold. As such this base requires a foundationthat can support the weight of the base and piping and takeside loading from pipeline pull-ins or from pipelay initiation.The offline rotary table can be employed to install both thefoundation pile and the manifold base while conductingnormal operations with the main rig. This is best done duringthe early part of drilling a well, when there is some short slackin the auxiliary rotary table use and when still rigged up fordrilling in top hole casing. Deploying the pile can betemporarily interrupted at any time except after its drilling hascommenced. There is a means to hang the pipe string on thespider beams under the auxiliary rig and skid it out of the way.When the auxiliary rotary table again becomes available thestring can be skidded back into position, and the operation canbe restarted. Drilling piles is a function for which the rig isspecifically specialized, and it is especially economical whenthis can be done offline to the main operation. The costs forthis are reduced to that of the hardware with the rig spreadcosts going against the well.

The moonpool of the Discoverer Enterprise is 60 ft x 30 ftand there is a clear height of 50 ft under the substructure. Thisallows manifolds of up to 25 ft x 35 ft to be easily run from theaft station. Again planning is required to minimize the risk ofinterference with the deployed

riser.Tree Jumper. Once the manifold base is set and the tree is

landed, it is possible to run the flowline jumper from the treeto the base with the auxiliary rig. Measurements are made bythe shipboard ROV and the jumper fabricated onboard.Jumpers less than 60 ft in length can be assembled on the piperack and run directly through the moonpool. Jumpers over 60ft but less than 125 ft in length can be assembled in one pieceand passed over the ship side for keelhauling to the runningstring of the auxiliary rotary table. Jumpers longer than 125 ftmust be assembled vertically in the moonpool under theauxiliary rotary table and then rotated 90 when clear of thehull. The jumper can be tested once it is installed.

Pipeline Jumpers. In addition to the tree jumpers thepipeline jumpers shown in Figure 7 can be run if the pipelinelanding skids are in place or if there is another pipelinemanifold base nearby. Even after the rig has moved offlocation the pipeline jumpers can continue to be fabricated onthe drillship and passed to a separate installation vessel. Thissaves on handling costs and, once set up, provides a veryeconomical means for jumper fabrication. Most importantly,


this gives the flexibility to cope with the inevitable changesthat come up in any short field construction period.

Flowline/Pipeline Installation with Dual-Activity RigThree methods of pipelaying are considered viable for fielddevelopment using the drillship: discrete lay, reverse lay anddedicated lay.

Discrete Lay. This method utilizes the second rotary table onthe drillship to construct flowline segments while the primaryrotary table is performing the drilling and completionsoperations. Figure 8 illustrates the concept. Pipeline segmentsless than the water depth are made up in the auxiliary rotarytable. First-end and second-end skids are made up at theextremities of the segment. The completed flowline segmentsare transferred to an anchor handling vessel (AHV) whichtransports and installs the segments. Subsea jumpers connectthe segments to form the completed flowline.

Disadvantages of this method are the additional cost of thejumpers and the limitation of the segment length to the waterdepth. Obviously this is not a useful method in shallow water.The hanging loads are quite large and, for lengths greater than6,000 ft, the weights approach the limit for standard AHVs.

The advantages are great. Most of the spread costs ischarged to the drilling operation and this results in aneconomic means to install a flowline, and the flowline isinstalled when needed rather than at the convenience of the laybarge. In the case where the wells are separated by distancesless than the water depth then this method becomes veryeconomical when the wells are daisy-chained together byflowlines. The lines can be laid while drilling and no extrajumper cost is suffered.

Reverse Lay. This is another method where pipe is laid fromthe auxiliary rotary table while drilling. The reverse laymethod uses the AHV to pull pipe away from the rig whilebeing assembled. Pipe lengths up to three times the waterdepth can be assembled, thereby eliminating one or twopipeline jumpers. To lay the tree end it is necessary to offsetthe rig in the direction of the lay and take a larger horizontalpull.

It is possible to run an even longer segment by combiningthe reverse lay with bottom tow. In this case the AHV pullsthe pipeline along the seabed while being made up by thesecond rotary table. Figures 9 and 10 illustrate the reverse layconcept.

Dedicated Lay. The dedicated lay technique uses the dual-activity drillship to install a flowline or pipeline in acontinuous manner, as does a pipeline lay barge. Both theprimary and secondary rotary tables can be usedsimultaneously to construct and install dual lines. Thededicated lay method assumes the rig is used strictly forpipelaying operations, with no drilling or completionsoperations being performed. Dedicated lay can be applied

simultaneously for two lines, or with one rotary table indedicated lay mode and the other rotary table in a discrete laymode. The most efficient use of the drillship is in a dedicateddual lay mode. The rotary table separation of 40 ft givessufficient clearance to run two lines simultaneously. Thehoisting capacity for both rigs are identical (2 million lb), andthe structure is designed to take maximum loadssimultaneously. The vessel would normally lay the pipelinetransverse to the hull centerline (sideways). In the event ofbad weather the rig can change heading 45 without dangerof the two pipes interfering. Possibly up to 60 can berealized without problems. The Discoverer Enterprise is muchlarger than a typical deepwater J-lay barge and has superiorstationkeeping capability. As such the downtime for weathershould be significantly less.

During times when simultaneous lay is not possible, say withseparated risers or single flowlines, it is possible to engage thesecond rotary table in the discrete mode or in helping thesingle lay rotary table by making up 160 ft-180 ft stands.Risers can be run discretely or in the normal forward lay. Thetwo 1 million lb motion compensators can be of benefit inemergency recovery operations. Use of the drilling riser as arunning string allows a 2 million lb lift load.

Dual lay of risers to a floating production facility is unlikelyowing to separation requirements of riser strings near theseabed. Nonetheless the second rotary table can be put togood use while laying a single riser with the other rotary table.For example, both rigs may be used for dual lay up to the timethat the single riser lay operation begins. From that pointonward the second rotary table pays out the second riser with adummy line (drill pipe). Upon connecting the first riser thesecond is hauled in and its lay recommenced.

During the period when the first rotary table is free, it can beusefully employed making up pipe segments for the discretelay. Pipe segments are passed to an adequately equipped AHVwhich in turn steams to the line under construction and laysthat section. Alternatively the idle rotary table can be used tomake up 180 ft stands (from two 90 ft sections) and pass thelonger stand vertically to the second rotary table using one ofthe rig floor pipehandlers. Savings would result from fasterrunning (180 ft joints vs. a standard quad joint length of 165ft) and from lower onshore fabrication costs (90 ft joints vs.165 ft joints).

Burial. The high pressure mud pumps used to power seabedequipment can also be utilized to bury flowlines, thusimproving their ability to maintain transported fluid heat whereflow assurance is a concern. A drill string can be run to thebottom by each rotary table and the pump hydraulics can beused to jet the pipe. The jets at the bottom of the string areoriented to keep the drill pipe pressed against the flowline.The drillship can travel along the flowline at up to 4-5 ktsduring this operation. An ROV maintains position just aheadof the forward string and above the unburied pipeline forinspection purposes. The drillship keeps station off the ROV


position.

ConclusionDual-activity drillships are unique in that they are capable ofperforming multiple functions in association with fielddevelopment. In the past, some of these functions haverequired mobilization and demobilization of one or moreadditional large construction vessels for completion.Implementation of the same vessel that performs well drillingand completion operations for other activities such as subseaequipment installation, central production facilitiesinstallation, and flowline/pipeline construction results inoverall project cost savings. Further, potential downtime andresultant cost occurring when one vessel waits for the other tocomplete their activities is negated. Finally, unexpected delaysoccurring from one field development operation do not resultin downtime and associated cost since the dual-activitydrillship can perform tasks as assigned.

The drillship Discoverer Enterprise has been featured in thispaper because of its first of a kind status. Future launchingsof additional dual-activity drillships will allow the practice ofsingle vessel usage for multiple field development tasks tobecome more prevalent. This capability is especially timely inlight of reduced oil prices from recent years, making vesselutilization and cost reduction prime factors in the ability todevelop new fields economically.

AcknowledgementsThe authors would like to acknowledge Transocean OffshoreInc. for their consideration and assistance in developing dual-activity drilling rig information, Mr. Andrew Macfarlane ofH.O. Mohr & Associates, Inc. for his assistance in developinggraphics associated with dual-activity drillship utilizationconcepts and Ms. Peggy Reyna of Aker Engineering, Inc. forcleanup and handling of this manuscript.

References1. Albaugh, E. Kurt: Offshore Pipeline Installation and Burial

Contractors and Vessels, Offshore (June 1998).


Table 1Comparison of Enterprise Capabilities with Typical Pipe J-Lay Barge

Characteristic J-Lay Barge EnterpriseDesign Load for J-Lay (kips) 775-2,000 (1) 2,000 x 2Capacity for Heave Compensation No Yes (2)Design Load for Pipe Abandonment/Recovery (kips) 775-1,210 (1) 2,000Ability to Allow Pipe-in-Pipe Installation Yes YesVessel Total Thruster Capacity (kips) Less than 800 (1) 1,200 (3)Number of Vertical Pipe Handlers 1 2Capacity of Quarters (# of persons) 200+ 200Maximum Number of Pipes that Can Be Laid at a Time 1 2

NOTES:

1. Maximum capacity currently anticipated after upgrade or construction.2. Heave-compensated design load is 1,000 kips.3. Vessel has continuous 52,000 horsepower capacity.

En t erpr ise Dua l Ac t ivi t y Floor Layou t

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Vessel Size Com pa r isonEn t erp r ise vs Typ ic a l La y Barge

Figure No 2

Dua l Ac t i vi t y Rig Pip e la y Adva n t a gesLa y Tw o Lines Sim u l t an eous ly

Fig u r e No 3

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Use of the Dual-Activity Drillship as a Field Development Tool

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