subsea completions

As early as 1996, Amoco engineershad determined they were going tocomplete a subsea well in the Gulfof Mexico from a dynamically positionedvessel. Last March, using the TransoceanSedco Forex dual activity drillshipDiscoverer Enterprise they did just that ontheir Nile well in the 3500ft waters of theViosca Knoll 914 area of the Gulf. A fewweeks later they repeated theperformance at their King well 15 milesaway in Mississippi Canyon Block 85 in aGOM record water depth of 5317ft.

While the Nile well was the first subseacompletion from a DP vessel in the Gulf,it was not a worlds first. Petrobras andothers have done several. But theaccomplishment is noteworthy not just asa first, but for having been done under

extreme operating constraints whichengineers at Amoco and then BP hadplaced upon themselves in the name ofsafety and environmental protection.Indeed, the parameters were so rigorousat the time they were set they weretechnologically unachievable.

The basic philosophy of the companyis that if you have a drift off or drive offof a DP ship you will maintain acompetent system all the way through thetree and the BOP stack. So you have toput the weak point above that, saysRodney Hensley, head of BPs wellsdelivery team, who has led the effort frominception. We took the worst case forwhatever reason nobody was able to pushany buttons or do anything and askedwhat is going to happen. And through all

the analyses we know that theintermediate flex joint is the weak point.

BP designed a spool horizontal tree towithstand all the loadings down to theconductor casing and then had Cameronconstruct a deepwater, high-strength,high-capacity wellhead profile andconnector that could withstand thebending moments associated with theirworst case scenario.

The Dril-Quip subsea wellhead wasalso redesigned to meet the design loadsand the size and capacity of the low-pressure housing and the high-pressurehousing increased to match, addsHensley. It had to be a system all the waydown. They were all new designs thathave become a standard at BP and are allDP compatible. 2

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Gulf team delivers on DP drillship promise

Nile recently found its way intothe industry annals as theUS Gulf of Mexico's firstdeepwater completion from adynamically positioned vessel.

Before the event disappearsunder today's virtual tidalwave of deepwater firsts, Rick von Flatern examinesits particular significance forthe industry and looks backover the five years oftechnology-stretching thatwent into its making.

SenTree 7 equipment on the deck of the DPdrillship Discoverer Enterprise.

to the next step where you have to do it allin 15 seconds required a whole newcontrol system not previously available.

At about the time BP was seeking todrastically reduce subsea disconnecttime, Schlumberger had begun work onwhat was to become its SenTREE subseacompletion and test tree. Having beeninvolved in subsea hydraulics since the1970s, the SenTREE series became viablewithin a relatively short time afterinception. But decreasing the timebetween when a command is initiated onthe drilling vessel and the operation ofthe specific function at the tree severalthousand feet away on the seafloor to 15seconds required what Frug terms arevolutionary step.

That 15 seconds is not just todisconnect the subsea test tree, saysSchlumberger senior engineer DavidHarrison, emphasizing the complexity ofthe task. That is to shut in the well, closeoff the landing string, bleed off pressure

between two valves and unlatch. The 15-second window was determined as theamount of time the test tree had toaccomplish its part of the disconnectingoperation in order for the rig to thendisconnect the BOP stack and still be freeof the well in 35-40 seconds.

Since the lag time in hydraulic systemsis due to the time it takes pressure appliedto one end of a long fluid-filled hose toaffect a valve or other component on theother end, the solution was to transmitand monitor commands via electronicsignals. The final multiplex version of thecontrol system for the SenTREE 7 wasqualified and accepted by BP in May 2000.

The heart of the solution, dubbed theCommander Telemetry control andmonitoring system, is a multiplex (MUX)system that permits signals to traveldownward from the drilling unit to thewell and information to travel back fromthe well to the surface.

The electronics control all thehydraulic functions including the tubinghanger running tool, opening and closingall the valves and the pressure andtemperature monitors, Frug explains.Since BP is utilizing horizontal trees wecan run the completion with the treealready set and have full monitoringcapability of all the functions.

Besides assuring quick disconnectactions at the wellhead, the MUX systemoffers some collateral advantages as well.With all-hydraulic systems, for instance,each function must have its ownhydraulic line, a requirement thatquickly leads to large, unwieldyumbilicals. Electro-hydraulic systems onthe other hand require only a singleelectrical line and one hydraulic line toperform up to 22 functions.

But most importantly, when signaled todo so, the new subsea test tree completesits part of the disconnect function in lessthan 15 seconds and that, according toFrug, has significant implications notonly for BPs ambitions, but fordeepwater development in general.

With the success of the SenTREE 7 andCommander telemetry system it isproven that subsea completions can bedone safely from a DP vessel, heconcludes. It allows us to go into deeperwater because we are getting to the depthlimitations of moored vessels. In waterdepths of much over 5500ft mooringbecomes very difficult and expensive.

Perf-and-surgeThe Nile well was drilled and cased andthen temporarily abandoned for twoyears with completion fluid left acrossthe formation and a bridge plug justbelow the mud line. As a precautionagainst possible drilling or completionfluid damage, completion engineers choseto perforate the well slightlyunderbalanced and to then clean theformation by surging the wellimmediately after it was perforated. 3

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subsea completions

But if someone were to push a button,that is if the drive off or float off wereunder control while the completion wasbeing flowed, BP mandated a disconnectsystem that would secure the well in lessthan one minute. In 1997, as theDiscoverer Enterprise was beginning totake shape, no such system existed.

BP engineers did the technical analysesand set the technical loads that the newtree systems would have to meet. Theyalso, according to Hensley, set such basictree functions as how fast it must operateand number and size of ball valves.

Everything then was controlledhydraulically, recalls Schlumbergersenior sales engineer Michael Frug, whohas been dedicated to the project since1999 and offices at BPs campus-likecomplex on the western edge of Houston.Depending on the water depth you havetwo to three minutes actuation time(using hydraulics control systems). Thatis fine with a moored vessel but taking it

We built this ship for thisproject says BP wells deliveryteam leader Rodney Hensley ofDiscoverer Enterprise, picturedhere preparing for sea trials twoyears ago (OE August 1999).

subsea completions

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Again, the task was made morecomplex by BPs suspender-and-beltsafety requirements. What BP asked usto do was come up with a perforating-surge technique so that the entire timethey were perforating and surging thewell we could do it with the downhole ballvalve closed, says Frug. That way, ifthey did have a drive off during thatoperation and were in a hurry to get offlocation and had to close [rams] andshear, a valve would be closed just abovethe formation.

Also, there was some concern thewellbore was out of gage from possiblewash out during drilling. That, combinedwith BPs expectations of high flow rates,required Schlumberger to develop a newcharge that could both penetrate deepenough to get past a thick cement sheathand at the same time create a large-diameter hole through the casing.

The test work string contained anumber of IRIS Dual Valves which arecoded to operate on a unique pressureimpulse, meaning one valve can beopened or closed without interfering withany other valve in the string. As theperforating guns were run into the hole,atmospheric pressure was trapped

between the lower valve just above theguns while kill weight fluid was trappedabove the second valve placed in the workstring about 3000ft up hole. Prior toperforating both valves were closed andonly a slight underbalance existed acrossthe formation.

The way we established anunderbalance was to displace the chokeand kill line on the rig with lighterseawater to reduce the hydrostaticpressure on the formation by about 200lbs, says Frug. So once we have thatunderbalanced condition, we set thepacker and trap the lower hydrostaticbelow it with kill weight completion fluidabove should worse come to worse.

Once it was determined the guns hadfired, a lower valve was opened. Due to apressure differential equivalent to about7600psi, the formation surged into theatmospheric chamber, filling it with mud,sand and other debris. It was a mixtureBP was reluctant to flow to the surface atany time while there existed a potentialfor loss of station.

[BP] said the only way to flow to theship was to have the formation isolated,explains Frug. So we then sent acommand to close the lower IRIS valve

and the formation is isolated. Anothercommand opens the upper valve andanother opens a circulating valve and wethen can circulate out all this stuff whileevery step of the way having full wellcontrol.

Included in the work string was anelectronic cable latched into the top valvethat permitted real time pressuremonitoring at the surface. Because thegages are reading in different placesthroughout the workstring we couldwitness all the pressure changes in realtime, explains Frug. This is importantbecause we wanted to be 100% sureeverything was under control before westarted to bring hydrocarbon and debristo the surface. If we had not been able tooffer this assurance to BP they wouldhave had to bullhead all that debris backinto the formation which would havegreatly reduced completion efficiencyeven if you ever did clean it up. Besidesincreasing production efficiency, Frugobserves, this one-trip technique savedBP more than $500,000 in rig time costs.

The well was fractured after a gravelpack assembly was put in the hole inorder to improve conductivity. But havinggone that far to assure a clean and

The Schlumberger teamprepares Commander modulefor Nile completion.

efficient formation, the last thing anyonewanted was fluid loss into the formation.To prevent that a formation isolationvalve was run with the gravel packassembly.

The formation isolation valve is justabove the gravel pack assembly, Frugsays. When the wash pipe is pulled out alatching mechanism closes the valve andyou then have a fully stimulatedformation that is isolated as you pull outof the hole so there is no possibility offluid loss.

The isolation valve also served anotherpurpose when it was decided to re-stressthe fracture. Since it opens and closesmechanically with a shifting tool, BP wasable to re-enter the well without having tofirst kill the well, risk formation damageand greatly extend clean up time. Then,once the completion was landed and thewell ready for production, the isolationvalve was opened by pressure cycles froman underbalanced condition.

I think we are getting good skinnumbers on these completions becausewe are very careful about getting cleancompletion fluids through filtering andthen surging the wells underbalancedduring perforating, says BPs Hensley.The Nile frac-pack went very well and wehad a net pressure increase of 750psi.

Nile is currently producing at 70million cubic feet of gas and about 9000barrels of condensate per day and as suchis in this years BP top ten list of mostefficient producers worldwide

Built to completeWhen plans were first announced for thebuilding of huge DP drillships like theDiscoverer Enterprise, some earlypromoters claimed they would reduce rigtime by as much as 40%. Whether thatlevel of efficiency will become standardremains to be seen and some observersremain skeptical. But the success of theNile completion job is proof to some thatthese claims were not overblown.

Our beyond the best target for thiskind of completion job four years downthe road was 38 days from move in tomove out, recalls Hensley, referring toBPs yardstick of operations efficiency.Our first well out of the box (the Nile) wedid in 44 days, which is very closeconsidering all the new technologies weemployed.

The second well, King, was completedin just 28 days, ten days under a target BPthought would take another four years toachieve. So confident is the Enterpriseteam in the ship and their ability to buildon what it has already learned that theyhave reset the Enterprises beyond thebest completion time target to 21 days.Much of the credit for these efficiencies,Hensley believes, is owing to the fact thatfive years ago engineers were designingand building the vessel with just thiskind of work in mind.

We built this ship for this project, says

Hensley. We have been with it since itsinception in 1995. We, with (rig manager)Larry McMahan and his team atTransocean Sedco Forex built this ship tobe doing what it is doing right now.

Among the features made possible bythe Enterprises immense deck space is afacility designed to allow wells to cleanup at production rates so that nodamaging fluids need be left on theformation once the completion has beeninstalled. That is particularly importantin subsea development wells that mayhave to wait months for a flow line to behooked up to a host facility.

But the real time-saver according toHensley and McMahon is the rigs dualactivity capability. With two complete rigfloors and derricks, operations that oncehad to be run successively can now berun concurrently. In that way, at leastsome major operations are beingcompleted in as much as half the time itwould take on a standard unit. And astotal expense to run a deepwater floater isabout $350,000/d, a few days saved quicklybuilds into millions of dollars over thefull span of a drilling and completionoperation.

We ran a full SIT (systems integrationtest) on the (Nile) subsea tree in openwater while we were drilling the CrazyHorse well, points out Hensley as oneexample of how his crews leveraged thevalue of the Enterprises dual activitycapability. You cant get a hyperbaricchamber large enough to test a tree thatsize so we ran the tree to 6500ft when theaft rotary became available.

In the deepwater we can eliminate a lotof the flat spots on the drilling curve,says McMahon, referring to times whenactual drilling must be halted while thecrew uses the rotary for other time-consuming activities. For instance wecan be setting the conductor pipe at thesame time we are running the BOPs tothe seafloor. Then when the conductor isin place we just move over and lower theBOP onto it.

Also, because they can run the spooltree with the drill pipe off the aft rotary,BP has discovered it need not invest theseveral weeks it takes to pull the drillingriser. On recent wells the company hasbeen conducting post-drilling operationson the drilling riser from one rotarywhile landing, testing and locking downthe subsea production test tree on theother. The company has even taken tomobilizing between jobs without pullingthe riser. After completing the Nile wellthe Enterprise moved 15 miles to the Kingwell and then six miles to the completionjob currently under way with the drillingriser fully deployed.

Personnel and merger affectsAt the time of the Enterprises keel-laying, Transocean and Sedco Forex wereseparate entities. And the five-yearcontract under which BP and the

Enterprise are currently working wassigned with Amoco. But other than adelay while the BP-Amoco merger wasbeing implemented, the project movedahead essentially in the manner in whichit was first designed and may hold lessonsfor an industry constantly at the mercy ofmergers and acquisitions.

My team has stayed intact, saysHensley who came from the Amoco sideof the merger. The team has grown but Ihave had people working in my teamsince 1996. The project had been set to goforward at the time of the merger so wedecided to slow that down and re-look atit until the companies had the time tocome together.

The setback lasted a year while ex-Amoco engineers acquainted themselveswith all the issues involved in a BPcompletion, he says. Technically therewere no differences between the [Amocoand BP completion] teams.

The new BP team then went throughtwo extensive hazardous operationsexercises to break the whole sequence ofplanned events into their smallestcomponents in order to determine allpossible what if scenarios. Theseexercises were facilitated by DNV,certifying authority for the Enterprise.

For its part, Schlumberger says the factthat it was able to bring several newtechnologies and processes to the tableessentially without a hitch is also due inlarge measure to personnel. There are agroup of people built into thisCommander system contract that haveworked this system from day one, saysHarrison. Some people from the clean upfacility to the perforating to theCommand system were there all the wayand had a lot to do with how smoothlyeverything went.

Onward and outwardWith two subsea wells now completedfrom an untethered floater and operatingprocedures, equipment and standardsbeing honed and polished, BP is movingto deeper water. The fourth completion onthe list at Kings Peak will be in about6300ft of water. World water depthrecords will likely be set there as someKings Peak wells will be located in watersup to 6975ft.

The second and third wells are the firstto have subsea production control panelsfitted with fiber optics while the fourthwell will also be a smart well withhydraulically operated sleeves to openand close zones.

Others in the industry have beencarefully watching the Kings Peak actionand have expressed interest in theSenTree 7 test trees and the Commandersystems that enable quick disconnectingDP subsea completions. BP now has twounder contract but the next ones havebeen purchased by TotalFinaElf andMarathon, reportedly for completions inwaters beyond 7000ft. OE

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subsea completions