OTC 7525
`
`Application of Horizontal Trees to Deep Water
`B.R. McConanghy and J.A. Gariepy, ABB Vetco Gray Inc.
`
`Copyright 1994, Offshore Technology Conference
`This paper was presented at the 26th Annual OTC in Houston, Texas, U.S.A., 2—5 May 1994.
`
`This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper,
`as presented have not been reviewed by the Olfshore Technology Conlerence and are subject to correction by the authorts). The material, as presented. does not necessarily reflect
`any position oi the Offshore Technology Conference or its officers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be cooled. The abstract
`should contain conspicuous acknowledgment of where and by whom the paper is presented
`
`INTRODUCTION
`
`As the search for new hydrocarbon reserves continues, it is
`inevitable that operations in deeper water will
`increase.
`In certain areas, such as Brazil and the Gulf of Mexico, deep
`water exploration and development is well underway. The
`speed at which deep water developments proceed worldwide
`will be dependent on their economic viability. Due to the
`requirement to use specialized equipment, operations in
`deep water are inherently more costly than conventional water
`depths. As is the case in more conventional water depths,
`standardization of equipment,
`interfaces and installation
`methods are seen as the keys to reduction of development
`costs.
`
`In Brazil, where a large percentage of the deep water
`developments have taken place,
`the main areas of
`standardization have been utilization of a tubing spool,
`flowline connection methods and ROV interfaces.
`In the
`
`Gulf of Mexico, the Deepstar group has been formulated
`primarily to look at areas where standardization can take
`place. To date, virtually all deep water developments have
`utilized what could be termed as conventional xmas tree
`
`technology. By this it is meant that the xmas tree utilizes gate
`valves in the vertical bore (master and swab) and wing valves
`in horizontally oriented outlets.
`
`The purpose of this paper is to evaluate the usage of Horizontal
`Tree Systems in deep water applications. Throughout this
`document, comparisons will be made to conventional systems.
`Issues which will be compared are equipment cost, installation
`times, workover times and risks associated with the Horizontal
`
`Tree System. There are numerous variations of conventional
`systems. For simplicity,
`it is assumed that a conventional
`system consists of a dual bore (4" x 2") system both with
`(Figure 1), and without (Figure 2) a tubing spool between the
`wellhead and the xmas tree. There is no intention within this
`document to cover areas where horizontal and conventional
`
`In particular,
`systems can be handled in a similar manner.
`flowline connection methods, ROV interfaces and gate
`valve design can be virtually the same regardless of the type
`of tree. For the equipment cost comparisons, water depth is
`assumed to be 3,000 ft. The drilling/completion vessel is
`assumed to be a dynamically positioned drillship which
`utilizes a guidelineless 16%” BOP system.
`
`HORIZONTAL TREE
`
`A horizontal tree can be most easily described as a tubing
`spool with outlets to which gate valves are mounted (Figure 3).
`The tubing hanger lands in the spool body and directs
`hydrocarbon flow out through the side outlet. The tubing
`hanger is equipped with a wireline plug profile above the
`side outlet. Once in place, this plug serves the same purpose
`as the swab valve in a conventional tree. There are two major
`advantages fora horizontal tree as compared to conventional.
`Firstly, since the tubing hanger is run after the tree is in place,
`a workover requiring the tubing to be pulled can be achieved
`without recovering the tree. Secondly, the tree and tubing
`hanger do not require a purpose built completion riser
`system. The tree can be run on drillpipe or connected to the
`BOP stack on marine riser. Though horizontal trees have not
`actually been used until recently (first subsea completion in
`1993), the idea for such a system has been around for over
`
`OSS Exhibit 2013, pg. 1
`FMC vs. OSS
`IPR2016-00495
`
`

`
`2
`
`APPLICATION OF HORIZONTAL TREES TO DEEP WATER
`
`OTC 7525
`
`idea came about for an electric
`five years. The original
`submersiblepump application. Thelarge powercable required
`would not fit through a conventional system. Additionally, the
`requirement for frequent (yearly) pump replacement makes
`the horizontal system particularly suitable, as the tubing can
`be recovered without pulling the tree.
`
`EQUIPMENT COST
`
`This section of the paper addresses equipment cost only.
`Equipment cost comparisons between systems can vary
`greatly depending on the number of wells to be completed.
`Due to the fact that less equipment is required to install a
`horizontal tree,
`it has a significant advantage, particularly
`whenalownumberofwellsisto be completed. As the number
`of wells in a development
`increases,
`it may make more
`sense to invest in a completion riser system. The following
`section addresses the required equipment for each system.
`As previously stated, comparisons are made to conventional
`systems both with and withoutatubing spool. Though atubing
`spool completion has an obvious equipment cost dis-
`advantage, approximately 90°/o of deep water guidelineless
`completions to date have utilized a tubing spool. The main
`reason for utilization of a tubing spool is to simplify orientation.
`The tubing spool
`is equipped with an internal helix which
`interfaces with a pin on the tubing hanger.
`It has been found
`that as water depths increase, orientation which utilizes the
`extendable BOP pin method, becomes more troublesome
`and, on average, significantly increases installation times.
`See Figure 4 for the results of the cost comparison.
`
`Wellhead
`
`It is
`The wellhead system can be the same for either system.
`assumed that any drilling required inside the 9-5/8" casing will
`be done with the horizontal tree installed.
`it should be noted
`that the horizontal system lends itself to the possibility of
`drilling for 9-5/8" casing with the tree in place which could save
`additional rig time. This technology does not currently exist
`but could be developed. The drawback is the possibility that
`drilling returns can damage gate valve internals if the pressure
`containing bore protector were to fail. The extent of drilling
`through the tree is up to Environmental/Governmental
`Authorities forthe area in question and operator philosophies.
`
`Tubing Spool
`
`In a
`A tubing spool is not required for a horizontal tree.
`conventional completion the tubing spool offers several
`advantages. Firstly, it provides a known profile for landing and
`locking ofthe tubing hanger negating any need for impression
`tools. Secondly, it can be equipped with an orientation helix
`which automatically orientates the tubing hanger (passively)
`as it lands. Lastly, flowlines and umbilicals can be made upto
`the tubing spool as opposed to the tree. A remote connection
`
`between the tree and tubing spool allows the tree to be
`recovered without breaking of the primary flowline/umbilical
`connection.
`
`Xmas Tree
`
`tree to a
`When comparing schematics of a horizontal
`conventional tree (Figures 1 , 2 and 3) it can be seen that less
`valves are required in the horizontal tree. Typically one less
`of each size. Obviously this is dependant on configurations
`required. In the UK sector ofthe North Sea where dual master
`valves are required, the difference increases to two on the
`production bore. The tree body is significantly simpler to
`machine, assuming there is no requirement for the master
`valves to be integral to the tree body. The large bore and
`cylindrical shape of the main body makes for simple cladding
`(inlay of stainless steel) and simplified machining, as most of
`the work can be done on a lathe as opposed to a mill.
`Assuming other aspects ofthe tree are the same, a horizontal
`tree can be from 15-20% less expensive than its conventional
`equivalent.
`
`Tubing Hanger System
`
`A horizontal tubing hanger is very similar to a conventional
`hanger except that it has a production bore side outlet and
`additional seals. The primary sealing system is more critical
`as the seals are directly exposed to produced fluids. The
`tubing hanger does not require vertical annulus access as the
`annulus is accessed through a side outlet in the tree spool
`below the hanger. The cost differential between the tubing
`hangers themselves is small with the advantage going toward
`the conventional. The big difference is in the installation
`systems. Assuming both systems utilize a multi-function
`hydraulic running tool (most operators preference in deep
`water), the running tools themselves can also be very similar.
`A Horizontal System requires a means of shutting in the well
`and emergency quick disconnecting as the well is vertically
`flow tested through the tubing hanger installation system
`(Figure 14).
`In current horizontal systems, subsea test trees
`have been utilized in much the same way as a drill-stem test.
`A subsea test tree is tubing deployed and consists of ball
`valves which can shear wireline and coil tubing and effect a
`seal and a quick disconnect facility. Utilization of currently
`available subsea test
`trees necessitates the use of a
`
`mechanical tubing hanger running tool as there is only one
`available hydraulic control line through the disconnectsection.
`Utilization ofa multi-function hydraulic running tool willtherefore
`require a subsea test tree with multiple hydraulic lines (and
`possiblyelectricalfordownho|egauge)throughthedisconnect
`section. Presently, this type oftesttree does notexist, though
`development is in progress. Existing subsea test trees can
`be rented on a day rate basis. For the cost comparison, it is
`assumed that a test tree with multiple hydraulic functions will
`have to be purchased. The cost for this tool is included in
`
`OSS Exhibit 2013, pg. 2
`FMC vs. OSS
`IPR2016-00495
`
`

`
`OTC 7525
`
`BRETT R. MCCONAUGHY AND JAMES A. GARIEPY P.E.
`
`Tubing Hanger Installation Tools. Thetubing hanger installation
`system for a conventional completion without a tubing spool
`requires an orientation spool. This spool sits above thetubing
`hanger running tool and interfaces with a hydraulically operated
`pin which penetrates the bore of the BOP through a choke or
`kill outlet. The hydraulically operated pin must be mounted to
`the BOP and the system must be preset prior to running the
`tubing hanger. Additionally, this system requires the BOP to
`be oriented. The most common way to achieve BOP orientation
`is a pin on the BOP connector which interfaces with a slot in
`the guide funnel. This system can be difficult operationally in
`deep water due to the weight of the BOP. The modifications
`and set up of the orientation system can be done while the rig
`is in transit. The BOP modification costs, therefore, include
`the cost of the hydraulically operated pin and an estimation of
`modifications required for orientation.
`
`Tree Cap
`
`The horizontal tree cap is very similar to the tubing hanger and
`it is in fact run with the same tool. The tree cap can be
`equipped with a through bore and a profile for a wireline plug
`if wireline workovers from a small vessel using a subsea
`Iubricator or subsea wireline unit is a future requirement.
`in
`either case, the tree cap is significantly less expensive than an
`external connector which would typically be used on a
`conventional system.
`
`Tree Running System
`
`In a conventional system the tree is run on a lower riser
`package and an emergency disconnect. The lower riser
`package is equipped with valves (gate or ram type) which are
`capable of shearing wireline and coil tubing and effecting
`a seal. The emergency disconnect is designed to quickly
`disconnect from the lower riser package allowing the rig to
`move off location without damaging the subsea equipment.
`
`It is proposed to run the horizontal tree on the BOP and marine
`riser. This may require rig modifications and will require that
`the xmas tree is equipped with hang off beams. The rig time
`which can be saved is significant. Rig modifications required
`will vary from rig to rig but in most cases would be minor.
`hydraulic control
`interfaces with the tree being the most
`critical.
`
`Completion Riser
`
`The fact that a horizontal system does not require specific
`completion riser is one of the major advantages of the system
`over conventional. The cost estimate for the completion riser
`in Figure 4 includes all required equipment such as a stress
`joint, tension joint, surface tree adaptor, etc. As the number
`of wells decreases the cost of the completeion riser becomes
`an even larger issue.
`
`Auxiliary Tools
`
`The Horizontal System again has the advantage here. The
`majority of auxiliary tools required for the conventional system
`are related to the completion riser which is the prime driver
`for the cost difference.
`
`Workover Control System
`
`The workover control system assumed herein for the
`conventional system is piloted hydraulic for all emergency
`quick disconnect functions. Additionally, it includes electrical
`linesfordownholegaugesthroughthe umbilical. The Horizontal
`System can be a simpler system as there are less functions
`required and no electrical
`line required to the tree.
`It is assumed that the tree will need its own umbilical and that
`
`it must be routed through the BOP. This requires that
`additional junction plates be mounted between LMRP and
`the BOP and at the bottom of the BOP to mate with the tree.
`The cost required to make this modification does not take
`into account rig time as it is a modification which could take
`place while the rig is in transit. An option that has not been
`addressed herein is to have no controls to the tree during
`installation or workover. This can be argued as acceptable
`as there are no functions within the tree that are required in an
`emergency shutdown during workover or installation. All tree
`functions would then be operated by an ROV as required.
`In
`both conventional and horizontal systems, a tubing hanger
`installation umbilical will be required complete with electrical
`line. The Horizontal System requires piloted hydraulics in the
`subsea test tree for valve closure and disconnect functions.
`
`This system is proposed to be built into the subsea test tree
`and is, therefore, included in the cost of the tubing hanger
`installation tools.
`
`Eguipment Cost Conclusion
`
`As can be seen on Figure 4, the equipment cost savings
`related to the horizontal tree is significant. A point of note is
`that any development costs relative to the Horizontal System
`are not considered in this comparison. The main area where
`development
`is required is the Subsea Test Tree and
`Emergency Disconnect System. Many assumptions had to
`be made so as not to be an overcomplicated comparison.
`The assumptions are based on what the writers feel would
`be the likely choices that most operators would make and is
`not necessarily the correct choice for all situations.
`
`INSTALLATION
`
`It is widely believed that horizontal trees offer advantages in
`reduced installation time. The purpose of this section is to
`realistically estimate installation times for all three systems
`and compare accordingly. Again, assumptions have been
`made (as per the Equipment Cost Section) so as to have a
`
`OSS Exhibit 2013, pg. 3
`FMC vs. OSS
`IPR2016-00495
`
`

`
`4
`
`APPLICATION OF HORIZONTAL TREES TO DEEP WATER
`
`OTC 7525
`
`direct comparison. The comparison begins after the 9" casing
`has been run, cemented and tested. Time estimates are
`based on information supplied primarily by drilling contractors
`with additional input from various operators with experience
`in deep water guidelineless techniques. The system utilized
`for time comparisons is a database such that inputs can be
`easily altered by operation (Figure 5). The “Fixed” column
`represents thetime required to rig up or rig down in preparation
`to perform that operation. “Joints per hour’ is an estimate as
`to how many of thattype ofjointcan be run in one hour. Length
`per joint is self explanatory.
`In the case of wireline, there is
`again a “fixed” or rig up/rig down time and a feet per hour rate.
`“Variable” isacalculated value which when multiplied by water
`depth and added to the fixed hours, yields the duration forthat
`operation in hours. This is a very useful program as variables
`can be easily changed yielding a different set of results in
`graphical form very quickly. There has been no attempt to
`estimate times which are the same in any case, such as flow
`testing the well or preparing the well for running the completion.
`Installation times have been considered for “Continuous" and
`“Complete Later‘ scenarios. “Continuous” meansthat the well
`is completed directly after drilling whereas "Compete Later’
`means that the well was suspended and completed at a later
`date. Following is a summary of the installation sequence for
`horizontal and conventional systems individually. See Figures
`6 through 12 for actual time estimates and graphical results.
`
`Horizontal System (Figure 6)
`
`Once the 9" casing is cemented and tested, the BOP stack is
`pulled and moved out of the moonpool so that the tree can
`be prepared to run. The tree is moved into the moonpool and
`hung off on beams such that it
`is below moonpool level.
`The BOP is then moved in and locked to the top of the tree
`and likewise the LMRP. The system is then run utilizing
`marine riser. The systemislanded and lockedtothe wellhead.
`Once the system is pressure tested, the cement inside the
`9-5/
`" casing is drilled out and the well is prepared for the
`completion string. The wear bushing which was run with the
`xmas tree is retrieved in preparation for running the completion
`string. The completion string is run which includes the
`downhole completion, tubing hanger, tubing hanger running
`tool, subsea test tree (complete with emergency disconnect),
`tubing, umbilical and surface tree.
`it should be noted that this
`system will be slightly quicker to run than the conventional
`system as the string above the tubing hanger is tubing as
`opposed to completion riser. Factors which will affect running
`times are the frequency at which connections are tested and
`whether the running string can be racked in stands or must be
`picked up as singles. This comparison assumes that all riser
`joints are run as singles (no racking).
`it should be noted that
`in deep water not all the completion riser can be racked as it
`takes more space than tubing. Once the tubing hanger is
`landed, locked and tested, the wireline sleeve which isolates
`the tubing hanger side outlet must be pulled so as to allow
`
`full bore access. The well can then be flow tested. Once flow
`testing is complete, a wireline plug is run and set in the tubing
`hanger bore above the side outlet. The tubing hanger running
`string can then be recovered. The internal tree cap is run,
`landed and locked into the tree above the tubing hanger.
`The tree cap is run with the tubing hanger running tool on
`tubing. After recovery of the tree cap running string is
`complete, the BOP stack can be recovered. Finally, the debris
`cap is run and locked to the top of the xmas tree.
`
`Conventional System With Tubing Spool (Figure 7)
`
`Once the 9-5/8" casing is cemented and tested, the BOP stack
`is pulled and moved out of the moonpool so that the tubing
`spool can be run. The tubing spool is run with the BOP and
`marine riser in the same way as the horizontal tree. Once the
`system is pressure tested, the cement inside the 9-5/8" casing
`is drilled out and the well is prepared for the completion string.
`The wear bushing which was run with the tubing spool is
`retrieved in preparation for running the completion string.
`The completion string is run which includes the downhole
`completion,
`tubing hanger,
`tubing hanger running tool.
`completion riser and umbilical. Once the downhole
`completion and tubing hanger are tested, a wireline plug is
`set below the tubing hanger and the running string is
`recovered. The BOP stack is recovered and moved out of the
`moonpool so that the xmas tree can be run. The xmas tree is
`run and locked to the tubing spool. The xmas tree running
`string consists of
`the lower riser package, emergency
`disconnect, stress joint, completion riser,
`tension joint.
`umbilical and surface tree. Once pressure integrity of the
`installed xmas tree and running string is confirmed, the well
`can be flow tested. Once flow testing is complete, the xmas
`tree running string is recovered. The tree cap is run on tubing
`or drillpipe, locked to the xmas tree and pressure tested. The
`tree cap running string is then recovered.
`
`Conventional System Without Tubing Spool (Figure 8)
`
`The installation sequence for the conventional system without
`the tubing spool is the same as for the system with tubing
`spool with the exception of running the tubing spool. There is
`significant time savings in the case where the completion is
`run directly after drilling (continuous) as a BOP stack trip is
`saved. Again, it should be noted that no additional time has
`been considered for orientation difficulties.
`
`WORKOVER
`
`Another perceived advantage of the Horizontal System is the
`reduction in time required to perform well workovers. There is
`unquestionably a major time savings if the workover requires
`that the tubing be pulled. This is specifically the time that it
`takes to pull and rerun the xmas tree.
`It can be argued that
`many operators would choose to recover the tree for
`
`OSS Exhibit 2013, pg. 4
`FMC vs. OSS
`IPR2016-00495
`
`

`
`OTC 7525
`
`BRETT R. MCCONAUGHY AND JAMES A. GARIEPY P.E.
`
`5
`
`inspection purposes (assuming a reasonable time period
`from installation or previous workover). The Horizontal System
`has an obvious advantage on wells where frequent (tubing
`pull) workovers are required whether it be to replace artificial
`lift systems or due to the reservoir itself.
`It should be noted,
`however, that wells that require frequent workover which are
`situated in deep water may not be economically feasible to
`begin with. Wireline workovers from small non-drilling
`vessels can be performed with either system though the
`horizontal is the more time consuming of the two as there are
`additional plugs to be pulled before downhole access can be
`achieved.
`It should be noted, however,
`that though it
`is
`deemed feasible, the technology does not currently exist to
`perform wireline workovers from a non-drilling vessel in these
`water depths. The same program as was used for installation
`times is utilized to estimate workover durations. Workover
`
`durations are broken down into two types which are tree
`recovery and tubing recovery.
`
`TECHNICAL RISKS
`
`Up to this point in this document, the focus has been on
`perceived advantages of the Horizontal System. Though
`assumptions must be made to eliminate options,
`the
`advantages are for the most part measurable and
`significant. The perceived disadvantages are much more
`difficult to quantify and will, in many cases, be proven correct
`or incorrect only by field experience. The basic purpose of
`the system (to control hydrocarbon flow) remains the same
`for both conventional and horizontal systems. The Horizontal
`System utilizes a different configuration which makes certain
`aspects critical from a failure mode and effect standpoint.
`The remainder of the paper attempts to briefly cover these
`areas and the impacts which they could have on the system
`reliability in the event of their failure.
`
`Tubing Hanger Seals
`
`in a horizontal completion, the tubing hangerseals are exposed
`to produced fluid, whereas in a conventional system they are
`only exposed to the tubing annulus. In a completion where the
`tubing is planned to be pulled frequently for workover, this
`is not seen as a problem as the tubing hanger seals can be
`replaced.
`in a completion that is expected to have a true 10
`to 20 year life (more likely case for deep water), tubing hanger
`seals are critical and must be addressed. The most commonly
`used seal configuration by virtually all subsea equipment
`suppliers is an elastomer with metal encapsulating end rings
`(Figure 13). These seals have proven very reliable and offer
`a metal seal with elastomer back up. The metal sealing
`mechanism on this type of seal is the end ring which is initially
`energized by compression of the elastomer. The downstream
`end ring is then further energized by increasing pressure. The
`concern with this type of seal is the exposure of the elastomer
`
`If the elastomer were to deteriorate due to
`to produced fluid.
`exposure to produced fluid, primary energization of the
`metal seal would be lost which eventually (with pressure
`fluctuations) could cause a metal seal failure. As in the case
`of a conventional completion, tubing hanger seal failure in
`itself is not a catastrophic event in that pressure can still be
`contained within the system.
`In both cases, the well must be
`killed with kill fluid before the tubing hanger can be recovered
`for seal replacement. The optimum solution to this problem is
`a full metal seal with no elastomer. The problem with this
`solution is that most metal seals are designed to plastically
`deform either the seal surface or the seal itself. A fundamental
`
`of this type of design is that the seal system will only function
`a limited number of times, even if the seal is replaced.
`It is an
`obvious requirement in a tubing hanger that the sealing
`system is capable of reliably sealing a large number of times.
`An all metal sealing system is currently being developed to
`address this problem.
`It should be noted that metal seals are
`not as reliable as elastomers when attempting to seal across
`a rough or scratched surface.
`it
`is, therefore, extremely
`important that the xmas tree wear bushing functions properly
`and is utilized at all times. Additionally, the metal seals should
`have as much damage tolerance as possible.
`
`Wireline Plug
`
`A horizontal tree utilizes a wireline set plug in place of a swab
`valve to direct flow horizontally through the wing valve.
`Though there has been some discussion of utilizing means
`other than wireline to run and retrieve the plug, it is currently
`the most widely accepted method. There are four basic
`problems with utilization of a wireline plug in this application.
`
`Firstly, there is a concern that overtime the bottom of the plug
`will be eroded due to constant
`impingement from the
`produced fluid. This is not perceived as a major problem as
`it is not any different than what a swab valve is exposed to in
`a conventional tree. The gate surface is typically hard faced
`with materials such as tungsten carbide. These systems have
`proven to be very reliable in field applications. Studies have
`shown that a major contributing factor to this is the dead fluid
`volume between the gate face and the side outlet which tends
`to act as a buffer. Similar hard facing can be applied to the
`bottom of the wireline plug in a horizontal tree application if
`deemed necessary.
`
`The second concern is the seal mechanism between the plug
`and the bore. Conventional wireline plugs which utilize
`Chevron type packing have proven to be quite reliable.
`Conventional wireline plugs are not
`typically exposed to
`produced fluid continuously for long periods of time as is the
`requirement in the horizontal tree system. The issue then
`becomes very similar to that of the tubing hanger seals which
`requires development of metal seals for the application.
`
`OSS Exhibit 2013, pg. 5
`FMC vs. OSS
`IPR2016-00495
`
`

`
`6
`
`APPLICATION OF HORIZONTAL TREES T0 DEEP WATER
`
`OTC 7525
`
`it could be argued that metal seal technology is not
`Again,
`required in a well which requires frequent workovers as the
`packing elements can be replaced each time a workover is
`done. An additional problem to metal seal development for
`a wireline plug is the limited setting force available in typical
`wireline installation systems and the effect that thermal
`loading may have.
`It is a fact that a preloaded metal seal will
`perform better than a pressure energized seal on a less than
`perfect seal surface. Additionally, the more preload available
`to set the seal, the better the seal will perform. As is the case
`with the tubing hanger, the seal surface cannot plastically
`deform as the sealing system must be truly repeatable. A
`major concern in development of this system is a means of
`protection of the sealing surface in the tubing hanger bore as
`wireline tools are run downhole. Metal sealing systems are
`currently being developed in conjunction with wireline
`equipment supply companies. The aim in this development is
`to utilize field proven systems for setting and pulling the plug
`and incorporating metal seal
`technology in similar
`geometries to those utilized in other applications.
`
`The third concern is that there is no way to perform a high
`pressure teston the plug from below once it is installed. When
`the plug is set in the tubing hanger, the production bore has to
`be open to the formation. The maximumtestpressurethat can
`be applied to the plug from below is that which the formation
`can take. This would obviously not be a very conclusive test.
`Testing from above is also inconclusive as current elastomer
`sealing systems and most metal seal systems are not bi-
`directional. Put another way, a successful test from above
`proves that the seals designed to hold pressure from above
`are functional, not necessarily those which are to hold pressure
`from below. It is, therefore, desirable to develop a bi—directional
`metal seal for this application.
`
`The last concern is that after years of exposure to pressure
`and temperature effects of produced fluid, the plug may prove
`difficult to retrieve. Of particular concern, is the effect that
`produced sand would have as it packs into the space
`between the plug outside diameter and the bore of the tubing
`hanger. This is a situation which would be difficult to model
`in the laboratory in order to prove that it would or would not be
`a problem in the field.
`If the plug proved to be unretrievable
`with wireline tools, equipment such as coiled tubing or a
`spaghetti string mill would be the only answer short of pulling
`the tubing with the plug in place.
`
`Internal Tree Cap
`
`The reliability of the internal tree cap itself is not perceived as
`a problem. The aspect that could cause problems is the
`installation of it. As the tree cap is run inside the marine riser,
`it will loosen debris on the inside of the riser.
`In the case of a
`conventional tubing hanger this debris ends up downhole.
`In a horizontal completion, there is nowhere for the debris to
`
`go but to land on top of the tubing hanger and wireline plug.
`The obvious solution to this potential problem is to have a
`clean riser and BOP. This may be easier said than done.
`How clean is clean and how much rig time should be spent?
`This issue obviously becomes a bigger issue as water depth
`increases as there is more riser and, therefore, a potential for
`more debris. It should be noted that no time has been set aside
`for cleaning of riser in the installation comparison.
`
`Flow Testing (Figure 14)
`
`Since virtually all subsea completed wells are flow tested or
`cleaned up by flowing vertically to the installation vessel. there
`is a need for a system which can safely shut in the flow and
`disconnect in a rig drive off situation.
`
`in and quick disconnect systems for
`Emergency shut
`conventional completions have evolved into what is now
`accepted as a reliable and safe system. These systems
`usually consist of a lower riser package (LRP) and an
`emergency disconnect. The lower riser package connects to
`thetop ofthetree and contains valves (gate or ram type) which
`can close and seal during full flow on wireline or coil tubing.
`The emergency disconnect sits on top of the LRP and, as
`the name implies, is the point at which the completion riser is
`disconnected from the subsea equipment. These systems
`commonly utilize piloted hydraulic control systems so as to
`have short response times.
`
`In a Horizontal Completion System, the BOP stack and marine
`riser act as the main conduit which connects the subsea
`
`system to the installation vessel. Flow testing to the vessel
`would then take place through the tubing hanger installation
`string. Current horizontal systems in relatively shallow water
`depths have successfully utilised a standard subsea test
`tree connected to the top of the tubing hanger running tool.
`The subsea test tree is equipped with ball values which are
`capable of shearing coil tubing or wireline and sealing, and
`a quick disconnect. Subsea test trees have been successfully
`utilized for many years for drill stem testing which is basically
`the same application.
`
`The problem with currently available subsea test trees is that
`they cater for only one hydraulic line right through the system.
`