Report on horizontal vertical christmas tree

Abstract

Christmas tree in oil and gas sector is not the same with the real christmas tree we use every end and

beginning of a new year. The name was given due to its resemblance and structure in the decorative

nature of a real christmas tree. Thousands of subsea Christmas trees have been installed since the first

subsea Christmas was installed in the Gulf of Mexico in 1961. Since then, the Christmas are an

essential part of the subsea fields. Christmas trees are used on both surface and subsea wells. It is

common to identify the type of tree as either “subsea tree” or “surface tree”. Each of these

classifications has a number of variations. Examples of subsea include conventional, dual bore, mono

bore, TFL (through flow line), horizontal mud line, mud line vertical, side valve, and TBT (through-

bore tree) trees. The primary function of a tree is to control the flow, usually oil or gas, out of the well.

A tree may also be used to control the injection of gas or water into a non-producing well in order to

enhance production rates of oil from other wells. Tree complexity and functionality has increased over

the last few decades. Subsea Christmas tree is the core equipment in offshore oil & gas production

system. Installed on subsea wellhead, the tree is used to connect and support tubing string, seal off

casing pipes and casing tubing annulus, isolate borehole fluids from external sea water, control

wellhead production pressure, and adjust borehole flow rates. When the well and facilities are ready to

produce and receive oil or gas, tree valves are opened and the formation fluids are allowed to go

through a flow line. Flow lines on subsea wells usually lead to a fixed or floating production platform

or to a storage ship , known as a floating storage offloading vessel (FSO), or floating processing unit

(FPU). A tree often provides numerous additional functions including chemical injection points, well

intervention means, pressure relief means, monitoring points such as pressure, temperature, corrosion,

erosion, sand detection, flow rate, flow composition, valve and choke position feedback, and

connection points for devices such as down hole pressure and temperature transducers.

CHAPTER 1Introduction

T120026423 AMAR MADUKAR GAIKWAD

In petroleum and natural gas extraction, a Christmas tree is an assembly of valves, spools, and fittings

used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate

well and other types of wells. It was named for its crude resemblance to a decorated tree. The purpose

of a production Christmas Tree is to control the flow of hydrocarbons from its respective well via

various control valves and choke, to receiving unit. This can be a fixed or floating vessel or produced

via pipeline to shore. The purpose of an injection Christmas Tree is to control the flow of water or gas

into its respective well via various control valves and choke, from a process installation off- or on-

shore.

Christmas tree are installed on both surface and subsea wells on the surface at the top of the casing

hanger to control the flow of fluid/gas out of the well. As the well production flows up the tubing, its

enters the Christmas tree. The equipment at the top of the producing wellhead is called a ‘‘Christmas

tree’’ and it is used to control flow. The ‘‘Christmas tree’’ is installed above the tubing head. An

‘‘adaptor’’ is a piece of equipment used to join the two. The ‘‘Christmas tree’’ may have one flow

outlet (a tee) or two flow outlets (a cross). The master valve is installed below the tee or cross. To

replace a master valve, the tubing must be plugged.

Subsea and surface trees have a large variety of valve configurations and combinations of manual and

pneumatic actuated valves. Subsea trees contain many additional valves and accessories compared to

Surface trees. Typically a subsea tree would have a choke (permits control of flow), a flowline

connection interface (hub, flange or other connection), subsea control interface (direct hydraulic,

electro hydraulic, or electric) and sensors for gathering data such as pressure, temperature, sand flow,

erosion, multi-Phase flow, single phase flow such as water or gas.

A basic surface tree consists of two or three manual valves (usually gate valves because of their flow

characteristics) A typical sophisticated surface tree will have at least four or five valves, normally

arranged in a crucifix type pattern.

CHAPTER 2Christmas Tree System

2.1 Main Components:Typical main components in an Christmas Tree assembly required to perform its functions include:


• Tubing Hanger • Tubing Head Spool • Tree piping • Flowline connector • Wellhead connector • Valves and fittings • Choke • Tree cap • Tree frame A typical Christmas tree composed of a master gate valve, a pressure gauge, a wing valve, a swab

valve and a choke is shown here. The Christmas tree may also have a number of check valves. The

functions of these devices are explained in the following paragraphs.

At the bottom we find the casing head and casing hangers. The casing will be screwed, bolted or

welded to the hanger. Several valves and plugs will normally be fitted to give access to the casing.

This will permit the casing to be opened, closed, bled down, and in some cases, allow the flowing well

to be produced through the casing as well as the tubing. The valve can be used to determine leaks in

casing, tubing or the packer, and will also be used for lift gas injection into the casing.

Figure 2.1. Schematic of Wellhead & Christmas Tree

The components are described following:

A) The Tubing Hanger: (also called a donut) is used to position the tubing correctly in the well. The

Tubing Hanger (TH) system is designed to suspend and seal the downhole tubing. The Tubing hanger

shall be possible to be installed through a BOP stack and locked into the internal landing profile of


either the casing hanger in the wellhead, the tree bore or in the THS. The tubing hanger shall provide

the means of communication between the Christmas Tree and the downhole hydraulic and electric

functionalities. Wet mate couplers/connectors are located on the top and bottom of the hanger and

engage with the Christmas Tree and the downhole equipment.

Tubing Hanger Configurations:The tubing hanger can be segmented into two types of configurations: monobore and dualbore Tubing

hanger. The monobore tubing hanger only have a production bore, with the annulus routed around the

bore. The dual bore tubing hanger is designed with a main production bore and an annulus bore. The

tubing hanger assembly consists of the hanger body, lockdown sleeves, locking dogs, gallery seals,

pump down seal, electrical penetrator receptacle, dry and wet mate connector and pup joint. These

components ensure that the tubing hanger is locked down and communicate with the systems around.

Figure 2.2 Monobore And Dual Bore Tubing Hanger

B) Wellhead Connectors: The wellhead connectors are the mechanism to lock and seal a Christmas

tree to the wellhead, Christmas tree to the THS and the THS to the Well head.

The connectors may be both mechanical and pressure connections. If remote operated, it may be

hydraulically actuated. Where possible, divers can actuate the screws in the mechanical connections.

It exists two types of tree connectors:• H4 connector • Collet connector


The H4 connector is the most commonly used connector. It is a hydraulically actuated connector

applicable for H4 type of wellhead profiles. The connector is used to land and lock a Christmas tree to

a subsea wellhead. The tree connectors can be both mechanical and pressure connections together with

orientation between the Christmas tree assembly and the wellhead.

C) Valves: Tree valves are designed in the Christmas tree assembly to control and safely stop the

fluid flow. The various valves are used for servicing, testing and regulating oil, gas, water or

chemicals. The most common type of valves in a Christmas tree is a gate valve. Gate valves are

operated either hydraulically, mechanically and/or by Remotely Operated Vehicles (ROVs). Christmas

tree valves should be designed, fabricated and tested in accordance with API 17D, API 6A and API

6D.

All main valves are power-operated fail-safe closed valves, which means that the valves will

automatically close if either the signal or the hydraulic control pressure is lost. Swab and control

valves are fail “as is” due to production regulations. All Christmas tree are configured to provide ROV

access to the principal main Christmas tree valves and isolation needle valves from the ROV panel.

ROV interfaces shall be configured per ISO 13628-8.

Typical valve sizes include:

Production and Injection valves (typ. 5-7” gate valves) for controlling the process medium Annulus or Injection Valves (typ. 2” gate valves) for annulus access Service valves (typ. 3/8” to 1”) for chemical injection Isolation valves (typ. 3/8 “to 1”) for pressure test and downhole lines Check valves (typ. ½-1”) for preventing back-flow of well fluid to service lines.

Master gate valve. The master gate valve is a high quality valve. It provides full opening, which

means that it opens to the same inside diameter as the tubing so that specialized tools may be run

through it. It must be capable of holding the full pressure of the well safely for all anticipated

purposes. This valve is usually left fully open and is not used to control flow.

The pressure gauge. The minimum instrumentation is a pressure gauge placed above the master gate

valve before the wing valve. In addition other instruments such as a temperature gauge will normally

be fitted.

The wing valve. The wing valve can be a gate or ball valve. When shutting in the well, the wing gate

or valve is normally used so that the tubing pressure can be easily read.

The swab valve. The swab valve is used to gain access to the well for wireline operations,

intervention and other workover procedures. On top of it is a tree adapter and cap that will mate with a


range of equipment.

The variable flow choke valve. The variable flow choke valve is typically a large needle valve. Its

calibrated opening is adjustable in 1/64 inch increments (called beans). High-quality steel is used in

order to withstand the high-speed flow of abrasive materials that pass through the choke, usually over

many years, with little damage except to the dart or seat. If a variable choke is not required, a less

expensive positive choke is normally installed on smaller wells. This has a built-in restriction that

limits flow when the wing valve is fully open.

This is a vertical tree. Christmas trees can also be horizontal, where the master, wing and choke are on

a horizontal axis. This reduces the height and may allow easier intervention. Horizontal trees are

especially used on subsea wells.


CHAPTER 3Overview Of Subsea Christmas Tree Systems

3.1 Industry RequirementsWhen a manufacturer is involved in a new project, the first step is to define the requirements and

specifications relevant for that particular project. The requirements are specified in the laws and

regulations of the countries involved, the standards are stipulated in the customer requirements

and in internal standards and requirements within the organization involved. Subsea production

system poses a hazard. It is therefore vital to have standards that give guidance to maintain secure

operations and prevent major accidents.

Applicable standards for the Christmas tree requirements include: API 6A: Specification for Wellhead and Christmas Tree Equipment. API 17D: Design and Operation of Subsea Production Systems-Subsea Wellhead and Tree

Equipment; ISO 13628-4: Petroleum and natural gas industries – Design and operation of subsea

production systems. Part 4: Subsea wellhead and tree equipment; ISO 10423:2009: Petroleum and natural gas industries - Drilling and production

equipment - Wellhead and Christmas tree equipment.

3.2 Christmas Tree Installation And Service Conditions

3.2.1 Christmas Tree Installation:An Christmas tree can be installed either by a drill-pipe or by a crane through a moon pool at a rig

or a vessel, depending on the size of the Christmas tree. The vessel may be a jack -up,

semisubmersible or a drillship. Both Vertical Christmas tree and Horizontal Christmas tree

systems use a landing string through the BOP stack to run the completion.

Typical procedures for installing the Vertical Christmas tree and the Horizontal Christmas tree

system are as follows (Bai & Bai, 2012):


Vertical Christmas Tree:

Perform pre-installation tree tests. Skid tree to moon pool. Push guide wired into tree guide arms. Install lower riser package and emergency disconnect package (EDP) on tree at moon pool area. Connect the installation and workover control system (IWOCS) Lower the tree to the guide base with tubing risers Lock the tree onto the guide base. Test the seal gasket. Perform tree valve functions with the Installation and Workover Control System (IWOCS). Retrieve the tree running tool. Rune the tree cap on the drill pipe with the utility running tool system. Lower the tree cap to the subsea tree. Land and lock the tree cap onto the tree mandrel. Lower the corrosion cap onto the tree

cap with a drill pipe (or lifting wires).

Some suppliers have developed ROV-

installed corrosion caps.

Horizontal Christmas Tree:

Complete drilling Retrieve the drilling riser and BOP stack; move the rig off Retrieve drilling guide base Run the Production Guide Base (PGB) and latch onto the wellhead Run the subsea Horizontal Christmas tree Land tree, lock connector, test seal function valves with an ROV, release tree running tool. Run the BOP stack onto the Horizontal Christmas tree; lock the connector Run the tubing hanger; perform subsea well completion; unlatch the Tubing Hanger Running Tool (THRT). Run the internal tree cap by wireline through the riser and BOP; retrieve THRT. Retrieve BOP stack. Install debris cap. Prepare to start the well.


3.2.2 Service Conditions:The pressure ratings for Christmas tree are standardized to 5000 psi, 10000 psi and 15000 psi.

Recently there are also Christmas tree constructed to apply for 20000 psi (ISO 13628-4, 2010).

Equipment shall be designed according to the material classes and temperature ratings required.

These ratings are specified in API SPEC 6A and 17D. For further information, see these

standards.

3.3 Christmas Tree Design And Analysis:Each Christmas tree design is driven by reservoir requirements, such as type of chemical

injection needed. As an example, a gas reservoir is in the need of a constant stream of Mono

Ethylene Glycol (MEG) to avoid formation of hydrates, while an oil reservoir require artificial

lift methods to be able to recover the full potential of the well as the pressure decrease along with

the extraction of hydrocarbons. For each reservoir, it is necessary to conduct analyses for

protection of the equipment.

The analyses shall include the means of:• Chemical injection • Cathodic protection • Insulation and coating • Structural loads • Thermal analysis

The kind of chemical injection chosen for a well depend upon the reservoir type and the fluid

characteristics. The final objective is to be certain that the equipment produces economically

from the reservoir to the production facilities throughout the whole lifecycle of the field

development. With the Christmas tree assembly constantly being exposed to the ambient sea

conditions, it is crucial with sufficient anodes for cathodic protection.

Thermal insulation is needed to ensure sufficient cool down time in the event of a production

stoppage. The main objective of thermal insulation is to have sufficient time to solve a shutdown

problem and avoid the burden of the launching preservation sequence with associated production

losses and to avoid dramatic consequences of hydrate formation with associated production

losses. Included in the insulation is a layer of corrosion coating suitable for working pressure,

specified by project requirements. The structures have to be designed so that they withstand

internal and external structural loads imposed during installation and operation.


According to the standard API RP 17D special Christmas tree load considerations should be analyzed

for:

• Dropped objects,• Marine riser and BOP loads, • Flow line connection loads,• Lifting loads,• Snagged tree frame, umbilical's or flow line,• Pressure induced loads. (American Petroleum Institute (API), 2011)

3.4 Test Program For Christmas TreesFactory Acceptance Test (FAT) shall be executed of all units pre-installation to ensure that the components of a unit and the unit itself satisfy all specified requirements to strength and functional performance (ISO 13628-4, 2010).All assemblies are required to pass FAT before they are passed to stock, prepared for Extended Factory Acceptance Test (EFAT) or delivered directly to site for installation. Whenever equipment is moved from one site to another it will be subjected to a Site Receipt Test (SRT). The aim is to verify equipment received at site is in the same as before transportation state, with no deterioration occurred during transportation.


CHAPTER 4Functional Requirements & Types Of Christmas Tree

4.1 Christmas Tree Functional Requirements:The subsea Christmas Tree is located on the top of the wellhead, providing an interface between the

completion string and the piping towards the process system. At its simplest, an Christmas tree can be

defined as an assembly of valves and fittings used for production or injection to control the flow of

product, chemicals, water or gas from a well. The injection system, production control system,

downhole control system and monitoring and flow control system are all systems controlled through

the Christmas tree assembly.

Typical functional requirements include:

• Control of flow by directing hydrocarbons from the well to the flowline (called production tree) or by canalizing water or gas into the reservoir to maintain reservoir pressure (called injection tree); • Regulate the fluid flow through a choke; • Monitor well parameters, such as temperature, annulus pressure, well pressure and flow composition; • Act as a barrier between the reservoir and the environment; • Safely open and shut down the fluid flow through the assembly of valves; • Inject protection fluids, such as inhibitors for corrosion and hydrate prevention, to protect the subsea

equipment and to assist the flow.

Each Christmas tree is designed for the individual reservoir conditions and for the possible facility

solutions available, which means that the configuration, size, weight and cost for a Christmas tree will

differ from one offshore field to another due to the specific design requirements. The optimum

Christmas tree will be driven by reservoir requirements and therefore never completely standardized.

However, there is a strong trend towards smaller, more compact Christmas tree in the industry.

4.2 Types And Configurations Of Christmas TreesChristmas tree may be segmented into two main types: Vertical Christmas Tree and Horizontal

Christmas Tree. The Subsea Engineering Handbook, written by Yong Bai and Qjang Bai in 2012, is

the main source for the background information about the trees in the following sections.

4.2.1 Vertical Christmas Tree:T120026423 AMAR MADUKAR GAIKWAD

The conventional Christmas tree, which is the Vertical Christmas tree, is the earliest and most

extensively used Christmas tree. The Vertical Tree concept is defined as having a vertical flow though

the tubing hanger. The master and wing valves in the tree body are located vertically above the tubing

hanger. The tubing hanger is landed in the subsea high-pressure (HP) wellhead. This concept allows

retrieval of the tree body with pulling the tubing hanger. A Vertical Christmas tree are installed either

on a wellhead or on a tubing head, after the subsea tubing-hanger has been installed through the

drilling BOP stack and landed and locked into the wellhead or in the tubing head. The production flow

path is through the valves mounted in the vertical bore and out of the top of the tree during workover

and testing or during production (injection) via the production outlet that branches off the vertical bore

(ISO 13628-4, 2010). The Vertical Christmas tree is identified by the location of the production and

annulus bore, that is placed vertically through the tree body with the primary valves placed in a

vertical configuration. The tree can have a concentric bore or multiple bores. Annulus access may be

through the bore or a side outlet in the tubing head, depending on the Christmas tree design. A typical

tree of this type is illustrated in figure 4.1 and 4.2.

Figure 4.1 Vertical Christmas Tree Configuration

4.2.2 Horizontal Christmas Tree:


The other main type of Christmas tree is the Horizontal Christmas tree design, also commonly known

as the spool tree. The Horizontal Tree is defined as diverting flow horizontally at the tubing hanger.

The master and wing valves in the tree body are located in the horizontal plane to the tubing hanger.

The tubing hanger is landed in the horizontal tree. This concept allows retrieval of the tubing hanger

with pulling the tree body. The Horizontal Christmas tree are distinguished from the conventional

design by the production and annulus valves being routed around the tubing hanger in a horizontal

configuration. One of the key functional features is that the Horizontal Christmas tree may be installed

after drilling and installation of the complete wellhead system, but prior to installation of the tubing

completion and tubing hanger.

Since the Christmas tree is installed prior to the tubing completion, the Blow Out Preventer (BOP)

stack is landed on top of the Horizontal Christmas tree and the tubing hanger and tubing completion is

run through the BOP and landed off on a landing shoulder in the bore of the Horizontal Christmas tree.

The production flow path exits horizontally through a branch bore in the tubing hanger between seals

and connect to the aligned production outlet.

An alternative arrangement is that the tubing hanger and internal tree cap are combined into a single

extended tubing hanger system suspended in the Horizontal Christmas tree. This doubles up on the

number of isolation plugs and annular seals for barrier protection and features a debris cap that can

also serve as a back-up locking mechanism for the tubing hanger (ISO 13628-4, 2010). A third

configuration, the drill-thru configuration, allow installation of the tree immediately after the wellhead

housing is landed, meaning that drilling and installation of the casing strings is performed through the

tree, minimizing the number of times it is necessary to run and retrieve the BOP stack.


Figure 4.2 Horizontal Christmas tree Configuration

4.3 Comparison Of Horizontal And Vertical Christmas Trees:An ongoing debate within the Christmas tree industry is comparing the relative merits of Vertical

Christmas tree and Horizontal Christmas tree systems. For the last 20 years the Horizontal Christmas

tree has been the preferred design for deepwater fields, while in recent years the focus in the industry

is returning to the conventional Vertical Christmas tree system. A key requirement when designing a

Christmas tree is that access to the annulus is enabled between the production bore and the casing.

This is an important feature for a number of reasons, such as pressure monitoring and gas lift means.

As an example, any pressure build-up in the annulus may be bled into the production bore via a

crossover loop.

The original design of the Vertical Christmas tree and the Tubing Hanger (TH) were a dual-bore

configuration. Prior to removal of the BOP it was then necessary to set plugs in both the production

bore and the annulus bore. Access to the bores is handled with a dual-bore riser or a landing string.

The handling and operation with dual-bore systems compared to mono-bore systems are more

complex and time-consuming, and then again more costly.


In an Horizontal Christmas tree configuration access to the annulus is incorporated in the tree design

and controlled by valves rather than plugs. This enables operation with mono-bore systems, which

means less complex riser or landing string. Easier access to the annulus enables operations that can

deliver significant advantages, particularly in deepwater (White, 2013).

Regarding installation and intervention, both Horizontal Christmas tree and Horizontal Christmas tree

systems use a landing string to run the completion through the BOP. In the Horizontal Christmas tree

configuration, the tree is normally run on a subsea test tree within the marine riser to carry out a

number of critical functions. Once the hanger is landed inside the landing shoulders in the tree, it is

critical to ensure communication of electric and hydraulic downhole functions. The TH is landed

passively inside the tree without relying on external input using an orientation sleeve.

Before production, after a well is completed, it is common practice to flow the well fluid to the drilling

rig to clean up the well or to carry out a well test. For the horizontal christmas tree systems this is

carried out through the subsea test tree and a marine riser. The primary function of the test tree ensures

that, if necessary to disconnect the rig from the BOP during testing or cleanup, the test tree will close

the valves and an emergency disconnect will be performed safely.

In the case of the Vertical Christmas tree system, the completion is run on a landing string

incorporating a tool that run, lock and orientate the TH. This orientation requires a tool to interface

with a pin installed inside the BOP. Once the TH is oriented and installed inside the wellhead, with the

understanding that when the tree is oriented and landed on the wellhead, the communication of all

electric and hydraulic down hole functions will function. Well cleanup and testing is then carried out

after a dedicated test package and an open-water riser replace the BOP. This test package comprises a

Lower Riser Package (LRP) and an Emergency Disconnect Package (EDP), enabling the rig or vessel

to disconnect safely in the case of an emergency in the same way as a test tree.


CHAPTER 5Tree Selection - Horizontal Vs Vertical Tree

5.1 Subsea Tree Selection:The decision to select the subsea tree concept usually occurs early in the project due to long

delivery times. It is typical for this phase of the project to occur 18 to 24 months prior to the start

of drilling operations. Also, the contract commitment for the trees can be one of the first large

CAPEX expenditures for a new project. Therefore, it is imperative that the subsea tree concept

has been fully integrated into the overall conceptual well plan and that potential well control

risks identified with each option have been considered. The subsea tree selection process is

typically led by the Subsea Group of the Development Company, but critical input and

endorsement from Drilling is required.

Decisions made during this development phase have significant implications on almost every

aspect of the well plan and completion installation (specifically the completion riser type). Early

integration of drilling, completion and subsea expertise during the concept selection phase of the

project enhance the planning and execution phases of deepwater subsea developments.

There are two fundamental subsea tree concepts available to industry – Vertical Trees (VT) and

Horizontal Trees (HT). Essentially all of the subsequent completion discussions are predicated

on the type of tree selected.

5.2 Vertical Trees:The nomenclature for these two tree concepts evolved from the tubing hanger design, depending

on the direction of the production flow stream relative to the tubing hanger (e.g., either vertical

or horizontal). With either subsea tree concept, the vertical production flow from the wellbore is

eventually diverted horizontally to enter the subsea flow line system. For the vertical tree

concept, the flow stream is diverted horizontally above the tubing. The production tubing is run

through the drilling BOP and landed in the 18-3/4 in. subsea wellhead or in the tubing hanger

spool. The drilling BOP must be pulled before the VT can be run. A special completion riser is

used to run the tubing/TH and the VT (Figure).The vertical tree concept can be further defined as

“Eccentric” or “Concentric” based on whether the production flow is routed eccentrically

(resulting in a dual bore configuration) or concentrically (a single bore configuration) through

the tubing hanger. These differences will be further discussed in this section.


5.2.1 Eccentric Dual Bore Vertical Tree:This tree concept, also known as a dual bore Vertical tree, has two vertical bores through the tree

body and tubing hanger as illustrated in Figure 5.2. In the Eccentric Vertical Tree concept, the

dual bore tubing hanger is eccentric and lands in the subsea wellhead body. The eccentric design

requires accurate orientation of the tubing hanger (TH) and the tree. The annulus bore provides

access to the production casing annulus and facilitates fluid circulation and well control during

tubing running or pulling operations when there are no downhole mechanical barriers. The

tubing hanger lands and seals in the subsea wellhead body. This makes the tubing hanger

independent from the VT itself. The Eccentric Vertical Tree concept has a minimum of two

subsea tree valves (e.g. master and swab valves) located vertically above each bore (e.g., the

production tubing bore and production casing annulus bore). It is also common for the tubing

hanger spool to have a crossover connection between the production and annulus bore, separated

by an X-over valve (XOV). This design provides vertical access through the vertical tree body to

run or pull wireline plugs that land into dedicated profiles in the dual bore tubing hanger. These

wire line plugs provide mechanical isolation to the tubing and production casing annulus for well

control. This becomes important during BOP removal/tree installation and during workover

operations. The Eccentric Vertical Tree concept has the production wing valve outlet in the tree

body at 90 degree to the production bore above the master valve. This diverts the production

flow stream horizontally, through the choke and mated connector to the subsea flowline system.


Figure 5.1 Dual Bore Vertical Tree

Figure 5.2 Eccentric Dual Bore

5.2.2 Summary Of Eccentric Vertical Tree:1. Two eccentric vertical bores through tubing hanger.


2. Tubing hanger lands in subsea wellhead (active alignment).

3. Flow diverted horizontally after master valve.

4. Tree installed with dual bore completion riser (open water).

5. Through tubing workovers – no plugs to pull.

The typical steps to run the tubing and Eccentric Vertical Tree are as follows:1. Production tubing hanger is run via a dual bore completion riser and landed in the subsea

wellhead. This system will be run through the drilling riser/BOP.

2. Flow barriers (wire line plugs) are run and set in the tubing hanger. The completion riser and

drilling riser/BOP are pulled.

3. The VT is run via a dual bore completion riser/BOP.

4. The flow barriers are removed, and either the well is allowed to flow back to surface via the

completion riser, or the tree cap is run and the well flows through the subsea manifold/pipeline.

The Eccentric Vertical Tree concept was used for the following projects:1 .Zinc (ExxonMobil – GOM)

2. Girossol (Total Fina Elf - Block 17 Offshore Angola)

3. Balder (ExxonMobil – North Sea Norway)

4 .Blackback (ExxonMobil – Austraila)

5.2.3 Concentric Mono Bore Vertical Tree:The Concentric Vertical Tree concept uses a mono bore tubing hanger that is concentric and sets

in a tubing hanger spool landed above the subsea wellhead. The concentric design does not limit

the production bore through the tubing hanger, therefore a larger tubing string can be used. For

this tree concept, the flowpath for the production casing annulus is around the concentric tubing

hanger through side outlets machined in the tubing hanger spool below and above the hanger as

shown in Figure. The tubing hanger lands and seals in the subsea wellhead body. This makes the

tubing hanger independent from the VT itself. The Concentric Vertical Tree concept has a

minimum of two subsea tree valves (e.g. master and swab valves) for both the production tubing

and casing. This design provides vertical access through the vertical tree body to run or pull

wireline plugs that land into dedicated profiles in the mono bore tubing hanger. These wireline

plugs provide mechanical isolation to the production tubing only. The production casing is

isolated with a valve in the tubing spool body. This becomes important during BOP removal/tree

installation and during workover operations.


Figure 5.3 Concentric Vertical Tree Concept & Tubing Hanger

5.2.4 Summary Of Concentric Vertical Tree:1. Single concentric bore through tubing hanger.

2. Tubing hanger lands in tubing hanger spool (passive alignment).

3. Flowbase design allows tree retrieval without pulling jumper.

4. Tree installed with mono bore completion riser (open water).

5. Additional permanent leak path (tree/tubing hanger spool).

The typical steps to run the tubing and Concentric Vertical Tree are as follows:1. Flow barriers (wire line plugs) are run and set in the production casing.

2. The drilling riser/BOP is pulled.

3. The tubing hanger spool is run with DP.

4. Once the spool is landed the DP is pulled.

5. The drilling riser/BOP is run and the flow barriers are removed.

6. Production tubing/TH is run via a mono bore completion riser and landed in the tubing spool.

This system will be run through the drilling riser/BOP.

7. Flow barriers (wire line plugs) are run and set in the tubing hanger.

8. The completion riser and drilling riser/BOP are pulled.

9. The VT is run via the mono bore completion riser/BOP with a hose or small string

of tubing to maintain access to the production annuals.

10. The flow barriers are removed and either the well is allowed to flow back to surface via the

completion riser or the tree cap is ran and the well flows through the subsea manifold/pipeline.


The Concentric Vertical Tree concept was used for the following projects:1. Bongo (Shell – Nigeria)

2. Mensa (BP – GOM)

3. Crazy Horse (BP – GOM)

4. Macaroni (Shell – GOM)

5.2.5 Vertical Tree SummaryThe main differences between the Eccentric and Concentric Vertical Tree concepts are:

1.The Concentric Vertical Tree body lands on a tubing hanger spool while the Eccentric Vertical Tree body land on the 18-3/4 in. subsea wellhead.2. The Concentric VT uses a mono bore tubing hanger. Therefore, the production annulus access is around the tubing hanger (i.e. through the tubing hanger spool). Although the Concentric VT system is designed to capture many of the advantages of both Eccentric VT and Horizontal Tree concepts, this concept has some significant limitations to be considered during subsea tree concept screening.The key limitations include:3. A separate trip in critical path (with a single derrick MODU) is required to install the tubing hanger spool located between the subsea wellhead and vertical tree body which results in additional installation costs and another potential leak path.4. More potential leak paths due to additional connections.

5.3 Horizontal Trees:As previously noted, the Horizontal Tree (HT) concept diverts the production flow stream out a

side outlet in the tubing hanger through production flow valves located 90 degrees from the

vertical run of the tree. The tubing hanger lands in the body of the HT. The HT uses a

hydraulically actuated connection to connect to the high pressure wellhead. The HT also has a

profile on top of the tree that allows the drilling BOP stack to connect to the tree.

The tubing hanger for this HT concept has an intersecting outlet at 90 degrees to the production

bore. This diverts the production flow stream through the production master and wing valves.

These valves may be either integral to the HT body or bolted on the side of the HT body. Access

to the production casing annulus is provided by a valved side outlet configuration that exits

below and re-enters above the tubing hanger as shown in Figures 5.5. and 5.6.

To secure the well for production operations, a permanent wireline plug must be installed in the

vertical bore of the tubing hanger above the flow outlet. Then, an internal tree cap is run and

landed in the tree above the tubing. A second permanent wireline plug is then run and landed in

the internal tree cap to fully isolate the vertical flow path. This plug allows future vertical access

without pulling the internal tree cap.


Since the tubing hanger lands in the HT body and not the subsea wellhead, as with a dual bore

vertical tree, the interface between the subsea wellhead and subsea tree is less critical than with a

Vertical Tree concept. This allows the drill team greater flexibility to more easily use different

vendor designs for the subsea wellhead or utilize an existing exploration well that is temporarily

suspended. The HT concept can be further defined as “Partial Drilling” or “Full Drilling” as

shown in Figures 5.5 and 5.6. Both horizontal tree concepts utilize a concentric, mono bore

tubing hanger. Tubing hanger orientation is still required, due to mating of all control line

connectors.

5.3.1 Partial Drilling Horizontal Tree:This concept (concentric vertical bore through the hanger and the production flow) is diverted

into an intersecting horizontal bore against a permanent wire line plug in the tubing hanger. A

fixed helix in the tree body provides passive tubing hanger alignment. Access to the tubing by

production casing annulus is around the tubing hanger through the tree body shown in Figure

5.5. This tree is typically run and landed using drill pipe. Once the HT is installed, it is not

possible to land casing hangers in the 18-3/4 in. subsea wellhead. However, since the HT is run

after setting a full string of 9-5/8 in. production casing, it is possible to run a drilling bore

protector and drill or sidetrack in an 8-1/2 in. hole through the HT. One of the key advantages of

this concept is that production tubing may be used as the completion riser. This eliminates the

need to have a costly dedicated completion riser.


Figure 5.4 A) – Mica Partial Drilling Horizontal Tree

Figure5.4 B )– Mica Partial Drilling Horizontal Tree


Figure 5.5 Partial Drilling Horizontal Tree

5.3.2 Summary Of Partial Drilling Horizontal Tree1. Concentric vertical and intersecting horizontal bore in hanger.

2. Tubing hanger lands in HT body (passive alignment).

3. Tree installed on drill pipe.

4. Drill 8-1/2 in. hole with HT installed using a bore protector.

5. Production tubing used as completion riser.

The typical steps to running the Partial Drilling HT are as follows:1. Run and cement the production casing.

2. Test the casing and install storm packer for well bore isolation.

3. The drilling riser/BOP is pulled.

4. The HT is run with a drill pipe landing string.

5. The HT coupled to the connection on top of the wellhead.

6. The drilling riser/BOP is run and coupled to the connection on top of the HT.

7. The storm packer is removed.

8. The production tubing/TH is run with a subsurface test tree (SSTT) and a mono bore

completion riser or landing string. This system will be run through the drilling riser/BOP.

9. The well is flowed back to surface via the completion riser (landing string), or the internal tree

cap is run and the well flows through the subsea manifold/pipeline.

10. Pull the subsurface test tree (SSTT) and a mono bore completion riser or landing string.

11. Pull the drilling riser/BOP.

12. Run the external tree cap on drill pipe.

The Partial Drilling HT concept was used for the following projects:1. Diana, Marshall, Madison (ExxonMobil GOM)

2. Mica (ExxonMobil GOM) (Figure 5.4)

3. Kizomba (ExxonMobil Angola)

5.3.3 Full Drilling Horizontal Tree:This Full Drilling HT concept is based on combining the existing 16-3/4 in. subsea wellhead and

18-3/4 in. HT tree designs. The concept is to manufacture a 16-3/4 in. subsea wellhead system

with an 18-3/4 in. hub. This would permit an 18-3/4 in. HT system to be run on drill pipe

immediately after running and cementing the subsea wellhead on 20 in. conductor casing. Or in

the case of the slim hole design, 13-3/8 in. with a crossover to 20 in. casing and an 18-3/4 in. hub

is run. The 18-3/4 in. BOP stack would then be run and landed on top of the 18-3/4 in. HT. A


drilling wear bushing would be run to protect the HT bore. Then drilling and casing running

operations would be conducted through the 18-3/4 in. HT body (see Figure5.6).

The Full Drilling HT concept eliminates the need to pull the 18-3/4 in. BOP stack by allowing

the 9-5/8 in. casing string to be run through the HT body. Even though this concept can save a

BOP trip on a single well basis, much of its incentive is lost if batch installation of trees is done.

One key advantage of this concept is the use of a flow base which allows the tree body to be

pulled without disconnecting the jumper between the flow base and the manifold. The tubing

hanger spool is typically run & landed using drill pipe, while the tree body is usually installed

with a mono bore completion riser in open water. However, a separate trip is required to install

the tubing hanger spool, and another permanent leak path is introduced.

Figure 14.6 - Full Drilling Horizontal Tree

5.3.4 Summary Of Full Drilling Ht Concentric Vertical And Intersecting Horizontal Bore In Hanger1. Tubing hanger lands in HT body (passive alignment).

2. Tree installed on drill pipe.

3. Drill 12-1/4 in. and 8-1/2 in. hole with HT installed using a bore protector (slim hole design).

Production tubing used as completion riser.

The typical steps to running the Full Drilling HT are as follows:1. Run and cement the surface casing. Test the casing and install storm packer for well bore

isolation.


The drilling riser/bop is pulled.

2. The tubing spool is run on mono bore completion riser.

3. The ht is run with a mono bore completion riser, and the ht is coupled to the tubing spool.

4. The drilling riser/bop is run and coupled to the connection on top of the ht.

5. The storm packer is removed.

6. The remaining hole sections are drilled.

7. The production tubing/TH is run with a subsurface test tree (SSTT) and a mono bore

completion riser or landing string. This system will be run through the drilling riser/BOP.

8. The well is flowed back to surface via the completion riser (landing string) or the internal tree

cap is run, and the well flows through the subsea manifold/pipeline.

9. Pull the subsurface test tree (SSTT) and a mono bore completion riser or landing string.

10. Pull the drilling riser/BOP.

11. Run the external tree cap on drill pipe.

The Full Drilling HT concept was used for the following projects:1. Dalia (Total Fina Elf – Block 17 Offshore Angola)

2. Ross (Talisman Energy – North Sea UK)

5.4 Tree Selection Summary:Since the cost of VT and HT tree components are very close, the key driver for the subsea tree

selection process becomes the cost, availability, and operation of the completion/workover riser

system during initial installation. Certain assumptions for well intervention versus subsea tree

failures can also contribute to the subsea tree selection process. If it is assumed that more tubing

pull work overs would be required due to down hole completion failures, the HT concept

becomes more attractive.

The subsea trees have generally proved more reliable than the overall down hole tubing and sand

face completion. Thus, ExxonMobil has preferred horizontal trees for recent deepwater

developments to capture the operational and financial benefits associated with installation

efficiencies and to eliminate the need for a dedicated completion riser system.

A significant advantage of the vertical tree is that routine interventions into the wellbore can be

accomplished without the risks associated with pulling and resetting wire line plugs, but a

dedicated completion riser system is needed. The major advantage of the horizontal tree is that

the production tubing can be pulled from the wellbore without pulling the HT body, (using the

MODUs well control system, e.g. no dedicated completion riser system is needed). In many

development areas this is a significant driver to the tree selection decision.

A qualitative and quantitative, development specific, intervention study will be needed to

determine the frequency of the expected intervention operations. This exercise will help


highlight which tree concept is most cost effective for a given scope of intervention work. In

general, if tree failures are the source of the majority of interventions, then vertical trees tend to

offer substantial benefits. If down hole failures have the higher potential, then horizontal trees

have inherent benefits.

For example, a quantitative intervention study for a particular development may determine that

artificial lift is required and/or the risk of sand control failure is high. In that case, the project will

likely benefit from the use of a horizontal tree that allows tubing string removal without the cost

of subsea tree removal.

In contrast, a development of naturally flowing wells with no sand control requirement and little

risk of down hole mechanical failure could likely benefit from use of a vertical tree. This is

because the relative frequency of tree related intervention with respect to down hole intervention

is likely greater. A major failure of a vertical tree requiring a pulling operation could be

conducted without pulling tubing. With a horizontal tree, the tubing must be pulled prior to

pulling the tree.

Other considerations are driven by rig selection and rig availability issues. The deck space,

variable deck load constraints and moon pool size/height under the rig floor is a critical aspect of

handling trees and completion equipment. The VT is typically taller than the HT and so requires

more height clearance under the rig floor. The HT is typically wider than the VT and therefore

requires a large moon pool area. If a dedicated completion/workover riser is utilized, the drilling

riser would likely need to be offloaded between wells. These size constraints and riser issues will

dictate the type of MODU (Fourth or Fifth Generation) is capable of conducting the operation.

For deepwater operations, horizontal trees provide a number of key advantages over

vertical trees.

1. When cleaning up and testing a subsea well to a Mobile Offshore Drilling Unit (MODU), two

independent well control systems are fully functional. First, the subsea test tree with shut-in and

disconnect functions, provides the primary well control system. Second, the rig blowout

prevention stack (BOP) provides shearing and disconnect capabilities in the event of a subsea

test tree failure.

As a result, well control risk is substantially reduced.

2. Horizontal trees can be batch set on the subsea wellhead before commencement of completion

operations; thus reducing overall installation times, equipment costs and complexities of vertical

trees.


CHAPTER 6Case Study – Tree Selection For Deepwater GOM Project

Horizontal trees were selected for all of the recent deepwater Gulf of Mexico subsea

developments (Diana, Marshall, Madison and Mica). The basis for the recommendation was a

single well comparison of an Eccentric Vertical Tree versus a Partial Drilling Horizontal Tree.

The time motion analysis of the vertical tree versus Horizontal tree installation sequence showed

that the HT concept would take about 2 days more of rig time than the vertical tree concept.

However, the lower leased cost of the SSTT more than offset the higher cost of a leased dual

bore completion/workover riser system.

Additionally, the future availability of the leased completion/workover riser system for

workovers was identified as a concern. Purchase of a dual bore completion/workover riser

system for the small number of wells in the development could not be justified. The analysis

suggested that selection of the Horizontal Tree would result in an average savings of about

$800k/well. Even though the horizontal tree concept had not previously been used at the required

water depth (~4,650 feet of water), the drilling organization supported the HT recommendation,

since it appeared that it had greater potential to reduce completion installation time and costs.

Use of the Eccentric Vertical Tree concept and dual bore completion/workover riser system

could expose operations to potential weather delays if offloading of the drilling riser was

required due to deck space limitations on MODU. Drilling estimated that weather delays

offloading or loading riser could add $1.0M per well if sufficient boats were not available to

conduct these operations out of critical path. Actual installation times for horizontal trees

installed at the Diana field were compared to the vertical trees installed at the Zinc field. This

comparison showed an average savings of 6.7 days per well for horizontal trees using a single

derrick MODU. Minimizing tripping of the BOP stack and batch installation operations were key

drivers to this savings in time achieved at Diana. Additionally, the elimination of the

completion/workover riser system by using the MODUs drilling riser increased available deck

space for other completion equipment and decreased overall cost.

The HT concept provided future flexibility by being able to use existing drilling risers from

many different MODUs. Assumptions for well intervention can also contribute to the subsea tree

selection decision. The ability to sidetrack and re-drill through the HT further reduces costs since

sidetracks can be performed without pulling the HT. One sidetrack re-drill through the horizontal

tree has already been performed at Diana, and the development plan anticipates future sidetrack

re-drill opportunities. Drilling operations personnel also preferred the SSTT system and the

MODU BOP stack for well control instead of a specialized completion/workover riser system

that included a workover BOP and EDP. The decision to use the production tubing as the landing

string also provided additional cost savings to the project.


Conclusion

The capability and flexibility offered by modern subsea Christmas trees and production systems

is, by any measure, truly impressive. The industry has moved from simple production systems to

more expansive ones incorporating complex controls and sensors and a range of monitoring and

diagnostic systems. For the last 20 years the Horizontal Christmas tree has been the preferred

design for deepwater fields, while in recent years the focus in the industry is returning to the

conventional Vertical Christmas tree system. The demands for the production of hydrocarbons

from deep water at higher pressures and temperatures, coupled with a range of additional design

constraints, ensure that subsea tree systems will continue to evolve to meet these challenges both

now and in the future. The design of the Vertical Christmas tree and the Tubing Hanger (TH)

were a dual-bore configuration. The handling and operation with dual-bore systems compared to

mono-bore systems are more complex and time-consuming, and then again more costly.


References

1. Towns, T. K., Deeken, D. G., Derby, L. M., Exxon Mobil Development Company, “Diana

Subsea Tree Selection and Installation Results in 4,653 Feet of Water” Deep Offshore

Technology held in Rio de Janeiro, Brazil, October 17-19, 2001.

2. Moyer, M.C., Barry, M.D., Tears, N.C., "Hoover-Diana Deepwater Drilling and

Completions", OTC 13081, Offshore Technology Conference, Houston, Tx, May 2001.

3. Havard Devold," Oil And Gas Production Handbook", An Introduction to Oil And Production,

ABB Oil And Gas, May (2009), Edition 2.0 Oslo,29-32.

4. Oda Ingeborg Stendebakken, NTNU Trondheim, "A reliability study of a Deepwater Vertical

christmas Tree with attention to XT retrieval rate", June (2014),11-32.

5. Randy J. Wester and Eric P. Ringle / FMC Energy Systems," Installation and Workover Time

Savings: Key Drivers for Deepwater Tree Selection" ,OCT 12943, Offshore Technology

Conference, May (2001).


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