Subsea completion and production systems

INTRODUCTION:

In this article we try to discuss subsea completion and production system.

There are many different types of platforms for offshore drilling activities, from shallow-water steel jackets and jackup barges , to floating Semi-submersibles and

drillships able to operate in very deep waters. The shift to subsea production systems represents a significant departure from conventional operations.(3)

History:

Subsea completions can be traced back to 1943 with the Lake Erie completion at a 35-ft water depth. The well had land-type christmas trees that required diver intervention for installation, maintenance, and flow line connections.

Shell completed its first subsea well in the Gulf of Mexico in 1961.1

Well completion definition:

Completion is doing a series of process like installing different equipments such as wellhead, chrismas tree, tubing , packers, safety valves and carrying out process like fracturing, perforating and acidizing.

In our definition we assume well completion as a process to make well ready for production, so it is mainly related to after drilling jobs, we don’t include cementing a casing part of well completion. there would be an emphasis on different subsea production systems and specific equipments needed.

Subsea oil field developments are usually split into Shallow water and Deepwater categories to distinguish between the different facilities and approaches will be applied.(1)

Usualy shallow water depths wells use bottom-founded facilities like jackup drilling rigs and fixed offshore structures. Deepwater well is located in water

depths greater than around 600 feet and another type of offshore drilling vessels is drillship which is capable of drilling in water depthes up to 13000 ft.3

Economy and technical complexities are two important factors which makes an offshore drilling very distinct.for example technical issues we run into are such as:

• Additional backpressure (resistance to a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving, such as piping or air vents, against its direction of flow)

• Processing fluids subsea

• Reducing wellhead pressure to allow effective recovery (muiltphase pumping and pressure boosting)

• Flow assurance (ensuring successful and economical flow of hydrocarbon stream from reservoir to the point of sale)

• Well monitoring and intervention

• Integrity of subsea and pipeline system

(4 and 1)

Importance of economy in subsea projects:

For example With deepwater drilling rig rates in 2010 of around $420,000/day, and similar additional spread costs, a deep water well of duration of 100 days can cost around US$100 million.

With high performance jackup rig rates in 2010 of around $150,000, and similar service costs, a high pressure, high temperature well of duration 100 days can cost about US$30 million.

Onshore wells can be considerably cheaper, particularly if the field is at a shallow depth, where costs range from less than $1 million to $15 million for deep and difficult wells.(1)

Environmental challenges:

Subsea production requires specialized equipment which must be enough safe for enviroment.For relatively shallow water depth we can use diving equipment while for deep water conditions we need robotic equipments. Any requirement to repair or intervene with installed subsea equipment is thus normally very expensive. This type of expense can result in economic failure of the subsea development.high pressure and high tempreture are common environmental challenges we have to deal with.

Subsea completion functions:

1.Sealing the wellhead from the environment by means of the tree connector. 2.Sealing the production bore and annulus from the environment. 3.Providing a controlled flow path from the production tubing, through the tree to the production flow line. Well flow control can be provided by means of tree valves and/or a tree-mounted choke. 4.Providing access to the well bore via tree caps and/or swab valves.(4)

Specialized equipment:

well heads: A wellhead is the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment ,

subsea wellheads from vetcogray company:

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Well head functions:

1.Protection against uncontrolled flowing

2.Well flow rate control( choking)

3.Periodic monitoring of well status and placing the well in safe condition by wireline tools run into well

4.Withstanding pressure and temperature during production, when the well is shut in or in exceptional operations(hydraulic fracturing for example),2

Christmas tree:

It is common to identify the type of tree as either "subsea tree" or "surface tree". Each of these classifications has a number or varieties within them. Examples of subsea include conventional, dual bore, mono bore, TFL (through flow line), horizontal, mudline, mudline horizontal, side valve, and TBT (through bore tree) trees.1

(in the offshore , dry trees sit on a production platform where the crew can use their hands to work them. Wet trees sit on the wellhead at the sea floor. An operator works them from a production platform connected through an umbilical)2

Types of Subsea Trees

There are various kinds of subsea trees, many times rated for a certain water depth, temperatures, pressure and expected flow.

The Dual Bore Subsea Tree was the first tree to include an annulus bore for troubleshooting, well servicing and well conversion operations. Although popular, especially in the North Sea, dual bore subsea trees have been improved over the years.

These trees can now be specified with guideline or guideline-less position elements for production or injection well applications.

Standard Configurable Trees (SCTs) are specifically tailored for company's various projects. A general SCT is normally used in shallower waters measuring up to 1,000 meters deep.

High Pressure High Temperature Trees (HPHT) are able to survive in rough environments, such as the North Sea. HPHT trees are designed for pressures up to 16,500 psi and temperatures ranging from -33 C to 175 C.(1)

Access to subsea equipments:

Remotely Operated Vehicles (ROVs) are robotic pieces of equipment operated from afar to perform tasks on the sea floor. ROVs are available in a wide variety of function capabilities and complexities from simple "eyeball" camera devices, to multi-appendage machines that require multiple operators to operate or "fly" the equipment.

Riser:

drilling riser is a conduit that provides a temporary extension of a subsea oil well to a surface drilling facility. Drilling risers are categorised into two types: marine drilling risers used with subsea blowout preventer (BOP) and generally used by floating drilling vessels; and tie-back drilling risers used with a surface BOP and generally deployed from fixed platforms or very stable floating platforms like a spar or tension leg platform (TLP).(4)

Subsea production system:

Subsea production system can range in complexity from:

1.a single satellite well with a flow linked to a fixed platform or FPSO

2.an onshore installation to several wells

3.clustered to a manifold and transferring to a fixed or floating facility

FPSO Transferring:

In order to transfer hydrocarbons from subsea wells to onshore facilities we use FPSO which is a giant unit vessel that can receive the hydrocarbons from several wells. FPSO is able to process and store the received hydrocarbon and then transfer it to onshore. This vessel can store huge amount of oil and gas until we get to onshore and needed to be offloaded. This massive vehicle works like a oil tanker and is suitable for frontier lands so we don’t have provide pipeline transportation so it decreases cost and save money because installing seabed pipeline is very expensive than onshore pipelines, especially for small fields which pipeline is not cost effective at all. FPSO is also movable to any rig which we need so after well is depleted we relocate it to a new rig.

1

Subsea Manifold:

• This is typically a tubular steel structure (similar to a template), consisting of valves and monitoring instruments.

• It is host to a series of remotely operated valves and chokes responsible for interconnecting the drainage/flow of several wells to the production unit

•

(4)

Transferring methodes:

Another system used to transfer hydrocarbons is pipeline system. This method has advantages and disadvantages. For example as mentioned before seabed pipeline is costly and difficult to be done but its suitable for large fields included several wells and close enough to onshore, because it can be buried under seabed to avoid massive waves , commercial fishing activities and it helps during hurricanes we don’t run into problems. But if pipeline was still uneconomical we can consider tankers for transportation purposes. It is suitable in small field and unstable sea floor condition.

. (7)

Subsea advantages:

• Produce from satellite wells.

• Extend field life of older platforms. • Faster development time • Scaleable developments

Economic viability

• Water depth • Resource density

Subsea Challenges:

• Operability and Integrity • Operability and integrity impact opex and downtime • Flow assurance challenges • Integrity of subsea and pipeline systems • Subsea valve integrity • Controls and distribution integrity • Aging equipment • Direct Failures: Corrosion, joint failure, splice failure, sensor failure,

solenoid valve • Indirect F ailures: Dropped object, umbilical dragged by anchor chain or

trawl board, wax or hydrate blocking sensor port (4)

Subsea completion sequences according to schlumberger guidline (20 steps):

1.completing drilling and install packers

2.retrieve drilling riser and BOP pack

3.retrieve drilling guidebase with ROV assistance

4.run the production flow base

5. run subsea horizontal tree

6.land the tree , lock connectors, test seals and function valves with ROV

7.run BOP stack onto horizontal tree, run BOP test tool

8. retrieve suspention packer , remove wearbushing from tree

9.run completion string, make up tubing hanger

10.land hanger in production tree and and test seals

11.rig up production test package( electric wireline and lubricator)

12.run guns ,correlate and perforate wells

13.carry out production test, acid stimulation and multirate test

14.unlatch running tools,rig down production test and packages

15.run internal tree cap 16.ROV closes tree valves

17.retrieve BOP stack, retrieve guidewires

18.install debris cap, deploy telescopic legs

19.suspend well

20.tie into pipeline for production

References:

1. Wikipedia (well completion, subsea and all definitions )2. Well completion and serving : oil and gas development techniques

(Denis Perinn, Michel Caron, Goerge Gaillot)3. Applied drilling engineering (Adam T.Bourgoyne)4. Subsea completion and production systems (steffan K.Jaroop)5. Vetcogray oil and gas business catalog6. Schlumberger subsea solutions brochure7. Transportation of oil and gas in Alaska (Shell Company )