Subsea completion and control
“Completion” is used in offshore oil and gas activities in two
different contexts. A well completion involves a set of
actions taken to convert an individual borehole into an
operational system for controlled recovery of underground
hydrocarbon resources. Those actions include installation
of the final well casings that isolate fluid migrations along
the borehole length while also establishing perforated
sections where needed to capture the hydrocarbons from
the geologic reservoir into the production casing.
A subsea completion refers to a system of pipes,
connections and valves that reside on the ocean bottom
and serve to gather hydrocarbons produced from
individually completed wells and direct those hydrocarbons
to a storage and offloading facility that might be either
A subsea completion is one in which the producing well
does not include a vertical conduit from the wellhead
back to a fixed access structure
A subsea well typically has a production tree to which a
flowline is connected allowing production to another
structure, a floating production vessel.
Subsea completions may be used in deep water as well
as shallow water and may be of any pressure and
temperature rating including high-pressure, high-
Subsea completions consist of a production tree sitting
on the ocean floor, an upper completion connecting the
production tree to the lower completion and the lower
completion which is installed across the producing
Subsea completions typically contain an upper completion, a
lower completion, and a production tree.
Production Tree. The production trees are typically available in
traditional vertical trees and horizontal trees. Those are further
characterized by their mode of operation (electric versus
hydraulic) and the number and types of penetrations through the
tree to control subsurface equipment and hydrocarbon production.
Upper Completion. The upper completion consists of production
tubing from the tree to the subsurface safety valve (SSSV) and
then production tubing down to the production packer installed in
the production casing.
The types of SSSVs vary by their method of installation. For
normal wells, the typical mode is within the tubing and installed
with the completion.
Other variations of SSSVs include the method of operation
(hydraulic versus electric), and various types depending on
methods of construction (opening method, sealing mechanism,
The production packer varies by the desired method of retrieval.
Permanent packers must be drilled out to remove them from the
The lower completion consists of a gravel-pack packer,
sand control screens, and a lower sump packer all
connected together by production tubing.
The gravel-pack packer is installed above the screens and
serves to anchor the lower completion inside the
Various types of packers are available depending on the
method of gravel packing the well and the desired release
The sand control screens and the accompanying gravel
pack or frac pack vary with the formation types and
desired productive interval placement.
Screens may be of various types including wire mesh; wire
wrapped, and pre-packed screens.
Expandable sand screens may also be installed to maximize
the remaining inside diameter of the screen base pipe
BENEFITS AND OPPORTUNITIES WITH SUB-SEA
A. Environmental and Economic Benefits
Subsea completions offer environmental benefits
that accrue during the development of the
resource (less time over the hole, fewer
resources used, less capital equipment requiring
resources to develop the field, etc.) as well as
continuing availability during the production and
eventual disposal of the production equipment
(platforms, manifolds, etc.).
Subsea completions have an economic
advantage compared to other field development
alternatives such as bottom-founded structures
(platforms, etc.). This advantage increases with
increasing water depth
the cost of a bottom-founded structure was
compared to a Floating Production, Storage, and
B. Barriers and Opportunities
The true success of a subsea completion lies in its
ability to continue to produce over time. Any
interruption of the production stream (particularly from
deepwater, high-producing wells) can quickly affect
the economic performance of a project. Fortunately,
subsea completions are relatively trouble-free after the
”. The new requirements are that when a lessee or operator
plans to install SSSVs and related equipment in an HPHT
environment, the lessee/operator must submit detailed
information with their Application for Permit to Drill (APD),
Application for Permit to Modify (APM), or Deepwater
Operations Plan (DWOP) that demonstrates the SSSVs and
related equipment2 are capable of performing in the
applicable HPHT environment. The detailed information must
include the following:
A discussion of the SSSVs’ and related equipment’s design
A discussion of the SSSVs’ and related equipment’s design
validation and functional testing process and procedures
An explanation of why the analysis, process, and procedures
ensure that the SSSVs and related equipment are fit-for-
service in the applicable HPHT environment
The safety management of different types of subsea
completions has been reviewed in previous industry
publications (Cooper, 2008; King, 2001; Fahlman, 1974). The
safety aspects can be distilled into the following categories: (1)
risks to personnel,
(2) risks to the environment
(3) risks to equipment or operations.
Economics. The primary economic advantage of a subsea
completion can evaporate instantly if a workover is required.
The subsea wellheads are designed so that workovers are
possible by re- entering the well but mobilization of floating
workover rigs and the day-rate costs of those vessels make all
but the most serious operations to be cost-prohibitive.
Advances in well intervention to reduce cost and improve
operational capability are required to further enhance the
economic attractiveness of subsea completions
General Technology Issues
General technological aspects of subsea completions are
concerned with the materials and environment of the wells.
Typically, the cost of interventions drives operators to select
materials which have known survival rates in the estimated
down hole environment. high alloy materials are generally
selected to insure life-of-the-well performance regardless of
their cost multiplier over conventional alloys.
Material availability in large-bore components can sometimes
be an issue as well as delivery in volumes as required for
subsea field development.
Since the completion of subsea wells began, the push to
deeper and deeper water to reach more and more
hydrocarbons seems to be an unstoppable march.
Drilling and completing exploratory wells is replete with risks
relative to unknown pressures, temperatures, and gradients of
pressure that may change quickly due to geologic conditions.
Shallow gas is one example of a drilling hazard that must be
adequately anticipated and managed during well construction.
The barriers and opportunities for subsea completions
relative to environmental aspects fall into two categories.
1)The first opportunity is reduction of overall resources
needed to develop the hydrocarbon production.
Considering the size and mass of steel required to
construct an offshore platform, the development of a
series of wells using subsea Completions make the
2)The second category of environmental effects is that on
the potential for reduction of spills, leaks, and other
releases of hydrocarbons during well construction and
production. The subsea completion by its nature is a
well-controlled activity as the equipment must be
designed to operate under water (at sometimes
significant pressures) which, in itself, requires sealed
connections to prevent water ingress and therefore
Subsea Control System
In order to ensure safe and efficient operation of the subsea
production systems, their various components such as
valves, chokes and connectors must be properly controlled.
Also it is often desirable to have feedback from these
components and other instrumentation indicating various
process parameters such as down hole/wellhead pressures
and temperatures and fluid flow rate and diagnostic
parameters of the control system itself.
Control systems currently employed utilize hydraulics and
often electronics to differing degrees. Of these most
commonly used is the electro-hydraulic multiplexed system.
The system requires hydraulic power supplied from the host
platform to actuate control devices. Coded signals for
operating these field devices and data signals from field
instruments to the host platform are transmitted through an
Electric power is usually supplied through a separate
cable.Research is underway for developing an all-electric
control system, which will eliminate the need for hydraulic
link between the subsea system and host platform