Subsea structures and equipment

The role of a subsea production system is to transport oil/gas production out of subsea wells to the host facilities for processing. It is composed of various subsea structures and equipment, typically including subsea trees, well jumpers, subsea manifold, flowline jumpers, PLET/PLEM, subsea flowline, risers, subsea umbilicals, etc

Subsea Manifolds

Subsea manifolds have been used in the development of oil and gas fields to simplify the subsea system,minimize the use of subsea pipelines and risers, and optimize the flow of fluid in the system. The manifold, is an arrangement of piping and/or valves designed to combine, distribute, control, and often monitor fluid flow. Subsea manifolds are installed on the seabed within an array of wells to gather production or to inject water or gas into wells. The numerous types of manifolds range from a simple pipeline end manifold (PLEM/PLET) to large structures such as a subsea process system. The manifoldmay be anchored to the seabed with piles or skirts that penetrate the mudline. Size is dictated by the number of the wells and throughput, as well as how the subsea wells are integrated into the system.

Pipeline Ends and In-line Structures

Pipeline end terminations (PLET)/pipeline end manifold (PLEM), and inline structure (ILS) are subsea structures designed to attach the pipeline end and then lowered to the seabed in the desired orientation. The PLET/PLEM is located at the end of a subsea pipeline, while the inline structure is located in the middle of the pipeline. The design and installation of PLET/ILS include first-end, middle, and second-end options. The components of them may include from a single hub with manual isolation valve, to two or three hubs with ROV actuated valves, chemical injection, pig launching capabilities and more. The foundation of PLET/ILS may be a mudmat, or a single suction pile. A rigid or flexible jumper is utilized to tie-in the PLET/ILS to the other subsea structures e.g. tree, manifold, or other PLET/PLEM.


Subsea PLET (Courtesy Shell)

In subsea oil/gas production systems, a subsea jumper, is a short pipe connector that is used to transport production fluid between two subsea components, for example, a tree and a manifold, a manifold and another manifold, or a manifold and an export sled. It may also connect other subsea structures such as PLEM/PLETs and riser bases. In addition to being used to transport production fluid, a jumper can also be used to inject water into a well. The offset distance between the components (such as trees, flowlines, and manifolds) dictates the jumper length and characteristics. Flexible jumper systems provide versatility, unlike rigid jumper systems, which limit space and handling capability.

Subsea Wellheads

Subsea Rigid Jumper

Wellhead is a general term used to describe the pressure-containing component at the surface of an oil well that provides the interface for drilling, completion, and testing of all subsea operation phases. It can be located on the offshore platform or onshore, in which case it is called a surface wellhead; it can also be settled down on the mudline, in which case it is called a subsea wellhead or mudline wellhead. Subsea wells can be classified as either satellite wells or clustered wells. Satellite wells are individual and share a minimum number of facilities with other wells. They are usually drilled vertically. Satellites wells can produce directly to a surface facility (the platform of a floating vessel) or through a subsea manifold that commingles the production of several satellite wells. The primary advantage of satellite wells is the flexibility of individual well location, installation, control, and service. Each well is handled separately, so that its production and treatment can be optimized. Exploration or delineation wells in a field can also be reused by completing them as satellite wells, thereby eliminating the drilling costs associated with a new well. When several subsea wellheads are located on a central subsea structure, the system is referred to as a clustered system. This arrangement provides the possibility of sharing common functions among several wells, such as manifolded service or injection lines and common control equipment, which then require fewer flowlines and umbilicals, thus reducing costs. In addition, because maintainable components are centralized on a clustered system, it is possible to service more than one well with a single deployment of a service vessel, thereby saving mobilization costs. On the other hand, shared functions can reduce the capability to treat each well separately. Clustered systems, however, introduce the need for subsea chokes to allow individual well control. Other disadvantages of clustered systems are that drilling or workover operations on one well of the cluster may interrupt production from others and special simultaneous drilling and production procedures need to be implemented.

Subsea Trees

The subsea production tree is an arrangement of valves, pipes, fittings, and connections placed on top of a wellbore. Orientation of the valves can be in the vertical bore or the horizontal outlet of the tree.The valves can be operated by electrical or hydraulic signals or manually by a diver or ROV.

Umbilical Systems

An umbilical is a bundled arrangement of tubing, piping, and/or electrical conductors in an armored sheath that is installed from the host facility to the subsea production system equipment. An umbilical is used to transmit the control fluid and/or electrical current necessary to control the functions of the subsea production and safety equipment (tree, valves, manifold, etc.). Dedicated tubes in an umbilical are used to monitor pressures and inject fluids (chemicals such as methanol) from the host facility to critical areas within the subsea production equipment.

Comparison of Vertical-Bore and Horizontal Subsea Production Trees (ABB Vetco Gray Inc.)
Subsea Steel Umbilical

Electrical conductors transmit power to operate subsea electronic devices. Umbilical dimensions typically range up to 10 in. (25.4 cm) in diameter. The umbilical will include multiple tubings normally ranging in size up to 2 in. (5.08 cm); the number of tubes is dependent on the complexity of the production system. The length of an umbilical is defined by the spacing of the subsea components and the distance these components are located from the host facility

Production Risers

The production riser is the portion of the flowline that resides between the host facility and the seabed adjacent to a host facility. Riser dimensions range from 3 to 12 in (76.2 to 304.8 mm). in diameter. Riser length is defined by the water depth and riser configuration, which can be vertical or a variety of wave forms. Risers can be flexible or rigid. They can be contained within the area of a fixed platform or floating facility, run in the water column.


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