Subsea Structure and Equipment

Subsea Manifolds

Subsea manifolds have been used in the development of oil and gas field 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 a 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 injection.

Pipeline Ends and In-Line Structures

As the oil and gas field developments move further away from existing subsea infrastructures, it becomes advantageous to consider a subsea tie-in of their export systems with existing deepwater pipelines systems offering spare transport capacity. This neccessitates incorporating pipeline end maniflods (PLEMs) at both pipeline ends to tie in the system. A PLEM is a subsea structure set at the end of pipeline. It is used to connect rigid pipeline with other subsea structures, such as manifold or tree, through a jumper. It is also called a pipeline end termination (PLET), especially to serve as support for one pipeline valve and one vertical connector.

Jumper

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. The offset distance between the components dictates the jumper length and characteristics.

Subsea Wellheads

Wellhead is a general term used to describe the pressure containing component at the surface of an oil/gas 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 mud-line, 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. Satellite wells can produce directly to a surface facility or through a subsea manifold that commingles the production of several wells. The primary advantages of satellite wells is the flexibility of individual well location, installation, control, and service.

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.

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. 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.

Production Riser

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. 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.