Serviço Nacional de Saúde (Servicio Nacional de Salud)

Once the four characteristics that influence the subsea tree design are defined, the next step in selecting the subsea tree for a deepwater development is to evaluate the different types of trees and how they best fit the application.

Subsea trees can be divided into two major types, horizontal trees and vertical trees (sometimes referred to as conventional trees).The major components and valve quantities are similar for both types of trees but they are arranged in a different manner. The major difference between the two types of trees is that for horizontal trees the tubing hanger is located in the body of the tree composite valve block and for the vertical tree the tubing hanger is located below the tree either in a wellhead or tubing head. The tubing hanger location drives the arrangement of the tree valves.

The graphic below shows how the tree valves are arranged for horizontal and vertical trees.

Horizontal Tree Vertical

Tree

WARNING: This information is provided to FMC customers solely to illustrate the operation of FMC equipment. It does not provide complete information for service or maintenance. Improperly performed service, maintenance, or installation could cause serious injury or death. FMC equipment is to be installed, serviced and maintained only by trained, authorized FMC personnel. © 2005 FMC Technologies, Inc

Other major considerations include the well control philosophy of the operator, the environment in which the tree will be installed, and the vessel(s) installing the tree.

Three example subsea tree designs are described below. All three of the following tree designs are used for deepwater field developments. These trees are fit-for-purpose, safe, reliable, and functionally equivalent in production service.

These tree designs can be configured for guideline (GL) and guidelineless (GLL) applications. For GL trees the tree guide frame would provide guide funnels located on an API standard 6ft radius to interface with guide posts on the wellhead or tubing head to guide and orient the tree to land in the desired orientation.

For vertical trees this guidance arrangement would also orient the tree production, annulus, hydraulic and electrical stabs with the mating profiles in the tubing

hanger. Guidelines are not normally used when completing a well in water depths that exceed 2,500 feet.

Other trees illustrated later provide large diameter re-entry funnels to allow orienting and landing of the tree in deep water without the need for guidelines.

This configuration is referred to as guidelineless, where a large guide funnel

captures the equipment and internal profiles (such as helixes) rotate the equipment into the proper orientation instead of guidelines.

Wellhead Completed Vertical Tree

The vertical tree system has the tubing hanger located in the wellhead or tubing head below the tree.

A tubing hanger is a component used in the completion of production wells. It is set in the tree or the wellhead and suspends the production tubing that provides a continuous bore from the production zone to the wellhead through which oil and gas can be produced.

Sometimes it provides porting to allow the communication of

hydraulic, electric and other downhole functions, as well as chemical injection. It also serves to seal-in the annulus and production areas.

Much more care in completion design is required when the tubing hanger is installed into a wellhead to account for casing hanger tolerance stack up and to ensure that correct orientation and alignment is achieved.

The use of a guideline/guidepost guidance system helps simplify in-the-wellhead completions because the wellhead permanent guidebase (PGB) becomes the

orienting keystone. The guide posts orient the blow out preventer stack assembly, or BOP stack, that in turn orients the tubing hanger typically using a hydraulic

WARNING: This information is provided to FMC customers solely to illustrate the operation of FMC equipment. It does not provide complete information for service or maintenance. Improperly performed service, maintenance, or installation could cause serious injury or death. FMC equipment is to be installed, serviced and maintained only by trained, authorized FMC personnel. © 2005 FMC Technologies, Inc

orienting pin connected to one of the BOP side outlets. When the subsea tree assembly is installed the same guideposts orient the tree to align the tree with the tubing hanger. In guide lineless operations, the BOP is typically not oriented, so other alignment tools and techniques must be employed. Installing the tubing hanger in the wellhead allows the well to be drilled and completed without the need to retrieve the BOP stack. The BOP stack does not have to be removed from the well to install the tubing hanger. However, all casing hangers, seal assemblies, and completion equipment must land and space-out exactly in the wellhead to ensure a successful completion operation because the tubing hanger metal seals are usually specified to seal between tubing bores, the wellhead, and tree.

Metal seals require precise alignment to within .003 inches and less than ¼ degree.

Therefore, an error free stack-up is essential. The UWD-15 wellhead system provides an indication of correct space out of the casing hangers and seal

assemblies in the wellhead when the seal assembly internal and external lock rings engage the lock down grooves in the casing hanger neck and the bore of the

wellhead housing.

If the casing hanger or seal assembly is landed high in the wellhead housing the seal assembly would not set correctly and be retrieved to surface with the running tool. Any debris, “gumbo”, silt, and/or scratches left behind by the drilling or completion process that lands on top of the tubing hanger may also cause problems in interfacing the tree to the wellhead/tubing hanger.

Vertical tree systems require two sets of landing strings or work strings for installation: one internal riser system for installing the tubing and tubing hanger through the BOP stack and drilling riser; the second via an open water

completion/workover riser system that connects to the top of the subsea tree to allow access from the surface through the tree’s production bore and into the well.

During the production phase, pressure containing tree caps are provided to add a second barrier above the tree’s swab valve. These tree caps can be run on drill pipe, riser pipe, wire rope, or ROV depending on water depth and installation conditions.

Open water completion risers are not available for water depths exceeding 7,500 feet, but industry development work is ongoing in that area. This issue may be a key decision driver in the tree selection process for ultra deep water.