Is it necessary to install a downhole safety valve in a subsea ... - NTNU

The wing valve is located on the horizontal outlet from the x-mas tree. If it becomes
necessary to close the well, the wing valve is the first to be closed. This is where the
produced fluid is flowing through.
The swab valve is located on the vertical bores of the tree above the wing valve. The
swab valve is used to perform safe vertical re-entries into the tree and well during
workover.
The crossover valve connects the production bore to the annulus bore via a crossover
service line.
The annulus master valve is located on the vertical annulus bore. It is normally closed
and opened only if fluid is to be injected to the annulus bore.
The annulus wing valve is normally closed. It closes the side outlet oft the tree block
to isolate the service line during production and intervention.
Gate valves are the most common type valves in the x-mas tree. The gate valves are normally
operated either hydraulically, by mechanical override or remotely operated vehicle (ROV).
A1.4 Subsea wellhead
The subsea wellhead system is the primary component of a subsea production well. It
functions both as a pressure vessel and a structural support at the seabed. During oil and gas
production the wellhead support and seal the x-mas tree and holds the casings and the tubing.
In the wellhead, the tubing hanger and casing hangers are connecting and carrying the load of
the tubing and casings.
The tubing head is attached to the uppermost casing head and supports the tubing string. The
tubing hanger is an integrated part of the wellhead. It seals and locks the tubing inside the
wellhead housing. Seals isolate the production and annulus fluids and prevent leakage. The
tubing hanger can either be locked to the wellhead housing or directly via a casing hanger
lock down profile, or locked in the last casing hanger suspended in the wellhead. Hydraulic
and electrical communication with downhole equipment requires the use of penetrators in the
tubing hanger and x-mas tree.
The casing head is attached to the wellhead on a casing hanger. A seal is provided to avoid
leakage of annulus fluids into the next casing or to the surroundings.
A1.5 Guiding system
Two different guiding systems have been developed for well completions and workover,
guidelines and guidelineless. These systems are used to guide the Blowout preventer (BOP)
down to the seabed and place it in the right position.
A1.5.1 Guideline system
The guideline system is the most conventional and experienced method used for subsea
equipment installation. Four guidelines are connected to guideposts of a square wellhead
landing base. The BOP is lowered down on the guidelines. When the BOP is closing in on the
seabed they are tightened. This helps the BOP to connect to the wellhead at a precise position.
The guidelines can be installed and retrieved by ROV or a diver.
A-4