When a well is drilled the
structure placed on the seabed is called a wellhead. There may
be a single wellhead, though often there may be several units
grouped together to form a block. Attached to the top of the
wellhead are the control units called subsea trees.
The wellhead assembly is usually
contained within a supporting structure called a template,
with many also having a protection structure covering all
the subsea equipment, as shown in the image to the right.
||Subsea wellheads within a protection
A single wellhead can be 3-4 metres square and up to 5 metres high.
When several are grouped together within a template they become
very large structures on the seabed.
|Wellhead and protection
The protection structures covering
wellheads and their associated equipment are extremely large
structures, which although offering some protection to / from
fishing activities, they should still be treated with extreme
caution. Below you can see a protection frame seen on the back
deck of a support vessel prior to being installed on the seabed.
From this picture you can see the scale of these structures by the
size of the crewman in the foreground.
|Protection structures on the deck of
a support vessel
After initial drilling, a well may
be temporarily suspended if an operator intends to carry out
further operations at a later date. In this event a guide base is
left on the seabed to facilitate re-entry if required (as seen in
the image on the right). The structure left in place resembles an
inverted table with legs approximately 4 metres high. A central
pipe about 20 inches in diameter normally projects about 4 - 5
metres from the seabed.
Suspended wellheads do not have any
safety zones attached to their location and are not usually marked
however some may have buoys attached. These structures are therfore
extremely vulnerable to fishing operations and should be avoided at
The positions of new suspended
wells are published in the Offshore News section of this
website and the Kingfisher
Fortnightly Bulletin. A complete list is
available through Kingfisher Information's Yellow Card listing and as fishing plotter