Page 318 - Hydrocarbon Exploration and Production Second Edition

Surface Facilities 305

The template will be constructed and fitted out at a fabrication yard and then
transported offshore to the drilling location. The template is lowered to the seabed
using a crane barge or, if small enough, lowered beneath a semi-submersible rig.
Prior to drilling the first well, piles are driven into the seabed to hold the template
in place.
As the first well is being drilled the template is connected to the host facility
with flowlines, umbilicals and risers. A chemical injection umbilical will also typically
be laid to the template or subsea facility and connected to a distribution manifold.
As soon as the subsea tree on the first well has been commissioned, production
can commence. The rig will then move to another template slot and start drilling
the next well.
If one or more clusters of single wells are required then an underwater manifold
system can be deployed and used as a subsea focal point to connect each well. The
subsea trees sit on the seabed and are tied back to the main manifold through subsea
flowlines.
Only one set of pipelines and umbilicals (as with the template) are required from
the manifold back to the host facility, saving unnecessary expense. Underwater
manifolds offer a great deal of flexibility in field development and can be very cost-
effective.
The manifold is typically a tubular steel structure (similar to a template) which is
host to a series of remotely operated valves and chokes. It is common for subsea tree
control systems to be mounted on the manifold and not on the individual trees.
A complex manifold will generally have its own set of dedicated subsea control
modules (for controlling manifold valves and monitoring flowline sensors).

11.2.5. Control systems

As subsea production systems are remote from the host production facility there
must be some type of system in place which allows personnel on the host facility to
control and monitor the operation of the unmanned subsea system.
Modern subsea trees and manifolds are commonly controlled via a complex
electro-hydraulic system. Electricity is used to power the control system and to allow
for communication or command signalling between surface and subsea. Signals
sent back to surface will include, for example, subsea valve status and pressure/
temperature sensor outputs. Hydraulics are used to operate valves on the
subsea facilities (e.g. subsea tree and manifold valves). The majority of the subsea
valves are operated by hydraulically powered actuator units mounted on the valve
bodies.
With the electro-hydraulic system the signals, power and hydraulic supplies
are sent from a master control station (or MCS) on the host facility down control
umbilicals (Figure 11.38) to individual junction boxes on the seabed or subsea
structure.
The MCS allows the operator to open and close all the systems remotely
operated valves, including tree and manifold valves and downhole safety valve.