Gas-over-oil pipeline valve actuation
and 75 psi (5bar-g) external pressure
( 1,090 cycles at 2400 psi [160bar-g]
on each test), final static torque monitoring, strip down and inspection. This
was a much shorter test, taking only
three days to complete, although the
combined total operating test, amounting to over 2,000 cycles, is representative of a much longer working life.
The real life application for this equipment involves units with pressure compensators and manual controls for divers,
supplied for Phase One of the Ashrafi Gas
Lift project on the Red Sea coast. The
project involves onshore facilities, gas
lines and metering systems for 10 gas
wells and associated platforms in the
Gulf of Suez and is the continuation of a
development that has been producing gas
since the 1990s. The subsea hydraulic
actuators are operating 8-inch (200mm)
Class 900 ball valves.
In common with the stringent
demands of all contracts involving sub-
Gas-over-oil test rig
sea equipment, these actuators have to
meet specific project requirements. In
this task the company can also draw on
the experience of an installed base of
designs encompassing pressure-compen-sated rack-and-pinion, scotch-yoke and
linear actuators for both double-acting
and spring-return duties, equipped with
ROV and/or diver-operated facilities.
Actuator companies often work with
gear manufacturers—particularly ones
with subsea experience—which produce
a range of subsea gearboxes with diver
or ROV-operable overrides. The combination of products, engineering knowledge and test facilities provided by the
actuator and gear companies provides a
comprehensive subsea solution for both
retrievable and non-retrievable applications. For example, on retrievable applications such as some SSIV (subsea isolation valve) duties the designs provide
for interchangeability between the actuator and a ROV-operated gearbox by
means of transition spools and quick
release methods.
For ROV-operated overrides another
option is the ability to fit adjustable
extensions between the ISO torque tool
receptacle (or “bucket”) and the override input drive shaft located on the
actuator. This enables the receptacle to
be remotely positioned from the actuator to facilitate ROV or diver access
with the tool.
Gas-over-oil Actuators
The basic principle of operation of the
gas-over-oil actuator involves taking gas
from the pipeline upstream of the valve
and storing it under pressure in a reservoir (accumulator). The gas is used to
power one side of gas/oil tanks (one for
each valve operating direction), controlling the flow of hydraulic oil to and from
the actuator cylinder to move the valve.
The gas-over-oil actuator is well suited to respond very quickly to changes in
the physical condition of the pipeline—
for example, a loss of pressure or
increase in velocity denoting a line
break—and utilizes the inherent speed
of hydraulic operation to immediately
close (or open) the valve. Because it
uses the pressurized pipeline gas as an
