Thursday, July 26, 2018

SSV AND ESD SYSTEM

Automatic safety systems are designed to automatically shut in the  well in an emergency.
An emergency could be a fire or abnormally high or low pressure fluctuations, due to
ruptured lines, faulty surface equipment, damaged wellheads or other dangerous situations. 

MONITORING SYSTEM

There are two basic monitoring systems for controlling the closing of the safety valves:

·       Remote Control: The valve is fitted with a monitor system sensitive to changes at points located away from the valve installation.

·         Direct Control: This uses a self‑operating valve and monitor system that is sensitive to changes in pressure at the valve installation point only.

Wellhead safety valves are normally gate valves with a pneumatic or hydraulic actuator.
Pneumatic or hydraulic pressure is needed, acting on a piston in the actuator to open the
valve.

Failure or loss of pneumatic or hydraulic pressure will cause powerful springs in the actuator
to close the safety valves.


All safety valves on wellhead safety systems are 'fail closed' valves. Fail closed means that
if there is any problem with the power supply that operates the valve then the valve will
move to the closed position. The power supply operating the valve could be electrical,
pneumatic or hydraulic.

Multi‑well Installation Combining Hydraulic and Pneumatic Equipment

Pilots

The sensors that monitor pressure changes in well safety systems are called pilots. 

A low pressure pilot detects abnormally low pressures.

·                     A high pressure pilot detects abnormally high pressures.

·                     A 'hi‑lo' pilot detects abnormal pressure outside of a preset operating
                      range.

On offshore platform installations there are many wells drilled from the same platform.
These wells are close together at the surface and there is very little space between the
producing wells and the production process equipment.

There is nowhere for people on the platform to run to get away from a dangerous situation so the possibility of a very serious and dangerous situation is therefore greater on offshore facilities than it is in the desert

Therefore, offshore facilities have Emergency Shut Down (ESD) systems.

Single well Control Panels with Pilot (Control SSV and SCSSV) on
Offshore Wells
Multi‑well Hydraulic Control Panels

EMERGENCY SHUTDOWN (ESD) SYSTEMS

These systems can be activated manually or by pilots and sensors. An ESD system will
automatically close in all the wells and shut down the plant.

If a well has a downhole safety valve that is controlled from the surface, this valve will also
be closed.

There is a time delay in the safety system that closes the surface safety valve several
seconds before the sub‑surface safety valve. This is called a 'sequential shut in'.

By closing the surface valves first, flow across the sub‑surface safety valve is stopped. This
removes the possibility of flow cutting or flow damage to the sealing elements of the
subsurface safety valve. 

ANCILLARY EQUIPMENT

Any emergency safety system has to operate quickly and efficiently to remove the
dangerous or potentially dangerous situation in the shortest possible time.

There are special pieces of equipment that help the system do this. They are described
below.

Fusible Plugs

Fusible Plugs
The fusible plug protects against extremely high temperatures. If a fire occurs at the well,
the fusible material in the plug melts and is blown out of the plug, causing a decrease of
control line pressure.

This decrease of control line pressure is sensed at the control panel which then releases pneumatic or hydraulic pressure from the safety valve actuator allowing the valve to close.

Emergency Shutdown Valves

Emergency Shutdown Valve
These are manually operated and have the same effect on the system once the loss of
control line pressure is sensed at the control panel.

The ESD valves can be placed at different remote locations and/or the limits of the plant or well tower.

Quick Exhaust Valves

Quick Exhaust Valves
Loss of control line pressure creates a pressure differential (∆p) across the quick exhaust
valve. This activates the quick exhaust valve.

The quick exhaust valve is located close to the actuator and is designed to relieve the pressure that is holding the actuator open. This causes immediate closure of the surface safety valve.

Block‑and‑Bleed Valves

Three Way Block‑and‑Bleed Valve
These are three way valves that receive the compressed air from the control panel and pass
it on to the actuator on the surface safety valve.

There is an internal diaphragm which has the pressure equalised across it.

A third port is connected to the control line that supplies air to the pilots.

When a pilot detects a pressure change beyond its preset values, the pilot releases (vents)
control line pressure. This creates a differential across the diaphragm which activates the
block‑and‑bleed valve.

The valve blocks (stops) incoming supply of control air and bleeds off the control pressure from the safety actuator. This causes immediate closure of the safety valve.

Flapper‑type Valves


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