Safety Considerations - Oil Production Platforms.  
By Mr. V.P. Raman (Chemtex Engg. )

 Oil Production – Unmanned Production Platform :     
The fluids emerging from any drilled oil/gas reservoir is a complex mixture of hydrocarbons, water, sand, salts together with methane & ethane which are in the gaseous form. The hydrostatic head drops rapidly as the fluid rises in the production tube string. This results in the lighter hydrocarbons to reach their saturation points and bubble out of the fluid. The gas/oil ratio generally increases dramatically, giving a two phase flow regime which is extremely complex to measure. This fluid then passes through the Down Hole Valve (DHV) alternatively called as Surface Controlled Subsurface Safety Valve (SCSSSV). Normally, the DHV is maintained in the open position by hydraulic pressure from the wellhead control panel. The fluid then passes through the Xmas tree traversing in turn through the Master Manual Valve, Hydraulic Master Valve alternatively called as Upper Master Valve or Surface safety valve (UMV/SSV) and Hydraulic Wing Valve (HWV). Both HWV and SSV is maintained in the open position by hydraulic pressure from the wellhead control panel. The fluid then passes through Manual or Hydraulically operated Choke Valve and this is used to control the production well pressure. The Choke Valve opening is controlled   Hydraulically from the well head control panel and the opening is determined with regard to the well production rate. In the event the flow line pressure rises above or below the designated set points, signals are derived from the well head control panel to close all the hydraulically operated valves, thus ensuring blowout of well. Finally, the flow line leads to a block valve, and thence to the production header with branch line to the wellhead oil test manifold. Normally, clusters of such production strings are connected to each production header which in turn feeds the production manifold. The piping is so arranged that an individual wellhead can be selected and routed to the oil test manifold for initial clean-up and analysis of reservoir performance in test separator. 
Each unmanned production platform can contain upto 4 or 6 single/ dual oil producers connected to the production header and depending upon the type of structure. Each cluster of wellheads is controlled from a single master well head control panel. Also this hydraulic control panel is interfaced to the Fire & Gas/MAC within this unmanned platform. 
Additionally, this hydraulic panel is interfaced to the SCADA system for remote transmission, monitoring and control from the manned platform and is also interfaced to the ESD system of the main manned platform. The hydraulic power for the various actuators are supplied either by pneumatically / electrically operated pumps. The electrical power is derived from solar panels charging a battery bank.  
Well Test Separator : 
Typically this has a conventional three phase separator, in which separation is enhanced by flashing the fluid to a lower pressure, which in turn lowers the fluid temperature and encourage the formation of gas hydrates. In consequence, the fluid is preheated giving the advantage of lowering fluid viscosity, which in turn enhances the separation process. 
This facility is either installed on the unmanned platform prior to drilling or is a mobile unit mounted on barge. 
This is used for the following: ¨   

  • Clean a well immediately after completion, disposing safely of its mud, sand and other contaminants prior to diverting the product to production train. ¨   

  • To establish initial product characteristics like Sp. Gravity, Salinity, Viscosity, Pour point, composition, characterization factor etc. ¨   

  • To develop reservoir characteristics like shut-in pressure, multiple flow rate tests, wellhead pressure & temperature.

  • To predict capabilities, potential and optimized oil recovery technique. 

 All the controls and the shutdown required for the operation of test separator inclusive of the ESD functions, are achieved from a local control panel which operates hydraulic/gas over oil actuators. Also this local control panel is interfaced to the Fire & Gas/MAC within this unmanned platform. This local control panel is also interfaced to the well head control panel. The control functions associated with this unit is driven either by the gas received from the well ( by suitable reducers / regulators ) or Nitrogen cylinders. 
Additionally, this hydraulic panel is interfaced to the SCADA system for remote transmission, monitoring and control from the manned platform and is also interfaced to the ESD system of the main manned platform. 

Production Safety System. 
Blowouts and fires in the oilfield can damage producing reservoirs, destroy equipment’s and take lives. Moreover they pose threat to environment also. 
Possibility of fire, natural calamities like earthquake, land slides, storm etc. or sabotage/terrorism etc., possibility of collision with marine vessels pose as main threat for any oilfield. Hence it is recommended to opt for automatic well shut-in system to insure against loss of life and property. Damage to sub-sea pipe line can seriously affect the environment and marine life. In fact the safety system represent less than 2% of the total well investment. 
The safety system is further divided into two categories:    

  • Surface safety valves (UMV/SSV) and Sub surface safety valves (SCSSSV). ¨   

  • The SSV’s are the first means of protection against most accidents or equipment failures in the production facility.

  • The SCSSSV are usually located 100 to 300 feet below the well head and are operated only if SSV’s fail. ¨   

  • The various types of valves used for these applications are Poppet type, Flapper type, Ball Valve and Sleeve type. 

Operation of SCSSSV:    

  • In case of direct control of SCSSSV, the valve is actuated by the well flow conditions at the valve.    

  • An excessive pressure drop must occur across the safety valve in order for it to close. mso-list:l17 level1 lfo6; ¨   

  • The differential type valve uses a spring and choke combination calculated to close the valve if the well flow rate should become greater than a predetermined rate. ¨   

  • The ambient pressure type, uses pre-charged chamber to actuate the valve if the tubing pressure falls below a certain value. ¨   

  • These valves are dependent on flow rate or pressure changes in   the well and hence the well characteristics must be known.    

  • For maximum safety the normal producing rate of a well must be below the closing rate of the valve and hence much lower than maximum production rate possible. These valves In case of remote control of these valves, they are operated from the surface via hydraulic control line and the hydraulic pressure must be greater than the well pressure, to hold the valve open. 

Operation of SSV   

  • This is the first means of shutting in a well. The valve is generally a reverse acting gate valve, which is opposite to a normal manual gate valve, with either hydraulic or pneumatic actuator. The reverse acting feature allows the well pressure pushing on the stem to assist in closing the valve thus increasing the reliability. Whenever the pressure is lost in the actuator the valve moves to safe position.    

  • The pneumatic actuator uses low pressure gas acting on a piston to push the stem and gate down into open position. The hydraulic actuator uses higher operating pressures thus making the actuator smaller in size than its pneumatic counterpart.   

Safety Sub Systems: 

  • The ability of a safety system depends on the ability of the safety valves to close in danger situations. The ability to perceive danger situations depends on a group of pressure pilots, known as monitors.          

  • Also fusible plugs are used to detect the fire situation in an unmanned platform. A low melting point alloy of lead, tin or bismuth plug will melt releasing the control line pressure, thus closing the safety valves. 

  • The erosion probe detects erosion from the fluid and abrasive particles before it erodes a flow-line and causing a major damage. 

  • Additional safety feature includes quick exhaust valves are installed on the safety valve actuators. Also the pneumatic tubing used for the separator control valve is made of plastic, so that this may fuse in case of fire and operate the safety valves.   

High Integrity Protection System – HIPS :  

A HIPS monitors the safety parameters and determines whether a process intervention or a total shutdown is necessary in order to avoid a dangerous situation and acts as a last line of defense in the safety system. Normally, in case of any oil production platform, the pressure is the most safety critical parameter. Similarly all the oil carrying pipelines arriving to the main manned production platform (risers) is equipped with HIPS to detect the trunk line failure. The common strategy for protection against over-pressure is to rely on multiple layers of protection (2 oo 3 – Voting logic) such as:-    

  • Control the pressure within normal operating limits by choke valve.    

  • HIPS to act immediately in case of control system failure or sudden sharp upward change in well pressure.   

Safety System Requirements:  
Typically in any of the manned & unmanned oil producing installations, it is recommended to use the safety shutdown systems including the field devices based on the IEC standards. 
Hence according to the criticality, a general guideline is defined below, related to selecting the type of safety systems for any manned/unmanned oil production platforms. Though this is not a set of standards, guidelines or norms, this only defines the minimum level of safety requirements for the intended application. 
(Note : Normally, the safety system requirements is determined only after a detailed HAZOP and the Operating Company’s operation / safety policy).      

  • ESD system shall meet TUV Class-7. ¨   

  • Process / Unit shutdown shall meet TUV Class-6. ¨   

  • Fire & Gas system shall meet TUV Class-7. ¨   

  • HIPS shall meet TUV Class-7. ¨   

  • Well head protection system shall meet TUV Class-7.   

 Conclusion :

•          Oil well disasters are catastrophic, both in terms of equipment and personnel loss. During the famous gulf war in early nineties, Iraq damaged a large number of Xmas trees and surface safety system from Kuwait’s oil producing wells and everyone witnessed the horrifying disaster of these wells, burning relentlessly for weeks.

•          Imagine a sub-sea oil trunk line getting damaged / ruptured and the amount of oil spill, which will create the ecological in-balance and environmental damage apart from the marine life.   “Safety is treated as a Company Policy, rather then dependent on individuals”. Hence, every individual of the Company should contribute to this policy in making the working place safe.    

Back