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1、Conceptual Design AnalysisApplied to Offshore Control SystemsAn Independent Learning Module from the ISA by Bill G. Tompkins, (summarized by Paul Gruhn)or Offshore Control Systems for DummiesBaseline CriteriaAn offshore control system monitors and controls the operations of drilling and production o
2、f oil and gas, yet it constitutes only 1 to 3% of the total cost of an offshore facility. The greatest impact of a control system component failure is not the expense of repair, but the loss of drilling time and/orlost production revenue. While there are a number of factors involved, the typical con
3、trol system payback period is measured in days.The building block method presented in this book begins with identifying the three groups o variables that must be considered, as shown in Figure 1: Facility types Control system conceptual design alternatives Influencing factorsFacility TypesThere are
4、three basic offshore facility types.Multilevel: This type of facility has the capacity for simultaneous drilling and production, and can accommodate 200 to 300 personnel. The supporting structure may be either stationary or floating. Typical examples of this type of platform are found in the North S
5、ea, Australia, Canada, and the Arctic.Multiplatform (Bridge-Connected): This type of facility also has the capability of simultaneous drilling and production and can accommodate around 100 personnel. The supporting structures are normally stationary. Typical examples are found in the US West Coast,
6、Middle East, and Southeast Asia.Single Platform： This minimum production facility does not have permanent drilling facilities, and may be either manned or unmanned. The supporting structure is stationary. Typical examples are found in the US Gulf of Mexico and Nigeria.Conceptual Design AlternativesA
7、ll offshore control systems consist of three basic subsystems: Monitoring and control subsystem Emergency shutdown (ESD) subsystem Safety subsystemFigure 1: Conceptual Design Data FlowThere are three general conceptual design alternatives: Local Monitoring and Control (LM&C): All indication instrume
8、ntation directly connects to the process. May be either pneumatic or electronic. Local Monitoring and Control with Remote Monitoring: Used to remotely monitor all shutdown and fire alarms in a central control room. May be either pneumatic, electronic, or computer based. Central Monitoring and Contro
9、l: All indication is local and monitored in a central control room. All control and recording instruments, as well as ESD and safety panels, are also located in the central control room. May be either pneumatic, electronic, or computer based.Influencing FactorsSome influencing factors are considered
10、 relatively constant, while others may unpredictably change with time, as shown in Table 1.Constant FactorsChanging FactorsSpaceDesign & Construction ScheduleWeightOperational ConsiderationsCostLocal RegulationsReliabilityAvailability of Trained PersonnelControl Room RequirementsAvailability of Spar
11、e Parts / ServiceTable 1: Influencing FactorsOffshore SystemsOffshore functional systems are typically divided into the following categories:1. Drilling2. Production Process3. Structural and Marine4. Piping5. Safety6. Facility ManagementDrilling SystemsDrilling systems are normally considered indepe
12、ndent and unique to any offshore facility. Operation of these systems is usually performed by personnel not skilled in the operation of other offshore systems. The drilling instrumentation (drillers console) is usually located in the same area as the drill equipment in order to monitor and control d
13、rilling operations.The drilling systems involve the portion of the drilling process concerned with safe, controlled perforation of the field reservoir. Four basic functions are generally included: Regulation of the drilling rate and rotary torque. Regulation of drilling fluid circulation. Blowout pr
14、evention (BOP), which are usually hydraulic systems. Drilling fluid circulation choke system.Instrumentation involved with drilling requires the measurement / control of weight, torque, and speed, and their association with automatic drilling and logging. Instrumentation involved with mud operations
15、 requires the measurement / control of pressure, temperature, level, flow, and density.Production Process SystemsThe production process area is the most significant contributor to the total offshore facility control system element (I/O) quantity, and has the greatest variety of variables to be measu
16、red and /or controlled. Variables include pressure, differential pressure, temperature, level, flow, speed, and other specialty measurements.Subsea/Platform Wellheads and Manifolds: The wellhead portion of the process can be divided into two basic schemes: subsea and surface (platform) wellheads. Ea
17、ch wellhead or group of wellheads is typically monitored/controlled with a hydraulic system. Other facility monitoring and control systems interface with these hydraulic subsystems either pneumatically or electrically.Oil Production: These systems basically consist of changing the product stream tem
18、perature (when required) and allowing gas/oil/water separation to occur. Control philosophy involves control of level, pressure, and temperature.Low-Pressure Gas Compression: Low-pressure gas (500 psig) is accumulated and compressed for injection back into the facility gas supply. The compression tr
19、ain is modular and most instrumentation is integral to the package.2500 psig Gas Compression: In order to efficiently dispose oflow-pressure gas or provide future gas lift, it is necessary to raise the pressure of this gas to at least that of the sales pipeline. These compression trains are also mod
20、ular and most instrumentation is integral to the package.6000 psig Gas Compression and Injection: In order to provide pressure maintenance for a field, the gas pressure must be greatly increase to allow well injection. These compression trains are also modular and most instrumentation is integral to
21、 the package.Gas Lift Wellheads and Manifolds: Gas lift is a means used for producing oil after natural flow ceases or for supplementing natural flow. High-pressure gas is introduced at the wellhead to displace or aerate the oil from the point of gas injection to the surface. These systems are usual
22、ly available with a packaged hydraulic control skid.Gas Dehydration: To prevent severe corrosion and hydrate problems, it is necessary to dehydrate gas prior to delivery to shore facilities. The gas is dehydrated to pipeline specifications by contacting it with glycol. Control philosophy involves co
23、ntrol of pressure, level, and temperature.Crude Sales Pipeline or Tanker Loading: Two methods of crude oil delivery are typical: pipeline- to-shore? and delivery to a ship for transport.Venting and Flaring System: These may operate at either atmospheric, low, or high pressures. Flare systems may req
24、uire pressure regulators and scrubbers may be regulated with a level controller.Wastewater Treating: These treating systems receive oily water effluent from the produced water header, process vessel drains, closed equipment drains, and open deck drains. The water is treated and disposed of. Control
25、philosophy involves control of level and pressure.Water Treating andlnj ection: When water is injected through wells into the reservoir as a secondary oil recovery technique, the water must be treated prior to injection to prevent reservoir plugging. This usually involves flow and back pressure cont
26、rol.Power Generation: There are typically two categories; a) main, interruptible AC power, and b) emergency power. Main facility power typically is provided by turbine-driven generators. Emergency power comes from engine driven generators and batteries. The control packages are usually integral with
27、 the systems.Other systems include: Firewater and Hypochlorite Generation Potable and Service Water Fuel Systems Service and Instrument Air Injection Chemicals Glycol and Lube Oil Storage and Distribution Quarters HVAC (Heating, Ventilating, and Air Conditioning)Process Shutdown SystemsThere are ess
28、entially four technologies used for process shutdown systems, which are really no different than other industries. Each uses a different principle in receiving data, executing the associated shutdown logic, and completing the process shutdown.Pneumatic; These systems are the largest and heaviest. Th
29、ey are considered the easiest to understand, and therefore only require a low operating and maintenance skill level.Relay; An electrical, rather than pneumatic system. Hermetically sealed relays are normally used.Solid state; An electrical system using printed circuit boards and solid state relays.
30、These systems do not use computers or software.PLC (Programmable Logic Controller); A software based system.Each system has been used offshore. Larger installations generally prefer solid state and software based systems, as they are smaller, lighter, and generally less expensive. Pneumatic systems
31、have historically been used on smaller platforms.Structural and Marine SystemsThese two systems are physically related by immersion of the structure in the water and by the effect of marine conditions on the structure. There are independent systems for monitoring controllable (e.g., buoyancy) and un
32、controllable (e.g., weather) variables.The structural subsystems include fixed-leg monitoring, critical support member monitoring, hull/compartment monitoring, collision damage monitoring, crane boom and drill mast monitoring and corrosion monitoring. Instrumentation involved may require the measure
33、ment/control of pressure, temperature, level, and load. Structural systems are monitored for integrity relative to the level of stress under which the members are placed. Fixed platform legs are compression members and are not normally monitored continuously, Semi-submersible facilities are buoyant
34、and held in place by tethers or tension members. Semi-submersible facilities are designed to move with differing environmental conditions, so the risers and drill string also have to be tensioned.Marine subsystems are normally related to seaworthiness and environmental compatibility. They typically
35、include ballast/stripping systems, bilge systems, and fuel dispensing. Semi-submersible facilities typically use a load and ballast computer to monitor and control stability. Marine systems typically require the measurement / control of pressure, temperature, level, flow, and acceleration.Piping Sys
36、temsLauncher Facilities: Pipeline efficiency is maintained by periodically sending spheres and / or pigs (with brushes, swabs, or scrapers) through the pipeline. Once launched, they are propelled by the pressure of the pipeline stream. The facility from which spheres and pigs enter the pipeline is r
37、eferred toas a launcher. The frequency of pigging depends on the pipeline fluid characteristics and pipeline pressure drop. These systems are typically equipped with local instrumentation.To prevent pipeline leakage or rupture due to severe corrosion, subsea pipelines are periodically internally ins
38、pected. This is usually accomplished with leased equipment.Safety SystemsActive safety systems include: Fire Detection； smoke, flame, and heat. Gas Detection； combustible, and sometimes toxic. Manual Detection； both emergency shutdown buttons and manual fire stations. Extinguishing； deluge (wet & dr
39、y), foam, Halon (if allowed) and CO2, dry powder. Personnel Warning; visual and audible alarms.Passive safety systems (e.g., life boats, rafts, fire hoses, etc.) are not covered.Facility Management SystemsThe quantity and complexity of these systems are closely linked to the size of the facility, an
40、d generally include the following subsystems. These subsystems are typically totally independent from the facility control systems or do not contribute significantly to it. Communications； platform to platform, platform to shore, platform to ocean vessel or aircraft. These are generally not part of
41、the overall control system and are not usually integrated into centralized facility monitoring/control equipment. However, if remote monitoring/control is required, control systems must interface with this communication equipment. Environmental Monitoring； to monitor weather and ocean data. Pollutio
42、n Monitoring； for produced water and drilling solids returned to the ocean. Surface Surveillance; active systems (e.g., radar) and passive systems (e.g., navigational aids, warning lights and horns). Subsurface Surveillance (e.g.? underwater cameras). Electrical Power System; switchgear and busses.
43、Operations Monitoring and Control; alarmandfaultindication, analogindication, recording, and control. Data Management; well test analysis, preventative maintenance information; personnel data, material inventory, manipulation of operating/ facility data.Control Panels and ConsolesPanel; A free-stand
44、ing upright structure for housing instruments intended for use by operators in a standing position. They may be pneumatic or electric.Console； A free-standing upright structure for housing instruments intended for use by operators in a seated position. They may be pneumatic or electric. They are typ
45、ically used in control rooms and frequently house computer display equipment.Three basic factors affecting design and operator performance are; Visibility; field of vision, and color. Accessibility; shape, arrangement, and operator reach. Data interpretation and response.Pneumatic panels are used fr
46、equently as they require less design consideration when placed in electrically classified areas. The construction characteristics are similar for electric and electronic panels, except when they are placed in electrically classified areas.Motive SystemsThere are three types of motive systems used of
47、fshore. In many applications, all three types are used. Motive systems may contribute lOto 40% of the space, weight and cost of the combined system (i.e., motive system plus control system). The three system types are hydraulic, pneumatic, and electric. Hydraulic systems use an incompressible fluid
48、(e.g., oil) to supply motive force. Pneumatic systems use a compressed gas (e.g., production gas or air), and electric systems obviously use electricity.Hydraulic systems are generally only used in three basic process systems, a) well valve hydraulic system, b) production manifold valve hydraulic sy
49、stem, and c) drilling hydraulic system.Pneumatic systems using air utilize a compressor, dryer, and receiving vessels.Electric systems provide main and emergency electrical power. The two levels of power on most facilities are 115 VAC and 24 VDC. Electric systems comprise; Generation； main generators can be turbine
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