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A ROUGH GUIDE TO OFFSHORE PLATFORM OR FPSO-2
- 1. 1. WATER TREATMENT •Water treatment varies depending on the produced oil quality • Heavy oils and tight emulsions need more treatment than light condensate found on gas platforms • Use Hydrocyclones if pressure > 8 barg • Heavy emulsion forming oils need other treatment e.g. induced or dissolved air flotation (DAF) and Walnut shell filtration
- 2. 2. WATER TREATMENT(cont.) • Typically for heavy oils – hydrocyclones achieve 200 ppm oil-in-water • DAF gives around 50 ppm • Walnut filters give 20 to 30 ppm TYPICAL WATER TREATMENT INSTALLATIONS HYDROCYCLONES DAF UNIT WALNUT FILTERS BULK SKIMMER DAF UNIT WALNUT FILTERS
- 3. 3. EQUIPMENT DESIGN •Heat Exchangers – Data from simulations incl. Heating/cooling curves • Specify all operating cases, not just maximum duty • Common practice is to add a safety margin – do this with caution – detrimental in high fouling cases • Cooling water exchanger – keep temperatures below 49ºC and velocities above 1.2 m/s • Use maximum 10ºC approach temperature
- 4. 4. EQUIPMENT –HEXs (cont.) • Try to make design pressure on low pressure side = min. 2/3 (or 4/5) DP on HP side to avoid tube rupture relief case • For Hot Oil Circuits, if possible, DP of Hot Oil side > DP of process side • For S/T HEX with high fouling service on shell side – prefer to use square tube pitch
- 5. 5. EQUIPMENT -VESSELS • Sizing – use Shell DEPs • Two phase sep with mostly gas, use vertical sep for space saving • Where there is a lot of liquid, use horizontal seps • Internals - momentum breaker, demisting pack, foam breaking baffles, vortex breakers, water weirs, baffles to stop pitch and roll effects on FPSOs, sand jets, plate packs for water removal
- 6. 6. EQUIPMENT -PUMPS • PD pumps for high head low flow • For low or high heads, high flow rates use centrifugal • centrifugal preferred if possible • For High viscosity, low shear pumps use low speed Screw Pumps – consider variable speed for level control • Design downstream piping for shutoff head – always specify head in preference to pressure • Power kW = m3/s x kPa. Efficiency from 50 to 80% depending on pump size
- 7. 7. PUMP SYSTEMDESIGN • All pumps must have the NPSHA specified so we have to calculate it. • In our processes, we often pump saturated liquids. Pf H H = static head above pump suction NPSHA = (P1 + atm - Pf - Pv)/ρg + H – Ha – Hv P1 P1 = Vessel pressure Pf = Friction pressure loss Pv = vapour pressure For a centrifugal pump, pumping a saturated liquid, Pv = P1 + atm and so NPSH = H - Pf/ρg NPSHA must be necessarily greater than NPSHR Ha = acceleration head (only for PD pumps) Hv = velocity head (normally ignored)
- 8. 8. PUMP SYSTEMDESIGN (cont) • It is not good practice to use a PD plunger pump for saturated liquids because of the acceleration effects. • Acceleration of the liquid can produce velocities 3 times the average (pressure drops 9 times average) – causing vaporization.
- 9. 9. EQUIPMENT - COMPRESSORS •Centrifugal compressors for low maintenance • Multistage with intercooling • Antisurge control for each stage • Surge is caused by flow reversal at low flows • Surge control is by flow control from downstream of discharge scrubber back to suction • Hot quick opening bypass may be required if dynamic simulation shows it is needed • Stonewall – when sonic velocity in compr.
- 10. 10. COMPRESSORS (cont.) •TYPICAL COMPRESSOR CONTROLS Suction Scrubber First stage compressor Intercooler Aftercooler Second stage compressor First Stage Scrubber Discharge Scrubber Antisurge valves Suction throttling
- 11. Centrifugal Compressor Curve Head InletVolume Flow Antisurge line algorithm Line A Centrifugal Line B Reciprocating Line C Axial Stonewall
- 12. 11. GENERAL DESIGN •Line Sizing • Velocity (erosional velocity for 2-ph lines) • Pressure drop • Flow regime (2-ph flow) – avoid slug flow • Don’t add too much design margin • Add design margin for high fouling services e.g. drains • Remember small sizes weigh less, cost less, take up less space – all important for offshore work
- 13. 12. LINE SIZINGRULES OF THUMB • Pump Suctions: no more than 20 kPa per 100m. Boiling liquids NPSHA>NPSHR. • Compressor Suctions and discharges: COOEC standard • Flare headers: design according to PSV back pressures. • For high pressure high flow PSVs, also calculate noise as special design may be needed
- 14. 13. LINE SIZINGRULES (cont.) • Pump Spillbacks, no criteria for pressure drop - keep below erosional velocity (API- 14E continuous flow). • All other lines: no more than 50 kPa per 100 m. • Avoid noise by restricting velocities such: Gas- v < 100/√ρ where ρ kg/m3 and v m/s Liquid - v < 9 m/s
- 15. 14. LINE SIZING– PRESSURE DROP CALCULATIONS • Liquids - Darcy formulae, or API 14E, or “Crane” equations • Compressible fluids with high pressure drops use Isothermal Flow equation • Can also use Panhandle and Weymouth formulae but these are theoretically not as accurate. • Commercial software like Pipesim • Friction factor - Colebroke White or Chen equation • Ensure you have the pipe spec
- 16. 15. CONTROL VALVESIZING • Determine the range of flow rates • Hydraulic calcs u/s and d/s to calculate valve u/s and d/s pressures at various flow rates • Get operating data from simulations or data base - viscosity, cp/cv, MW (gases), S.G., critical properties and vapour pressures (for liquids).
- 17. 16. CONTROL VALVESIZING • Size the control valve to allow piping layouts to be progressed • If Fisher valves used – ‘e’ body globes are most common • Calculate Cv using the Fisher Firstvue program or manually from cat. 10 for the various flow rates • For the range of flow rate, select a control valve that is between 25 and 70% open
- 18. 17. CONTROL VALVESIZING • For good turndown select globe valves with eq% trim (10% turndown) • For higher turndowns, consider using v- ball valves (2% turndown) • For very high turndowns, use on-off control, or perhaps two or more different sized valves in parallel with split range control
- 19. 18. RELIEF VALVESIZING • Determine relief contingencies API-521 • Select the contingency with the highest flow rate • Data from simulation • Select setpoint (design pressure) API-521 • Where relieved to? Atmosphere or flare? • Relieve HCs to flare, air etc. to atmos • Atmosphere – use conventional valves • Flare – use balanced bellows or pilot op.
- 20. 19. RELIEF VALVESIZING (cont.) • Bursting discs – use for rapid relief of high flow e.g. on tube rupture • Line Sizing – upstream pressure drop < 3% of set pressure. Downstream pressure drop to give back pressure < 35% of set pressure. • For high flow high set pressure, calculate noise to see if there are acoustic vibration problems
- 21. 20. RELIEF VALVESIZING (cont.) • For valve sizing, get data from simulations and use API methods for calculating orifice area required • Fire relief for multicomponent mixtures, you can use HYSYS. If you can’t use dynamic simulation, then simulate the fire as a series of flashes, inputting heat calculated per API-521, separate gas and liquid, split the gas stream and make one split actual volume equal to the gas volume in the vessel. The other stream is the relief stream for that flash. Use the highest flow as the size determining flow
- 22. 21. MULTICOMPONENT FIRE RELIEFVALVE SIZING H1 H2 H3 H4 Calculate heat per API-521 R1 R2 R3 R4 Vv Vv Vv Vv Vv = volume of vessel simulated as Am3/h H = heat input based on vessel wetted area R = relieving flow rate Typical HYSYS PFD for multicomponent fire relief sizing Q = 21000FA0.82 Vg = Vv - Vl Vl Vg
- 23. Blocked Outlet Relief FromPipeline P = 5600 kPag PT PT PT Class 600# Class 150# Full flow relief To Flare To Process P = 1000 kPag 2” startup bypass 2oo3 voting SDV
- 24. 22. GAS DISPERSIONAND FLARE RADIATION • Done so height/position of the flare or vent stack in relation to operating areas can be determined • Gas dispersion - maximum gas concentration in working areas < 25% of the LEL to ensure prevention of vent ignition • If ignited, radiation levels should not exceed those defined in API-521 (Table 8) • To do these calcs, you can use Cirrus which is available free of charge from B.P.
- 25. 23. DRAWINGS -P&IDs • Use Distribution Drawings for Utilities • Lines going from one drawing to the next is messy. Neater to use a distribution drawing. • Use a check list when checking P&IDs • Ensure that Fonts are consistent • When modifying plant, use “Demolition Drawings” to show removed equipment and lines
- 26. 24. CONTROLLING THE PROCESS •Use the KISS principal when designing control systems • Offshore process - simple • In most cases, control systems are simple • Pressure controlled in the Production Sep • Flow controlled manually on well chokes • Flow to WI Wells is by flow or pressure • High pressure override can be used on WI wells to guard against well fracturing – use a signal selector
- 27. 25. TYPICAL CONTROLSYSTEM Production Separator PC LC TC Cooler using cooling water TC Hot Oil Supply Hot Oil Return Produced water to treatment LC Control valve with e/p positioner INT
- 28. 26. HOMEWORK TC Cooling water Supply ProcessFluid Process Cooler TV Cooling water Return Question: The exchanger outlet temperature is controlled by bypassing hot process fluid around the heat exchanger. Cooling water flow rate is constant. 1. What is a possible problem in sizing the control valve? What is the approximate expected maximum process turndown achievable with this design? How can it be improved? 2. What things will affect the sizing of this control valve i.e. what things will affect the required flow rate through this valve? 2b. What determines maximum flow through this valve? 3. If this is a gas exchanger, is there anything else we need to be careful of with this control system? 4. What if it is a waxy oil exchanger? To Flare
- 29. 27. HOMEWORK ANSWER TC Coolingwater Supply Process Fluid Process Cooler TV Cooling water Return Answers: 1. As demand for bypass increases, pressure drop thru the exchanger decreases. The most turndown you will achieve would be about 50% assuming full flow pressure drop through the heat exchanger is a bar or less. You could add a globe valve in the inlet or better still a control valve or 3-way control valve, if higher than 50% turndown is required. 2. Cooling water flow, heat exchanger fouling, cooling water temperature, process inlet temperature, process flow rate all affect the valve required throughput. 2b. Lowest heat duty, cleanest condition 3. We need to ensure that the process side does not go below hydrate formation temperature. 4. We need to ensure that the process side does not go below wax formation temperature.
- 30. 28. TEMPERATURE CONTROL •Heating is used on platforms to stabilize crude and break emulsions – temperature controlled by adjusting heat medium flow • For Sea Water Coolers – use a hot process bypass. Do not control water flow • Keep cooling water velocities > 1.2 m/s • Keep cooling water temperature < 48ºC to prevent calcium salts scale (reverse sol)
- 31. 29. LEVEL CONTROL •Various level controllers are used • For heavy waxy emulsion forming crudes, profilers can be used to control level • If S.G. is constant, a bubbler, dP or pressure transmitter (for atmospheric tanks) can be used. • Other types – displacers, capacitance, ultrasonic
- 32. 30. MORE COMPLEXCONTROL • Design out Process upsets using controls • Upsets can cause shutdown • Aim of Operators is to keep plant running -design accordingly • Homework problem
- 33. 31. TEG UNITS Q1.What can go wrong with this control system design? (Hint: What happens if one TEG Train shuts down Q2. What controls can be added to solve the problem? QUESTIONS PC PC PC From Gas Production Wells Production Separator Filter Separator Gas Cooler To Flare TEG Contactor FT FT Set @ 11,700 kPag
- 34. 32. TEG UNIT A1.If one train shuts down, all of the flow tries to go through the other train – PV2 will go wide open. High pressure drop through the train causes high pressure in the Production Sep and the PV1 to flare will open but not before the flow through the train has increased substantially. The increased flow would cause the TEG Contactor to flood with loss of TEG to the pipeline. A2. The existing flow transmitter was used to send a signal to a flow controller set at train design flow. The control signals from the PC and FC go to a low signal selector to restrict flow to design flow. ANSWER PC PC From Gas Production Wells Production Separator Filter Separator Gas Cooler To Flare TEG Contactor FT FC FY PC FT FC FY PV1 PV2 To Pipeline To Pipeline SR SR PY
- 35. 33. SYSTEMS • processsystem and the Utility Systems • process consists of a number of discrete systems that interact with each other • utility systems provide infrastructure to the process system that allows it to operate • Utility Systems include: Utility and Instr. Air, Cooling Medium, Heating Medium, Open Drains, Closed Drains, Relief and Blowdown System, Sea Water, Fresh Water, Fuel Gas, Diesel, Fuel Oil, Chem. Injection, Electric Power, Fire Water System (not really utility)
- 36. 34. UTILITY ANDINSTRUMENT AIR • Utility air - air driven tools and equipment • Instrument air is filtered and dried utility air • Instr. Air is used to drive control valve, SDVs and BDV actuators. • SDVs and BDVs are fail safe • SDVs generally fail closed • BDVs generally fail open • BDVs and SDVs are actuated by failsafe solenoid vavles
- 37. 35. AIR COMPRESSORSAND DRYERS • Air is filtered, compressed, cooled, separated and stored in the Utility Air Receiver (buffer for Instrument and Utility Air System) • Air for instruments is filtered, dried, filtered again stored in a Receiver then distributed • Air compressors normally 2 x 100% operating in duty standby mode • Instrument air normally 700 kPag • Air compressors discharge at 1100 kPag
- 38. 36. AIR COMPRESSIONSYSTEM Drying Skid Utility Air Receiver Instrument Air Receiver Water Drain Air Compressors Aftercooler Lead Compressor Lag Compressor 900 kPag Lag Compressor loads 950 kPag Lead Compressor loads 1050 kPag Lag Compressor unloads 1100 kPag Lead Compressor unloads Set @ 700 kPag Set @ 700 kPag Set @ 850 kPag 100 kPa dP 50 kPa dP Inlet Air Filters PC To Instrument Air Distribution header To Utility Air Distribution header
- 39. 37. INSTRUMENT AIR(cont.) • The Compressor flow rate must account for Dryer regeneration air • Compressor sized on basis of maximum continuous instrument air requirement • Receivers are sized to give at least 15 minutes plant operation with the compressors shut down
- 40. 38. COOLING MEDIUM •cooling water (sea water), air (fin-fan cooling), or secondary medium cooled by sea water • In China we use direct Sea Water Cooling • special materials are needed for piping and exchangers – monel, titanium, hastelloy C etc. • Piping is duplex, cunifer, plastic (GRP) • If plastic pipe is used, pump discharges normally metal to protect plastic against shock and vibration
- 41. 39. HEATING MEDIUM •In China, thermal oils (Hot Oils) are used • Heat source - Fired heaters or waste heat • Expansion Vessel – sized to allow expansion due to density difference between cold and hot oil. • Expansion vessel can be run hot or cold • Expansion Vessel requires blanketing • Pumps – preferably to give hot oil pressure > process pressure
- 42. 40. HEAT MEDIUMSYSTEM (cont.) • System Design Pressure – consider tube rupture case. Increase Des P to avoid this • Hot Oil heaters – fired by waste heat with supplemental duct burners. • Radiant heaters – keep skin temperatures below thermal oil degradation temp. • Fuel – gas, crude oil, diesel (start-up or emergencies only)
- 43. 41. TYPICAL HOTOIL SYSTEM Process Stream TC TV TC FC From Fuel Gas Expansion Vessel Hot Oil Heater To Flare From Fuel Gas Hot Oil Circulation Pumps
- 44. 42. TYPICAL HOTOIL SYSTEM WITH VARIOUS USERS TC Process Stream TV Process Stream Hot Oil Expansion Vessel Blanket Gas To Flare Hot Oil Circulation Pumps Hot Oil Heaters Fuel FC TC TC TV FV
- 45. 43. TYPICAL HOTOIL SYSTEM WITH TWO HEATERS EXERCISE 1. Mark in the minimum flow heater controls 2. Mark in a control system that will allow either heater to run at 100% to supply the majority heat demand, and the other to supply the remainder of the heat demand. Work as a team - discuss it with your colleagues. Hot Oil Expansion Vessel Blanket Gas To Flare Hot Oil Circulation Pumps Hot Oil Heater Fuel Hot Oil Heater Fuel Process Stream Process Stream TV TV FT TC FT TC TC TC
- 46. 44. Typical HotOil System showing controls for duty heater and “floating” heater TV Process Stream Process Stream Hot Oil Expansion Vessel Blanke t Gas To Flare Hot Oil Circulation Pumps Hot Oil Heater TC Fuel Hot Oil Heater Fuel HS FY Duty Heater Selector TC FC FY FC FC FC FT FT > > TC TC TV TV
- 47. 45. HOT OILSYSTEM (cont.) • System Filtration – filters ~ 10% of flow to remove mill-scale and cracked hot oil. • Pickle system to minimise filter debris • System Drainage – to blanketed tank (sized for max. drainage) with return pump • System make-up – from drums – utilize the drain tank for this purpose
- 48. 46. HOT WATERSYSTEM • Similar to Hot Oil except the system pressure is controlled and the hot water is saturated • The Surge tank is the highest point in the system to ensure the water in the rest of the system is below the saturation temperature
- 49. 46a HOT WATERSYSTEM PC TV TC From Fuel Gas Surge Vessel Hot Water Heater Hot Water Circulation Pumps Users Distribution Header Process Stream Surge Vessel at highest point in system PV PC