ZenLearning Process Module: Offshore Oil and Gas Processing Basics

Purpose, Scope, and Module Objectives

  • Primary Purpose: This specialized slide pack provides essential information and critical considerations for the design of crude oil processing systems within typical offshore facilities.

  • Scope of Content: The module serves as a definitive summary of processing schemes and typical crude oil specifications. It provides foundational data necessary for developing the preliminary phase of offshore design.

  • Governing Regulations: The guide explicitly states that specific requirements from the Project, Client, or Local regulations must always prevail over the contents provided in this mate rial.

  • Key Objectives:

    • Encourage Self-Learning: Highlight industry information that is freely available online to foster independent professional development.

    • Target Audience Identification: This material is specifically tailored for non-process engineers, interns, and newcomers to the oil and gas industry.

    • Basic Introduction: Provide a comprehensive entry-level understanding of offshore oil processing to improve client interactions and multidisciplinary collaboration.

Essential Definitions and Physical Properties

  • API Gravity: This parameter measures how heavy or light petroleum is compared to water. It is calculated using the following formula:   Deg API=(141.5SG(@60∘F))−131.5\text{Deg API} = \left(\frac{141.5}{SG(@60^\circ \text{F})}\right) - 131.5

  • Asphaltenes: These are heavy molecules found in crude oil that are insoluble in light hydrocarbons. They are characterized by the presence of aromatic rings and sulfur.

  • Cloud Point: The specific temperature at which waxes in the crude oil begin to precipitate, causing the oil to take on a cloudy appearance.

  • Pour Point: The temperature threshold below which crude oil loses its flow characteristics and becomes semi-solid.

  • Reid Vapor Pressure (RVP): A measure of the vapor pressure of crude oil at 100∘F100^\circ \text{F}, used primarily to assess the volatility of the product.

  • Stabilization: The specific process of removing volatile components to reduce the vapor pressure of crude oil for safe transport and storage.

  • Sweet and Sour Crude:

    • Sweet Crude: Contains less than 0.5% S0.5\% \text{ S} (sulfur). It is highly desirable and easier to refine.

    • Sour Crude: Contains greater than 0.5% S0.5\% \text{ S}. It is generally less desirable due to processing complexity.

  • Stock Tank Conditions: The reference conditions used to specify flowrates (measured in STBOPD):

    • Atmospheric Pressure: 14.7 psia14.7\,\text{psia} or 101,325 Pa101,325\,\text{Pa}.

    • Temperature: 60∘F60^\circ \text{F} or 16∘C16^\circ \text{C}.

Crude Oil Growth and Categorization

  • Classification by API Gravity:

    • Light Crude: API gravity > 31.1^\circ (density less than 870 kg/m3870\,\text{kg/m}^3).

    • Medium Oil: API gravity between 22.3∘22.3^\circ and 31.1∘31.1^\circ (density range of 870–920 kg/m3870\text{--}920\,\text{kg/m}^3).

    • Heavy Crude: API gravity between 10.0∘10.0^\circ and 22.3∘22.3^\circ (density range of 920–1000 kg/m3920\text{--}1000\,\text{kg/m}^3).

    • Extra Heavy Oil: API gravity < 10.0^\circ (density greater than 1000 kg/m31000\,\text{kg/m}^3).

  • Commercial Benchmarks: The main types of commercial crude oil include:

    • West Texas Intermediate (WTI)

    • Brent Crude

    • Dubai Crude

  • Note on Grading: The module notes that grading standards vary; for example, the United States Geological Survey uses slightly different ranges than those listed above.

Common Industry Abbreviations

  • FTHP: Flowing Tubing Head Pressure

  • BS&W: Basic Sediment & Water

  • GOR: Gas Oil Ratio

  • PTB: Pounds of salt per thousand barrels of oil

  • ppm: Parts per million

  • RVP: Reid Vapour Pressure

  • TVP: True Vapour Pressure

  • TEG: Triethylene Glycol

Production Facility Overview and Process Flow

  • Main Role: The primary function of a production facility is to separate well fluids into saleable oil, gas, and water while ensuring the safe disposal of remaining waste.

  • Separation Train: Well fluids enter a multi-stage separation train. Volatile components are vaporized, and crude is stabilized to meet RVP standards.

  • Water Treatment:

    • Produced water is treated for oil removal, often using hydrocyclones because oil-water emulsions are difficult to clean due to small particle size and emulsifying agents.

    • Treated water is either disposed of in the sea or re-injected into injection wells after filtration and biocide treatment.

  • Gas Processing:

    • Compression: Necessary to allow economic transport through small-diameter pipelines.

    • Dehydration: Uses a TEG (Triethylene Glycol) contactor unit and regeneration system to remove water and prevent hydrate formation and corrosion.

    • Sweetening: Removes acid gases (CO2CO_2 and H2SH_2S). This is usually performed upstream of dehydration. Offshore facilities typically export sour gas to onshore plants for this specific treatment.

  • Downstream Output Examples: Refineries process the crude into gasoline, jet fuel, lube oil, and bitumen; gas fractionation plants produce ethane, propane, and butane.

Product Specifications and Crude Quality Standards

  • Water Content (BS&W):

    • Must be limited because shipping emulsified oil wastes transportation capacity.

    • Mineral salts corrode equipment, pipelines, and storage tanks.

    • Dissolved sediments cause plugging and scaling in heat exchangers and refinery column trays.

    • Regional Standards: The Gulf of Mexico accepts up to 1% BS&amp;W1\%\,BS\&amp;W, while other global regions may require less than 0.5%0.5\%.

  • Vapor Pressure Specifications:

    • Storage/Export: RVP is typically set at 10–12 psia10\text{--}12\,\text{psia} for safety and to minimize gas release during transport.

    • Pipeline Export: Crude may be partially stabilized (TVP≈6.9 baraTVP \approx 6.9\,\text{bara} at 38∘C38^\circ\text{C}) with final stabilization occurring onshore.

  • Contaminant Limits:

    • H2S: Levels are controlled during separation, typically achieving 50 ppmw50\,\text{ppmw}, with a maximum range of 10–100 ppmw10\text{--}100\,\text{ppmw}.

    • Salt: Maximum concentration is usually limited to 10–30 PTB10\text{--}30\,\text{PTB} to prevent catalyst damage and corrosion.

Separation Equipment: Wellhead and Separators

  • Wellhead and Manifold: Production begins at the wellhead where flow is governed by a choke valve. Manifolds route and test fluids from multiple wells.

  • Separator Function: Employs gravity to split fluids into gas, oil, water, and contaminants.

    • Two-phase: Gas and liquid.

    • Three-phase: Gas, oil, and water.

  • Internal Components of a Separator:

    • Inlet Deflector and Inlet Distribution Baffles.

    • Foam Breaker.

    • Gas Flow Straightening Device and Mist Eliminator.

    • Submerged Weir and Vortex Breakers.

    • Sand Jetting System.

    • Coalescer.

Comparative Analysis: Horizontal vs. Vertical Separators

  • Horizontal Separators (Preferred for low GOR and 3-phase separation):

    • Advantages: Sufficient residence time for liquid-liquid separation; large surface area for foam dispersion; large surge volume capacity.

    • Disadvantages: Large footprint; only part of the shell is available for gas flow; liquid level control is critical; difficult to clean (sand/wax).

  • Vertical Separators (Preferred for high GOR well fluids and 2-phase separation):

    • Advantages: Full diameter for gas flow; small plot area; easy bottom drain/clean-out for sand and wax; less critical level control.

    • Disadvantages: Occupies significant vertical space (deck spacing); difficult to skid mount; hard to service top-mounted instruments; not suitable for bulk liquid-liquid separation.

Crude Oil Stabilization Strategy

  • Multi-stage Separation: Involves a series of separators at sequentially reduced pressures. This maximizes hydrocarbon liquid recovery and minimizes gas compression power requirements.

  • Flash Losses: If high-pressure liquid were moved directly to a stock tank, the resulting vaporization would lead to the loss of heavy ends. Multi-stage separation prevents these losses.

  • Thermal Management:

    • Heating: Required for emulsion breaking, improving separation, and adjusting final product RVP/H2SH_2S content.

    • Cooling: Necessary for high-temperature well streams to avoid excessive vaporization.

    • Wax Control: Skin temperatures inside coolers must be at least 5∘C5^\circ\text{C} above the crude oil cloud point. Strategies include cooling water recycle or wax inhibitor injection.

  • Offshore Limitations: Due to space and weight constraints, offshore systems are typically limited to a maximum of 3 stages.

Detailed Breakdown of 3-Stage Separation

  1. HP (1st Stage) Separator:

    • Operates at high pressure to control well behavior via choke operation.

    • Performs bulk water removal (remaining oil has ≈5–10% water\approx 5\text{--}10\%\,\text{water}).

    • Gas is sent to compression/dehydration for pipeline export.

  2. MP (2nd Stage) Separator:

    • Operates at lower pressure than the HP stage.

    • Oil water content is reduced to ≈2% vol\approx 2\%\,\text{vol}.

    • Vaporized gas is sent to an inter-stage compressor to join HP gas.

  3. LP (3rd Stage) Separator:

    • Operates just above atmospheric pressure.

    • Controls the final export crude RVP.

    • Gas is typically sent to flare or a vapor recovery unit.

Dehydration and Desalting Operations

  • Electrostatic Coalescer Principles: Uses electric fields for dewatering through three steps: Destabilization of emulsion, Coalescence of droplets, and Sedimentation.

  • Operational Targets:

    • Reduces water from up to 10\%\text{ to }<0.2\%\,BS\&W.

    • Ideal for offshore because of compact size and lower chemical use.

  • The Desalting Process:

    • Formula: Crude Oil + Fresh Water →\rightarrow Desalter Vessel →\rightarrow Treated Oil + Separated Water (with Salt).

    • Wash Water: 4–8% wash water4\text{--}8\%\,\text{wash water} by volume is injected after the crude pump.

    • Mixing Valve: Typical pressure drop is ≈25 psi\approx 25\,\text{psi} to ensure efficient mixing prior to the desalter.

    • Temperature: Feed must be at least 70∘C70^\circ\text{C} for efficiency.

    • Technology: Dual Polarity (AC/DC) fields offer higher efficiency than pure AC fields.

    • Two-Stage Systems: Water from the 2nd stage is recycled to the 1st stage to reduce freshwater demand. Heat integration uses outlet oil to preheat inlet crude.

Roles and Deliverables of the Process Engineer

  • Key Responsibilities: Gathering project requirements, translating needs into design, defining process conditions, selecting materials, and specifying safety requirements (HAZOP/HAZID).

  • Crucial Software Tools:

    • Aspen HYSYS (Steady State & Dynamics): Simulation and transient behavior analysis.

    • UniSim / ChemCAD: Chemical process simulation.

    • PipeSim: Multiphase flow simulation in pipelines.

    • FlareSim: Radiation and dispersion modeling for flare systems.

    • Aveva E3D: 3D modeling of offshore facilities.

    • Caesar II / StaadPro: Pipe stress and structural analysis.

  • Primary Deliverables:

    • Process Flow Diagrams (PFDs) and Piping and Instrument Diagrams (P&IDs).

    • Process Description Narratives and Line Lists.

    • Master Project Equipment Lists and Mechanical Data Sheets.

    • Process Safety/Shutdown Philosophies and Instrument Setpoint Tables.

    • Plant Operating Manuals.