Gas Oil Separation Plants (GOSPs) are critical facilities in the oil and gas industry, designed to separate crude oil, gas, and water from well production fluids. The process design of a grassroots GOSP and the debottlenecking of an existing GOSP require meticulous planning to ensure operational efficiency, safety, and compliance with product specifications. This article outlines the key requirements for both scenarios.
1. Scope of GOSP Process Design
The process design requirements for GOSPs encompass both grassroots facilities and the debottlenecking of existing plants. Grassroots GOSPs are new facilities constructed to handle oil field production, while debottlenecking involves modifying existing GOSPs to increase capacity or address operational constraints. The scope excludes crude oil stabilization units, produced water treatment and disposal units, and auxiliary systems such as fire water, fire and gas detection, and flare systems.
2. Requirements for Grassroots GOSP Process Design
The design of a grassroots GOSP involves a structured approach to ensure the facility meets production, safety, and environmental requirements. Key aspects include:
2.1 Design Phases and Inputs
The design process progresses through structured phases:
- Process Study: Establishes the design basis using inputs such as production forecasts, reservoir compositions, crude arrival pressure and temperature, and water salinity. These inputs are provided by relevant reservoir and facilities planning teams.
- Design Basis Scoping Paper (DBSP): Refines the design with detailed data, including emulsion characterization studies and delivery pressures for crude, gas, and water.
- Project Proposal and Detailed Design: Finalizes the design with comprehensive inputs, including transient flow studies and hydraulic calculations.
2.2 Product Specifications
The GOSP must produce desalted dry crude, stabilized crude (if applicable), and treated produced water meeting the following specifications:
- Desalted Dry Crude: Maximum 10 PTB salt and 0.2 Vol% BS&W.
- Stabilized Crude: Maximum 70 ppm H2S by weight (30 ppm for design) and 13 psia True Vapor Pressure at export/storage temperature.
- Produced Water: Oil-in-water content as specified by reservoir management (default 50 ppm if unspecified) for disposal wells, or per environmental standards for marine disposal.
Chemical Cleaning Standards for Industrial Systems
2.3 Process Simulation and Analysis
- Steady-State Simulation: Developed during DBSP and updated through Detailed Design using approved simulation software. Simulations cover summer and winter conditions for various water cut and Gas Oil Ratio (GOR) scenarios.
- Transient Dynamic Simulation: Required for gas compression systems before 50% detailed design completion.
- Computational Fluid Dynamics (CFD): Used to validate equipment internals design, such as inlet devices and mist eliminators, with approved software.
- Deliverables: Process Flow Diagrams (PFDs), Piping and Instrumentation Diagrams (P&IDs), and final simulation models are mandatory, with PFDs showing Heat & Material Balances for multiple operating conditions.
2.4 Equipment Design Requirements
The design of key GOSP equipment must adhere to specific guidelines:
- Production Manifold and Inlet Header: Designed for maximum well shut-in pressure, with top-entry flowlines at a 45-degree angle to reduce turbulence.
- Production Traps: Horizontal vessels (two-phase or three-phase), equipped with inlet devices, anti-wave baffles, and mist eliminators. Three-phase traps require a minimum 30% water cut design and specific oil and water retention times based on crude gravity and temperature.
- Low Pressure Degassing Tanks (LPDTs): Designed with significant hold-up times (1 hour for oil, 30 minutes for water) to accommodate surges.
- Dehydrators and Desalters: Use approved technologies (Conventional AC, AC/DC, or Modulated AC/DC) with specific grid configurations and electrical requirements. A minimum of two-stage dehydration/desalting for lighter crudes and three-stage for heavier crudes.
- Pumps and Heat Exchangers: Charge pumps, water draw-off pumps, and heat exchangers (Wet Dry Crude and Trim) designed for staged expansion to handle future water load increases.
- Gas Compression: Equipped with Suction KO Drums, mist eliminators, and after-coolers, with CFD validation for internals.
2.5 Auxiliary Systems
- Wash Water Systems: Designed for 5% of dry crude production rate, with High Efficiency Mixing Systems and de-aeration to remove oxygen.
- Chemical Systems: Include storage and injection for demulsifiers, corrosion inhibitors, and scale inhibitors, with provisions for biocides and oxygen scavengers as needed.
- Hot Oil Systems: Use fluids with high auto-ignition temperatures and inert gas-blanketed expansion vessels.
- Drain Systems: Closed Drain Systems (CDS) and Oily Water Drain Systems for onshore and offshore facilities.
- Instrument Air/Plant Air: Designed with utility station connections.
2.6 Piping and Instrumentation
- Piping: Designed with non-slam check valves at critical points and self-draining bypass lines.
- P&IDs: Include detailed instrument data such as orifice bores, control valve shut-off requirements, and level alarm settings.
2.7 Corrosion Control and Safety
- GOSPs are designed for sour service unless confirmed otherwise, with corrosion monitoring systems and material selection to mitigate corrosion risks.
- Safety features include Emergency Shutdown (ESD) systems and emergency eyewashes/showers.
3. Requirements for Debottlenecking Existing GOSPs
Debottlenecking aims to enhance the capacity or efficiency of an existing GOSP without compromising safety or product quality. The process involves:
3.1 Process Study
A comprehensive study identifies equipment modifications based on current and future production forecasts. This includes assessing bottlenecks in production traps, dehydrators, desalters, and gas compression systems.
3.2 Flare and Relief Assessment
Evaluates the flare and blowdown system’s capacity to handle increased throughput or modified process conditions, ensuring compliance with safety standards.
3.3 Utility Systems Assessment
Reviews utility systems (e.g., wash water, chemical injection, hot oil, and drain systems) to identify necessary upgrades to support process modifications.
3.4 Hydraulic Study
If applicable, a hydraulic study assesses the impact of increased production on downstream pipeline systems, ensuring adequate pressure and flow capacity.
3.5 Plant Test and Capacity Assessment
If exceeding nominal capacity without a full process study, a Flare and Relief Capacity Assessment determines the maximum crude capacity, followed by a Plant Test to identify equipment limitations.
3.6 Management of Change (MOC)
All modifications are tracked through an MOC process, updating plant documentation to reflect changes in design capacity or equipment.
4. Key Considerations for Both Scenarios
4.1 Compliance with Standards
Both grassroots and debottlenecking designs must comply with applicable engineering standards and industry codes. Conflicts are resolved through designated authorities, and deviations require formal approval.
4.2 Turndown and Flexibility
GOSPs must accommodate turndown (maximum to minimum flow ratio), with stabilizer column turndown being the controlling factor for GOSPs with stabilization units. Provisions for future expansion, such as spare manifold connections and plot space for additional equipment, are critical.
4.3 Emulsion Handling
If tight emulsions are confirmed or no characterization study is available, designs must account for full water cut in piping and reserve space for future equipment to handle emulsion challenges.
4.4 Mass Balance and Monitoring
Flowmeters on main process lines ensure accurate mass balance. Online BS&W analyzers and corrosion monitoring systems enhance operational control and maintenance.
5. Conclusion
The process design of grassroots GOSPs and the debottlenecking of existing GOSPs require a systematic approach to meet stringent product specifications, operational efficiency, and safety standards. Grassroots designs focus on comprehensive planning through structured phases, precise equipment sizing, and robust auxiliary systems, while debottlenecking emphasizes targeted modifications to address capacity constraints. By incorporating advanced simulation and monitoring tools, GOSPs can achieve reliable performance and adaptability to future production demands.