Title : Re-evaluation of a brownfield offshore depressurization system to updated API 521 requirements
Abstract:
Background:
The primary motivations for reevaluating the existing depressurization system are recent facility modifications and new installations at the Sabratha Platform (an offshore platform that produces natural gas and is run by Mellitah Oil and Gas Company-Libya), particularly compressor module upgrades for low pressure wells. A system that was initially designed in accordance with previous standards requires being reevaluated in light of these advancements and the criteria of API 521 (Seventh Edition).
The system was developed using 25 °C as the minimum depressurization temperature. However, API 521 now mandates the minimum ambient temperature, which might affect depressurization behavior, phase changes, and minimum metal temperatures. As a brownfield facility, identified problems must be remedied while taking into account existing infrastructure limits, operational continuity, and cost-effective mitigating strategies.
Objectives:
The main objectives of the present study are to examine the existing cold depressurization system in compliance with API 521 (Seventh Edition), with the minimum ambient temperature as the starting condition and the maximum operating pressure. The study's purpose is to identify, determine, and evaluate any minimum design metal temperature (MDMT) breaches on both the process and flare sides resulting from the changed design basis. Besides, the study aims to assess the impact of recent operational changes and provide feasible mitigation strategies appropriate for a brownfield facility, assuring system integrity, safety, and compliance.
Methodology:
The depressurization case investigations were carried out using gFLARE, an API 521 (7th Edition) compatible modeling platform that employs proven first principles transient models with rigorous thermodynamics and heat transport processes. Standard cubic equations of state (e.g., Peng-Robinson, SRK) were used, with precise models of ambient and wall heat transmission, equipment internals, secondary phases, and solid formation (hydrates and dry ice).
Process and flare systems were solved simultaneously to determine transient capacity, back-pressure, and facility-wide blowdown behavior. Including temperature-dependent material characteristics and stress calculations during depressurization.
System models were created by reviewing PFDs, P&IDs, HMBs, GAs, pipe classes, and isometrics, as well as identifying elevations and low points. Representative gFLARE models were examined for certain scenarios, with the worst-case fluid compositions chosen for low temperature risk evaluation.
Key finding and mitigation:
The assessment indicated minimum design metal temperature (MDMT) breaches in several locations throughout both plants. These breaches were discovered both on the process side and the flare side, implying that elements of the current infrastructure may be exposed to metal temperatures below their specified MDMT limits under the examined scenarios.
Various mitigations are proposed, involving immediate blowdown, low temperature trips, and partial depressurization of the system.
Conclusion:
Reassessing the depressurization and flare systems under API 521 (7th Edition) conditions revealed localized MDMT violations on both the process and flare sides. These findings demonstrate that existing brownfield facilities may be vulnerable to low temperature exposure under altered design assumptions, thereby emphasizing the importance of selective mitigation measures and operational or design modifications to maintain sustained integrity and compliance
Key words: API 521 (7th Edition), Depressurization System, MDMT Violation, Flare System,
