Abstract
In Oil & Gas facilities, emergency depressurization is a prime mitigation to reduce risk to personnel/assets during fire and avoids catastrophic failure. The conventional approach considers standard criteria for vessels/pipes for establishing depressurization rates without assessing dynamic stress changes due to actual material properties/thicknesses. The paper discusses latest API approach of Fire Response Analysis (FRA), which evaluates rupture possibility, consequences and mitigation to ensure the integrity of process/flare system and establishes more accurate depressurization rates.
The conventional approach considers depressurization to 50%/100 psig in 15 minutes for material thicknesses one inch & above and faster depressurization for thicknesses below one inch. As standard engineering practice, depressurization of facilities in 15 minutes is normally followed irrespective of material thickness. Latest API approach determines depressurization rate based on FRA, which accounts transient thermo-physical properties along with heat transfer and consider reduction in material strength when exposed to fire. FRA is an exhaustive study requiring detailed inputs such as type of fire, fire duration, heat flux, rupture acceptance criteria, in addition to inputs considered in conventional approach.
Emergency depressurization rate in FRA Study is established based on adequacy of ultimate material tensile strength against the stresses developed under fire scenario. In house case studies compare the results of emergency depressurization rates based on FRA Study and conventional approach for various isolatable system of process complex. Emergency depressurization rates in FRA Study are found to be dependent on material thickness as well as tensile strength and usually results in lower or higher than 15 minutes to ensure vessel/pipe survivability.
FRA Study follows a multi-discipline approach to conclude depressurization rates based on various parameters such as acceptability of facility rupture consequences, search/rescue time of field personnel. If rupture criterion is not acceptable from Safety Risk Analysis, FRA study re-establishes the emergency depressurization rates by accelerating the depressurization rates and/or increasing vessel/pipe thickness and/or providing Passive Fire Protection (PFP).
FRA study results are utilized to finalize emergency depressurization / blowdown line sizes including Restriction Orifice (RO) size, flare headers sizes and flare system design capacity. RO sizes per FRA Study are utilized to finalize the non-fire case blowdown and minimum metal design temperature of facilities.
NPCC has executed many Oil & Gas projects involving flare system. This paper discusses the challenges and "Lessons Learned" by EPC Contractor in applying new FRA approach to ensure integrity of safety critical systems through case studies from recent project. The paper also highlights the benefits of FRA study including effective utilization of existing flare spare capacity and proposes way forward to assist Operators in decision making process for up-gradation of existing facilities.
