Optimizing PFP through Risk Based Structural AnalysisJames Loudoun

Onder Akinci, Mike Stahl

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Agenda

• What is PFP?

• Approaches to Identifying PFP Application

• Utilizing Existing Safety Studies to identify Time Dependent Thermal Loading

• Optimizing PFP using Structural Analysis

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What is PFP and When Do We Apply It?

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Passive Fire Protection

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Passive Fire Protection has three main aims:

• To prevent load-bearing structure reaching elevated temperatures, where the steel structure may become impaired.

• To protect process vessels and piping (no disproportionate escalation).

• To prevent heat transfer through walls into habitable spaces by limiting the inner wall temperatures.

PFP Criteria Definition

• Protected element: barrier or load-bearing

• Type of fire and associated heat flux

• Required duration of protection effectiveness

• Resistance to environment

– Installation: transport, erection

– Operating: vibration, mechanical, chemical, ageing, weather, abrasion, erosion, hosing, impact (non-accidental), temperature (high: flare, low: frost)

– Accidental: thermal shock, blast, projectiles / missiles

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Determination of PFP Requirements

Prescriptive Method

• Code/Standard based

• Systematic Method (not optimized, and sometimes conservative)

• May not be conservative (e.g. top flange exposed etc.)

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Determination of PFP Requirements

Screening Methods:

• Conservative/High level checks to initially identify issues

• Element-by-Element Method

• Conservative Method

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Determination of PFP Requirements

Strength Level Analysis:

• Based on global structure

• Employs Linear elastic methods combined with checks of utilization with respect to a critical temperature

• Generally applied offshore

• Less Conservative

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Structural Response

• Determine a structure’s behavior given it is subjected to fire loading.

• Identify Global Collapse Fire/Load Limits using Finite Element Analysis (FEA)

• Identify the integrity of structural supports for safety critical elements

• Identify potential for escalation

• Identify PFP and other mitigation requirements

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Identifying Thermal Loading

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Identifying Thermal Loading

• First Step in PFP optimization is identification of the thermal loads.

• This builds on existing safety studies routinely produced as part of design process:

– Fire and Explosion Analysis

– Quantitative Risk Assessment

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Fire and Explosion Analysis

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Value

• Specification of minimum fire/ explosion ratings for buildings and/or critical structures

• Assist in optimization of fire and explosion protection and design

• Identify the potential need for Additional Analysis (QRA, FEA, Computational Fluid Dynamics (CFD) analysis)

Purpose

• Identify Major Accident Hazards

• Identify of potential fire and explosion scenarios

• Assess potential consequences arising from these Fire and Explosion events

Data Requirements

General arrangements and Layouts, P&IDs, PFDs, Heat and Material Balance, Blowdown and Isolation Data, Cause and Effects, Inputs from HAZOP & HAZID for outlier events, Structural analysis to determine survivability criteria.

Quantitative Risk Assessment (QRA)

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Value

• Determine current, future, and sensitivities to the facility risk profile

• Determine how to address any recommendations for changes to the layout or the process in order to reduce the facility risk (Assess these recommendations -Goal)

• Identify high risk equipment, process, activities

• Influence the design – reduce critical elements

• Inherently safe design

Purpose

• Evaluate the current state of the design

• Identification and classification of all hazards (hydrocarbon, non-process, environmental, transportation, etc.)

• Identify potential cases of structural and process impairment (Offshore)

• Classify Individual Risk, Facility Risk, Societal Risk (Risk Profile)

Data Requirements

Population data and distribution across the site, potential external hazards (HAZOP, HAZID), operating philosophy, safety design measures, FEA results, General Arrangements and layouts, Heat and Material Balance, Blowdown and isolation data, ESD philosophy, Critical outlying questions?

Fire Events

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3D modelling from Phast

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Fire Impinged Areas/Durations

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Identify impinged structural members

Associate a heating duration (heat-flux time profile)

Structural Assessment and PFP Optimization

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Overview

• Establish the minimum PFP coating requirements while satisfying the following criteria:

- No global collapse,

- No disproportionate escalation

• Convert linear structural model e.g. STAAD to NLFEA model

• Heat flux applied to the entire structure to determine failure time

• Develop initial PFP scheme based on experience and failure time

• Iterate analysis until PFP minimized and criteria is met

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Example, PFP on Piperack

• Generated detailed Abaqus FEA model• PFP’d members and pipes temp limited to

400 °C• Non-PFP’d members were removed and

their load was redistributed to protected members

• PFP’d members are highlighted in red

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Thermal Loads and Structural Members

Example, PFP on Piperack

• Screening method required to PFP entire structure

• The figures show the optimized PFP scheme (PFP coated members shown in magenta)

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A B

Example, PFP on Piperack

• Plastic utilization plots

• Critical members are PFP’d

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A B

Example, PFP on Piperack

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Deflections and Strain

Vertical DeflectionsPlastic Strain

Assessment of Vessel Failure under Thermal Load

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Thermal Load on Vessel

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Phase Plots (Propane)150 kW/m2

250 kW/m2 350 kW/m2

Initial

LiquidVapor

Thermal Load on Vessel

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Vessel Failure

Failure of vessel in less than 6 minutes.

PFP Detailing

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• PFP termination on top flange of lateral members. The exposed top flange causes the section stiffness and capacity to decrease.

• To be conservative, the top flange is reduced from the section in the extreme condition analysis.

Detailing

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Exposed Top Flange

Detailing

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Coatback

Detailing

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Coatback

Thank you, Questions?

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