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Dynamic modelling and simulation of Flare Systems Flow assurance and Dynamic Simulations Seminar, Stavanger 23.10.2012 Henriette Raddum | Process Specialist Engineer, Aker Solutions MMO Public © 2012 Aker Solutions
Typical flare system ■ Purpose: ■ Protect equipment against overpressure from system malfunction or fire ■ Collect hydrocarbon gases and liquids from relief and depressurisation ■ Conduct relieved fluid to a location were it can be safely discharged
Flare tip Flare sub header
Pressure Safety Valve (PSV)
Blowdown valve (BDV)
Flare main header Flare KO Drum Pressure Control Valve (PV)
From various process equipment
■ Typical relief sources: ■ Pressure Safety Valves (PSVs) ■ Blowdown valves (BDVs) ■ Pressure Control Valves (PVs) Public © 2012 Aker Solutions
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Typical simulation of flare systems ■ Steady state (SS) considerations traditionally used for flare system
load evaluations (Flarenet) ■ Steady State (SS) simulations: ■ Initial peak rate “goes on forever” ■ Flare system volume not
accounted for ■ No accumulation in system ■ Peak rate in = peak rate out ■ Why dynamic simulation (DS)? ■ ■ ■ ■
More realistic Less conservative Includes initial accumulation due to volume in flare network Peak flow out typically delayed and smaller than peak flow in
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Rules and regulations ■ API Standard 521 / ISO 23251 (Pressure-relieving and Depressuring
Systems) about Dynamic Simulations (DS): ■ § 5.22 (Single system): ■ DS can be used to calculate transient pressure increases ■ DS can be used to calculate relief rates (PSVs) ■ Conventional methods are conservative and can give oversized relief and flare system designs. ■ Dynamic simulations are better to define the realistic relief loads and improves the understanding of what happens during relief ■ If the physical phenomena are not well understood, the dynamic model shall include conservative assumptions ■ At steady-state conditions the dynamic model shall closely match the steady-state ■
§ 7.1.4.2 (Disposal system load): ■ DS is particularly useful in analysing existing flare systems ■ DS allows user to predict timing of individual system peak loads ■ Peak load for various parts of disposal system can occur at different times
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Typical flare simulation tools used by Aker Solutions ■ Flarenet (steady state) ■ OLGA (dynamic) ■ Hysys Dynamics (dynamic) ■ K-Spice (dynamic) ■ Selection criteria – choice of tool: ■ ■ ■ ■
Number of sources Compositions Gas or multiphase flow Customer preference
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Ongoing study ■ Background: ■ ■ ■ ■ ■ ■
Modification project on existing platform with limited flare capacity Total Platform depressurisation → maximum gas to flare Sequential depressurisation introduced Tie-in of new fields → new equipment → increased blowdown volumes Increased Inlet separator pressure → higher initial blowdown rate Static calculations → Flare KO drum pressure close to max pressure allowed by design code (design pressure + 10%)
■ Main purpose – dynamic flare model: ■ ■ ■ ■
Include initial “packing effect” in flare system More realistic rates and pressures in different parts of flare system Include effect of flare back pressure on segment blowdown times Minimise Project Execution cost (modification requirement)
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Ongoing study – methodology ■ Flare model built in Hysys (v7.3) ■ Built “from scratch” (isometrics) ■ 67 Blowdown Valves (BDV) ■ 87 Pressure Safety Valves (PSV) ■ 12 Pressure Control valves (PV)
■ Headers and lines downstream Blowdown Valves modelled in detail ■ Lines without Blowdown Valves simplified
■ Lines <4 inch not included unless connected to Blowdown Valve ■ Blowdown segments modelled as vessels
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Ongoing study – Flare network in Hysys
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Ongoing study – Flare network in Hysys
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Ongoing study – challenges ■ Large model – many sources ■ Large amount of input data required ■ Choice of settings and parameters in Hysys ■ Existing input data for blowdown segments incomplete ■ Existing Flarenet model has errors → challenging to tune the
dynamic model ■ Model detail level ■ Simulation time
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Ongoing study – current status ■ Flare network and KO drum / flare tip modelled in Hysys ■ Test / verification of dynamic model against Flarenet model ■ Aspentech Support consulted for advice and trouble-shooting ■ Compared “Depressuring Utility” (Hysys SS) with depressuring of ■ ■ ■ ■
equivalent vessel in Hysys Dynamics Re-calculated several blowdown segments Tested blowdown of 5 identical “dummy segments” connected to flare network Re-run of Depressuring Utility in Hysys SS for all BD segments Dynamic modelling of vessels representing blowdown segments ongoing
■ 1st priority: Depressuring without heat input from fire ■ Depressuring with fire may be included later
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Ongoing study – preliminary test results ■ Result from “dummy test” ■ 5 identical segments connected at different locations in flare network ■ Blowdown valves opened simultaneously ■ Graph shows mass flow from BD valves (blue) and out of flare tip
(pink)
Steady State mass flow
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Ongoing study – “the End” ■ What we hope to achieve: ■ ■ ■ ■ ■
Find available margins to maximum allowable pressure in KO drum Find available flare capacity for blowdown of future tie-ins Clarify need for sequential depressuring on new or existing segments Better understanding Reduced modification scope and cost for future tie-ins
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Disclaimer This Presentation includes and is based, inter alia, on forward-looking information and statements that are subject to risks and uncertainties that could cause actual results to differ. These statements and this Presentation are based on current expectations, estimates and projections about global economic conditions, the economic conditions of the regions and industries that are major markets for Aker Solutions ASA and Aker Solutions ASA’s (including subsidiaries and affiliates) lines of business. These expectations, estimates and projections are generally identifiable by statements containing words such as “expects”, “believes”, “estimates” or similar expressions. Important factors that could cause actual results to differ materially from those expectations include, among others, economic and market conditions in the geographic areas and industries that are or will be major markets for Aker Solutions’ businesses, oil prices, market acceptance of new products and services, changes in governmental regulations, interest rates, fluctuations in currency exchange rates and such other factors as may be discussed from time to time in the Presentation. Although Aker Solutions ASA believes that its expectations and the Presentation are based upon reasonable assumptions, it can give no assurance that those expectations will be achieved or that the actual results will be as set out in the Presentation. Aker Solutions ASA is making no representation or warranty, expressed or implied, as to the accuracy, reliability or completeness of the Presentation, and neither Aker Solutions ASA nor any of its directors, officers or employees will have any liability to you or any other persons resulting from your use. Aker Solutions consists of many legally independent entities, constituting their own separate identities. Aker Solutions is used as the common brand or trade mark for most of these entities. In this presentation we may sometimes use “Aker Solutions”, “we” or “us” when we refer to Aker Solutions companies in general or where no useful purpose is served by identifying any particular Aker Solutions company.
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6 November, 2012
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