Designing Robust MEG Distribution Systems

Jeff Zhang, Snr Flow Assurance Consultant.Paul Oakley, Staff Flow Assurance Consultant.

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Continuous MEG injection is commonly used for hydratemanagement in deepwater gas developments such as the Gorgonand Ichthys fields.MEG Distribution System (MDS) design appears deceptively simpledue to single phase operation. However, over 20 yrs in combinedexperience in MEG distribution system design has highlighted:

• Design complexity due to multi-discipline involvement;• Flow Assurance discipline is arguably best placed to

co-ordinate the MEG distribution system design.This presentation provides guidance on MDS design and typical flowassurance design challenges.

Introduction

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MEG

Reservoir• Gas Composition• Water Production• Production Profile

Onshore/Topsides Design• MEG Regeneration Units• MEG Storage Tanks• MEG Pumps & Controls

MEG Pipeline Design• Size • Design Pressure

Subsea Design• Wet Gas Meter• MEG Distribution Valve

Production Flowline

MDS Design & Interfaces

What is the role of Flow Assurance?

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Flow Assurance MDS Design

• Identification of key design decisions & stakeholder engagement in late concept /early FEED is paramount to a robust design;

• Flow Assurance considerations strongly influence the MDS design and operational robustness.

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• Minimising the MEG regeneration unit design capacity against thedesire to ensure conservative MEG overdose margins;

• Reducing the MDS design pressure against the desire to minimizeMEG pipeline sizes;

• Selecting the MEG injection location to suit the reservoircharacteristics and production profile against the desire to enhanceMDS operability.

Typical MDS Design Challenges

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Variable Unit P 1 P 2 P 3

MEG Overdose Margin % 30 30 28.6

MEG Pipeline Size inch 8 8 8

MEG Injection Location - All

DS-ChokeUS-ChokeDS-Choke

AllUS-Choke

Max. Dry Raw Gas Flowrate Bscf/d 1.9 1.6 2.4

Max. Lean MEG Rate m3/h 93 24 105

Max. SITHP bara 365 285 317

MDS Design Data Comparison

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• It is a necessary design practice to add margins to the requiredMEG injection rate to allow for uncertainties and minimise therisk of under injection.

• Excessive MEG overdose margin:• MDS over-loading from onshore/topsides to wellhead;• Production system slugging and liquid management issues.

• Typical MEG overdose margin values:• 30% for complex networks and lower for simple ones;• Further optimization requires the margin breakdown to focus

on the key contributing factors.

Optimal MEG Overdose Margin

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Composition Uncertainty

FluidDefinitionWaterProductionHydrateCurve

Metering and System Uncertainty

Water Meter

MDV

ProductionSystem

MEG Overdose Margin Contributions.

• Recent advances in MEG Injection valves (closed-loop control)provide an opportunity to significantly reduce overdose margins;

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Multiphase production flowline sizing criteria:• Pressure management: Backpressure;• Liquids management: Liquid holdup;• Solids management: Liquid film velocity.

Single phase MEG pipeline sizing criteria?• CAPEX (e.g. Pipeline standardisation and installation)• MEG deliverability and pressure safeguarding• Operability and controllability considerations

Optimal MEG Pipeline Sizing

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MEG Pipeline Sizing Trade-offsVariable Larger Pipeline Smaller Pipeline

FrictionalPressure Drop Negligible Higher

MEG Mal-distribution Lower Higher

Pressure Surge Lower Higher

MEG Pump ControlStrategy Discharge Pressure Control Discharge Pressure Control with Flow

Ratio Over-riding

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Highly dependent on subsurface data:• Reservoir characteristics (composition/P/T);• Production profiles (flowrate).

Potential for a significant impact on the subsea and onshoredesign:• Subsea Tree functionality and configuration;• MEG pipeline supply pressure;• MEG injection pump pressure setting;• Operational complexity.

MEG Injection Location

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Item Upstream of Choke Downstream of Choke

Reservoir Characteristics

Homogeneous reservoir depletion

Discrete reservoirs (High FTHP variability)

MEG Supply Pressure

High(SITHP + 20 bar)

Med.(Intermittent HP supply req’d)

XT Hydrate Blockage Risk Low Risk Med. Risk

(Hydrate kinetics analysis)

Pressure Safeguarding

High Consequence(Potential inter-connections)

Low Consequence

MEG Injection Location Trade-offs

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Simple MDS DesignUnfortunately there is no simple way to design an optimal MEDDistribution System:• Each project will likely have a unique combination of design drivers

due to the selected development concept and reservoir properties;• Each project will have a unique set of individual stakeholders, often

these are in overseas design offices;• Multi-discipline involvement results in an iterative (and sometimes

slow) design process.

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Keys to Robust MDS Design

Subsea Architecture Pressure

Safeguarding

Topsides/ Onshore Design

PipelineDesign

Well & Completion

Design

Subsurface Data

Operating Philosophy

• Early recognition that the MDS is not just a chemical injectionsystem – it is the lifeblood of the production system;

• Pre-FEED identification of key design decisions and stakeholders,to allow adequate time for recycle;

• Experienced technical Flow Assurance or “Systems” Leadershipthat understand the trade-offs being made in design decisions.

Subsea Design

Flow Assurance

Thank you!

Questions?