LNG Cargo Ageing and Transport Simulation

The neqsim.process.equipment.lng package provides a physics-based simulation framework for LNG cargo quality evolution during storage and marine transport. It models boil-off gas (BOG) generation, preferential evaporation, stratification, rollover risk, quality KPI tracking, and multi-tank ship operations.

Package Overview

Class Purpose
LNGAgeingScenario Top-level orchestrator for a single-tank ageing simulation
LNGShipModel Multi-tank ship model with common BOG header
LNGTankLayeredModel Core physics engine: multi-layer VLE, heat ingress, boil-off
LNGTankLayer Single horizontal layer with composition and density tracking
LNGVaporSpaceModel Vapor space pressure evolution and relief valve logic
LNGRolloverDetector Stratification rollover risk assessment
LNGBOGHandlingNetwork BOG disposition: fuel, reliquefaction, GCU, venting
LNGVoyageProfile Route environment: ambient temperature, waves, wind, solar
LNGHeelManager Heel retention, spray cooling, cargo mixing
LNGAgeingResult Snapshot of quality KPIs at each time step
TankGeometry Physical geometry for membrane, Moss, Type-C, SPB tanks
TankHeatTransferModel Multi-zone heat transfer: bottom, sidewalls, roof, cofferdam
MethaneNumberCalculator Methane Number via EN 16726, MWM, or simplified correlation
LNGSloshingModel Sloshing mixing and BOG enhancement from sea state

Quick Start

Java

import neqsim.process.equipment.lng.*;
import neqsim.process.equipment.stream.Stream;
import neqsim.thermo.system.SystemSrkEos;

// 1. Define LNG composition
SystemSrkEos lng = new SystemSrkEos(111.0, 1.013);
lng.addComponent("methane", 0.92);
lng.addComponent("ethane", 0.05);
lng.addComponent("propane", 0.02);
lng.addComponent("nitrogen", 0.01);
lng.setMixingRule("classic");

// 2. Create feed stream
Stream feed = new Stream("LNG feed", lng);
feed.setFlowRate(140000.0, "m3/hr");
feed.run();

// 3. Configure scenario
LNGAgeingScenario scenario = new LNGAgeingScenario("Qatar-Japan", feed);
scenario.setTankVolume(174000.0);
scenario.setInitialFillingRatio(0.98);
scenario.setSimulationTime(480.0);  // 20 days
scenario.setTimeStepHours(1.0);
scenario.setOverallHeatTransferCoeff(0.045);
scenario.setAmbientTemperature(308.15);  // 35 degC
scenario.setTankPressure(1.013);

// 4. Run and get results
scenario.run();

for (LNGAgeingResult r : scenario.getResults()) {
    System.out.printf("t=%6.1f h  T=%.2f K  rho=%.1f kg/m3  WI=%.2f MJ/Sm3  MN=%.1f  BOR=%.4f %%/day%n",
        r.getTimeHours(), r.getTemperature(), r.getDensity(),
        r.getWobbeIndex(), r.getMethaneNumber(), r.getBoilOffRatePctPerDay());
}

Python (Jupyter)

from neqsim import jneqsim

SystemSrkEos = jneqsim.thermo.system.SystemSrkEos
Stream = jneqsim.process.equipment.stream.Stream
LNGAgeingScenario = jneqsim.process.equipment.lng.LNGAgeingScenario

lng = SystemSrkEos(111.0, 1.013)
lng.addComponent("methane", 0.92)
lng.addComponent("ethane", 0.05)
lng.addComponent("propane", 0.02)
lng.addComponent("nitrogen", 0.01)
lng.setMixingRule("classic")

feed = Stream("LNG feed", lng)
feed.setFlowRate(140000.0, "m3/hr")
feed.run()

scenario = LNGAgeingScenario("Qatar-Japan", feed)
scenario.setTankVolume(174000.0)
scenario.setInitialFillingRatio(0.98)
scenario.setSimulationTime(480.0)
scenario.setTimeStepHours(1.0)
scenario.setOverallHeatTransferCoeff(0.045)
scenario.setAmbientTemperature(308.15)
scenario.run()

for r in scenario.getResults():
    print(f"t={r.getTimeHours():6.1f}h  WI={r.getWobbeIndex():.2f}  MN={r.getMethaneNumber():.1f}")

Core Concepts

Preferential Evaporation and Ageing

LNG is a cryogenic mixture (predominantly methane with nitrogen, ethane, propane, and heavier hydrocarbons). During storage and transport, heat leaks into the tank, causing boil-off. Lighter components (nitrogen first, then methane) evaporate preferentially, enriching the liquid in heavier components. This “ageing” effect changes the cargo’s quality KPIs over time:

Multi-Layer Stratification

The tank model supports multiple horizontal layers to simulate stratification. When LNG of different compositions co-exists (e.g., heel + new cargo, or ship-to-ship transfer), layers with different densities can form. Key physics:

Boil-Off Gas (BOG) Handling

The LNGBOGHandlingNetwork allocates generated BOG across:

  1. Engine fuel — DFDE, MEGI, or steam turbine consumption
  2. Reliquefaction — partial or full BOG recondensation
  3. Gas Combustion Unit (GCU) — excess BOG combustion
  4. Venting — emergency only (last resort)

Advanced Features

Tank Geometry

Use TankGeometry to define the physical containment:

TankGeometry membrane = TankGeometry.createQMax();    // 174,000 m3 membrane
TankGeometry moss = TankGeometry.createMossSingle();   // 36,000 m3 Moss sphere
TankGeometry typeC = TankGeometry.createTypeC();       // 7,500 m3 Type C

scenario.setTankGeometry(membrane);  // Wires geometry into tank model

The geometry provides surface areas for heat transfer zones, wetted wall areas, liquid height calculations, and insulation U-values specific to each containment type.

Multi-Zone Heat Transfer

The TankHeatTransferModel replaces the simple UAΔT calculation with zone-specific heat transfer:

TankHeatTransferModel htModel = new TankHeatTransferModel(geometry, 308.15);
// Automatically creates zones: bottom, sidewalls, roof, cofferdam

// Update for varying ambient conditions
htModel.updateBoundaryConditions(310.0, 600.0, 285.0);
// (ambientTemp, solarRadiation, seaWaterTemp)

Heat distribution to layers: bottom heat goes to the bottom layer, roof heat to the top 30%, sidewall heat distributed proportionally.

Methane Number Calculation

Three methods are available:

MethaneNumberCalculator calc = new MethaneNumberCalculator();

// EN 16726 (European standard)
calc.setMethod(MethaneNumberCalculator.Method.EN16726);
double mn = calc.calculate(composition);

// MWM (engine manufacturer method)
calc.setMethod(MethaneNumberCalculator.Method.MWM);

// Simplified (fast heuristic)
calc.setMethod(MethaneNumberCalculator.Method.SIMPLIFIED);

// All at once
Map<String, Double> allMN = calc.calculateAll(composition);

// Specification check
boolean ok = calc.meetsSpecification(composition, 70.0);

Sloshing Model

Sea state affects LNG tanks through sloshing, which:

LNGSloshingModel sloshing = new LNGSloshingModel(TankGeometry.ContainmentType.MEMBRANE);
double mixingFactor = sloshing.calculateMixingFactor(2.5, 0.5); // Hs=2.5m, 50% fill
double bogFactor = sloshing.calculateBOGEnhancement(2.5, 0.5);

Fill level effect follows a bell curve centered at ~40% fill (worst for sloshing).

Rollover Detection

The LNGRolloverDetector monitors stratified tanks for rollover risk:

LNGRolloverDetector detector = new LNGRolloverDetector();
detector.setDensityWarningThreshold(2.0);  // kg/m3
detector.setDensityAlarmThreshold(5.0);     // kg/m3

RolloverAssessment result = detector.assess(tankModel.getLayers());
System.out.println("Risk: " + result.getRiskLevel());
System.out.println("Max density diff: " + result.getMaxDensityDifference());

// Time-to-rollover prediction (after multiple assessments)
double ttr = result.getEstimatedTimeToRolloverHours();

Voyage Profile

Define time-varying environmental conditions along the route:

LNGVoyageProfile profile = new LNGVoyageProfile("Qatar to Japan");
profile.addSegment(new Segment(0, 120, 308.15, 1.0, 10.0, 800.0));    // Persian Gulf
profile.addSegment(new Segment(120, 240, 305.15, 1.5, 15.0, 700.0));   // Indian Ocean
profile.addSegment(new Segment(240, 400, 300.15, 2.0, 20.0, 500.0));   // South China Sea
profile.addSegment(new Segment(400, 480, 293.15, 1.0, 10.0, 400.0));   // Arrival

scenario.setVoyageProfile(profile);

Heel Management

Model the mixing of residual heel with new cargo:

LNGHeelManager heel = scenario.getHeelManager();
heel.setHeelState(heelComposition, 112.0, 425.0);
// Heel typically 3% of tank volume, aged/methane-rich

Multi-Tank Ship Model

The LNGShipModel runs multiple tanks in parallel and aggregates results:

LNGShipModel ship = new LNGShipModel("Q-Max Carrier");

// Add 4 membrane tanks
for (int i = 1; i <= 4; i++) {
    LNGAgeingScenario tank = new LNGAgeingScenario("Tank " + i, feed);
    tank.setTankVolume(43500.0);    // 174,000/4
    tank.setInitialFillingRatio(0.98);
    // ... configure each tank
    ship.addTank(tank);
}

// Shared BOG network
ship.getBogNetwork().setHandlingMode(HandlingMode.FUEL_PLUS_RELIQUEFACTION);
ship.getBogNetwork().setBaseFuelConsumption(3000.0);
ship.getBogNetwork().setReliquefactionCapacity(5000.0);

ship.setSimulationTime(480.0);
ship.setTimeStepHours(1.0);
ship.run();

System.out.println("Cargo loss: " + ship.getTotalCargoLossPct() + "%");
System.out.println(ship.getShipSummary());

GERG-2008 Equation of State

For high-accuracy density calculations, enable GERG-2008:

scenario.setUseGERG2008(true);
// The tank model will use SystemGERG2008Eos for density
// when available — falls back to ISO 6578 Klosek-McKinley otherwise.

Operational Events

Track loading, unloading, and port operations:

scenario.addOperationalEvent(new OperationalEvent(
    EventType.LOADING, 0.0, 12.0));            // 12h loading
scenario.addOperationalEvent(new OperationalEvent(
    EventType.LADEN_VOYAGE, 12.0, 480.0));     // 480h laden voyage
scenario.addOperationalEvent(new OperationalEvent(
    EventType.UNLOADING, 492.0, 18.0));        // 18h unloading

Quality KPIs Tracked

KPI Unit Method
Temperature K Mass-averaged across layers
Density kg/m³ ISO 6578 (Klosek-McKinley) or GERG-2008
Wobbe Index MJ/Sm³ ISO 6976
Gross Calorific Value MJ/Sm³ ISO 6976
Methane Number — EN 16726, MWM, or simplified
Boil-Off Rate %/day Mass loss / initial mass × 24
BOG mass flow kg/hr From VLE flash
Heat ingress kW Multi-zone heat transfer
Rollover risk Level Density inversion + Rayleigh number

Class Diagram

LNGShipModel
  ├── LNGAgeingScenario (per tank)
  │     ├── LNGTankLayeredModel
  │     │     ├── LNGTankLayer[] (1..N layers)
  │     │     ├── TankGeometry (optional)
  │     │     ├── TankHeatTransferModel (optional)
  │     │     ├── MethaneNumberCalculator (optional)
  │     │     └── LNGSloshingModel (optional)
  │     ├── LNGVaporSpaceModel
  │     ├── LNGRolloverDetector
  │     ├── LNGBOGHandlingNetwork
  │     ├── LNGHeelManager
  │     └── LNGVoyageProfile
  └── LNGBOGHandlingNetwork (shared header)

Example Notebooks