Riser Mechanical Design

Comprehensive documentation for riser mechanical design in NeqSim, including catenary mechanics, VIV analysis, fatigue life estimation, and dynamic response calculations per industry standards.

Table of Contents

Overview

The riser mechanical design system provides:

Location: neqsim.process.mechanicaldesign.pipeline

Classes:

Architecture

Class Hierarchy

RiserMechanicalDesign extends PipelineMechanicalDesign
├── RiserMechanicalDesignCalculator extends PipeMechanicalDesignCalculator
│   ├── Catenary calculations (top tension, TDP stress)
│   ├── TTR calculations (tension, stroke)
│   ├── VIV analysis (frequency, amplitude, fatigue)
│   ├── Dynamic response (wave, heave stress)
│   └── Fatigue life (combined sources)
├── RiserMechanicalDesignDataSource
│   ├── TechnicalRequirements_Process (company-specific)
│   └── dnv_iso_en_standards (industry standards)
└── Inherits from PipelineMechanicalDesign
    ├── Wall thickness calculations
    ├── Stress analysis
    └── Weight and buoyancy

Design Flow

1. Create Riser with type and water depth
2. Set environmental conditions (current, waves, heave)
3. Run riser simulation
4. Initialize mechanical design
5. Load design parameters from database
6. Calculate riser-specific parameters
7. Perform fatigue analysis
8. Export to JSON

Riser Types

Steel Catenary Riser (SCR)

Free-hanging catenary configuration from FPSO or semi-submersible.

Key calculations:

Riser scr = Riser.createSCR("Production SCR", inletStream, 800.0);
scr.setTopAngle(12.0);        // Degrees from vertical
scr.setDepartureAngle(18.0);  // Degrees from horizontal at TDP

Top Tensioned Riser (TTR)

Tensioned from TLP or Spar platform with tensioner system.

Key calculations:

Riser ttr = Riser.createTTR("Export TTR", inletStream, 500.0);
ttr.setAppliedTopTension(2500.0);       // kN
ttr.setTensionVariationFactor(0.15);    // 15% variation
ttr.setPlatformHeaveAmplitude(2.5);     // meters

Lazy-Wave Riser

SCR with buoyancy modules creating wave shape to reduce TDP stress.

Key calculations:

Riser lazyWave = Riser.createLazyWave("Gas Export", inletStream, 1200.0, 400.0);
lazyWave.setBuoyancyModuleLength(150.0);  // meters
lazyWave.setBuoyancyPerMeter(500.0);      // N/m

Flexible Riser

Unbonded flexible pipe for dynamic applications.

Key features:

Riser flexible = Riser.createFlexible("Water Injection", inletStream, 300.0);

Hybrid Riser

Tower riser with flexible jumper, for ultra-deep water.

Riser hybrid = Riser.createHybrid("Deepwater Export", inletStream, 2000.0);

Design Standards

Supported Standards

Standard Version Scope
DNV-OS-F201 2010 Dynamic Risers - main design standard
DNV-RP-F204 2010 Riser Fatigue
DNV-RP-C203 2021 Fatigue Design of Offshore Structures
DNV-RP-C205 2021 Environmental Conditions and Loads
API RP 2RD 2013 Riser Design for FPS
API RP 17B 2014 Flexible Pipe

Key Parameters from Database

DNV-OS-F201 (Dynamic Risers)

Parameter Value Range Description
Usage Factor 0.77 - 0.83 Resistance utilization
Safety Class (Low) 1.046 Low consequence
Safety Class (Medium) 1.138 Medium consequence
Safety Class (High) 1.308 High consequence
Dynamic Amplification Factor 1.1 - 1.3 DAF for dynamic loading
Max Utilization 0.80 Combined loading limit

DNV-RP-F204 (Riser Fatigue)

Parameter Value Description
Fatigue Design Factor 3 - 10 Per safety class
S-N Parameter (seawater) 12.164 log(a) for D-curve
S-N Slope 3.0 m parameter
SCF (girth weld) 1.2 - 1.5 Stress concentration

DNV-RP-C205 (Environmental Loads)

Parameter Value Description
Strouhal Number 0.18 - 0.22 VIV frequency
Drag Coefficient 0.9 - 1.2 Bare cylinder
Added Mass Coefficient 1.0 Hydrodynamic mass
Lift Coefficient 0.8 - 1.0 VIV lift

Catenary Mechanics

Top Tension Calculation

For a catenary riser, the top tension is calculated from:

\[T_{top} = \frac{w \cdot H}{\sin(\theta_{top})}\]

Where:

RiserMechanicalDesignCalculator calc = design.getRiserCalculator();
calc.calculateCatenaryTopTension();

double topTension = calc.getTopTension();       // kN
double bottomTension = calc.getBottomTension(); // kN
double catenaryParam = calc.getCatenaryParameter(); // m

Touchdown Point Stress

Bending stress at the touchdown point:

\[\sigma_b = \frac{E \cdot D_o}{2 \cdot R_{TDP}}\]

Where:

calc.calculateTouchdownPointStress();

double tdpStress = calc.getTouchdownPointStress();     // MPa
double tdpRadius = calc.getTouchdownCurvatureRadius(); // m
double tdpLength = calc.getTouchdownZoneLength();      // m

Top Tensioned Risers

TTR Tension

For TTR, the applied tension must exceed the riser weight plus environmental loads:

\[T_{applied} > T_{riser} + T_{environmental}\] 
calc.setAppliedTopTension(2500.0);  // kN
calc.setTensionVariationFactor(0.15);

calc.calculateTTRTension();

double topTension = calc.getTopTension();
double minTension = calc.getMinTopTension();
double maxTension = calc.getMaxTopTension();

Stroke Requirement

Tensioner stroke required for heave compensation:

\[Stroke = 2 \cdot A_{heave} \cdot (1 + \text{margin})\] 
calc.setPlatformHeaveAmplitude(3.0);
calc.calculateStrokeRequirement();

double stroke = calc.getStrokeRequirement();  // m

VIV Analysis

Vortex Shedding Frequency

\[f_v = \frac{St \cdot V}{D}\]

Where:

Lock-In Detection

Lock-in occurs when the vortex shedding frequency is close to a natural frequency:

\[0.7 < \frac{f_v}{f_n} < 1.3\] 
calc.calculateVIVResponse();

double vortexFreq = calc.getVortexSheddingFrequency();  // Hz
double naturalFreq = calc.getNaturalFrequency();         // Hz
boolean lockIn = calc.isVIVLockIn();
double amplitude = calc.getVIVAmplitude();               // A/D ratio

VIV Fatigue Damage

Annual fatigue damage from VIV:

\[D_{VIV} = \frac{n \cdot f_v \cdot 3.15 \times 10^7}{N_{cycles}}\] 
calc.calculateVIVFatigueDamage();

double vivDamage = calc.getVIVFatigueDamage();  // per year

Fatigue Analysis

Combined Fatigue Life

Total fatigue life combines multiple damage sources:

\[\text{Life} = \frac{1}{D_{VIV} + D_{wave} + D_{TDP} + D_{heave}}\] 
calc.calculateRiserFatigueLife();

double fatigueLife = calc.getRiserFatigueLife();  // years
double totalDamage = calc.getTotalFatigueDamageRate();  // per year

S-N Curve

Fatigue life per DNV-RP-C203:

\[\log N = \log a - m \cdot \log \Delta\sigma\]

Where:

Dynamic Response

Wave-Induced Stress

Stress from wave loading:

\[\sigma_{wave} = DAF \cdot \frac{H_s \cdot \rho \cdot g \cdot D}{16 \cdot t}\] 
calc.setSignificantWaveHeight(4.0);
calc.setPeakWavePeriod(12.0);
calc.calculateWaveInducedStress();

double waveStress = calc.getWaveInducedStress();  // MPa

Heave-Induced Stress

For TTR, heave motion induces axial stress variation:

calc.setPlatformHeaveAmplitude(3.0);
calc.setPlatformHeavePeriod(10.0);
calc.calculateHeaveInducedStress();

double heaveStress = calc.getHeaveInducedStress();  // MPa

Database Integration

Loading Design Parameters

Parameters are loaded from TechnicalRequirements_Process and dnv_iso_en_standards tables:

RiserMechanicalDesign design = riser.getRiserMechanicalDesign();
design.setDesignStandardCode("DNV-OS-F201");
design.setCompanySpecificDesignStandards("Equinor");

design.readDesignSpecifications();  // Loads from database

Configurable Parameters

Parameter Method Source
Strouhal Number setStrouhalNumber() DNV-RP-C205
Drag Coefficient setDragCoefficient() DNV-RP-C205
Added Mass setAddedMassCoefficient() DNV-RP-C205
S-N Parameter setSnParameter() DNV-RP-C203
S-N Slope setSnSlope() DNV-RP-C203
Fatigue Design Factor setFatigueDesignFactor() DNV-RP-F204
DAF setDynamicAmplificationFactor() DNV-OS-F201
SCF setStressConcentrationFactor() DNV-RP-F204

JSON Reporting

Full Design Report

String json = design.toJson();

Output includes:

Calculator Results

String calcJson = design.getRiserCalculator().toJson();

Examples

Example 1: Complete SCR Design

import neqsim.thermo.system.SystemSrkEos;
import neqsim.process.equipment.stream.Stream;
import neqsim.process.equipment.pipeline.Riser;
import neqsim.process.mechanicaldesign.pipeline.RiserMechanicalDesign;

// Production fluid
SystemSrkEos fluid = new SystemSrkEos(350.0, 100.0);
fluid.addComponent("methane", 0.80);
fluid.addComponent("ethane", 0.10);
fluid.addComponent("propane", 0.05);
fluid.addComponent("n-heptane", 0.05);
fluid.setMixingRule("classic");

Stream production = new Stream("Production", fluid);
production.setFlowRate(50000.0, "kg/hr");
production.run();

// SCR configuration
Riser scr = Riser.createSCR("Export SCR", production, 1000.0);
scr.setDiameter(0.3048);           // 12 inch
scr.setTopAngle(12.0);
scr.setDepartureAngle(15.0);

// Environmental conditions
scr.setCurrentVelocity(0.8);
scr.setSeabedCurrentVelocity(0.3);
scr.setSignificantWaveHeight(4.0);
scr.setPeakWavePeriod(12.0);
scr.setPlatformHeaveAmplitude(3.0);

scr.run();

// Mechanical design
RiserMechanicalDesign design = scr.getRiserMechanicalDesign();
design.setMaxOperationPressure(150.0);
design.setMaterialGrade("X65");
design.setDesignStandardCode("DNV-OS-F201");
design.setCompanySpecificDesignStandards("Equinor");

design.readDesignSpecifications();
design.calcDesign();

// Results
var calc = design.getRiserCalculator();
System.out.println("=== SCR Design Results ===");
System.out.println("Top Tension: " + calc.getTopTension() + " kN");
System.out.println("TDP Stress: " + calc.getTouchdownPointStress() + " MPa");
System.out.println("VIV Lock-In: " + calc.isVIVLockIn());
System.out.println("Fatigue Life: " + calc.getRiserFatigueLife() + " years");
System.out.println("Design OK: " + design.isDesignAcceptable());

Example 2: TTR for TLP

Riser ttr = Riser.createTTR("Gas Export TTR", production, 600.0);
ttr.setDiameter(0.254);
ttr.setAppliedTopTension(3000.0);
ttr.setTensionVariationFactor(0.12);
ttr.setPlatformHeaveAmplitude(2.0);
ttr.run();

RiserMechanicalDesign ttrDesign = ttr.getRiserMechanicalDesign();
ttrDesign.setMaxOperationPressure(200.0);
ttrDesign.setMaterialGrade("X65");
ttrDesign.calcDesign();

var ttrCalc = ttrDesign.getRiserCalculator();
System.out.println("TTR Tension: " + ttrCalc.getTopTension() + " kN");
System.out.println("Min Tension: " + ttrCalc.getMinTopTension() + " kN");
System.out.println("Stroke Req: " + ttrCalc.getStrokeRequirement() + " m");

Example 3: Deepwater Lazy-Wave

Riser lazyWave = Riser.createLazyWave("Ultra-Deep", production, 2500.0, 800.0);
lazyWave.setDiameter(0.3048);
lazyWave.setBuoyancyModuleLength(200.0);
lazyWave.setCurrentVelocity(0.5);
lazyWave.run();

RiserMechanicalDesign lwDesign = lazyWave.getRiserMechanicalDesign();
lwDesign.setMaxOperationPressure(180.0);
lwDesign.setMaterialGrade("X70");
lwDesign.calcDesign();

System.out.println("Lazy-Wave Top Tension: " + 
    lwDesign.getRiserCalculator().getTopTension() + " kN");