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17th INTERNATIONAL CONFERENCE & EXHIBITION

ON LIQUEFIED NATURAL GAS (LNG 17)

LNG VESSEL CASCADING DAMAGE

STRUCTURAL AND THERMAL ANALYSES By: Jason Petti, Sandia National Laboratories

17 April 2013

17th INTERNATIONAL CONFERENCE & EXHIBITION ON

LIQUEFIED NATURAL GAS (LNG 17)

LNG Cascading Damage Scenario

Can an LNG tank breach event cascade throughout the

tank, extend to other tanks, and experience

further damage due to an external fire?

Membrane LNG Carrier

Multi-Physics Cascading Damage Analysis Approach

Heat Transfer

Tests

Cryo Damage

Testing/Model

Development

Fire Load

F(x,t)

Structural Damage

Model

Exterior

LNG Pool

F(x,t)

Tank Draining

F(t)

LNG Flow

Analysis

Fire Tests & Analysis

Stress, Damage,

& Stability Analysis

Cryo Interior

Flow, F(x,t)

Temperatures

F(t)

LNG Cascading Damage Components

Structural Damage Analysis – Assess Cryo Damage State

Assess Effects from Fire

LNG Flow Analysis

Internal Flow External Flow

Vessel Model Development Basic Material Tests

Large Scale Fracture Tests

Damage Model Development & Calibration

LNG Heat Transfer Tests

Cooling Rates (Air and Water)

Heating Rates

Fire & Insulation Tests

Vessel Fire Analyses

LNG Ship Structural Steel Thermal Damage Issues and Concerns

LNG spills known to cause brittle fracture of ship deck plates

– Fracture testing showed LNG ship steels vulnerable to fracture at LNG temperatures

Fires reduce steel strength

– Testing showed LNG ship steel strength will be reduced to ~15% at LNG fire temperatures

Cryogenic Damage Testing TESTING GOAL: Perform tests to explore thermally induced crack

propagation. Results provide input for Damage Model.

Phase I – Exploratory small plate tests, subjected to LN2 and designed to explore testing procedures

Phase II – Small scale tests to examine crack propagation, cooling region changes, marine steels

Phase III - Large scale tests with representative vessel hull features

Also performed basic material tests (stress-strain, Charpy, CTE) for ABS Grade A and ABS Grade EH36 over -191C to 800C.

Cryogenic Damage Test Setup

Cryogenic Damage Testing

Cracked Plate Example

Cryogenic Damage Testing - Phase III Structure

Cryogenic Damage Testing - Phase III Example

Cryogenic Damage Testing - Phase III Example

Cryogenic Damage Testing - Phase III Example

Summary of Cryogenic Testing Results

Fractures occurred mainly in the LN2 flow area

– Fractures did propagate slightly outside the LN2 flow area due to cooling

Fracture initiation was observed at machined notches and structural discontinuities

– Many initiation sites exist within vessels

Mechanical pre-load (initial stress) not required to initiate/propagate fracture

– Localized cryo temperatures are enough to generate fractures given stress concentrations

Fractures propagate through all cold structural members - webs, stiffeners, flanges

Fractures can occur in structural elements in contact with water

– Depends on the current strength and other factors

Damage Model

• Fracture model produces similar qualitative fracture and damage directions and results

• Damage occurs across plates and members and secondary cracks occur

• Damage progresses up and through the side wall

Full Vessel Model Development

Used detailed drawings and information on Membrane and Moss vessels

Structural components have been explicitly represented with some simplifications in aft and forward regions

Weight distribution for non-structural items and LNG cargo are represented with appropriate loading functions

~4.5 million elements used for Moss and Membrane vessels

Moss Regions

Fore (less detail)

Mid ship (detailed

section

Aft (less detail)

Membrane

Meshing

Moss and Membrane Flow Analyses

External Flow Analyses

External LNG Pool Formation

MOSS Membrane

Above WL

~ 180m ~ 350m

Near WL

~ 205m ~ 330m

External Pool Sizes

Cooling Rate Studies

-162

-142

-122

-102

-82

-62

-42

-22

-2

18

38

0 300 600 900 1200 1500 1800 2100 2400 2700 3000 3300 3600

Tem

pe

ratu

re (

°C)

time (s)

Temp-mid (400)

Temp-edge (400)

Temp-out (400)

Temp-mid (1080)

Temp-edge (1080)

Temp-out (1080)

Two vessels examined: Membrane and Moss

Several breach scenarios evaluated for each class of ship

– based on a range of small to large breach events, these define the initial conditions for this study

Conditions analyzed are near shore, calm water

The initial load state was computed for each vessel (Summer Arrival – full load – no ballast)

Regions of LNG flow identified for both vessels

Above waterlines analyzed, but not significantly different that near waterline

The temperature of the LNG flow regions was decreased based on LNG heat transfer tests

Damage model removes elements after reaching strain/temperature criterion

LNG Cascading Damage Initial Conditions & Assumptions

Moss - Damage

Moss - Damage

Moss Section Analysis

Original Section

Fire

Cryo

Membrane – Damage

Membrane Damage

Fire and Heating Regions

MOSS: 180m

Mem.: 210m

Heating Regions

Membrane Section Analysis

Original Section

Fire

Cryo

LNG Cascading Damage Summary

Large scale fracture testing program conducted to calibrate computational damage model

Assessments performed for a number of assumed breach scenarios for each class of ship: Membrane and Moss

Each scenario included an assessment of the final damage states, small holes may be manageable, medium and large holes lead to significant damage within the vessel