thomas lamb

FLNG 2011

REDUCING THE SLOSHING PROBLEM IN LARGE FLNGPSOS WITH UNIQUE

TANK DESIGN

Thomas LambInnovative Marine Product Development, LLC

Lynnwood, Washington, USA

Regu RamooMohan Parthasarathy

Julien SantiniALTAIR Engineering, Inc.Troy, Michigan, USA

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FLNG 2011June 15, 2011

Before we get into the sloshing problem a brief description of the CDTS will be given.

For further details please see following references:LAMB, T., and RAMOO, R., “The Application of a New Tank Containment System to ULTRA‐Large LNG Carriers," OTC 2009RAMOO, R., PARTHASARATHY, M., SANTANI, J., and LAMB, T., "The use of Advanced Structural Analysis and Simulation Tools to Validate a New Independent LNG Tank Containment System,"ICCAS 2009LAMB, T., and RAMOO, R., “A New Concept for CNG Carriers and Floating CNG/Oil Processing and Storage Offshore Platforms," CNG Forum, London 2009LAMB, T, and RAMOO, R, “A New Tank Containment System for Floating LNG and CNG Processing and Storage Offshore Platforms," OTC 2010

Introduction

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Current Geometry of the CDTS

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Cylinders and Inner Volume

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Tank Type Volume Efficiency*Prismatic Self‐Standing IHI‐SPB 46,162 0.96Membrane 43,706 0.88Membrane PRISM 38,304 0.78CDTS 40,000 0.87Spherical 25,713 0.53

*Efficiency compared to solid cube of 49,108 c m

Comparison of Tank Volumetric Efficiency

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Tank Space Required by Tank Containment System for 300,000 m3 Capacity - Membrane, Spherical and CDTS

SPHERICALLength 272.5mBreadth 52.5mDepth 52.5m

SPHERICALLength 232mBreadth 56mDepth 56m

MEMBRANE and IHI‐SPBLength 218mBreadth 53mDepth 32m

CDTSLength 206mBreadth 49.5mDepth 49.5m

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Current Tank Arrangement Design

Length Overall 370 m, Beam 70 m, Depth 36 m Parallel Tank Arrangement

Length Overall 350 m, Beam 74 m, Depth 34 m

Tank Arrangement with CDTS

Length Overall 264 m, Beam 80 m, Depth 39 m at Side & 44 m at Center

Comparison of CDTS OIL/LNG FPSO with other Arrangements and Containment Systems

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Concept FONGPSO Arrangement

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Comparison of FLNG Designs CHARACTERISTIC IHI SPB CDTSLOA m 488 425Beam m 74 85Depth m 44 46Draft m 20 19Displacement t 602000 578000Lightship Weight t 316000 288000Deadweight t 286000 290000LNG t 205920 205920Condensate t 57200 57200LPG t 22880 22880LNG cm 460000 460000Condensate cm 65000 65000LPG cm 50000 50000Tank Length m 350 290Number of LNGTanks 7x2 12Tank Dimensions 50x64x34 38x38x38Tank Weight t 50000 18000COST RATIO Platform 100 95Cost Ratio Tanks 100 70Note Shell Tanks Steel - CDTS Aluminum

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Seakeeping evaluation involves 3 parts

• Expected Sea Conditions• Resulting Vessel Motion• SeaKeeping Design Criteria

Seakeeping Considerations

For LNG Carriers, we are also interested in the

• Resulting Liquid Motion in the LNG Tanks• Tank Resonant Frequencies at different Liquid Fill levels

• Tank Structural Design Criteria• Affect of Sloshing Motion on Vessel Motion, i.e., the coupling effect

Tank Sloshing

Vessel Hydrodynamics

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Estimation of the likely environmental conditions encountered by the vessel, based on hind cast or predicted weather data as applicable (e.g. wind, wind wave, swell);

Expected Sea Conditions

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Compute Ship’s Response (RAO) using:•Prior Experience•Tow‐Tank Testing• Numerical methods:

Time Domain AnalysisFrequency Domain Analysis

Most of the Commercial Codes (Seakeeper, Hydrostar, SEALAM) use frequency domain Strip Theory:

•Divide vessel into a number of transverse sections•Compute hydrodynamic properties of these section assuming 2D inviscid flow assuming no interference from upstream section•Calculate coefficient inequation of motion

•which yields vessel’s response to waves

Resulting Vessel Motion

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The RAOs shown on the right are typical:•At low frequency (long wave), the vessel follows the wave profile, riding up and down like a cork

•At the high frequency end of the scale (very short waves), there are so many waves along the length of the ship that their effect cancel and the vessel is unaffected by the waves

•Some where in between, there is a resonant peak which occurs at the natural frequency of the vessel

•At resonance the vessel motion can be several times that of the wave

• The amplitude of the peak depends on the amounting of damping associated with that motion

•Motions such as heave and pitch are highly damped compared to roll motion

Resulting Vessel Motion

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Relevant Normal Design Criteria would be as follows:• Added Resistance

• Propeller Emergence

• Slamming

• Deck Wetness

• Vertical Accelerations (MSI)

• Velocities and Accelerations

Seakeeping Design Criteria

For an LNG Carrier or FLNG the Design Criteria would also include:

• Design Sloshing loads on Containment Tank

• Affects of Tank Sloshing on Vessel’s Motion

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• Sloshing is a typical resonance phenomenon; it is not necessarily the most extreme ship motions or external wave loads that cause the most severe sloshing

• This implies that external wave induced loads can in many practical cases be described by linear theory, however, nonlinearities must be accounted for in the tank fluid motions

• Since it is the highest sloshing period (natural period) that is of prime interest, vertical tank excitation is of secondary importance

• Generally speaking the larger the tank size is and the less internal structures obstructing the flow in the tank are present, the more severe sloshing is, because:

a) Increased tank size tends to increase the highest natural sloshing period and hence higher sea states and larger ship motions will excite the severe sloshing.

b)Internal structures dampen the fluid motions.Faltinsen, O.M. And Rognebakke O. F.Department of Marine HydrodynamicsNorwegian University of Science and Technology

Physical Considerations of Sloshing Phenomena

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It seems generally accepted that CFD codes have difficulties in predicting impact loads. This was also the conclusion of the load committee of 13th ISSC in 1997. A reason is rapid changes in time and space occurring even for relatively large local angles between the impacting free surface and the body surface ([17])

Analytical Methods are limited to simplified geometries and high fill volumes

FSI methods appear to be the most suitable approach for estimating sloshing impact loads:

• Smooth Particle Hydrodynamics• ALE non‐linear transient dynamic simulation• CEL non‐linear transient dynamic simulation

Theoretical Considerations of Sloshing Phenomena

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Classification Societies require sloshing model test with irregular tank motions corresponding to the most severe sea conditions that can occur during the lifetime of the LNG Carrier

The severity of the sea condition should be judged based on the severity of the sloshing load on the containment system

A group of sea states are selected for the model test based on:

• The probability of occurrence• Tank motion response• Proximity of encountering wave period to tank natural period

Qualified sloshing simulation tools can also be used to pre‐screen the sea‐states to cause the most significant sloshing loads

Selection of Critical Wave Conditions for Design Sloshing Load

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Validation of Simulation Program

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The roll motion with a period of 1.91s and amplitude of 4 degrees.

Center of rotation

Pressure sensor

18.4 cm

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Standing Waves Movements of the liquid particles are predominantly verticalThe surface has one of more nodes where no vertical displacement takes placeGenerally occur when F/Ls => 0.2High pressure is imparted on tank top

Sloshing Waves

Traveling Waves The surface has no nodes, a wave crest travels back and forth between tank boundariesGenerally occur when F/Ls =< 0.2High pressure is imparted on both side walls and tank topHydraulic JumpThis is a special case of a standing waveCharacterized by a discontinuity (jump) in surface forming a vertical front which travels back and forth in the tankUsually occur when fill levels is 20% or less of horizontal free surfaceCombination WaveThis is a combination of standing and traveling waves

Source: lloyd’s Register, 2008

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Comparison of Tank Geometry RADIOSS SPH Sloshing Simulation –Model Setup

LNG (50% filled)

Model setup was similar for other tanks but only the model setup for the New CDTS Tank is shown

Enforced angular velocity: 30o rotation about with a period of 10sGravity was constant during the entire simulation

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Tank Faces for Sloshing Load ExtractionIHITank

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Sloshing Load on the Tank –50% Filled, Roll

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Sloshing Load on the Tank with Swash Bulkheads– 50% Filled, Roll

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Membrane Tank Sloshing Animation -50% Filled, Roll, 10s Period



Membrane Tank Sloshing Animation -50% Filled, Pitch, 10s Period

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Spherical Tank Sloshing Animation - 50% Filled, 10s Period IHI Tank Sloshing Animation

- 50% Filled, Roll, 10s Period

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CDTS Tank Sloshing Animation - 50% Filled, 10s Period



IHI Tank Sloshing Animation - 50% Filled, Roll, 10s Period

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Lloyds Rules for Approximating Roll & Pitch Periods

Ship & Tank Roll & Pitch Periods

Source: lloyd’s Register, 2008

39m

27m

19m

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Total Sloshing Load on the Tank - 30% Filled

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Peak Sloshing Load (kN) at 2900 ms

Von Mises Stress (MPa) due to Sloshing Loads – Baseline

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Baseline ModelThickness: 100mmTank Mass: 1954 T (base excluded)Material: Aluminum 5083_O, Ultimate: 290 MPa

Allowable Stress: 121 MPa(50% of Ultimate with a factor safety of 1.2)

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Topology Optimization

Objective: Minimize compliance

Stress constraint: 121 MPa

Tank thickness fixed at 75 mm

Free Size on Base: 15 mm to 250 mm

Volume fraction: < 60%

Topology design Space

Topology Load Path(symmetry enforced)

Only a conceptual design (too heavy)

A design which is lighter and feasible for manufacturing needs to be developed using this conceptual design as a guideline

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Gauge Optimization (Internal Bulkheads)



Thickness: 15 mm to 250 mm, Base fixed at 100mm

Tank Mass: 1304T

Thickness (mm) after Gauge Optimization3

Elements in the areas of high stress concentration were excluded from the stress constraint

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Gauge Optimization (Internal Bulkheads)



Thickness: 15 mm to 250 mm, base fixed at 100mm

Tank Mass: 1304TElements in the areas of high stress concentration were excluded from the stress constraint

Von Mises Stress (MPa) after Gauge Optimization3

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Conclusion The CDTS offers a superior and cost effective solution to the sloshing problem for large LNG FPSOs and other applications where there can be no filling restrictions

The IHI SPB has the lowest sloshing loads due to centerline watertight bulkhead

The CDTS has no peak sloshing loads as found in other tank containment systems. If a swash bulkhead is installed in the cylinders at their center lines then the CDTS has the same sloshing loads as the IHI SPB

The combination of the unique geometry of the CDTS as well as the reduced sloshing forces and resulting loads on the tank sides, reduces the tank manufacturing and installation cost compared to other containment systems

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thomas lamb

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