Experimental Study of Floating Roof

Integrity for Seismic Sloshing

October 8, 2008Haruki NISHI, Dr.Eng.

National Research Institute of Fire and DisasterFire and Disaster Management Agency

Background

Oil storage tanks are regulated by the Fire Service Law (FSL) in Japan.

Technical standards of Earthquake-proof OST are defined in the FSL.• ≒ the standard in API 650 App. E

20.7% of large earthquakes are occurred in Japan.

Six floating roofs sank and two tank fires occurred in the 2003 Tokachi-oki earthquake.

Collapse of a floating roof may lead to a large tank fire, FDMA decided to reinforce the standard to prevent a similar disaster in the future earthquake.

Distribution of Hypocenter of Large

Earthquakes(>M5.0,1998-2007)

Sloshing

Any motion of the free liquid surface inside its containers

Depending on the type of disturbance and container shape, the free liquid surface can experience different types of motion; simple planar, nonplanar, rotational, irregular beating, symmetric, asymmetric.

Actual FRT has a floating roof on the surface.

→Floating roof may collapse by the sloshing.

Liquid sloshing in cylindrical tank

Under sinusoidal lateral excitation, free surface

Seismic Sloshing of Oil Storage Tank

Resonance between liquid (oil) and ground motion

Larger diameter – longer sloshing period

Higher liquid height – shorter sloshing period

D=10mT=3s

D=50mT=8s

D=100mT=13s

Damage to the Oil Storage Tanks due to

Liquid Sloshing in the Past Earthquakes

1964 Niigata Earthquake

Nihonkai-chubu Earthqukae(1983)

2003 Tokachi-oki Earthquake

Detail of the Damage

• Overflow of the Oil

• Tank Fire Ring-type, Open-top

• Sinking of the Floating Roof

昭和石油新潟製油所

タンクのリング火災（秋田火力発電

所）

Ring-type (Rim) fire on crude oil tank

（2003.9.26）

Open-top (Full surface) fire on Naphtha tank

（2003.9.28）

(2003/9/28 14:26)

Two floating roofs sank. (2003/9/30 16:43)

Sinking of the floating roof of the ４0,000ｋ

ｌ kerosene tanks

Fractured welding part between the outer rim and the lower deck of the pontoon(Bottom view)

40062 Kerosene Tank

40061 Kerosene Tank

30063 Naphtha Tank

40062 Kerosene Tank

40061 Kerosene Tank

30063 Naphtha Tank

Oil spill onto the deck of the 100,000kl crude oil

tank（2003/9/26）

Diameter 80m, Estimated sloshing height 1.4m

Buckling of the pontoons

Buckled pontoon(P40&P41) of the #100654 crude oil tank（2nd mode dominant, sloshing height 1.4m）

Fractured welding

Floating roof of the #100654 crude oil tanksank completely in the end(2003/9/30, four days after the earthquake)

D

H

g

DTs

68.3coth

68.321 ・

V

S

S T

π.

g

D

1

max)1( 2

83702

（１） Natural period of the 1st order mode of sloshing

Here, D：Tank diameter, H：Liquid height,

g：gravitational acceleration

（２）Maximum sloshing height

D

H

Here, Sv：Velocity response spectrum （105cm/s→200cm/s present）

（１）

（２）

(1)max

Based on the velocity potential theory with small sloshing height assumption

Evaluation method of the Sloshing Height

in the Fire Service Law

Strength Evaluation of Floating

Roofs for Seismic Sloshing

Standard before the earthquake in the FSL

API650 Appendix C

C.3.4.1

Floating roofs shall have sufficient buoyancy to remain afloat on liquid

a. 250mm of rainfall in a 24-hour

b. Two flooded pontoon compartments plus center deck in single-deck pontoon roofs

Effect of the Large sloshing height and non-linear sloshing

Evaluation of the effect of the out of plane

deformation of the deck

Building a structural evaluation system of a floating roof

（Problem to build a simpler method）

Construct technical standards regarding a structural evaluation

in the Fire Service Law

（Simplification）

（１） Effect of the 1st mode

（２） Effect of the 2nd mode

(1)Linear response could not explain the damaged to the floating roof

(2) Floating roof of the 100,000kl tank damaged even with small sloshing heightProblem

Velocity response spectrum & damping factor

-2 0 2

-2

0

2

-1

0

1

-2 0 2

-2

0

2

-2 0 2

-2

0

2

-1

-0.5

0

0.5

1

-2 0 2

-2

0

2

石油タンクは消防法で規制、現行では雨水滞水時の浮力に関する規定があるだけ

４0,000kl、４ｍ

100,000kl

To prevent the same damage in the future earthquake from occurring

Normal

Sloshing（Elevation angleθ）

Side view at sloshing（rotate C.C.W.）

θ

Model tank experiment using

shaking tableCapacity Diameter Height Roof type Liquid

height

Content

300kl 7.6m 5m Single-deck type 3m Water

Natural period１st mode ３(sec)２nd mode 1.7(sec)

Mode Floating Roofs

Carbon steel （thickness 4.5mm） Aluminum（thickness3mm）

Stainless steel（thickness１mm）Stainless steel（thickness 0.3,0.6mm）

4Rg

DD

710≒D

Sloshing Experiment of Free Surface

1st mode excitation experiment

using the aluminum floating roof

Stainless Steel Floating Roof (thickness 0.3mm)

Experiment exciting 2nd mode of Sloshing

Central part of the deck of the floating roof vibrates heavily.（Amplitude of shaking table 22mm、20waves、period1.7s、Sv=2800mm/s）

Floating Roof Shaking Experiment using an

Actual Tank

38000

DeckAir Cylinder

A

A:magnificationShell plate

Pontoon× 20 Annular deck

Deck

32500

38000

6t 8t4.5t4.5t

200 2400150

Diagram of the actual tank(capacit 15,000kl)

Floating roof excitation equipment

Eight air cylinders are installed onto the floating roof

Shell plate

Air cylinder

Jig

Air compressor

Controller

Valve

Inner diameter 250mmStroke 2mMax. rated pressure 0.7MPaDouble acting cylinder→

Push pull loading※RestrictionOnly 1st mode can be excited since the points of loading are

At the pontoon.

Point ofloading

Point of loading

Damping Factor and Sloshing Height

0

1

2

3

4

5

6

7

0 500 1000

Sloshing height(m m )

Dam

ping factor(%)

0.25M Pa_600s

0.2M Pa_600s

0.15M Pa_600s

0.3M Pa_300s

0.25M Pa_300s

0.2M Pa_300s

0.15M Pa_300s

0.1M Pa_300s

Relationship between sloshing height and damping factor at each air pressure and excitation duration

Measurement of the Circumferential bending

strain at the pontoon

Cross section of the pontoon

歪みゲージ

Shell plate

Pontoon

Annular deck

Deck

200 2400

Strain gauge

-160

-120

-80

-40

0

40

300 310 320 330 340 350

Elapsed tim e(s)

Ben

ding strain(

st)

-3200

-2400

-1600

-800

0

800

Vertical displace

men

t (m

m)

Pontoonlow er end

Pontoonupperend

C al.

η1m ax

Strain Measured at the Pontoon and Calculated

Strain from the FSL

2)1(

max

4

11

8

26.2

mm

m

eff

bb

RR

I

kR

EI

k

ZE

TimeInterval7.5s

Strian b1 becomes large when sloshing height η(1)max

is both at max. and min.

η(1)max：0.813(m)

β1m：3.23×10－１

k：2.35×10４(N/m２)

E：2.06×10１１(N/m2)

Iθ：0.00287 (m４)

Rm：17.6(m)

ρ：1.00×10３(kg/m3)

Ｂ：2.4(m)

Mθ：5.23×10４(Nm)

Ze：2.08×10－３(m３)

σb1：2.52×10７ (N/m２)

Conclusions

Elucidate the mechanism of damage to large oil storage tank during an earthquake

Regarding large sloshing height for the 1st mode and the 2nd mode

→ Reinforcing Seismic Design of Floating Roof

Sloshing Height Estimationmore than 2m → 4m

(depending the region and tank diameter)Strain at Pontoon < 0.9 y

Fire Service law and related ordinance were revised in 2005

Enforced in 2005→Plan of reinforcement by 2007

→Reinforcement by 2017

End

Shell plate

Pontoon

Annular deck

Deck

Shell plate

Pontoon

Annular deck

Deck

補強案Ａ

補強案Ｂ

z

a

a

Linear

Non-linear

Shape of wave surface cross section when smallsloshing height assumption dose not hold

Non-linearity of the sloshing

When sloshing height is large, upward height and downward height differ.

Upward：

z＋＝z＋a

Downward：

z－＝z－a

Wire displacement meter on the rack

Shaking direction

Wave form for excitation

-10

-5

0

5

10

0 10 20 30

Tim e(s)

Displacement(mm)

experim ent

Inputw ave data

Laser displacement meter

Real Sloshing（Non-linear）

Out-of-plane bending

Fracture of welded part of the pontoon by out-of-plane bending

a

a

Side view at sloshing（rotate C.C.W.）

Strain at the pontoon 2

)1(

max1max

26.2mm

mRR

EIM

kR

EI

k

m

m

4

18

eff

bZ

M1

Max. circumferential bending moment of the pontoon modeled asa ring beam

Ratio of the rigidity of the outer pontoon and the springcorresponding to the buoyancy

Circumferential bending stress of the pontoon

Mθ

Mθ

2)1(

max

4

11

8

26.2

mm

m

eff

bb

RR

I

kR

EI

k

ZE