Seismic Analysis for offshore structures

8/10/2019 Seismic Analysis for offshore structures

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SEISMIC ANALYSIS

To generate forces on the structure due to base motion arisingfrom earthquake.

Structural

response

Verticalmotion

or zon amotion

14 June 2009Dr. S. Nallayarasu

Department of Ocean EngineeringIndian Institute of Technology

Madras-36

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SEISMIC ANALYSIS

API RP 2A IS 1893

STRENGTH LEVEL EQ

(200 YEARS)

DESIGN BASIS EQ

(DBE)

DUCTILITY LEVEL EQ(2000 YEARS)

MAXIMUMCONSIDERED EQ



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SEISMIC ANALYSISGEOMETRY

ANDSEASTATE

LINEARISE

SOIL MODELPSI INPUT

FILE

SUPERELEMENT

DYNAMIC ANALYSIS

& FREQUENCIES

DYNAMIC RESPONSERESPONSE

SEISMICSPECTRASPECTRA METHOD API IS 1893

LOAD COMBINATIONMODEL: CQC

NUMBER CHECKPRST 1.0



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SEISMIC ANALYSIS

Peak Ground Acceleration (PGA) Indication of Strengthof earthquake

PGA = 0.3 X Spectral average between period 0.1 to 0.3sec.

Structure

PGA



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SEISMIC ANALYSIS

AS PER IS: 1893-2000

ZONE SEISMIC

INTENSITY

ZONE

FACTOR.

III Moderate 0.16

.

V Very 0.36



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SEISMIC ANALYSIS

ar qua e es gn r er a

Specific design criteria depends on the country, location andtype of soil stratum etc.

IS 1893

.

Maximum Considered Earthquake (MCE)

API RP 2A

Strength Level Earthquake (SLE)

Ductilit Level Earth uake DLE



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SEISMIC ANALYSIS

ar qua e es gn r er a

OPERATIONAL EARTHWUAKE (200 YEARS RETURN PERIOD)

conventional structural design Allowable stresses can be increased

EXTREME EARTHQUAKE (2000 YEARS RETURN PERIOD)

structure to be stable

Redundant framing



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SEISMIC ANALYSIS

e pea groun acce era on o a par cu ar s e epen s onthe following parameters.

Soil stratum

Distance from epicenter

.

IS 1893 defines PGA as

Sa Sa

0.1 0.30.4

2

G Gs sPGA =



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SEISMIC ANALYSISResponse SpectraSpectra Normalized to 1.0 Gravity (API RP 2A)



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SEISMIC ANALYSIS

Response spectra for rock and soil sites for 5% damping (IS 1893)



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SEISMIC ANALYSIS

e o s o na ys s

Seismic co-efficient method

Time series method

Response spectra method

esponse cons era on

Horizontal acceleration both orthogonal directions

Vertical acceleration

Number of modes to be combined by complete quadraticcombination (CQC)

.(Square Root of Sum of Sequences)

e.g. ( ) ( ) ( )2 2 2

X Y ZER ER ER ER= + +



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SEISMIC ANALYSIS

e sm c o-e c en me o

Design horizontal co-efficient

2h

ZISaA

Rg=

Where

Z =zone factor

I =importance factor

=average response acceleration co-efficient

R =response reduction factorg



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SEISMIC ANALYSIS

Total base shear can be calculated as

= .

Where

Ah=design horizontal seismic co-efficient

W=total weight of structure

Distribution of Design Forces

Design seismic force at each floor is to be calculated by

i ii B n

W hQ V=

1

j j

j

W h=

n=number of storage or points of mass located along the height.



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i=se sm c we g o oor,

Seismic Analysis for offshore structures

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