Design of Tubular Members

Desi gnofTubul arMemb mber s

Earthquake Engineering Research Centre Earthquake International Institute of Information Technology IIIT Hyderabad, Gachibowli, Hyderabad -

Tubular members: Tubular members are commonly used for jacket structures. Because these are resisting various forces. Following are the reasons: 

Good dynamic properties (low  d ! m"



Good resistance against hydro static pressure.



Good buoyancy to weight ratio



#niform property across section



$o torsional buckling



Good ultimate strength compared to others



Full moment connections

Tubular members: Following factors affect the strength of the member: 

&aterial properties (' F y Ft"



)mperfections and residual stresses



,roduction method of tubular sections



Boundary conditions



-oading



Geometric properties (-/ /t"

(an*t be +ero".

(This is based on e0perience and code guidelines. Both parameters will play major role in design". 

1tiffeners: circumferential or longitudinal (hoose stiffeners rather than increasing thickness of pipe. (a" #neconomical and (b" ,lates are already purchased".

Material properties of Steel: 

/ensity: 3456 kgm2 or 34.5 k$m 2



Tensile stress (Ft": 786966 &pa



;ield stress (Fy": %569766 &pa



&odulus of 'lasticity ('": %669%<6 Gpa



1train in elastic range: 6.%=



,oisson*s ratio: 6.296.7



o.efficient of friction: 6.296.7

Imperfections: Following are some imperfections that need to be included 

>ariation in cross section



>ariation in thickness (-arger the bending radius smaller the stresses. -arger the /t ratio strain will be smaller. ?eat induced stresses during welding

could be large due to restraint provided by the joining components". @educe yield stress by 5= as per /$>.  @esidual stresses   ,) @, % is silent on residual stresses 

Aut9off roundnessstraightness



&isalignment of thicknesslength

Imperfections: 

Aut9off straightness The tolerance shall be measured at all points along the length of member and the ma0imum shall be taken for consideration.

 

/$> code specifies a ma0imum limit of 6.66<5- or - as the limit.  ,) specifies a ma0imum limit of -86 or 8.5 mm

The tolerance is important as this deviation will lead to eccentric load and corresponding moment.

Imperfections: 

Aut9off roundness Aut of roundness is normally specified as δ D D max D min −

=

D  

%

D mean

 ,) 1pec %B specifies that the above tolerance shall not e0ceed %= and /$> specifies that the tolerance shall not e0ceed <= /ma0 /mean

/min

Imperfections: 

&isalignment of joints ()t includes additional eccentricity in a0ial loads and stresses".



 ,) allows an eccentricity Ce* of (a" 6.% t


/$> allows an eccentricity of 6.<5 t < or 7 mm whichever is less

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-ocal ! global buckling 

-ocal buckling is governed by /t ratio



Global buckling is governed by k-r ratio

Design Methods: /esign of structural member  

1elect a suitable material with sufficient strength



1atisfy the functional reuirements with adeuate safety margin

/esign is to be satisfied when

!apacit" # Demand Design Methods: 

Allowable Stress Design (ASD)



Load and Resistance actor Design (LRD)

Load $ Resistance

Load % Resistance

Load & Resistance

Allowable Stress Design (ASD) Demand (Applied stress)

!apacit" (Allowable stress)

'robabilit" 

Load and (LRD)

Resistance Demand (Design loads)

actor

!apacit" (Resistance)

Design

@esistance factor based on probability of failure for different methods of estimating the resistance.

'robabilit" HHH

'robabilit" distribution of  * R:

'robabilit" distribution of  * R:

Load combinations: -oad combinations and the associated load factors reuired as per ,) @, %9-@F/ 

Factored gravity loads:

<.2/
Jind wave and current loads:

/<9dead load< (eg: self weight" /%9dead load% (eg: euip load" -<9live load< (eg: fluid wt" -%9live load% (eg: operating loads"

<.
Je9e0treme wind wave and current loads

6.8/
Jo9operating wind wave and current loads

<.2/
'arthuake loads

/n9inertia loads w.r.t Jo

/-: 6.89<.2

<.
--: <.29<.5

6.8/
'n-: <.29<.7

ASD Design Members: 

 



 

'rocedure

for

Tubular

/ivide the member into sections and calculate a0ial bending and shear forces in each section along the length. t least 2 sections shall be checked. 'stablish geometric properties such as sectional area moment of inertia effective length factors radius of gyration for each section. alculate the applied a0ial (f a" bending (f b0 f by" hoop (f h" and shear stresses (f s" using the geometry of the section and the applied a0ial bending hydrostatic and shear forces. 'stablish the slenderness ratio (k-r" and calculate the allowable a0ial stress (F a" and calculate the elastic buckling stress (F 0e" and inelastic buckling stress (F 0c". 'stablish the /t ratio and calculate the allowable bending stress (F b". The combined effect of loads is obtained using interaction of these loads in an appropriate manner using a0ial bending hoop and shear interaction formulae.

ASD Design 'rocedure: Applied Stresses Following method shall be used in calculation of applied stresses in members.

Axia$ Stress, f a

P =

A

#en!in" Stress, f  %x Shear Stress, f s Hoop Stress, f h

=

Mxy I xx

 f  %y

=

V =

=

0.5A Ph D 2t

( Ph

=

γ  h )

Myy I yy

ASD Design 'rocedure: Allowable Stresses Following method shall be used in calculation of allowable stresses in members. (Source: A'I R' +A ,SD cl- .-+) A/ial !ompression:

D0t 1 23

ASD Design 'rocedure: Allowable Stresses

For members with a /t K 6 substitute the critical local buckling stress (F0e or F0c whichever is smaller" for F y in determining c and Fa.

ASD Design 'rocedure: Allowable Stresses  <. 'lastic local buckling stress

The elastic local buckling stress F 0e  should be determined from

The theoretical value of  is 6.. ?owever a reduced value of  L 6.2 is recommended to account for the effect of initial geometric imperfections within ,) 1pec %B tolerance limits.

ASD Design 'rocedure: Allowable Stresses  %. )nelastic local buckling stress

The inelastic local buckling stress F 0c should be determined from

A/ial Tension:

FaL6. Fy

ASD Design 'rocedure: Allowable Stresses 4ending stress:

ASD Design 'rocedure: Allowable Stresses

For /t ratios greater than 266 refer to ,) Bulletin %#. &a0imum value is limited to 3-56 " &ost of the time we don*t permit /t beyond 6 for offshore structures. )t is mandatory because of local buckling effects.

ASD Design 'rocedure: Interaction of a/ial compression and bending

ylindrical

members

subjected

to

combined

compression and bending should be proportioned to satisfy following reuirements at all points along their length. For f aFa K 6.<5

For f aFa E 6.<5

General case

ASD Design 'rocedure: Interaction of a/ial compression and bending For asymmetric sections

ASD Design 'rocedure: Moment Reduction actor 

&eLm&B

>alues of the reduction factor C m referred to in the above table are as follows (with terms as defined by )1":

e&

+2π 2 * =

(  ' r ) 2

ASD Design 'rocedure: 7lastic hoop buc8ling stress The elastic hoop buckling stress determination is based on a linear stress9strain relationship from:

The critical hoop buckling coefficient  h  includes the effect of initial geometric imperfections within ,) 1pec %B tolerance limits.

ASD Design 'rocedure:  !ritical 9oop 4uc8ling Stress The material yield strength relative to the elastic hoop buckling stress determines whether elastic or inelastic hoop buckling occurs and the critical hoop buckling stress F hc , in ksi (MPa) is defined by the appropriate formula.

ASD Design 'rocedure: Interaction euation for a/ial tension and h"drostatic pressure Jhen member longitudinal tensile stresses and hoop compressive stresses (collapse" occur simultaneously the following interaction euation should be satisfied:

ASD Design 'rocedure: Interaction euation for a/ial tension and h"drostatic pressure 1afety Factors

ASD Design 'rocedure: Interaction euation for a/ial compression and h"drostatic pressure Jhen longitudinal compressive stresses and hoop compressive stresses occur simultaneously the following euations should be satisfied:

The following euation should also be satisfied when f  x  > 6.5 F ha

Ring Design: ircumferential stiffening ring si+e may be selected on the following appro0imate basis

Ring Design: