Lecture 15A.7-Tubular Joints in Offshore Structures

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ESDEP WG 15A

STRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.7: Tubular Joints in Offshore

StructuresOBJECTIVE/SCOPE

To present methods for the design of large tubular joints typically found on offshorestructures.

PREREQUISITES

Lecture 15A.1: Offshore Structures: General Introduction

RELATED LECTURES

Lecture 15A.8 : Fabrication

Lecture 15A.12: Connections in Offshore Deck Structures

SUMMARY

The lecture defines the principle terms and ratios used in tubular joint design. Itpresents the classifications for T, Y, X, N, K and KT joints and discusses the significanceof gaps, overlaps, multiplanar joints and the details of joint arrangements. It describesdesign methods for static and fatigue strength, presenting some detailed information onstress concentration factors.

1. INTRODUCTION

The main structure of a topside consists of either an integrated deck or a modulesupport frame and modules. Commonly tubular lattice frames are present, however asignificant amount of rolled and built up sections are also used.

This lecture refers to the design of tubular joints. These are used extensively offshore,particularly for jacket structures. Connection of I-shape sections or boxed beamswhether rolled or built up, are basically similar to those used for onshore structures.Refer to the corresponding lectures for appropriate design guidance.

Two main calculations need to be performed in order to adequately design a tubularjoint. These are:

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Static strength considerations1.Fatigue behaviour considerations2.

The question of fatigue behaviour always has to be addressed, even where simpleassessment of fatigue behaviour shows this will not be a problem. The joint designermust therefore always be "fatigue minded".

2. DEFINITIONS

The following definitions are universally acknowledged [1]: (refer to Figure 1 forclarification):


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The CHORD is the main member, receiving the other components. It is necessarily athrough member. The other tubulars are welded to it, without piercing through the


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chord at the intersection.

Other tubulars belonging to the joint assembly may be as large as the chord, but theycan never be larger.

The CAN is the section of the chord reinforced with an increased wall thickness, orstiffeners.

The BRACES are the structural members which are welded to the chord. They physicallyterminate on the chord skin.

The STUB is the extremity of the brace, locally reinforced with an increased wallthickness.

Different positions have to be identified along the brace - chord intersection line:

CROWN position is located where the brace to chord intersection crosses the planecontaining the brace and chord.SADDLE position is located where the brace to chord intersection crosses the planeperpendicular to the plane containing the brace and chord, which also contains thebrace axis.

2.1 Geometrical definitions

Refer to Figure 1

L is the length of the chord can

D is the chord outside diameter

T is the chord wall thickness

d is the brace outside diameter

t is the brace wall thickness (where there are several braces, a subscript identifies thebrace)

g is the theoretical gap between weld toes

e is the eccentricity × Positive when opposite to the brace side, Negative when on thebrace side

q is the angle between brace and chord axis.

2.2 Geometrical ratios

a = Can slenderness ratio

b = Brace to chord diameter ratio (always £ 1)


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g = Chord slenderness ratio

t = Brace to chord thickness ratio

z = Relative gap

These are non-dimensional variables for use in parametrical equations.

3. CLASSIFICATION

Load paths within a joint are very different, according to the joint geometry. Thefollowing classification is used, see Figure 2.


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3.1 T and Y Joints


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These are joints made up of a single brace, perpendicular to the chord (T joint) orinclined to it (Y joints).

In a T joint, the axial force acting in the brace is reacted by bending in the chord.

In a Y joint, the axial force is reacted by bending and axial force in the chord.

3.2 X Joints

X joints include two coaxial braces on either side of the chord.

Axial forces are balanced in the braces, which in an ideal X joint have the same diameterand thickness. In fact, other considerations such as brace length, which can be verydifferent on each side of the chord, may lead to two slightly different braces. Anglesmay be slightly different as well.

The important point to note is the balance of forces in the braces. If the axial force inone brace is far higher than the one in the other brace, the joint may be classified as a Y(or a T) joint rather than an X joint.

3.3 N and K Joints

These joints include two braces. One of them may be perpendicular to the chord (Njoint) or both inclined (K joint).

The ideal load pattern of these joints is reached when axial forces are balanced in thebraces, i.e. net force into chord member is low.

3.4 KT Joints

These joints include three braces.

The load pattern for these joints is more complex. Ideally axial forces should bebalanced within the braces, i.e. net force into chord member is low.

3.5 Limitations

For a joint to be able to be fabricated and to be effective, the geometrical ratios given inSection 2.2 have limitations. Table 3.1 shows these limits and their typical ranges.

ParameterTypicalrange

Limitations

min max

0,4 - 0,8 0,2 1


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12 - 20 10 30

0,3 - 0,7 0,2 1 (2)

q 40° - 90° 30° (3) 90° (1)

(1) Physical limitation

(2) Brace shall be less or equal to chord thickness (see punching shear)

(3) Angle limitation to get a correct workmanship of welds.

Table 3.1 Geometrical Limits and Typical Ranges

3.6 How to classify a joint

This classification deals only with braces located in one plane.

It must always be remembered that this classification is based on load pattern as wellas the geometry. Engineering judgement must therefore be used to classify a joint. Forexample a geometrical K joint may be classified as.

a K joint when forces are balanced within braces.a Y joint when the force in one brace is reacted predominantly by the chord, ratherthan by the second brace.

4. GAP AND OVERLAP

4.1 Definitions

The GAP is the distance along the chord between the weld toes of the braces (Figure 3).


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The theoretical gap is the shortest distance between the outer surfaces of two braces,measured on the line where they cross the chord outer surface. The real gap is the onemeasured at the corresponding location, between actual weld toes.

A brace OVERLAPS another brace when one brace is welded to the other brace.

The overlapping brace is always the thinner brace.

The overlapped brace is always completely welded to the chord.

4.2 Limitations

The minimum gap allowed is 50mm. This limitation is set to avoid two welds clashing.This is important because the gap is a highly stressed zone.


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4.3 Multiplanar Joints

The same definitions and limitations apply to multiplanar joints.

5. JOINT ARRANGEMENT

As a rule, welds in a joint have to be kept away from zones of high stressconcentration.

The following practice, see Figure 4, should be followed:

The chord circumferential welds are to be located at either 300mm or a quarter ofthe chord diameter, whichever is the greater, from the nearest point of abrace-chord connection.

1.

The brace circumferential welds are to be located at either 600mm or a bracediameter, whichever is the greatest, from the nearest point of the brace-chordconnection.

2.

The actual gap shall not be less than 50mm. To achieve this, most designers use a70 or 75mm theoretical gap.

3.

Eccentricity and offset are to be kept within a quarter of the chord diameter. Whenhigher values can not be avoided, secondary moments have to be introduced inthe structural analysis by introducing extra nodes.

4.

Thickness transitions are smoothed to a 1 in 4 slope, by tapering the thicker wall.5.


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6. STATIC STRENGTH


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6.1 Loads taken into account

The loads considered in a joint static strength design are the axial force, the in-planebending moment and the out-of-plane bending moment for each brace.

The other components (transverse shear and brace torsion moment) are usuallyneglected since unlike the preceding loads, these loads do not induce bending in thechord wall. Nevertheless, their presence must never be forgotten and in some specificcases, their effects must be assessed. The axial load, in-plane and out-of-plane bendingmoments are normally the dimensioning criterion for tubular joints.

6.2 Punching shear

6.2.1 Acting punching shear

The acting punching shear is the shear stress developed in the chord by the brace load.

The acting punching stress vp is written as:

vp = t f sin q

where f is the nominal axial, in-plane bending or out-of-plane bending stress in thebrace (punching shear for each kept separate), see Figure 5.

6.2.2 Allowable punching shear

Allowable punching shear values in the chord wall are determined from test results


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carried out on full scale or on reduced scale models.

Tests are performed on experimental rigs such as the one shown in Figure 6. They areperformed for a single load-case (axial force, in-plane bending, or out-of-planebending).

The ultimate static strength obtained through these tests can then be expressed interms of punching shear, as defined above.

Statistical treatment of results allow formulae to be defined for the allowable punchingshear stress.

6.2.3 The API method

Several offshore design regulations are based on the punching shear concept [1,2]. Thefollowing method is presented in API RP2A [2]:

A. Principle

This method applies to a single brace without overlap, for a non-stiffened joint.When the joint includes several braces, each brace connection is checkedindependently.Punching shear for each load component (axial force, in-plane bending, and out ofplane bending) is calculated and compared to the allowable punching shear stressfor the appropriate load and geometry.


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Interaction formulae are given for combined loading, combining the three punchingshear ratio calculated for each component.

B. Allowable punching shear stress

The allowable punching shear stress for each load component is:

Vpa = Qq Qf

where: Fyc is the yield strength of the chord member

Qq is to account for the effects of type of loading and geometry, see Table 6.1.

Qf is a factor to account for the nominal longitudinal stress in the chord

Qf = 1 - l g

fAX, fIPB, fOPB are the nominal axial, in-plane bending and out of plane bending stressesin the chord

Value for l and Qq are given in Table 6.1

Load componentAxial load In-plane bending Out of plane bending

Stress in brace fax fby fbz

Acting punching shear Vpx = t fax sin q Vp = t fby sin q Vp = t fbz sin q

Qq K joints

T & Y Joints

w/o diaphragm

X

w diaphragm

Tension Compression

l 0,030 0,045 0,021


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Table 6.1 Values of Qq for allowable punching shear stress from APIRP2A

Qg = 1,8 - 0,1 for g £ 20

Qg = 1,4 - 4 g/D for g > 20

but Qg must be ³ 1,0

Qb = for b > 0,6

QB = 1,0 for b £ 0,6

C. Loading Combination

For combined loadings involving more than one load component, the followingequations shall be satisfied:

where: IPB refers to in-plane bending component

OPB refers to out-of-plane bending component

AX refers to axial force component

and

ax

where: arc sin term is in radians.

6.3 Overlapping joints

The parametric formulae discussed in Section 6.2 were specifically established fornon-overlapping joints with no internal reinforcement. These formulae cannot be usedfor overlapping joints.

In an overlapping joint, part of the load is transferred directly from one brace to theother through the overlapping section, without that part of the load transferring throughthe chord. The static strength of an overlapping joint is higher than a similar jointwithout an overlap.

API RP2A, [2] allows the static shear strength of the overlapping weld section to beadded to the punching shear capacity of the brace-chord connection, see Figure 7.


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The allowable axial load component perpendicular to the chord, P^ (in Newtons) shouldbe taken to be:

P ̂= (vpa T l1) + (2vwa tw l2)

where:

vpa is the allowable punching shear stress (MPa) for axial stress.

l1 is the circumference for that portion of the brace which contacts the chord (mm), seeFigure 7.

vwa is the allowable shear stress for weld between braces (MPa).

tw is the lesser of the weld throat thickness or the thickness t of the inner brace (mm).

l2 is the projected chord length (one side) of the overlapping weld, measuredperpendicular to the chord (mm), see Figure 7.

6.4 Reinforced joints

6.4.1 Definition

Large chord wall thickness may be reduced by stiffening the chord. The most usualreinforcement consists of ring stiffening inside the chord.

Some joints may require more complex stiffening. This is the case for large diameterchords which would otherwise require an un-economic chord wall thickness.

There are very many different stiffening solutions for a large diameter chord. Thereforethere are no parametric formulae available for these designs. Specific analyses musttherefore be carried out for an accurate solution. This may involve finite element


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analysis.

6.4.2 Ring Stiffening

Ring stiffening consists of ring plates welded in the chord can prior to welding thebraces to it.

The punching shear capacity of the chord still may be taken into account whencalculating the forces acting on the stiffeners.

Ring stiffeners can be justified through parametric formulae available in variouspublications, the best known being published by Roark [3].

7. STRESS CONCENTRATION

As in any mechanical body presenting discontinuities, stresses are not uniform alongthe connecting surface of a brace and chord. Figure 8 shows an example of the stressdistribution in a joint with local discontinuities at and in the vicinity of the brace chordintersection.

7.1 Stress concentration factor

The stress concentration factor (SCF) is defined as the ratio of the highest stress in the


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connection (or hot spot stress fHS) to the nominal brace stress fNOM:

SCF = fHS/fNOM

7.2 Kellog equation

This approximate formula can be used for rapidly assessing SCF, for preliminaryanalyses.

fHS/vp = 1,8 g

vp being the punching shear.

7.3 Parametric formulae

SCF parametric formulae have been determined based on a large number of finiteelement analyses and cross-checked with either full scale or model tests. They arebased on many man years of work by numerous research teams.

A large number of parametric formulae have been published [4]. Sections 7.3.1 to 7.3.3give, as an example, the most commonly used and acknowledged formulae.

In using any set of formulae, care should be taken in classifying the situation andascertaining any limitations that apply.

The only alternatives to these formulae are to perform model tests (full size or atreduced scale) or finite element analyses.

No parametric formulae are presently available for stiffened joints. The only onespublished to date concern non-stiffened, non overlapping joints.

7.3.1 Kuang equations for T/Y joints [4]

Axial load

SCFCHORD = 1,981 g0,808 t1,333exp(-1,2b3 a0,057 sin1,694 q

SCFBRACE = 3,751 g0,55 texp(-1,35b 3) a0,12 sin1,94 q

Out-of-plane bending

SCFCHORD = 1,024 g1,014 t0,889 b0,787 sin1,557 q 0,3 £ b £ 0,55

SCFCHORD = 0,462 g1,014 t0,889 b(-0,619) sin1,557 q 0,55 £ b £ 0,75

SCFBRACE = 1,522 g0,852 t0,543 b0,801 sin2,033 q 0,3 £ b £ 0,55

SCFBRACE = 0,796 g0,852 t0,543 b(-0,281) sin2,033 q 0,55 £ b £ 0,75


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In-plane bending

SCFCHORD = 0,702 g0,60 t0,86 b(-0,04) sin0,57 q

SCFBRACE = 1,301 g0,23 t0,38 b(-0,38) sin0,21 q

7.3.2 Kuang equations for K joints [4]

Balanced axial load

SCFCHORD = 1,506 g0,666 t1,104 b(-0,059) (g/D)0,067 sin1,521 q

SCFBRACE = 0,92 g0,157 t0,56 b(-0,441) (g/D)0,058 Exp(1,448 sin q )

In-plane bending (bending moment applied to one brace only)

SCFCHORD = 1,822 g0,38 t0,94 b0,06 sin0,9 q

SCFBRACE = 2,827 t0,35 b-0,35 sin0,5 q

7.3.3 Kuang equations for KT joints [4]

Balanced axial load Outer braces only loaded

SCFCHORD = 1,83 g0,54 t1,068 b0,12 sin q 0° < q £ 90°

SCFBRACE = 6,06 g0,1 t0,68 b-0,36 {(g1+g2)/D}0,126 sin0,5 q 0° < q ³ 45°

SCFBRACE = 13,8 g0,1 t0,68 b-0,36 {(g1+g2)/D}0,126 sin2,88 q 45° £ q ³ 90°

SCFBRACE = 4,89 g0,123 t0,672 b-0,396 {(g1+g2)/D}0,159 sin2,267 q

In-plane bending - as for K joint

Validity range

The above equation for T/Y, K and KT joints are generally valid for joint parameterswithin the following limits:

8,333 £ g £ 33,3

0,20 £ t £ 0,8

0,3 £ b £ 0,8 unless stated otherwise

6,667 £ a £ 40 unless stated otherwise

0° £ s £ 90° unless stated otherwise.


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8. FATIGUE ANALYSIS

A fatigue analysis of a joint consists of the following steps:

Calculation of nominal stress ranges in the brace and the chords1.Calculation of hot-spot stress range2.Calculation of joint fatigue lives using S-N curves for tubular members at joints.3.

8.1 Nominal stress range

Nominal stress ranges in braces and chords are calculated by a global stress analyses.

8.1.1 Wave histogram

A wave histogram has to be obtained for each direction around the platform. A simpleform of a wave histogram is as follows:

Wave height(metres) Average

number peryear

0-1,5

1,5-3

3-4,5

4,5-6

6-8

8-10

3 100 000

410 000

730 000

5 000

800

20

8.2.2 Nominal stress ranges

Nominal stress ranges can be calculated by following the steps below:

Wave heights are grouped in "blocks", for which just one stress range will becalculated. Different wave directions need to be considered with a minimum ofthree "blocks" per wave direction.

1.

For each block one representative wave is chosen, whose action is supposed torepresent the action of the whole block. The highest wave of the block is normallychosen.

2.

Nominal stresses for each joint component are then calculated for different phaseangles of the chosen wave, for one complete cycle (360°). The nominal stressrange for the joint component is defined as the difference between the highest andthe lowest stress obtained for a full wave cycle. Four to twelve phase angles perwave are usually considered.

3.


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8.2 Hot spot stress ranges

Hot spot stress ranges are then evaluated for each chosen joint location by applyingparametric formulae [4] (or by applying the SCF calculated from a detailed analysis).

When using parametric formulae, stress components (axial, in plane bending and out ofplane bending) have to be distinct throughout the calculations, as the SCF formulaeapply individually for each load component.

Where a chord and brace intersect, four to eight locations are usually chosen aroundthe intersection line. For each of these locations the stress response for each sea stateshould be computed, giving adequate consideration to both global and local stresseffects.

8.3 S-N Curves

S-N curves to be used for offshore structures are given by statutory regulations [1,2].APIRP2A uses the curves shown in Figure 9.


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The X and X1 curves should be used with hot spot stress ranges based on suitablestress concentration factors. The permissible number of cycles is obtained from the S-Ncurve by taking the hot spot stress range, and entering the graph.

It should be noted that Curve X presumes welds which merge smoothly with theadjoining base metal. For weld without such profile control, the X¢ curve is applicable.

8.4 Cumulative Fatigue Damage Ratio

The stress responses should be combined into the long term stress distribution, whichshould then be used to calculate the cumulative fatigue damage ratio, D, given by:

D =

where


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n is the number of cycles applied at a given stress range

N is the number of cycles to cause failure for the given stress range (obtained fromappropriate S-N curve).

In general the design fatigue life of each joint and member should be at least twice theintended service life of the structure, i.e. a safety factor of 2,0.

For critical elements whose sole failure would be catastrophic, use of a larger safetyfactor should be considered.

9. CONCLUDING SUMMARY

Terminology, geometric ratios and joint classifications are now standardised fortubular joints.The presence of gaps and overlaps significantly influence joint behaviour.Determination of static strength is generally based on the concept of punchingshear, with the allowance of overlapping joints.Special analysis are required for reinforced joints.Stress concentration factors (SCF) are defined for most commonly occurringjoints.Determination of fatigue strength is based on nominal stress range multiplied byappropriate SCF.

10. REFERENCES

[1] Offshore Installations: Guidance on Design, Construction and Certification. FourthEdition, HMSO, 1990.

[2] Recommended Practice for Planning, Designing and Constructing Fixed OffshorePlatforms, API RP2A Nineteenth Edition.

[3] Young, Warren C, Roark's Formulae for Stress and Strain. Sixth Edition,McGraw-Hill.

[4] Stress Concentration Factors for Simple Tubular Joints, 1989, Volumes 1 to 5,Lloyds Register of Shipping-Offshore Division.

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Lecture 15A.7-Tubular Joints in Offshore Structures