Stress Distribution along the Weld Toes of Tubular KT and KDT

Joints under Balance Axial Loads and In-Plane-Bending Moments

Rudi Walujo Prastianto, Yoyok Setyo Hadiwidodo, Sofyan Wahyu Widhestomo

and Rizky Yazhahir

Department of Ocean Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS Sukolilo Surabaya 60111, Indonesia

Keywords: Stress Distribution, Tubular KT, Tubular KDT.

Abstract: Jacket offshore structures are constructed from tubular members that consist of several types of multi-planar

tubular joints. So far, very few investigations have been performed on stress characteristics of such joints

due to their complexity. The present research is focused on the study of stress distribution along the weld

toe of brace-chord intersection for most critical brace due to the joint loading. In this paper tubular-KT and

KDT joints as elements of an offshore jacket platform are modelled as finite element models. The effect of

multi-planarity caused by adding a brace to the stress distribution along the weld toes is investigated under

two different loading conditions. To ensure validity of the model, Stress Concentration Factor (SCF) of the

KT-joints model was validated by Efthymiou SCF equations. An additional brace has been added within the

validated KT-joints models to form a multi-planar KDT joint, and the stress distribution along the weld toes

of the joints are investigated under balance axial load and in-plane bending moments. The results showed

that under balance axial loading, maximum stress occurred at a point of Crown 1 on the KDT-joints were

smaller than maximum stress occurred in the KT-joints as well as the case of in-plane bending moment

1 INTRODUCTION

Jacket offshore platforms that frequently used for oil

and gas exploitation in shallow water areas, during

their operation life will hold wave forces which

introduce variable loading on the structure. This

variable loading causes fatigue damage to the

structure members which usually initial crack will

appears at weld toes region of the tubular joints

where maximum stress occurs. Therefore, in the

fatigue design it is important to determine the stress

distribution along the weld toe of tubular joints.

In this paper, finite element analysis of tubular

KT and multi-planar KDT joints will be presented.

The results are stress distribution along the weld toe

of both type of the tubular joints. Multi-planarity

effect to the stress distribution along the weld toe of

the tubular KDT joint will be investigated. The

multi-planar tubular joint to be analyzed in the

present study is depicted in Fig. 1.

Numerous researches have been performed to

investigate the stress distribution for various multi-

planar tubular joints under several loading

conditions. Ahmadi and Zavvar (2016) numerically

studied the chord-side SCFs in two- and three-planar

tubular KT-joints under in-plane and out-of-plane

bending moments. Ahmadi and Nejad (2017)

proposed a new parametric formulas to calculate the

local joint flexibility of two-planar tubular DK-joints

subjected to four types of out-of-plane bending

(OPB) loads. Derivation the SCF formula for DT-

joints under axial loads was conducted by Jiang et

al. (2018). They used two types of error analysis to

verify the reliability of the formula. Using another

type of tubular joint which is XX-joints, Chiew et al.

(2000) investigated stress concentration factors

within the joints due to the axial, in-plane-bending

(IPB), and out-of-plane bending (OPB) loads. Based

on 64 finite element models they proposed general

SCF design equations for the joints. Recently

Prastianto et al. (2018) have conducted numerical

study on stress concentration factor distribution of

60 degrees two-planar DKT tubular joints subjected

to axial and in-plane bending loads.

Although several types of multi-planar tubular

joints as parts of offshore structures were already

been the subject of recent researches, but still no

170

Prastianto, R., Hadiwidodo, Y., Widhestomo, S. and Yazhahir, R.

Stress Distribution along the Weld Toes of Tubular KT and KDT Joints under Balance Axial Loads and In-Plane-Bending Moments.

DOI: 10.5220/0008650101700175

In Proceedings of the 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2018), pages 170-175

ISBN: 978-989-758-455-8

research on the multi-planarity effect in the stress

distribution at the brace-to-chord intersection areas

of the tubular-KDT joint. Therefore, this research

will investigate the effect of additional one brace to

the KT tubular joint to form a tubular-KDT joint (as

shown at Fig. 1) on the stress distribution along the

weld toes of the joint.

Figure 1: Multiplanar tubular K double T (KDT) joint to

be analyzed.

2 FINITE ELEMENT

MODELLING

In this research, structure of tubular-KT and KDT

joints are modelled using a finite element method-

based software with the geometry as presented at

Fig. 1 for the KDT joint. Dimensions and material

properties of the models can be seen in Table 1 and

Table 2. Weld profile along the brace/chord

intersection of the model satisfies the AWS D1.1

(AWS, 2002).

Table 1: Geometry of the model of the tubular-KT and

KDT joints.

Thickness

(inc)

Outside

Diameter

(OD)

Intside

Diameter

(ID)

Length

(inc)

Chor

0.688 24 22.62 1032.68

Diagonal

Brace 1

0.5 14 13 487.19

Diagonal

Brace 2

0.5 14 13 611.55

Horizontal

Brace 1

0.364 10.75 16.5 278.22

Horizontal

Brace 2

0.364 10.75 16.5 149.80

Table 2: Material properties of the model of the tubular-

KT and KDT joints.

Spec

and

Grade

Yield

Strength

(ksi)

Modulus

Young

(ksi)

Shear

Modulus

(ksi)

Poisson’s

Ration

Chord

API

Grade

290

29007.5

449

11603.01

0.3

Brace

API

Grade

241

29007.5

449

11603.01

0.3

The finite element model of the tubular joints

using element type of solid three-dimensional with a

linear element tetrahedron for the model of braces,

chord, and weld profiles. The tubular joint models

are divided into two different zones according to the

computational requirements with sub-zone mesh

generation is used to ensure good quality meshing.

Meshing size is made smaller on the region around

brace-chord intersection rather than meshing size on

the areas that far from the brace-chord intersection.

The model with the meshing can be seen at Fig. 2.

Figure 2: Solid three-dimensional tetrahedron elements

used for the model.

According to Efthymiou (1988), fixity condition

of the chord ends in tubular joints of offshore

structures ranges from almost fixed to almost pinned

with generally being closer to almost fixed. For the

present study, the chord end fixity condition of the

joint models are also set to fixed support on both

ends of the chord. The braces of the structures will

be loaded with two different types of loading

conditions, namely balance axial loading and in-

Stress Distribution along the Weld Toes of Tubular KT and KDT Joints under Balance Axial Loads and In-Plane-Bending Moments

171

plane bending moment. The loading and boundary

condition applied to the models are illustrated at

Figs. 3 and 4 for the KT joint model and the KDT

joints models, respectively.

(a) Balance axial load

(b) In-plane bending load

Figure 3: Loads and boundary conditions for the KT-joint

models.

(a) Balance axial load

(b) In-plane bending load

Figure 4: Load and boundary conditions for the KDT-joint

models.

3 RESULTS AND ANALYSIS

3.1 Meshing Sensitivity Analysis

Meshing sensitivity analysis is performed to ensure

the consistency of the output from the finite element

analysis. This approach is done by changing element

size in the region around brace-chord intersection

line from 1.0 inch to 0.5 inch until stress at a

particular location reached a constant value as the

function of the size change.

(a)

(b)

Figure 5: Result of meshing sensitivity analysis due to

balance axial loading condition on: (a) KT-joint; (b) KDT-

joint.

From the results of the meshing sensitivity

analysis (see Figs. 5 and 6), for both cases of KT and

KDT joints it is found that stress output at particular

point nearly constant when element size reached

0.65 inch.

ISOCEEN 2018 - 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

172

(a)

(b)

Figure 6: Result of meshing sensitivity analysis due to in-

plane-bending moment loading condition on: (a) KT-joint;

(b) KDT-joint.

3.2 Validation of Stress Concentration

Factor (SCF) for KT-joint

Stress Concentration Factor (SCF) is a ratio between

hot-spot stress in chord and nominal stress in brace.

In this research hot-spot stress calculation is

determined using two linear extrapolation points as

mentioned in DNVGL-RP-C203 (2001) by using the

maximum principal stresses occurred in the joint

model.

In this research, nominal stress occurred in brace

is obtained by reviewing stresses in 96 elements in

the middle of brace length to be analyzed which

located far enough from geometrical discontinuity

(weld toes area) and will be validated with following

equations.

For axial load condition:

𝜎









(1)

For in-plane-bending moment load condition:

𝜎

























(2)

where,

σn : nominal stress for axial load or in-plane-

bending moment (MPa)

Mi : in-plane-bending moment (Nm)

d : brace diameter (m)

t : brace wall thickness (m)

F : axial load (N)

A : area (m2)

The obtained nominal stress and hot-spot stress

are used to calculate the stress concentration factor

of the joints. Prior to analyze stresses of the KDT

tubular joint, as a validation step the SCF from finite

element analysis for the KT joint will be compared

to Efthymiou SCF formula (Efthymiou, 1988) with

the results as presented in Table 3.

Table 3: Stress Concentration Factor Validation.

Model

FEM Efthymiou Error)%)

SCF

KT-

Joint

3.47 2.39 3.44 2.33 0.87 2.58

Both errors of the comparison for the chord and

brace stress concentration factor (SCFc and SCFb)

are under 5% which are 0.87 and 2.58, respectively.

Therefore, KT-joints model was good and can be

accepted, then a horizontal brace 2 can be added to

the KT-joint to become a KDT tubular joint for later

analysis.

3.3 Stress Distribution along Weld

Toes of the Tubular-KT and KDT

Joints

In this research, the stress in chord side is considered

to analyse stress distribution along the weld toes of

the joint model. The maximum principal stress was

used and the weld toes to be investigated are on joint

between the brace with the chord which have the

biggest stress among the other braces (see Fig. 7).

As shown at Fig. 7, maximum stress due to in-plane-

bending moment and balance axial loading occurs at

Horizontal Brace 1 and at Diagonal Brace 1,

respectively.

Stress Distribution along the Weld Toes of Tubular KT and KDT Joints under Balance Axial Loads and In-Plane-Bending Moments

173

(a)

(b)

Figure 7: Location of the maximum stress occurs due to:

(a) in-plane-bending moment; (b) balanced axial load.

Description of stresses along the weld toes of the

brace-chord intersection line is based on the

nomenclature depicted at Fig. 8. There are four main

points namely Crown 1, Outer Saddle, Crown 2, and

Inner Saddle where are located at 0o, 90o, 180o, and

270°, respectively. Stress distribution along the weld

toes of the tubular-KT and KDT joint models are

presented at Figs. 9 and 10 for two loading

conditions which are balanced axial load and in-

plane-bending moment, respectively.

Fig. 9 depicts a comparison of stress distribution

along brace-to-chord intersection line between the

tubular-KT and KDT joints occurred at diagonal

brace 1 due to balance axial loading. Among all

points observed, the stress significantly decreased at

two points namely Crown 1 and Crown 2 for the

KDT joint.

Meanwhile, Fig.10 shows a stress comparison for

the two different tubular joints occurred at horizontal

brace 1 under in-plane bending moment loading. For

this mode of loading, the stress significantly

decreased at Crown 1 and two other points at

positions of 45O and 315O, respectively. Patterns of

the stress distribution are very much different to that

for axial loading case.

Generally after the uniplanar KT joint is added

by one horizontal brace to the center of the chord to

become a multi-planar tubular-KDT joint, stress

along brace-to-chord intersection line mostly

becomes smaller in all points reviewed (0o to 360o),

especially at a point of Crown 1. This results occur

for both two cases of loading conditions applied,

balance axial load and in-plane bending moment.

Figure 8: Nomenclature of the observed points along the

brace-to-chord intersection line.

Figure 9: Stress distribution along the weld toes of the

joint under balance axial load.

Figure 10: Stress distribution along the weld toes of the

joint under in-plane-bending moment.

4 CONCLUSIONS

From the present study, the following conclusions

can be drawn:

• The maximum stress along the weld toe of the

KT and KDT tubular joints under balance axial load

ISOCEEN 2018 - 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

174

occurs in a member of Diagonal Brace 1 at the

Crown 1 point (0o).

• For in-plane-bending moment case, the

maximum stress along the weld toe of the KT and

KDT tubular joints can be found also at the Crown 1

point (0o), but in critical member of Horizontal

Brace 1.

• The multi-planarity caused the KDT-joint has

smaller maximum principal stress than the KT-joint

under both loading cases of balance axial load and

in-plane bending moment. This is due to partly, the

occurred stress redistributed onto the additional

plane where the new brace laid. In turn, this

condition will also make the value of SCF of the

KDT-joint smaller than the SCF of the KT-joint.

ACKNOWLEDGEMENTS

The authors would like to thank the Institute for

Research and Community Services (LPPM), Institut

Teknologi Sepuluh Nopember (ITS) Surabaya for

supporting this research by a grant of “Laboratory

Research 2018”.

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