Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Effects of geometric parameters on SCF distribution of three-planar tubular Y-joints under axial force
Shiliu Bao1,*; Ruonan Li2; Xiyao Zhao1; Yun Tian1; Yating Tai1
1

College of Civil and Transportation Engineering, Hohai University
Nanjing, Jiangsu, China
2
Faculty of Infrastructure Engineering, Dalian University of Technology
Dalian, Liaoning, China

evaluation index to calculate fatigue life combined with S-N curves
according to the cumulative damage criterion.
As an essential parameter in fatigue analysis, the HSS is commonly
obtained by multiplying stress concentration factor (SCF) with nominal
stress which evenly distributes on the cross-section of braces far awayfrom the weld area and can be calculated by von Mises stress definition.
The SCF is defined as the ratio of HSS and nominal stress. Some codes
recommend SCF formulas for simple tubular joints, such as T-, Y-, Kand X-joints.
However, the above formulas all only give attention to the SCF of thecrown or saddle points, or at most the peak value with unknown
location, which is not applicable to calculating HSS of tubular joints
under combined load conditions of several basic loads applied together.
Directly superposing peak values of SCF under each basic load toacquire HSS of a tubular joint is not in line with the actual situation, as
the locations of peak value vary considerably with load type. Even theimproved method is given in DNV (2016) emphasizing eight points
along a weld toe curve has room for improvement in the accuracy.
Because the SCF value of points other than crown and saddle points are
extracted through interpolation instead of independent formula.
Bao et al. (2019, 2020a, 2020b) already offered the HSS formula ofmultiplanar tubular joints under conditions of combined loads applied
on multiplanes, however, the premise of applying the formula is toknow SCF distribution along weld toe curves of intersections of chord
and braces of multiplanar tubular joints under conditions of basic load
applied on a single plane.
Compared to formulas for SCF on crown or saddle points, the amountof formulas for predicting SCF distribution is much less, though, somevaluable researches are published. Morgan and Lee (1998a, 1998b,1998c) provided SCF distribution formulas for K-joints under threebasic loads (axial force, in-plane bending moment and out-of-planebending moment) according to simulating results using thin-walledshell elements. Rahmanli and Becque (2021) proposed SCF distribution
formulas for DKT-joints under balanced axial forces based on
numerical studies.
Formulas in these papers can only be used to calculate SCF of 8 points
with 45° separation each on weld toe curves rather than a full

ABSTRACTThe three-planar tubular Y-joint is normally multiplanar loaded withsevere multiplanar interaction effect, which may cause the hot spot
stress (HSS) to occur other than crown or saddle points of the weld
seam. Thus, the distributions of stress concentration factor (SCF) along
weld seam are needed to calculate HSS rather than formulas that only
care about values of crown or saddle points. To find the distribution
patterns, a numerical database is established using the validated finite
element (FE) analysis method. Sensitivity analyses for five geometricparameters affecting distribution along weld toe curves of SCF are
conducted.

KEY WORDS: three-planar tubular Y-joint; hot spot stress; stressconcentration factor; distribution; axial force

INTRODUCTION
The development of ocean energy carries a massive weight in the areaof clean energy resources. Many marine structures are welded by steel
tubular components which make lots of tubular joints. Fatigueevaluation is inevitable in the structure design of marine engineering as
it has to resist long-term cyclic loading caused by harsh environmental
and machine operation loads. Geometric discontinuities in theintersection of chord and brace members lead to high stressconcentration making tubular joints critical for fatigue design. As tripod
substructures of offshore wind turbine (OWT), shown in Fig. 1, arebroadly adopted in the offshore wind farms in the intertidal zone ofChina, the three-planar tubular Y-joint incorporating multiplanarinteraction between braces is focused in this study.
The hot spot stress (HSS) method, approved by codes such as DNV(2016), API (2014), AWS (2010), CIDECT (2001), IIW (2016),
Lloyd's Register (1992, 1997), is currently the most mainstream among
many methods for a fatigue assessment of welded tubular joints due toits high accuracy and good applicability. This method calculates
amplitudes of HSS by extrapolation considering effects of both crosssection and geometric sizes of the joint. The HSS is thus be taken as the2937