Page 185 - Practical Design Ships and Floating Structures

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2 DESIGN RECOMMENDATIONS
2.1 Hot Spot Stress Analysis

The hot spot stress in tubular joints has ...e last 5 years been derived by some extrapolation of the
measdcalculated stress at the hot spot region back to the weld toe. The local stress effect due to the
weld is accounted for in the S-N curve. Similar concepts were also investigated in the present project,
ref. Fricke (2001). Three different methods for derivation of hot spot stress (geometric stress) were
investigated:
Linear extrapolation of stresses to the weld toe from stress at distances 0.4*t and t from the toe (t =
plate thickness). This method is recommended by the International Institute of Welding (IIw).
Linear extrapolation of stresses to the weld toe from stress at distances OSft and lSft from the toe.
This method is used by some of the Classification Companies.
Stress at a distances 0.5*t from the weld toe. (No extrapolation).
For analysis by shell elements the distance to the stress read out points is measured from the
intersection lines as the weld is not normally included in the model. For analysis by solid elements the
distance to the stress read out points is measured from the weld toe.
It should be noted that the finite element modelling might influence the calculated stress at the hot spot
region. Parameters effecting this are:
type of element used,
size of elements at the hot spot region,
how the stresses are derived from the analysis (Gaussian stress, nodal stress etc.).
See Fricke (2001) for recommendations on fmite element modelling for derivation of hot spot stress at
different positions shown in Figure 1.

2.2 S-N Data
Available S-N data from the The Welding Institute were reviewed in terms of hot spot stress S-N
curves. In addition a number of typical ship details were fatigue tested by Hyundai Heavy Industries in
Korea (HHI), (Kim et al., 2000,2001). Five full scale test specimens as shown in Figure 2 were tested
at DNV laboratories. (Lotsberg et al., 2001). The purpose of these tests was to achieve test data for
calibration of the FEA and for verification of S-N data. Based on the present work the following main
recommendations were drawn by Maddox (2001):
An extensive database for transverse butt welded joints suggests FAT90 as a suitable hot-spot
stress design curve for plate thicknesses up to 25 mm. This S-N curve corresponds to the D- curve in
DNV (2000) and is approximately the same as the D curve in HSE (1 995).
This proved to be consistent with the HHI data and some published data for hot-spot types (a) and
(c) in Figure 1 (i. e. weld end and toe on plate surface respectively) provided the hot-spot stress is
obtained by one of the extrapolation methods.

Lower FAT80 is recommended if the hot-spot stress is assumed to be that 03 from the weld toe.
This S-N curve corresponds to the E- curve in HSE (1 995) and DNV (2000).
Fillet welds that may fail in the throat should be assessed based on the FAT36 design curve when
analysed using the standard method based on the ‘engineering shear stress’ on the weld throat. This S-
N curve corresponds to the W’ curve in HSE (1995) and the W3- curve in DNV (2000).
The fatigue endurance to a 12 mm crack in the full scale tests are shown in Figure 3 for the same
ballast loading. It is observed that the geometry with double bracket design (specimen 1) is favourable
in terms of fatigue life. This comparison is based on sideways pressure. In the case of longitudinal