10 Design application
10.1 General
10.1.1 The relevant load conditions and design conditions should be
established in accordance with section 4.18 of the IGC Code or section 6.4.12 of the
IGF Code. Guidance on special considerations for high manganese austenitic steel is
described below.
10.1.2 For the selection of relevant safety factors for high manganese
austenitic steels (see paragraphs 4.21 to 4.23 of the IGC Code or section 6.4.15 of
the IGF Code), the safety factors specified for "Austenitic Steels" should be
applied both for the base material and for as welded condition.
10.2 Ultimate design condition
(Reference: section 4.18.1 IGC Code or section 16.3.3 IGF Code)
It should be noted that high manganese austenitic steels normally have
under-matched welds and, therefore, it is of great importance that the design values
of the yield strength and tensile strength are based on the "minimum mechanical
properties" for the base material and as welded condition (see section 6 on
Mechanical Properties). Note the limitation for under-matched welds defined in
section 4.18.1.3.1.2 of the IGC Code or section 16.3.3.5.1 of the IGF Code.
10.3 Buckling strength
10.3.1 Buckling strength analysis should be carried out based on
recognized standards. Functional loads as defined in section 4.3.4 of the IGC Code
or section 6.4.1.6 of the IGF Code should be considered. Note that design tolerances
should be considered where relevant and be included in the strength assessment as
required in section 6.6.2.1 of the IGC Code or section 16.4.2 of the IGF Code.
10.3.2 It should be noted that the acceptance criteria for the flooding
load cases are different from other buckling load cases. Furthermore, the acceptance
criteria for flooding load cases, as defined in the IGC Code and the IGF Code, are
also different, as the IGF Code requires the tank to "keep its integrity after
flooding to ensure safe evacuation of the ship" (section 6.4.1.6.3.3 of the IGF Code), while
the IGC Code only refers to endangering the integrity of the ship's hull (section
4.3.4.3.3 of the IGC Code).
10.4 Fatigue design condition (Reference: 4.18.2 IGC Code and
6.4.12.2 IGF Code)
The fatigue design curves for base material and for welded conditions
have been documented as a comparison with recognized S-N curves, as provided by the
D-curve in reference 11.4 (table 4) and FAT 90 provided by reference 11.5 (figure
1). Fatigue tests have been carried out for butt welded joints only. However, for
other details, the application of other S-N curves should be to the satisfaction of
the Administration. Section 4.18.2.4.2 of the IGC Code and section 6.4.12.2.4 of the
IGF Code specify the design S-N curves to be based on a 97.6% probability of
survival corresponding to the mean-minus-two-standard-deviation curves of relevant
experimental data up to final failure.
Table 4: (S-N curves in air): High manganese austenitic steel has
been documented to be equal or better than the D-curve (reference 11.4) for as
welded condition without stress concentration from any structural
details
Figure 1: Reference S-N curve to high manganese austenitic steel is
the FAT 90 curve (reference 11.5). The FAT 90 curve is as welded condition
without stress concentration from any structural details.
10.5 Fracture mechanics analyses
10.5.1 For a cargo tank or fuel tank where a reduced secondary barrier is
applied, fracture mechanics analysis should be carried out in accordance with the
IGC or IGF Code.
10.5.2 Fracture toughness properties should be expressed using recognized standards.
Depending on the material, fracture toughness properties determined for loading
rates similar to those expected in the tank system should be required. The fatigue
crack propagation rate properties should be documented for the tank material and its
welded joints for the relevant service conditions. These properties should be
expressed using a recognized fracture mechanics practice relating the fatigue crack
propagation rate to the variation in stress intensity, ΔK, at the crack tip. The
effect of stresses produced by static loads should be taken into account when
establishing the choice of fatigue crack propagation rate parameters.
10.5.3 Note that for the application where very high static load utilization is
relevant, alternative methods such as ductile fracture mechanic analyses should be
considered.
10.5.4 An example of a typical Crack Tip Opening Displacement (CTOD) value at
cryogenic condition can be found in figure 2.
10.5.5 A fracture mechanics analysis is required for type B tanks
(section 4.22.4 of the IGC Code and section 6.4.15.2.3.3 of the IGF Code) where a
reduced secondary barrier is applied. Fracture mechanics analysis may also be
required for other tank types as found relevant to show compliance with fatigue and
crack propagation properties. Note that CTOD values used in fracture mechanics
analysis may in any case be an important property to analyse to ensure that
materials are considered suitable for the application.
| CTOD TEST REPORT
|
|
|
REPORT
NO.
|
|
| Test Method
Standard
|
ISO
12135/15653 Specimen No.
|
FCAW-2
|
Test
Date
|
|
| Specimen
configuration
|
Square Cross-Section 3 Point Bend(W=B)
|
Crack
plane orientation
|
L-T
|
| Specimen Dimensions
|
|
1
|
2
|
3
|
Average
|
| Thickness, B (mm)
|
40
|
40
|
40
|
40
|
| Width, W
(mm)
|
80
|
80
|
80
|
80
|
| Span, S
(mm)
|
320
|
Knife
edge thickness, z (mm)
|
0
|
| Test Material
|
Young's Modulus of
Elasticity, E (MPa)
|
182,000
|
| YS (0.2% proof),
σYSP (MPa)
|
474
|
| TS, σTSP
(MPa)
|
780
|
| YS (0.2% proof),
σYS (MPa)
|
655
|
| Machined
Notch (mm)
|
Width, N
|
Length, Lmc
|
Root
Radius
|
|
| 4.7
|
32.4
|
0.1
|
|
| Test
Condition
|
Temperature
(℃)
|
-165
|
| Test Result
|
|
| Crack Length to Tip of Fatigue Pre crack
(mm)
|
| a1
|
a2
|
a3
|
a4
|
a5
|
a6
|
a7
|
a8
|
a9
|
a0
|
| 37.62
|
39.28
|
39.36
|
38.95
|
39.24
|
38.27
|
38.55
|
38.67
|
37.21
|
38.72
|
| a0/W
|
0.54
|
Plastic Component of V, Vp (mm)
|
1.53
|
| Critical CTOD (mm)
|
| Type
of CTOD
|
Total
CTOD
|
| δm
|
0.53
|
Figure 2: Example of typical values for CTOD test at -165°C
10.6 Welding
10.6.1 Welding should be carried out in accordance with section 6.5 of
the IGC Code or section 16.3 of the IGF Code, and to the satisfaction
of the Administration.
10.6.2 For welding, the following points should be considered:
-
.1 for reducing the heat input during production:
-
.1 special attention should be given to the first root
pass when applying flux-cored arc welding (FCAW); reduced amperage
should be considered; and
-
.2 welding heat input of maximum 30 kJ/cm should be used
as guidance for 3G position, as that has less heat input for 1G
position.
-
.2 distance between the weld and nozzle should be kept to a
minimum to reduce the oxygen content at the vicinity of the weld pool;
-
.3 weld gas composition of FCAW should normally be an 80/20 mix
of argon and carbon dioxide; and
-
.4 appropriate ventilation should be provided to reduce exposure
to hazardous welding fumes.
10.7 Non-destructive testing (NDT)
The scope of non-destructive testing (NDT) should be as required by
section 6.5.6 of the IGC Code or section 16.3.6 of the IGF Code. NDT procedures should
be in accordance with recognized standards to the satisfaction of the
Administration. For high manganese austenitic steel suitable NDT procedures normally
applicable for austenitic steels should be used.
10.8 Corrosion resistance
10.8.1 Appropriate measures with respect to corrosion protection and
avoidance of a corrosive environment should be taken. Particularly for LNG fuel
tanks that may not be in operation, appropriate precautions should be taken at all
times to ensure that empty tanks are filled with inert gas or dry air when not in
use.