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DNV-OS-C201 Structural Design of Offshore Units (WSD method)
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SECTION 4
Structural Categorisation, Material Selection and Inspection
PrinciplesSec.4
A. General
Sec.4
A 100
Sec.4 A
101 This section describes the structural categorisation, selection
of steel materials and inspection principles to be applied in design
and construction of offshore steel structures.Sec.4
B. Temperatures for Selection of
Material
Sec.4
B 100 General
Sec.4 B
101 The design temperature for a unit is the reference temperature
for assessing areas where the unit can be transported, installed
and operated.
The design temperature shall be lower or equal to the lowest mean daily temperature in
air for the relevant areas. For seasonal restricted operations the lowest mean daily temperature in
air for the season may be applied.
Sec.4 B
102 The service temperatures for different parts of a unit apply
for selection of structural steel.
Sec.4 B
103 The service temperature for various structural parts is given
in B200 and B300. In case different service temperatures are defined
in B200 and B300 for a structural part the lower specified value
shall be applied. Further details regarding service temperature
for different structural elements are given in the sections for different
types of units.
Sec.4 B
104 In all cases where the temperature is reduced by localised
cryogenic storage or other cooling conditions, such factors shall
be taken into account in establishing the service temperatures for
considered structural parts.
Sec.4
B 200 Floating units
Sec.4 B
201 External structures above the lowest waterline shall be designed
with service temperature not higher than the design
temperature for the area(s) where the unit is to operate.
Sec.4 B
202 External structures below the lowest waterline need not be
designed for service temperatures lower than 0°C. A higher service
temperature may be accepted if adequate supporting data can be presented
relative to the lowest mean daily temperature applicable to the
relevant actual water depths.
Sec.4 B
203 Internal structures in way of permanently heated rooms need
not be designed for service temperatures lower than 0°C.Sec.4
B 300 Bottom fixed units
Sec.4 B
301 External structures above the lowest astronomical tide (LAT)
shall be designed with service temperature not higher than the design temperature.
Sec.4 B
302 Materials in structures below the lowest astronomical tide
(LAT) need not be designed for service temperatures lower than of
0°C. A higher service temperature may be accepted if adequate supporting
data can be presented relative to the lowest
mean daily temperature applicable for the relevant water
depths. Sec.4
C. Structural Category
Sec.4
C 100 General
Sec.4 C
101 The purpose of the structural categorisation is to assure
adequate material and suitable inspection to avoid brittle fracture.
The purpose of inspection is also to remove defects that may grow
into fatigue cracks during service life.Guidance note:
Conditions that may result in brittle fracture are sought
avoided. Brittle fracture may occur under a combination of:| - | presence of sharp defects such
as cracks| - | high tensile stress in direction normal to planar defect(s)| - | material with low fracture toughness. | | |
Sharp cracks resulting from fabrication may be found by inspection
and repaired. Fatigue cracks may also be discovered during service
life by inspection.
High stresses in a component may occur due to welding. A complex
connection is likely to provide more restraint and larger residual
stress than a simple one. This residual stress may be partly removed
by post weld heat treatment if necessary. Also a complex connection
shows a more three-dimensional stress state due to external loading
than simple connections. This stress state may provide basis for
a cleavage fracture.
The fracture toughness is dependent on temperature and material
thickness. These parameters are accounted for separately in selection
of material. The resulting fracture toughness in the weld and the
heat affected zone is also dependent on the fabrication method.
Thus, to avoid brittle fracture, first a material with a suitable
fracture toughness for the actual service temperature and thickness
is selected. Then a proper fabrication method is used. In special
cases post weld heat treatment may be performed to reduce crack
driving stresses, see D501 and
DNV-OS-C401. A suitable amount of inspection is carried out to remove
planar defects larger than acceptable. In this standard selection
of material with appropriate fracture toughness and avoidance of
unacceptable defects are achieved by linking different types of
connections to different structural categories and inspection categories.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4
C 200 Selection of structural category
Sec.4 C
201 Components are classified into structural categories according
to the following criteria:| — | significance of component in
terms of consequence of failure |
| — | stress condition at the considered detail that together
with possible weld defects or fatigue cracks may provoke brittle
fracture. |
Guidance note:
The consequence of failure may be quantified in terms of residual
strength of the structure when considering failure of the actual
component.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4 C
202 Structural category for selection of materials shall be determined
according to principles given in Table C1. Further details regarding
selection of structural categories for different types of units
are given in Sec.11 to 14.Sec.4 C
| Table C1 Structural
categories for selection of materials |
| Structural category | Principles for determination
of structural category |
| Special | Structural parts where failure will have substantial
consequences and are subject to a stress condition that may increase
the probability of a brittle
fracture.1) |
| Primary | Structural parts where failure will have substantial
consequences. |
| Secondary | Structural parts where failure will be without
significant
consequence. |
- In complex joints a tri-axial or bi-axial stress pattern will
be present. This may give conditions for brittle fracture where
tensile stresses are present in addition to presence of defects
and material with low fracture toughness.
|
Sec.4
C 300 Inspection of welds
Sec.4 C
301 Requirements for type and extent of inspection are given in
DNV-OS-C401 dependent on assigned inspection category for the welds.
The requirements are based on the consideration of fatigue damage
and assessment of general fabrication quality.
Sec.4 C
302 The inspection category is by default related to the structural
category according to Table C2.Sec.4 C
| Table C2 Inspection
categories |
| Inspection category | Structural category | I | Special | | II | Primary | | III | Secondary | |
Sec.4 C
303 The weld connection between two components shall be assigned
an inspection category according to the highest of the joined components.
For stiffened plates, the weld connection between stiffener and
stringer and girder web to the plate may be inspected according
to inspection category III.
Sec.4 C
304 If the fabrication quality is assessed by testing, or well
known quality from previous experience, the extent of inspection
required for elements within structural category primary may be reduced, but not
less than for inspection category III.
Sec.4 C
305 Fatigue critical details within structural category primary and secondary shall
be inspected according to requirements in category I.
Sec.4 C
306 Welds in fatigue critical areas not accessible for inspection
and repair during operation shall be inspected according to requirements
in category I during construction.
Sec.4 C
307 Inspection categories determined in accordance with the above
provide requirements for the minimum extent of required inspection.
When considering the economic consequence that repair may entail,
for example, in way of complex connections with limited or difficult
access, it may be considered prudent engineering practice to require
more demanding requirements for inspection than the required minimum.
Sec.4 C
308 When determining the extent of inspection, and the locations
of required NDT, in addition to evaluating design parameters (for
example fatigue utilisation) consideration should be given to relevant
fabrication parameters including;| — | location of block (section)
joints |
| — | manual versus automatic welding |
| — | start and stop of weld etc. |
Sec.4 C
309 The extent of NDT for welds in block joints and erection joints
transverse to main stress direction shall not be less than for IC
II.Sec.4
D. Structural Steel
Sec.4
D 100 General
Sec.4 D
101 Where the subsequent requirements for steel grades are dependent
on plate thickness, these are based on the nominal thickness as
built.
Sec.4 D
102 The requirements in this subsection deal with the selection
of various structural steel grades in compliance with the requirements
given in DNV-OS-B101. Where other, agreed codes or standards have
been utilised in the specification of steels, the application of
such steel grades within the structure shall be specially considered.
Sec.4 D
103 When considering criteria appropriate to material grade selection,
adequate consideration shall be given to all relevant phases in
the life cycle of the unit. There may be conditions and criteria,
other than those from the in-service, operational phase, that provide
the design requirements in respect to the selection of material, e.g.
design temperature and/or stress levels during the construction
phase or marine operations.
Sec.4 D
104 The steel grades selected for structural components shall
be related to calculated stresses and requirements to toughness
properties. Requirements for toughness properties are in general
based on the Charpy V-notch test and are dependent on service temperature,
structural category and thickness of the component in question.
Sec.4 D
105 The material toughness may also be evaluated by fracture mechanics
testing in special cases, see D401 and DNV-OS-C401.
Sec.4 D
106 In structural cross-joints where high tensile stresses are
acting perpendicular to the plane of the plate, the plate material
shall be tested to prove the ability to resist lamellar tearing,
e.g. by Z-quality, see 203.
Sec.4 D
107 Requirements for forging and castings are given in DNV-OS-B101.Sec.4
D 200 Material designations
Sec.4 D
201 Structural steel of various strength groups will be referred
to as given in Table D1.
Sec.4 D
202 Each strength group consists of two parallel series of steel
grades: | — | steels of normal weldability |
| — | steels of improved weldability. |
The two series are intended for the same applications. However,
the improved weldability grades have in addition to leaner chemistry
and better weldability, extra margins to account for reduced toughness
after welding. These grades are also limited to a specified minimum
yield stress of 500 N/mm2.
Sec.4 D
| Table D1 Material
designations |
| Designation | Strength
group | Specified minimum yield stress
fy (N/mm2)1) | NV | Normal strength steel
(NS) | 235 | | NV-27 | High strength
steel (HS) | 265 | | NV-32 | 315 | | NV-36 | 355 | | NV-40 | 390 | | NV-420 | Extra
high strength steel (EHS) | 420 | | NV-460 | 460 | | NV-500 | 500 | | NV-550 | 550 | | NV-620 | 620 | | NV-690 | 690 | - For steels of improved weldability the required specified
minimum yield stress is reduced for increasing material thickness,
see DNV-OS-B101.
| |
Sec.4 D
203 Different steel grades are defined within each strength group,
depending upon the required impact toughness properties. The grades
are referred to as A, B, D, E, F or AW, BW, DW, EW for improved weldability
grades as shown in Table D2.Additional symbol:
| Z | = | steel grade of proven through-thickness properties. This symbol
is omitted for steels of improved weldability although improved
through-thickness properties are required. |
Sec.4 D
| Table D2 Applicable
steel grades |
| Strength
group | Grade | Test
temperature 2)
(ºC) | Normal weldability | Improved
weldability 1) | NS | A | - | Not tested | B 3) | BW | 0 | D | DW | -20 | E | EW | -40 | | HS | A | AW | 0 | D | DW | -20 | E | EW | -40 | F | - | -60 | | EHS | A | - | 0 | D | DW | -20 | E | EW | -40 | F | - | -60 | - For steels with improved weldability, through-thickness properties
are specified, see DNV-OS-B101.
- Charpy V-notch impact tests, see DNV-OS-B101.
- Charpy V-notch tests are required for thickness above
25 mm but are subject to agreement between the contracting parties
for thickness of 25 mm or less.
| | |
Sec.4
D 300 Selection of structural steel
Sec.4 D
301 The grade of steel to be used shall in general be related
to the service temperature and thickness for the applicable structural
category as shown in Table D3.
Sec.4 D
302 Selection of a better steel grade than minimum required in
design shall not lead to more stringent requirements in fabrication.
Sec.4 D
303 Grade of steel to be used for thickness less than 10 mm and/or
service temperature above 10°C may be specially considered.
Sec.4 D
304 Welded steel plates and sections of thickness exceeding the
upper limits for the actual steel grade as given in Table D3 shall
be evaluated in each individual case with respect to the fitness
for purpose of the weldments. The evaluation should be based on
fracture mechanics testing and analysis, e.g. in accordance with BS
7910.
Sec.4 D
305 For structural parts subjected to compressive and/or
low tensile stresses, consideration may be given to the use of lower
steel grades than stated in Table D3.
Sec.4 D
306 The use of steels with specified minimum yield stress greater
than 550 N/mm2 (NV550)
shall be subject to special consideration for applications where
anaerobic environmental conditions such as stagnant water, organically
active mud (bacteria) and hydrogen sulphide may predominate.
Sec.4 D
307 Predominantly anaerobic conditions can for this purpose be
characterised by a concentration of sulphate reducing bacteria,
SRB, in the order of magnitude >103 SRB/ml (method according
to NACE TPC Publication No.3).
Sec.4 D
308 The steels' susceptibility to hydrogen induced stress
cracking (HISC) shall be specially considered when used for critical
applications (such as jack-up legs and spud cans). See also Sec.10.Sec.4 D
| Table D3 Thickness limitations (mm) of structural
steels for different structural categories and service temperatures
(°C) |
| Structural Category | Grade | ³ 10 | 0 | -10 | -20 | -25 | -30 | Secondary | A
B/BW
D/DW
E/EW
AH/AHW
DH/DHW
EH/EHW
FH
AEH
DEH/DEHW
EEH/EEHW
FEH | 35
70
150
150
60
120
150
150
70
150
150
150 | 30
60
150
150
50
100
150
150
60
150
150
150 | 25
50
100
150
40
80
150
150
50
100
150
150 | 20
40
80
150
30
60
150
150
40
80
150
150 | 15
30
70
120
20
50
120
*)
30
70
120
*) | 10
20
60
100
15
40
100
*)
20
60
100
*) | | Primary | A
B/BW
D/DW
E/EW
AH/AHW
DH/DHW
EH/EHW
FH
AEH
DEH/DEHW
EEH/EEHW
FEH | 30
40
70
150
30
60
120
150
35
70
150
150 | 20
30
60
150
25
50
100
150
30
60
150
150 | 10
25
50
100
20
40
80
150
25
50
100
150 | N.A.
20
40
80
15
30
60
150
20
40
80
150 | N.A.
15
35
70
12.5
25
50
*)
17.5
35
70
*) | N.A.
10
30
60
10
20
40
*)
15
30
60
*) | | Special | D/DW
E/EW
AH/AHW
DH/DHW
EH/EHW
FH
AEH
DEH/DEHW
EEH/EEHW
FEH | 35
70
15
30
60
120
20
35
70
150 | 30
60
10
25
50
100
15
30
60
150 | 25
50
N.A.
20
40
80
10
25
50
100 | 20
40
N.A.
15
30
60
N.A.
20
40
80 | 17.5
35
N.A.
12.5
25
50
N.A
17.5
35
70 | 15
30
N.A.
10
20
40
N.A.
15
30
60 | | *) | For service temperature below -20°C the upper limit for use
of this grade must be specially considered. |
N.A. = no
application | |
Sec.4
D 400 Fracture mechanics (FM) testing
Sec.4 D
401 For units which are intended to operate continuously at the
same location for more than 5 years, FM testing shall be included
in the qualification of welding procedures for joints which all
of the following apply:| — | the design temperature is lower
than +10°C |
| — | the joint is in special area |
| — | at least one of the adjoining members is fabricated
from steel with a SMYS larger than or equal to 420 MPa. |
For details on FM testing methods, see
DNV-OS-C401 Ch.2 Sec.1 C900.
Sec.4
D 500 Post weld heat treatment (PWHT)
Sec.4 D
501 For units which are intended to operate continuously at the
same location for more than 5 years, PWHT shall be applied for joints
in C-Mn steels in special areas when the material thickness at the
welds exceeds 50 mm. For details, see DNV-OS-C401 Ch.2
Sec.1 F200.If, however, satisfactory performance
in the as-welded condition can be documented by a fitness-for-purpose assessment
applying fracture mechanics testing, fracture mechanics and fatigue
crack growth analyses, PWHT may be omitted.
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