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DNV-OS-C103 Structural Design of Column Stabilised Units (LRFD method)
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SECTION 2
Structural Categorisation,
Material Selection and
Inspection PrinciplesSec.2
A. General
Sec.2
A 100 Scope
Sec.2 A
101 This section describes the structural categorisation, selection
of steel materials and inspection principles to be applied in design
and construction of column-stabilised units.
Sec.2 A
102 The structural application categories are determined based
on the structural significance, consequences of failure and the
complexity of the joints. The structural application category set
the selection of steel quality and the inspection extent of the
welds.
Sec.2 A
103 The steel grades selected for structural components shall be
related to weldability and requirements for toughness properties
and shall be in compliance with the requirements given in the DNV-OS-B101.Sec.2
B. Structural Categorisation
Sec.2
B 100 Structural categorisation
Sec.2 B
101 Application categories for structural components are defined
in DNV-OS-C101 Sec.4. Structural members of column-stabilised units
are grouped as follows:
Special category
- Portions of deck plating, heavy
flanges, and bulkheads within the upper hull or platform which form «box» or «I» type
supporting structure which receive major concentrated loads.
- External shell structure in way of intersections of
vertical columns, decks and lower hulls.
- Major intersections of bracing members.
- «Through» material used at connections
of vertical columns, upper platform decks and upper or lower hulls which
are designed to provide proper alignment and adequate load transfer.
- External brackets, portions of bulkheads, and frames which
are designed to receive concentrated loads at intersections of major
structural members.
- Highly utilised areas supporting anchor line fairleads
and winches, crane pedestals, flare etc.
Guidance note:
Highly stressed areas are normally considered to be areas
utilised more than 85% of the allowable yield capacity.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Fig.1 to Fig.4 show typical examples of special
structures.
Primary category
- Deck plating, heavy flanges,
and bulkheads within the upper hull or platform which form «box» or «I» type
supporting structure which do not receive major concentrated loads.
- External shell structure of vertical columns, lower
and upper hulls, and diagonal and horizontal braces.
- Bulkheads, decks, stiffeners and girders which provide local
reinforcement or continuity of structure in way of intersections,
except areas where the structure is considered for special application.
- Main support structure of heavy substructures and equipment,
e.g. anchor line fairleads, cranes, drillfloor substructure, life
boat platform, thruster foundation and helicopter deck.
Secondary category
- Upper platform decks, or decks
of upper hulls except areas where the structure is considered primary
or special application.
- Bulkheads, stiffeners, flats or decks and girders in
vertical columns, decks, lower hulls, diagonal and horizontal bracing,
which are not considered as primary or special application.
- Deckhouses.
- Other structures not categorised as special or primary.
Sec.2
C. Material Selection
Sec.2
C 100 General
Sec.2 C
101 Material specifications shall be established for all structural
materials. Such materials shall be suitable for their intended purpose
and have adequate properties in all relevant design conditions.
Material selection shall be undertaken in accordance with the principles
given in DNV-OS-C101.
Sec.2 C
102 When considering criteria appropriate to material grade selection,
adequate consideration shall be given to all relevant phases in
the life cycle of the unit. In this connection 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. (Such criteria may, for example, be design temperature
and/or stress levels during marine operations.)
Sec.2 C
103 In structural cross-joints essential for the overall structural
integrity where high tensile stresses are acting normal to the plane
of the plate, the plate material shall be tested to prove the ability
to resist lamellar tearing (Z-quality).
Sec.2 C
104 Material designations are defined in DNV-OS-C101.Sec.2
C 200 Design and service temperatures
Sec.2 C
201 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.2 C
202 The service temperatures for different parts of a unit apply
for selection of structural steel. The service temperatures are
defined as presented in 203 to 206. In case different service temperatures
are defined in 203 to 206 for a structural part the lower specified
value shall be applied.
Sec.2 C
203 External structures above the light transit waterline shall not
be designed for a service temperature higher than the design temperature
for the unit.However, for column-stabilised units of conventional type,
the pontoon deck need not be designed for service temperatures lower
than 0ºC.
Sec.2 C
204 External structures below the light transit waterline need not
be designed for service temperatures lower than 0ºC.
Sec.2 C
205 Internal structures of columns, pontoons and decks shall have
the same service temperature as the adjacent external structure,
if not otherwise documented.
Sec.2 C
206 Internal structures in way of permanently heated rooms need
not to be designed for service temperatures lower than 0ºC.
Sec.2
D. Inspection Categories
Sec.2
D 100 General
Sec.2 D
101 Welding and the extent of non-destructive testing during fabrication,
shall be in accordance with the requirements stipulated for the
appropriate inspection category as defined in DNV-OS-C101, Sec.4.
Sec.2 D
102 Inspection categories determined in accordance with DNV-OS-C101,
Sec.4 provide requirements for the minimum extent of required inspection.
When considering the economic consequence that repair during in-service
operation 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.2 D
103 When determining the extent of inspection and the locations
of required NDT, in additional 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.2
E. Categorisation and Inspection Level
for
Typical Column-Stabilised Unit Details
Sec.2
E 100 General
Sec.2 E
101 Fig.1 to Fig.4 illustrate minimum requirements
for structural categorisation and extent of inspection for typical
column-stabilised unit configurations.
Sec.2 E
102 In way of the pontoon and column connection as indicated in Fig.1 and Fig.2,
the pontoon deckplate should be the continuous material. These plate
fields should be of material with through-thickness properties (Z-quality
material).
Sec.2 E
103 Shaded areas indicated in the figures are intended to be three-dimensional
in extent. This implies that, in way of these locations, the shaded
area is not only to apply to the outer surface of the connection,
but is also to extend into the structure. However, stiffeners and
stiffener brackets within this area should be of primary category
and the bracket toe locations on the stiffeners should be designated
with mandatory MPI.
Sec.2 E
104 Stiffeners welded to a plate categorised as special area should
be welded with full penetration welds and no notches should be used.
Sec.2 E
105 The inspection categories for general pontoon, plate butt welds
and girder welds to the pontoon shell are determined based upon,
amongst others, accessibility and fatigue utilisation.
Sec.2 E
106 Major bracket toes should be designated as locations with
a mandatory requirement for MPI. In way of the brace connections
as indicated Fig.3, the brace and
brace bracket plate fields should be the continuous material. These
plate fields should be material with through-thickness properties
(Z-quality material).
Sec.2 E
107 In way of the column and upper hull connection as indicated
in Fig.4 the upper hull deckplate
should be the continuous material. These plate fields should be
material with through-thickness properties (Z-quality material).
Fig. 1 Pontoon and column connection, twin pontoon design
Fig. 2 Column and ring pontoon connection, ring-pontoon design
Fig. 3 Brace connection
Fig. 4 Connection column and upper hull
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