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DNV-OS-C105 Structural Design of TLPS (LRFD method)
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SECTION 2
Structural Categorisation, Material Selection and
Inspection
PrinciplesSec.2
A. Introduction
Sec.2
A 100 General
Sec.2 A
101 Selection of materials and inspection principles shall be
based on a systematic categorisation of the structure according
to the structural significance and the complexity of the joints
or connections as given in DNV-OS-C101 Sec.4.
Sec.2 A
102 In addition to in-service operational phases, consideration
shall be given to structural members and details utilised for temporary
conditions, e.g. fabrication, lifting arrangements, towing and installation arrangements,
etc.
Sec.2 A
103 For TLP structures that are similar to column stabilised units,
the structural categorisation and extent of inspection for the structural
components should follow the requirements as given in DNV-OS-C103.
For TLPs that are similar to deep draught floaters, the structural
categorisation and extent of inspection for the structural components
should follow the requirements as given in DNV-OS-C106.Sec.2
B. Structural Categorisation
Sec.2
B 100 General
Sec.2 B
101 Application categories for structural components are defined
in DNV-OS-C101 Sec.4. Structural members of TLPs are grouped as
follows, see Figure 1 and Figure 2.
Special category
- External shell structure in
way of intersections of columns, topside deck, lower hull and tendon
porch etc.
- "Through" material used at connections
of columns, topside decks and lower hull 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.
- Tendon interfaces with the foundation and the TLP hull.
- Tendon and tendon connectors.
- Highly utilized areas supporting crane pedestals, flare
booms etc.
Guidance note:
Highly stressed areas are normally considered to be areas
utilized 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---
Primary category
- External shell structure of
columns, lower and upper hulls.
- 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.
- Truss rows and horizontal diagonal bracings on the deck.
- Main support structure of heavy substructures and equipment,
e.g. cranes, life boat platform, thruster foundation and helicopter
deck.
Secondary category
- Bulkheads, stiffeners, flats
or decks and girders in columns, decks and lower hulls, which are
not considered as primary or special application.
- Horizontal braces and members on the decks.
- Well-bay trusses and reaming members.
- Other structures not categorised as special or primary.
Sec.2 B
102 When using composite materials the structural categories (special,
primary and secondary) as defined in B101 are equivalent to safety
class high, normal and low in DNV-OS-C501 Sec.2.
Fig. 1 Principles of the extent of structural categorisation special
and inspection categories at tendon foundation
* Special if damaged
condition is not fulfilled.
Fig. 2 Example of tendon connections
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 Examples of considerations with respect to structural categorisation
of tendons and tendon interfaces are given in the Figure 1 and Figure
2. These examples provide minimum requirements.
Sec.2 C
103 Material selection is defined in DNV-OS-C101.Further detailed information about material designation is
defined in DNV-OS-B101.
Sec.2 C
104 Composite materials shall be designed in accordance with DNV-OS-C501.
Sec.2
C 200 Design temperatures
Sec.2 C
201 For TLPs, materials in structures above the lowest astronomical
tide (LAT) shall be designed for service temperatures lower or equal
to the lowest daily mean temperature in air for the area(s) where
the unit is to operate.
Sec.2 C
202 Materials in structures below the LAT are normally to be designed
for service temperatures of 0°C. A higher service temperature may
be used if adequate supporting data shows relative to the lowest
daily mean temperature applicable to the relevant actual water depths.Sec.2
D. Fabrication Inspection Categories
Sec.2
D 100 General
Sec.2 D
101 Welding and the extent of non-destructive testing (NDT) during
fabrication, shall in general 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
provide requirements for the minimum extent of required inspection.
When considering the consequences during in-service operation, it may
be necessary to specify more demanding inspection requirements than
the required minimum. Examples are in way of complex connections
with limited or difficult access, or special material/process
without proven characteristics.
Sec.2 D
103 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. |
| — | materials and criticality of location |
| — | types of NDT used |
| — | first time welds or repair welds. |
Sec.2 D
104 The Figure 1 and Figure 2 shows examples of structural
categorisation and inspection category (IC).
Sec.2 D
105 Inspection of composite components is described in DNV-OS-C501
Sec.12 B. Quality aspects regarding fabrication are described in
DNV-OS-C501 Sec.11.
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