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DNV-OS-C201 Structural Design of Offshore Units (WSD method)
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Sec.12
C. Design and Loading Conditions
Sec.12
C 100 General
Sec.12 C
101 The general definitions of design and loading conditions are
given in Secs.2 and 3. whilst
the loading conditions within each design condition are defined
in 102.
Sec.12 C
102 The structure shall be designed to resist relevant loads associated
with conditions that may occur during all stages of the life-cycle
of the unit. The conditions that should be considered are:| — | transit condition(s) |
| — | installation condition |
| — | operating condition(s) |
| — | survival condition |
| — | retrieval condition. |
Sec.12 C
103 Relevant loading conditions for the different design condition
are shown in Table C1.Sec.12 C
| Table C1 Relevant
design and loading conditions |
| Design conditions | Loading conditions | a) | b) | c) | d) | e) | | Transit | X | X | | | X | | Installation | | X | | | | | Operation | X | X | X | X | | | Survival | X | X | | | | | Retrieval | | X | | | | |
Sec.12 C
104 Load cases shall be established for the various design conditions
based on the most unfavourable combinations of functional loads,
environmental loads and/or accidental loads. Analysis should
include built in stresses due to assembly of the structure during
fabrication.
Sec.12 C
105 Limiting environmental and operating conditions (design data)
for the different design conditions shall be specified by the builder.
Sec.12 C
106 Limiting design criteria for going from one design condition
to another shall be clearly established and documented.
Sec.12 C
107 If the unit is intended to be dry docked the footing structure
(i.e. mat or spudcans) shall be suitably strengthened to withstand
such loads.Sec.12
C 200 Transit
Sec.12 C
201 The present standard considers requirements for wet transits.
Requirements in case of dry transit on a heavy lift vessel are considered
to be covered by the warranty authority for the operation.
Sec.12 C
202 Wet transits are characterised as either| — | a field
move requiring no more than a 12-hour voyage to a location
where the unit could be elevated, or to a protected location |
| — | an ocean transit requiring
more than a 12-hour voyage to a location where the unit could be
elevated, or to a protected location. |
Sec.12 C
203 A detailed transportation assessment shall be undertaken for
wet transits. The assessment should include determination of the
limiting environmental criteria, evaluation of intact and damage
stability characteristics, motion response of the global system
and the resulting, induced loads. The occurrence of slamming loads
on the structure and the effects of fatigue during transport phases
shall be evaluated when relevant.Guidance note:
For guidance on global analysis for the transit condition
see Classification Note 31.5, 5.3 and for environmental loading
see DNV-RP-C205.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.12 C
204 The structure shall be analysed for zero forward speed in
analysis of wet transit.
Sec.12 C
205 The legs shall be designed for the static and inertia forces
resulting from the motions in the most severe environmental transit
conditions, combined with wind forces resulting from the maximum
wind velocity.
Sec.12 C
206 The leg positions for both field moves and ocean moves shall
be assessed when considering structural strength for transit condition.
Sec.12 C
207 In lieu of a more accurate analysis, for the ocean transit
condition the legs shall be designed for the following forces considered
to act simultaneously:| — | 120% of the acceleration
forces caused by the roll and pitch of the platform |
| — | 120% of the static forces at the maximum amplitude
of roll or pitch |
| — | wind forces from a 45 m/s wind velocity. |
Guidance note:
The effect of heave, surge and sway are implicitly accounted
for by use of the 20% upscaling of the motions.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.12 C
208 For the field move position the legs may be designed for the
acceleration forces caused by a 6° single amplitude roll or pitch
at the natural period of the unit plus 120% of the static
forces at a 6° inclination of the legs unless otherwise verified
by model tests or calculations.
Sec.12 C
209 Dynamic amplification of the acceleration forces on the legs
shall be accounted for if the natural periods of the legs are such
that significant amplification may occur.
Sec.12 C
210 If considered relevant, the effect of vortex shedding induced
vibrations of the legs due to wind shall be taken into account.Guidance note:
For guidance relating to vortex induced oscillations see DNV-RP-C205.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.12 C
211 The hull shall be designed for global mass and sea pressure
loads, local loads and leg loads during transit.
Sec.12 C
212 Satisfactory compartmentation and stability during all floating
operations shall be ensured, see DNV-OS-C301.
Sec.12 C
213 Unless satisfactory documentation exists demonstrating that
shimming is not necessary, relevant leg interfaces (e.g. leg and
upper guide) shall be shimmed in the transit condition.
Sec.12 C
214 All aspects of transportation, including planning and procedures,
preparations, seafastenings and marine operations should comply
with the requirements of the warranty authority.Sec.12
C 300 Installation and retrieval
Sec.12 C
301 Relevant static and dynamic loads during installation and
retrieval shall be accounted for in the design, including consideration
of the maximum environmental conditions expected for the operations
and leg impact on the seabed.Guidance note:
Guidance relating to simplified analytical methodology for
bottom impact on the legs is given in Classification Note 31.5,
5.8.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.12 C
302 The capacity of the unit during pre-loading must be assessed.
The purpose of pre-loading is to develop adequate foundation capacity
to resist the extreme vertical and horizontal loadings. The unit
should be capable of pre-loading to exceed the maximum vertical
soil loadings associated with the worst storm loading.Guidance note:
Guidance relating to pre-loading is given in Classification
Note 30.4, 1 and 8.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.12 C
303 The hull structure shall be analysed to ensure it can withstand
the maximum pre-loading condition.
Sec.12 C
304 The structural strength of the hull, legs and footings during
installation and retrieval shall comply with the strength condition
given in Sec.5.Sec.12
C 400 Operation and survival
Sec.12 C
401 The operation and survival conditions cover the unit in the
hull elevated mode.
Sec.12 C
402 A detailed assessment shall be undertaken which includes determination
of the limiting soils, environmental and weight criteria and the
resulting, induced loads.
Sec.12 C
403 Dynamic structural deflection and stresses due to wave loading
shall be accounted for if the natural periods of the unit are such
that significant dynamic amplification may occur.
Sec.12 C
404 Non-linear amplification (large displacement effects) of the
overall deflections due to second order bending effects of the legs
shall be accounted for whenever significant.
Sec.12 C
405 The effect of leg fabrication tolerances and guiding system
clearances shall be accounted for.
Sec.12 C
406 The leg/soil interaction shall be varied as necessary
within the design specifications to provide maximum stress in the
legs, both at the bottom end and at the jackhouse level.
Sec.12 C
407 Critical aspects to be considered in the elevated condition
are structural strength, overturning stability and air gap.
Sec.12 C
408 The structural strength of the hull, legs and footings during
operation and survival shall comply with the requirements of this
section and Sec.5. The strength
assessment should be carried out for the most limiting conditions
with the maximum storm condition and maximum operating condition
examined as a minimum.Guidance note:
The hull will typically comprise the following elements:| - | decks| - | side and bottom plating| - | longitudinal bulkheads| - | transverse frames| - | longitudinal girders and stringers| - | stringers and web frames on the transverse bulkheads| - | jackhouses. | | | | | | |
---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.12 C
409 The strength of the hull shall be assessed based on the characteristic
load conditions that result in maximum longitudinal tension and
compression stresses (for yield and buckling assessment) in deck
and bottom plating.
Sec.12 C
410 The effect of large openings in the hull (e.g. drill slot)
which affect the distribution of global stresses should be determined
by a finite element model accounting for three-dimensional effects.
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