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DNV-OS-C102 Structural Design of Offshore Ships
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SECTION 4
Ships Pt.3 Sec.4
A. Introduction
Sec.4
A 100 General
Sec.4 A
101 The requirements in this section define and specify load components
to be considered in the overall strength analysis as well as design
pressures applicable for local scantling design.
Sec.4 A
102 Design load criteria given by operational requirements shall
be fully considered. Examples of such requirements may be:
| — | drilling, production, workover
and combinations thereof |
| — | consumable re-supply procedures and frequency |
| — | maintenance procedures and frequency |
| — | possible load changes in most severe environmental conditions. |
Sec.4
B. Static Loads
Sec.4
B 100 General
Sec.4 B
101 The still water loads consist of the permanent and variable
functional loads.
Sec.4 B
102 Permanent functional loads relevant for offshore units are:| — | mass of the steel of the unit
including permanently installed modules and equipment, such as accommodation,
helicopter deck, cranes, drilling equipment, flare and production
equipment. |
| — | mass of mooring lines and risers. |
Sec.4 B
103 Variable functional loads are loads that may vary in magnitude,
position and direction during the period under consideration.
Sec.4 B
104 Typical variable functional loads are:| — | hydrostatic pressures resulting
from buoyancy |
| — | crude oil |
| — | ballast water |
| — | fuel oil |
| — | consumables |
| — | personnel |
| — | general cargo |
| — | riser tension. |
| — | mooring forces |
| — | mud, brine and drill water. |
Sec.4 B
105 The variable functional loads utilised in structural design
shall normally be taken as either the lower or upper design value,
whichever gives the more unfavourable effect.
Sec.4 B
106 Variations in operational mass distributions (including variations
in tank filling conditions) shall be adequately accounted for in
the structural design.Sec.4
B 200 Still water hull girder loads
Sec.4 B
201 All relevant still water load conditions shall be defined
and permissible limit curves for hull girder bending moments and
shear forces shall be established for transit and operating condition
separately.
Sec.4 B
202 The permissible limits for hull girder still water bending
moments and hull girder still water shear forces shall be given
at least at each transverse bulkhead position and be included in
the loading manual. Separate limits will normally be given for sagging
and hogging bending moments, and positive and negative shear forces.
Sec.4 B
203 Actual still water shear forces shall be corrected for structural
arrangement according to the procedures given in the DNV Rules for
Classification of Ships Pt.3 Ch.1 Sec.5 D.
Sec.4 B
204 The shape of the limit curves for the still water bending
moments and shear forces are defined in the DNV Rules for Classification
of Ships Pt.3 Ch.1 Sec.5. The permissible limit curve is to envelop
the main class minimum values and actual still water bending moments
and shear forces calculated for transit and operating conditions.
The permissible limit curve shall be applied to all relevant load
combinations.Sec.4
C. Environmental Loads
Sec.4
C 100 General
Sec.4 C
101 Environmental loads are loads caused by environmental phenomena.
Environmental loads which may contribute to structural damages shall
be considered. Consideration should be given to responses resulting
from the following listed environmental loads:| — | wave induced loads |
| — | wind loads |
| — | current loads |
| — | snow and ice loads, when relevant |
| — | green sea on deck |
| — | sloshing in tanks |
| — | slamming (e.g. on bow and bottom in fore and aft ship) |
| — | vortex induced vibrations (e.g. resulting from wind
loads on structural elements in a flare tower). |
Sec.4
C 200 Wave induced loads
Sec.4 C
201 If wave induced loads are not based on the DNV Rules for Classification
of Ships Pt.3 Ch.1 Sec.4 and Sec.5, the wave induced loads shall
be calculated by the hydrodynamic wave load analysis using three dimensional
sink source (diffraction) formulation.
Sec.4 C
202 The wave loads shall be determined for the site specific environment
in which the unit is intended to operate, see DNV-RP-C205 for environmental
data.
Sec.4 C
203 The following wave induced responses shall be calculated:| — | motions in six degrees of freedom |
| — | vertical wave induced bending moment at a sufficient
number of positions along the hull. The positions shall include
the areas where the maximum vertical bending moment and shear force
occur and at the turret position. The vertical wave induced bending
moment shall be calculated with respect to the section's neutral
axis |
| — | horizontal bending moment |
| — | accelerations |
| — | axial forces |
| — | external sea pressure distribution. |
Sec.4 C
204 The wave induced bending moments and shear forces may be calculated
considering the weather vaning characteristics of the unit, see Sec.3 B500.
Sec.4 C
205 Torsional moments may normally be disregarded, unless found
relevant.Sec.4
C 300 Wind loads
Sec.4 C
301 Wind loads shall be accounted for in the design of topside
structures subject to significant wind exposure, e.g. flare tower,
derrick, modules, etc. The reference mean wind load is often given
as 1 hour period at 10m above sea level, but for the calculations
the mean wind speed over 1 minute period at actual position above
the sea level shall be used.
Sec.4 C
302 The calculation procedure of wind loads may be found in DNV-RP-C205.
The pressure acting on vertical external bulkheads exposed to wind
shall not be less than 2.5 kN/m2,
unless otherwise documented.
Sec.4 C
303 For slender structures, 3s duration shall be used. See DNV-RP-C205
for details.
Sec.4 C
304 The wind velocity for transit, operating
and survival condition should normally be not less than the following,
unless otherwise documented:| — | Transit condition: 36 m/s (1 hour period
at 10m above sea level) |
| — | Operating and survival condition
(at site specific): |
Guidance note:
Some typical 1 hour mean wind speeds with a return period
of 100 year at different locations:Sec.4 C
| Norwegian sea | Haltenbanken | 37.0 m/s |
| North
Sea | Troll field | 40.5 m/s |
| Greater Ekofisk area | 34.0 m/s |
| Mediterranean | Libya | 25.3 m/s |
| Egypt | 25.1 m/s |
| Gulf
of Mexico | Hurricane | 48.0 m/s |
| Winter storm | 23.9 m/s |
| West Africa | Nigeria | 16.0 m/s |
| Gabon | 16.6 m/s |
| Gabon (squall) | 24.1 m/s |
| Ivory Coast | 16.0 m/s |
| Ivory Coast (squall) | 29.5 m/s |
| Angola (squall) | 21.8 m/s |
| South America | Brazil (Campos basin) | 35.0 m/s |
| Timor
Sea | Non typhoon | 16.6 m/s |
| Typhoon | 23.2 m/s |
| South
China Sea | Non typhoon | 28.6 m/s |
| Typhoon | 56.3 m/s |
Additional information can be found in ISO19901.1 and API
RP95F.
1 minute mean wind speed is approximately = 1.25 ´ 1
hour mean wind speed---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4
C 400 Green sea
Sec.4 C
401 The green sea is the overtopping by sea in severe wave conditions.
The forward part of the deck and areas aft of midship will be particularly
exposed to green sea. Short wave periods are normally the most critical.
Sec.4 C
402 Appropriate measures should be considered to avoid or minimise
the green sea effects on the hull structure, accommodation, deckhouses,
topside modules and equipment. These measures include bow shape design,
bow flare, bulwarks and other protective structure. Adequate drainage
arrangements shall be provided.
Sec.4 C
403 Structural members exposed to green sea shall be designed
to withstand the induced loads. Green sea loads are considered as
local loads.
Sec.4 C
404 When lacking more exact information, e.g. from model testing,
green sea loads specified in unit specific provisions Sec.11 and Sec.12 shall
be used.
Sec.4 C
405 Shadow effects from either green water protection panel or
other structure may be accounted for.Sec.4
C 500 Sloshing loads in tanks
Sec.4 C
501 In partly filled tanks sloshing occurs when the natural periods
of the tank fluid is close to the periods of the motions of the
unit. Factors governing the occurrence of sloshing are:¾ tank dimensions
¾ tank filling level
¾ structural arrangements inside the tank (wash bulkheads,
web frames etc.)
¾ transverse and longitudinal metacentric height
(GM)
¾ draught
¾ natural periods of unit and cargo in roll (transverse)
and pitch (longitudinal) modes.
Sec.4 C
502 The pressures generated by sloshing of the cargo or ballast
liquid and the acceptance criteria shall comply with the requirements
given in the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.4
C300.
Sec.4
C 600 Bottom slamming
Sec.4 C
601 When lacking more exact information, e.g. from model testing,
relevant requirements to strengthening against bottom slamming in
the bow region are given in the DNV Rules for Classification of
Ships Pt.3 Ch.1 Sec.6 H.
Sec.4 C
602 The bottom aft of the unit shall be strengthened against stern
slamming according to DNV Rules for Classification of Ships Pt.3
Ch.1 Sec.7 E200.Sec.4
C 700 Bow impact
Sec.4 C
701 The bow region is normally to be taken as the region forward
of a position 0.1 L aft of F.P. and above the summer load waterline.
The design of the bow structure exposed to impact loads shall be
carried out according to DNV Rules for Classification of Ships Pt.3
Ch.1 Sec.7 E.
Sec.4 C
702 The speed V in knots used in the formulas shall not be less
than 8.0.
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