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Sec.2: cture and elements not covered by Rules for [Table of Contents] Sec.4: Ships Pt.3

DNV-OS-C102 Structural Design of Offshore Ships

[-] Sec.3: Classification of

SECTION 3
Classification of

Sec.3
A. Introduction

Sec.3
A 100   Overall design principles

Sec.3 A
101   This section defines the principles for design of the hull, topside structures and topside support structures.

Sec.3 A
102   The overall principles are based on the following:
—safety of the structure can be demonstrated by addressing the potential structural failure mode(s) when the unit is subjected to loads scenarios encountered during transit, operation and in harbour.
—structural requirements are based on a consistent set of loads that represent typical worst possible loading scenarios
—unit has inherent redundancy. The unit's structure works in a hierarchical manner and as such, failure of structural elements lower down in the hierarchy should not result in immediate consequential failure of elements higher up in the hierarchy
—structural continuity is ensured. The hull, topside structures and topside interface to the hull structure should have uniform ductility.

Sec.3
B. Design Conditions

Sec.3
B 100   Modes of operation

Sec.3 B
101   All relevant modes of operation shall be considered. Typically, the assessment of the unit shall be based on the following operational modes:
—all transit conditions
—all operating conditions, intact and damaged, at the design location(s)
—all inspection and repair conditions.


Sec.3 B
102   Changes in the design conditions of offshore ship-shaped units are usually accompanied by significant changes in draught, ballast, riser connections, mooring line tension, etc. Limited variation of some of these parameters may be contained within a specific design condition.

Sec.3 B
103   The suitability of offshore ship-shaped units is dependent on the environmental conditions in the areas of the intended operation. A well intervention/drilling unit may be intended for World Wide operation or operation in a specific region or site(s). A production unit may be planned to operate at a specific site. Such a site may be harsh environment or benign waters.

Sec.3
B 200   Transit and non-operational conditions

Sec.3 B
201   Unrestricted transit is defined as moving the unit from one geographical location to another, and shall be based on the DNV Rules for Classification of Ships Pt.3 Ch.1.

Sec.3 B
202   Non-operational conditions like e.g. survey condition are considered to be covered by DNV Rules for Classification of Ships Pt.7.

Sec.3 B
203   The design accelerations for the topside structures and topside interface to hull may be taken from either a wave load analysis or DNV Rules for Classification of Ships Pt.3 Ch.1.

Sec.3
B 300   Operating conditions

Sec.3 B
301   Operating conditions are defined as conditions wherein a unit is on location for purposes of production, drilling or other similar operations, and combined environmental and operational loadings are within the appropriate design limits established for such operations.

Sec.3 B
302   The maximum allowable sea state (max Hs) shall be established based on the unit's criteria for aborting the operation.The sea state shall be stated in the structural design brief document.

Sec.3 B
303   The operating condition shall account for the combination of wave effects and wind effects.

Sec.3 B
304   All the operating limitation used for the design and safe operation shall be stated in the operating manual.

Sec.3
B 400   Survival condition

Sec.3 B
401   A condition during which a unit may be subjected to the most severe environmental loadings for which the unit is designed. Drilling or similar operations may have been discontinued due to the severity of the environmental loadings. The unit may be either afloat or supported on the sea bed, as applicable.

Sec.3 B
402   The survival condition shall account for the combination of wave effects and wind effects.

Sec.3
B 500   Extreme condition

Sec.3 B
501   Extreme condition is defined as a condition during which a unit may be subjected to the extreme environmental loadings such as hurricane or typhoon for which the unit is designed. The operation e.g. drilling or production is normally stopped due to the severity of the environmental loadings. The unit may be either afloat or moored to the sea bed, or leave the site under its own propulsion or assistance from tugs, as applicable.

Sec.3 B
502   Extreme scenario is to be established, if applicable. For units with disconnectable mooring system e.g. submerged turret, the environmental limiting conditions for disconnection or reconnection of the mooring system are to be specified.

Sec.3 B
503   If the vessel intends to leave the site and seek for sheltered waters in extreme weather, this actual extreme condition can be omitted from the design condition, i.e. the extreme condition needs not to be considered.

Sec.3
B 600   Wave load analysis

Sec.3 B
601   The wave load analysis may be carried out for operating conditions with specific wave environments at the considered site, and may also be carried out for transit conditions as alternative to the requirements given in the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.5.

Sec.3 B
602   The design loading conditions for ultimate strength shall be based on the units loading manual and shall in addition include part load conditions as relevant for the specific type of unit. For selection of still water load conditions to be used as basis for the wave load analysis (with a return period of 20 or 100 years), the most demanding loading conditions defined in the loading manual shall be determined. The most demanding loading conditions are normally selected as those giving maximum still water bending stresses in longitudinal material in different parts of the unit.

Sec.3 B
603   If short term analysis is carried out for ultimate strength, the combination of significant wave height (Hs) and spectral peak period (Tp) or zero-crossing period (Tz) for all sea states along a contour line shall be considered to determine the short term extreme values.

Sec.3 B
604   The wave heading profile given in Table B1 and B2 should be used for the ultimate strength and fatigue strength respectively, unless otherwise documented.

Sec.3 B
605   The sectional loads are normally calculated at the neutral axis of the section considered.

Sec.3 B
606   The wave shear forces shall be determined at a sufficient number of sections along the hull to fully describe the limit curve for the maximum value.

Sec.3 B
Table B1 Design basis of wave load analysis for ultimate strength 
 Transit Operation Survival 
Wave environment North Atlantic Maximum significant wave height (Hs) and corresponding period (Tp or Tz) to be defined by the project  Site specific 
Return period 1) 20 year  100 year  100 year  
Wave spectrum PM spectrum Specified spectrum 2) Specified spectrum 2) 
Wave heading profile 3) All headings included
(0° - 360°) 		Head sea: 60% 3)
± 15 degrees: 30%
± 30 degrees: 10%  		Head sea: 60% 3)
± 15 degrees: 30%
± 30 degrees: 10%  
Wave spreading  Cos 2 Cos 2 None 
  1. See unit specific provisions Sec.11 and Sec.12
  2. JONSWAP spectrum is normally used, ref. DNV-RP-C205
  3. For weather vanning units. Other heading profile for operation and survival condition may be used, if documented. For units with spread mooring arrangement, all heading with same probability may be considered
 

Sec.3 B
Table B2 Design basis of wave load analysis for fatigue strength 
 Transit Operation 
Wave environment World Wide 
 Site specific 
Probability of exceedance 1) 10-4 10-4 
Wave spectrum PM spectrum PM spectrum 
Wave heading profile All headings included
(0° - 360°) 		Head sea: 60%
± 15 degrees: 30%
± 30 degrees: 10%  
Wave spreading 2) Cos 2 Cos 2 
  1. Recommended value (approximately daily return period)
  2. Cos 2 is to be used, unless otherwise documented.
  3. For weather vanning units. Other heading profile for operation condition may be used, if documented. For units with spread mooring arrangement, all heading with same probability may be considered
 



Sec.3 B
607   Non-linear correction factor shall be included for the survival and the extreme condition when using the wave bending moments and shear forces from a linear wave load analysis. Typical non-linear correction factors for ships with traditional hull shape are given in Table B3 and shall be used, unless otherwise documented.

Sec.3 B
Table B3 Non-linear correction factor for ultimate strength 
Item Sagging Hogging 
Wave bending moment 1.1 0.9 
Wave shear force 1.1 0.9 



Sec.3 B
608   If the unit is intended to stay on a specific location during an extreme condition such as a hurricane, the actual extreme wave height and the extreme wind speed shall be defined by the project. The 100 year condition shall be used.

Sec.3
C. Working Stress Design, WSD

Sec.3
C 100   General

Sec.3 C
101   The WSD principles and the acceptance criteria for the different combinations are applicable to hull and topside structure for direct calculations when the structural requirements are not covered by DNV Rules for Classification of Ships Pt.3 Ch.1, or if direct calculations are used to replace the structural requirements in the Rules for Classification of Ships Pt.3 Ch.1.

Sec.3
C 200   Load combinations

Sec.3 C
201   Each structural member shall be designed for the most unfavourable of the loading conditions given in Table C1.

Sec.3 C
Table C1 Load combinations 
Combination Description Basic usage factor, h0 
a) static loads 0.60 
b) maximum combined static and dynamic loads  0.80 
c) accidental loads and associated static loads  1.00 
d) maximum combined operational static loads and dynamic loads from extreme environmental situations, e.g. hurricane or typhoon  1.00 
Notes:
b)Represent operational and survival conditions.
c)Represent accidental conditions with low probability of occurrence such as explosions, fire, dropped objects etc.
d)Represent for units intended to stay on location during extreme weather condition, e.g. permanently moored, with a 100 year return period. For units intended to escape during extreme weather condition, e.g. not permanently moored or with disconnectable mooring system, this load combination can be omitted.
 

Sec.3
C 300   Permissible usage factor

Sec.3 C
301   The permissible usage factors depend on load combination, failure mode and importance of strength member.

Sec.3 C
302   The usage factor is defined as the ratio between the calculated nominal stress and the corresponding minimum specified yield stress of the material used. The calculated usage factor based on the von Mises equivalent membrane stress at centre of a plane element (shell or membrane) shall not exceed the permissible usage factors defined in 303.

Sec.3 C
303   The permissible usage factor, hp, is calculated by:

hp = b h0

where: 
h0 = basic usage factor, see Table C1 
b == for material yield and buckling checks in general
for specific type of structure, see Sec.6 

The basic usage factor h0 accounts for:
—possible unfavourable deviations of specified or expected loads
—uncertainties in the model and analysis used for determination of load effects
—possible unfavourable deviations in the resistance of materials
—possible reduced resistance of the materials in the structure, as a whole, as compared to the values deduced from test specimens
—deviation from calculated strength resistance due to fabrication.

Sec.3
C 400   Usage factor for peak stress

Sec.3 C
401   The peak usage factor is defined as the ratio between the calculated peak stress and the corresponding minimum specified yield stress of the material used.

Sec.3 C
402   Local peak stress by fine mesh Finite Element (FE) analysis in areas with pronounced geometrical changes, such as moonpool corners, frame corners etc., may exceed the permissible usage factor in C203 provided plastic mechanisms are not developed in the adjacent structural parts.

Sec.3 C
403   Local peak stress criteria are based on the element mesh size of either 50 ´ 50 mm or 100 ´ 100 mm or 200 ´ 200 mm depending on the actual thickness and geometrical complexity of local details to be checked. The calculated usage factor based on the von Mises equivalent membrane stress at centre of a plane element (shell or membrane) shall not exceed the permissible peak usage factors given in Sec.5 Table E1.

Sec.3 C
404   For fatigue critical connections local peaks are not accepted, ref. Sec.5 F.
Sec.2: cture and elements not covered by Rules for [Table of Contents] Sec.4: Ships Pt.3