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A: Introduction [Table of Contents] C: Inspection Principles

DNV-OS-C102 Structural Design of Offshore Ships

[-] Sec.2: cture and elements not covered by Rules for
[-] B: Selection of Material

Sec.2
B. Selection of Material

Sec.2
B 100   General

Sec.2 B
101   A material specification shall be established for all structural materials. The materials shall be suitable for their intended purpose and have adequate properties in all relevant design conditions.

Sec.2 B
102   In structural cross-joints where high tensile stresses are acting perpendicular to the plane of the plate, the plate material shall be tested according to Rules for Classification of Ships Pt.2 Ch.2 Sec.1 to prove the ability to resist lamellar tearing (Z-quality). Continuous deck plate under crane pedestal shall have Z-quality steel with minimum extension of 500mm. If the pedestal is continuous through the deck plate within 0.6L amidship, the pedestal plate shall have Z-quality steel with minimum extension of 500 mm above and below the deck.

Sec.2 B
103   For stiffeners, the grade of material may be determined based on the thickness of the web.

Sec.2 B
104   Structural elements used only in temporary conditions, e.g. fabrication, are not considered in this standard.

Sec.2
B 200   Design and service temperatures

Sec.2 B
201   The design temperature for a unit is the reference temperature in air for assessing areas where the unit can be transported, installed and operated. The design temperature is to be lower or equal to the lowest mean daily average temperature in air for the relevant areas. For seasonal restricted operations the lowest mean daily average temperature in air for the season may be applied for external structures above the lowest ballast waterline shall be set equal to the design temperature for the area(s) in which the unit is specified to operate.

Sec.2 B
202   The service temperature is a reference temperature on various structural parts of the unit used as a criterion for the selection of steel grades.

Sec.2 B
203   The service temperature for external structures above the lowest ballast waterline shall be set equal to the design temperature for the area(s) in which the unit is specified to operate. External structure is defined, with respect to design temperature, as the plating with stiffening to an inwards distance of 0.5 metre from the shell plating.

Sec.2 B
204   The service temperature for external structures below the lowest ballast waterline needs normally not to be set lower than 0°C.

Sec.2 B
205   The service temperature for internal structures in way of permanently heated rooms needs normally not to be set lower than 0°C. Crude oil tanks may be considered to be permanently heated.

Sec.2 B
206   The service temperature for internal structures in oil storage tanks need normally not to be set lower than 0°C except for the upper strake in longitudinal bulkheads and top wing tanks.

Sec.2
B 300   Hull structure

Sec.2 B
301   The material grade shall be selected according to the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.2 if the design temperature is equal or above -10°C based on lowest mean daily average temperature.

Sec.2 B
302   Lower Design Temperatures than -10°C may be specified. The requirements for material class and grade shall then be based on the DNV Rules for Classification of Ships Pt.5 Ch.1 Sec.7.

In the Rules for Classification of Ships Pt.3 Ch.1 materials are categorized into Material Classes. The purpose of the structural categorization is to ensure adequate material and suitable inspection to avoid brittle fracture, and to ensure sufficient fracture resistance of a material (stress intensity factor) to avoid crack sizes which may develop into brittle fracture at certain stress situations.

Sec.2
B 400   Topside structure and elements not covered by Rules for Classification of Ships Pt.3 Ch.1

Sec.2 B
401   Structural members are classified into Material Classes according to the following criteria:
—significance of member in terms of consequence of failure
—stress condition at the considered detail that together with possible weld defects or fatigue cracks may provoke brittle fracture.


Sec.2 B
402   Materials for topside modules, topside supporting structures, foundations and main supporting structures of heavy equipment attached to deck and hull shall be selected according to the principles in B300.

Sec.2 B
403   Materials for the specific structural members are not to be of lower grade than those corresponding to the material classes specified in Table B1.

Sec.2 B
Table B1 Material Classes 
Material class Structural member Equivalent structural category in the DNV-OS-C101 
I 
—Outfitting steel
—Mezzanine decks, platforms
—Pipe support structure
—Laydown platforms
—Doubler plates, closer plates and support infill steels in topside structures 1)
 		Secondary 
II 
—Stair towers
—Topside modules (plate, web frame, girder) 2)
—Main structures in riser hold
—Longitudinal and transverse bulkheads in way of moonpool and setback area
—Offshore Crane boom rest and main supporting structures
 
III 
—Main girders and columns in truss work type modules
—Topside support stools and main supporting structures
—Pipe/riser rack stanchions
—Main structures in drill-floor and substructure
—Main supporting structures for helideck 3)
—Crane pedestal (shipboard and offshore)
—Flare tower
—Foundations and main supporting structures of heavy machinery and equipment: e.g. thruster, gantry and rail, chain stoppers, offloading riser fairleads, anchor line fairleads, winches, davits, hawser brackets for shuttle tanker, towing brackets etc.
 		Primary 
IV 
—Deck and bottom corner plates in way of moonpool
—Foundations and main supporting structures for derrick (substructure)
—Foundations and main supporting structures for flare tower and offshore crane pedestal
—Main supporting structures for turret
 		Special 
  1. To have the same minimum yield strength as the material to which they are attached
  2. Stiffener and other local members (brackets, collar plates, web stiffeners) are classified as Class I.
  3. For material selection of helideck and substructure, see DNV-OS-E401
Note:Main structures and main supporting structures are primary load bearing members such as plates, girders, web frames/bulkheads and pillars
 



Sec.2 B
404   Alternatively materials for the specific structural members specified in Table B1 may be selected based on equivalent structural category in accordance with the principles given in DNV-OS-C101.

A: Introduction [Table of Contents] C: Inspection Principles