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C: Permanent Functional Loads [Table of Contents] E: Environmental Loads

DNV-OS-C201 Structural Design of Offshore Units (WSD method)

[-] Sec.3: Loads and Load Effects
[-] D: Variable Functional Loads

Sec.3
D. Variable Functional Loads

Sec.3
D 100   General

Sec.3 D
101   Variable functional loads are loads which may vary in magnitude, position and direction during the period under consideration, and which are related to operations and normal use of the installation.

Sec.3 D
102   Examples of variable functional loads are:
—personnel
—stored materials, equipment, gas, fluids and fluid pressure
—crane operational loads
—loads from fendering
—loads associated with installation operations
—loads associated with drilling operations
—loads from variable ballast and equipment
—variable cargo inventory for storage vessels
—helicopters
—lifeboats.


Sec.3 D
103   The variable functional load is the maximum (or minimum) specified value, which produces the most unfavourable load effects in the structure and design condition under consideration.

Sec.3 D
104   The specified value shall be determined on the basis of relevant specifications. An expected load history shall be used in fatigue design.

Sec.3
D 200   Variable functional loads on deck areas

Sec.3 D
201   Variable functional loads on deck areas of the topside structure, e.g. hull and superstructures, shall be based on Table D1 unless specified otherwise in the design basis or design brief. The intensity of the distributed loads depends on local and global aspects as shown in Table D1.

The following notations are used:
Local design: e.g. design of plates, stiffeners, beams and brackets 
Primary design: e.g. design of girders and columns 
Global design: e.g. design of deck main structure and substructure 

Sec.3 D
Table D1 Variable functional loads on deck areas 
Area Local design Primary design Global design 
Distributed load, q(kN/m2) Point load, P(kN) Apply factor to
distributed load 		Apply factor to
primary design load 
Storage areas q 1.5 q 1.0 1.0 
Lay down areas q 1.5 q f  f  
Lifeboat platforms 9.0 9.0 1.0 may be ignored 
Area between equipment 5.0 5.0 f  may be ignored 
Walkways, staircases and platforms, crew spaces 4.0 4.0 f  may be ignored 
Walkways and staircases for inspection only 3.0 3.0 f  may be ignored 
Areas not exposed to other functional loads 2.5 2.5 1.0 - 
Notes:
-Wheel loads to be added to distributed loads where relevant. (Wheel loads can normally be considered acting on an area of 300 x 300 mm.)
-Point loads, P, may be applied on an area 100 x 100 mm, and at the most severe position, but not added to wheel loads or distributed loads.
-The distributed loads, q, to be evaluated for each case. Lay down areas should not be designed for less than 15 kN/m2.
-The factor f may be taken as: raster , where A is the loaded area in m2.
-Global load cases should be established based upon "worst case", characteristic load combinations, complying with the limiting global criteria to the structure. For buoyant structures these criteria are established by requirements for the floating position in still water, and intact and damage stability requirements, as documented in the operational manual, considering variable load on the decks and in tanks.
-The right column of the table, i.e. "Global design", presents variable functional loads to be included in load model for the global analysis. In the capacity checks, stresses from the global analysis shall be combined with the effect of local loads, i.e. tank pressures, weight of equipment, etc.
 

Sec.3
D 300   Tank pressures

Sec.3 D
301   The structure shall be designed to resist the maximum hydrostatic pressure of the heaviest filling in tanks that may occur during fabrication, installation and operation.

Sec.3 D
302   Hydrostatic pressures in tanks should normally be based on a minimum density equal to that of seawater, r = 1.025 t/m3. Tanks for higher density fluids (e.g. mud) shall be designed on basis of special consideration. The density, upon which the scantlings of individual tanks are based, shall be given in the operating manual.

Sec.3 D
303   Pressure loads that may occur during emptying of water or oil filled structural parts for condition monitoring; maintenance or repair shall be evaluated.

Sec.3 D
304   Hydrostatic pressure heads shall be based on tank filling arrangement by for example pumping, gravitational effect, accelerations as well as venting arrangements.

Sec.3 D
305   Pumping pressures may be limited by installing appropriate alarms and auto-pump cut-off system, i.e. high level and high-high level with automatic stop of the pumps. In such a situation the pressure head may be taken to be the cut-off pressure head. Descriptions and requirements related to different tank arrangements are given in DNV-OS-D101 Ch.2 Sec.3 C300.

Sec.3 D
306   Dynamic pressure heads due to flow through pipes shall be considered, see 308.

Sec.3 D
307   The internal pressure in full tanks should be defined by the formula:

raster

hop = vertical distance (m) from the load point to the position of maximum filling height. For tanks adjacent to the sea that are situated below the extreme operational or transit draught, the maximum filling height should not be taken lower than the extreme operational draught  
av = maximum vertical acceleration, (m/s2), being the coupled motion response applicable to the tank in question. 
r = density of liquid (t/m3) 
g0 = 9.81 m/s2 



Sec.3 D
308   For tanks where the air-pipe may be filled during filling operations, a special tank filling design condition shall be checked according to loading condition a). The following additional internal design pressure conditions shall be used:

raster

pdyn = pressure (kN/m2) due to flow through pipes, minimum 25 kN/m2 



Sec.3 D
309   In cases where the maximum filling height is less than the height to the top of the air pipe, it shall be ensured that the tank will not be over-pressured during operation and tank testing conditions.

Sec.3 D
310   In a situation where design pressure head might be exceeded, should be considered as an accidental condition.

Sec.3 D
311   Requirements for testing of tank tightness and structural strength are given in DNV-OS-C401 Ch.2 Sec.4.

Sec.3
D 400   Lifeboat platforms

Sec.3 D
401   Lifeboat platforms shall be checked for the strength and accidental design conditions if relevant. A dynamic factor of 0.2 g0 due to retardation of the lifeboats when lowered shall be included in both strength and accidental design conditions.
C: Permanent Functional Loads [Table of Contents] E: Environmental Loads