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

App.B: Methods and Models for Design of Column-Stabilised Units

APPENDIX B
Methods and Models for Design of Column-Stabilised Units

App.B
A. Methods and Models

App.B
A 100   General

App.B
A 200   World wide operation

App.B
A 300   Benign waters or restricted areas

App.B A

Table B1 Methods and models which should be used for design of typical column-stabilised units
		Two-pontoon semisubmersible			Ring-pontoon semisubmersible
		Hydrodynamic model, Morison	Global structural strength model	Fatigue method	Hydrodynamic model, Morison	Global structural strength model	Fatigue method
Harsh environment, restricted areas or world wide	X	1	4	6	1	5	7
	Y	1	4	7	1	5	7
Benign areas	X	2	3	6	1	5	7
	Y	1	4	6	1	5	7
Definitions X-unit following normal class survey intervals (survey in sheltered waters or drydock every 4 to 5 years). Y-unit located for a longer period on location - surveys carried out in-water at location. Hydrodynamic models 1) Hybrid model - Sink-source and/or Morison (when relevant, for calculation of drag forces). 2) Morison model with contingency factor 1.3 for strength and 1.1 for fatigue. Global structural models 3) Beam model. 4) Combined beam and shell model. The extent of the beam and shell models may vary depending on the design. For typical beam structures a beam model alone may be acceptable. 5) Complete shell model. Fatigue method 6) Simplified fatigue analysis. 7) Stochastic fatigue analysis, based on a screening process with simplified approach to identify critical details. Harsh environment, restricted areas or world wide ¾ Units (X) designed for operation based on world wide requirements given in DNV-RP-C205. ¾ Units (Y) designed for operation based on site specific requirements. Benign waters ¾ Units (X) designed for operation based on site specific criteria for benign waters. ¾ Units (Y) designed for operation based on site specific criteria for benign waters.

Guidance note:
Benign area:

Simplifications with respect to modelling procedures required for design documentation may be accepted for units intended for operations in benign areas, where the environmental design conditions dominate for the design of the unit, are less strict than for world-wide operation.

Units operating in benign areas are less dominated by environmental loads. Therefore the load condition b) and the fatigue capacity for standard performed detail are of minor importance for the design, and simplifications as described in the table above may be accepted.

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