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DNV-OS-C103 Structural Design of Column Stabilised Units (LRFD method)

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

A: Methods and Models

App.B
A. Methods and Models

App.B
A 100   General

For further details reference is made to DNV-RP-C103.

App.B A
102   Table A1 gives guidance on methods and models normally applied in the design of typical column-stabilised units. For new designs deviating from well-known designs, e.g. by the slenderness of the structure and the arrangement of the load bearing elements, etc., the relevance of the methods and models should be considered.

App.B
A 200   World wide operation

App.B
A 300   Benign waters or restricted areas

App.B A

Table A1 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 1.1 for ULS and FLS. The contingency factors shall be applied in addition to the relevant load factors. 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 — Units (X) designed for operation based on world wide requirements given in DNV-RP-C205. — Units (Y) designed for operation based on site specific requirements. Benign waters — Units (X) designed for operation based on site specific criteria for benign waters. — 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 i 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 ULS-b condition and 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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