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App.B: Methods and Models for Design of Column-Stabilised Units [Table of Contents] App.D: Certification of Tendon System

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

[-] App.C: Permanently Installed Units

APPENDIX C
Permanently Installed Units

App.C
A. Introduction

App.C
A 100   Application

App.C A
101   The requirements and guidance given in this Appendix are supplementary requirements for units that are intended to stay on location for prolonged periods, normally more than 5 years, see also DNV-OSS-101 and DNV-OSS-102 for requirements related to in-service inspections.

App.C A
102   Permanently located units shall be designed for site specific environmental criteria for the area(s) the unit will be located.

App.C
B. Inspection and Maintenance

App.C
B 100   Facilities for inspection on location

App.C B
101   Inspections may be carried out on location based on approved procedures outlined in a maintenance system and inspection arrangement, without interrupting the function of the unit. The following matters should be taken into consideration to be able to carry out condition monitoring on location:
—arrangement for underwater inspection of hull, propellers, thrusters and openings affecting the unit's seaworthiness
—means of blanking of all openings
—marking of the underwater hull
—use of corrosion resistant materials for propeller
—accessibility of all tanks and spaces for inspection
—corrosion protection
—maintenance and inspection of thrusters
—ability to gas free and ventilate tanks
—provisions to ensure that all tank inlets are secured during inspection
—testing facilities of all important machinery.

App.C
C. Fatigue

App.C
C 100   Design fatigue factors

App.C C
101   Design Fatigue Factors (DFF) are introduced as fatigue safety factors. DFF shall be applied to structural elements according to the principles in Sec.7. See also Fig.1.

raster

Fig. 1   Example illustrating considerations relevant for selection of DFF in a typical section


App.C C
102   Fatigue safety factors applied to the unit will be dependent on the accessibility for inspection and repair with special considerations in the splash zone, see 200.

App.C C
103   When defining the appropriate DFF for a specific fatigue sensitive detail, consideration shall be given to the following as applicable:
—evaluation of likely crack propagation paths (including direction and growth rate related to the inspection interval), may indicate the use of a higher DFF, such that:
—where the likely crack propagation indicates that a fatigue failure affect another detail with a higher design fatigue factor
—where the likely crack propagation is from a location satisfying the requirement for a given 'Access for inspection and repair' category to a structural element having another access categorisation.

App.C
C 200   Splash zone for floating units

App.C C
201   For fatigue evaluation of floating units, reference to the draught that is intended to be utilised during condition monitoring, shall be given as basis for the selection of DFF.

App.C C
202   If significant adjustment in draught of the unit is possible to provide satisfactory access with respect to inspection, maintenance and repair, account may be taken of this possibility in the determination of the DFF. In such cases, a sufficient margin in respect to the minimum inspection draught should be considered when deciding upon the appropriate DFF in relation to the criteria for 'Below splash zone' as opposed to 'Above splash zone'. Where draught adjustment possibilities exist, a reduced extent of splash zone may be applicable.

App.C C
203   Requirements related to vertical extent of splash zone are given in Sec.10 B200.
App.B: Methods and Models for Design of Column-Stabilised Units [Table of Contents] App.D: Certification of Tendon System