phase 4 report - DNV

RN02: DESIGN OF FLOATING STRUCTURES IN ICE // PART 2 GAP Analysis based on ISO/FDIS 19906Barents 20206.5 Sea ice and icebergsEstimating statistical distributions for sea ice and icebergsbased on fairly limited databases should be addressed.6.5.4 Ice movementFor floaters, in particular ship-shaped, local ice movements(variable ice drift) is of great interest7.1.5 Limit statesabnormal limit states (ALS) Difference to other standards ISO 19-series defining ALSas accidental limit states not clear. Why not using sameterminology?Table 7-4 ULS and ALS action factors and action combinationsAs an alternative to Table 7-4, the partial action factors may be derivedfrom a calibration analysis using a full probabilistic description of actionsif this analysis demonstrates that the reliability targets of A.7.2.4 areachieved- EL load factor of 1.35 has been “calibrated” based onthe load distribution that approximates floater by a fixeddownward breaking slope. Dynamic interaction and icevaning with associated coupling between actions andaction effect (could be highly non-linear) has been ignored.- Unclear if a different factors shall be used if e.g.representative load action includes altering ice regime.8 Action and action effects8.1 General; Guidance on actions and action effects is found in […] and19904-1.[…] Structures or components subjected to ice interaction events…[…] there is a reasonable probability ofInteraction…Guidance on action effects of floaters are covered elsewhereand not part of ISO19906. This statement undermines theapparent influence of the hull response to ice loading.Suggest further elaborating on this for floater (mooring,risers, umbilicals, hull appendages)Better be defined, otherwise it could lead to a verysubjective interpretation8.2.1 General principles for calculating ice actionsDirect ice actions and actions arising from the interaction betweenthe ice and the structure shall be considered for both global and localconsiderations. Such actions can include; a) static, quasi-static, cyclicand dynamic actions (EL and AL); b) cyclic and dynamic actions that cancause structural fatigue, liquefaction and personnel discomfort; and c)spatial actions such as rubbling, pile-up, ride-up and similar ice behaviourthat can hinder operations.Both static and dynamic actions of floaters are included. Forice actions on floaters reference is made to clause 13.Further requirements relating to ice actions are found in 10.3 for manmadeislands, in 13.4 for floating structures and in 14.3 for subseastructures.8.2.2 Representative values of ice actionsRepresentative values for ice actions shall be calculated usingprobabilistic methods or deterministic methods for ELIE and ALIE.- … or using a combination thereof (i.e. application of semiprobabilisticmethods need to be addressed)- For some locations in the Barents Sea (where floaterswill be used), special care shall be given to deterministicmethods for calculating “worst conceivable” based on thelittle data at hand.Provide recommendations with regard to the dataextrapolation of sea ice and icebergs (from fairlylimited statistics) in the Barents Sea.More specific guidance shall be provided as to howand what to do with regard to characterising the icedrift on the site.Including a note addressing particulars of abnormallimit state opposed to accidental limit state.A recipe of how partial load factors may be estimatedfor floaters shall be developed and included.Address ice load dependence on structural response.Add a note that A.8.2.2 pertains to load models forfixed structures and that the need to be modified foruse in relation to floating structures.Add a note that coupled analyses shall be performedfor assessing the effect of ice loading on mooring,risers, umbilicals, etc.No recommendationPractical use of probabilistic methods shall beoutlined, particularly for the case of limited availabledata to perform a fully probabilistic design.Report no 2012-0690 25