phase 4 report - DNV

RN02: DESIGN OF FLOATING STRUCTURES IN ICE // PART 4 ANNEX ABarents 2020The effective buoyancy is given by Equation (A.8-51):(A.8-51)whereeρ wρ iis the keel porosity;is the water density;is the ice density.Guidance for the specification of ridge keel parameters is provided in A.8.2.8.8.Alternative equations can be derived to obtain the unconsolidated keel action component, F k, on a slopingstructure. Measurements indicate that the keel cohesion varies from zero at the base of the keel to a maximumimmediately beneath the consolidated layer. An average value over the keel depth is appropriate for use inEquation (A.8-49).The application point for the action of a first-year ridge keel can be assumed to be at one third of the keel depthbelow the base of the consolidated layer.To calculate the keel action on a multi-leg structure, the sum of the keel action from each individual leg should bechecked against the action on the effective width of the structure and the lower action selected. In addition, thevertical action of ice rubble should be considered if the ice acts against a submerged portion of the structure.Equations (A.8-49) to (A.8-51) represent a limit stress approach for actions due to ice ridges. Other failuremodes, such as the ridge building process, plug shear failure and out-of-plane ridge failure (see A.8.2.4.2), canlimit the design action. The plug failure case tends to occur when the ice blocks are loose or when the cohesion isuniformly distributed in the vertical direction. The model described in References [A.8-15] and [A.8-23] can beused in such conditions.For floating structures with a conical shape (also bow shape) in the water line passive failure of the ice rubbletakes account of surcharge. In this case, h kin Equation (A.8-49) is substituted by h eff;h eff= h k+ S·w ein which the term h effrefers to the effective keel depth which is a sum of the keel draft, h k, and the surcharge, S,applied over the effective width of the structure, w e, at the water depth where surcharge of the keel takes place.In most of the calculations for realistic ridge widths, a surcharge factor of 0,1 is typically sufficient which wouldrelate to a 10% increase of effective keel draft due to surcharge. Model tests can be consulted in the detail designphase to assess the surcharge on the actual structure. In addition, numerical methods can be used in addition toice model tests to analyse shape/width and friction effects.Report no 2012-0690 89