Influence of Iceskegs on Head-On Ramming Forces
Richard Hayward, DNV GL SE, Hamburg
During the head-on ramming of ice features (a design collision scenario), polar class ships need to maintain sufficient positive stability when riding up onto the ice. Towards this end, bow designs sometimes incorporate an ice skeg to inhibit beaching. In case of ice skegs which are integrated into the run of the bow lines (so-called “continuous” ice skegs), the stopping force is built up gradually. In case of ice skegs which are not integrated into the run of the bow lines (so-called “appendage” ice skegs), the stopping force is built up quickly. In both cases, ice forces at the bow due to head-on ramming are affected. In the longitudinal strength requirements of the IACS polar class ship rules (Unified Requirement I2), these ice forces are defined using energy methods where the initial kinetic energy of the ship is equated to potential energy and crushing energy. Both the potential and crushing energy can be stated in terms of the maximum vertical ice force at the bow which occurs when the ship comes to rest at the end of its penetration into the ice. Using this vertical ice force, global ice bending moments and shear forces are obtained. However, in the IACS polar class ship rules it is assumed that the bow of the ship can be idealised as a (perfect) wedge, i.e. the definition of design ice loads ignores the effects of ice skegs (or assumes that these effects are negligible). For this reason, the numerical model used in the development of these design ice loads is modified to take into account the influence of an “appendage” ice skeg. The original and modified numerical models are firstly described, followed by a comparison of results for a benchmark ship with and without an ice skeg.