Review of IACS Unified Requirements for Design of Polar Ships against Ice Loads

Abstract

This master thesis is a review of the IACS Unified Requirements for Design of Polar Ships against Ice Loads. The transversely stiffened plate requirements in IACS URI are based on a roof-top yield line mechanism where the rectangular yield lines corresponds to the load patch boundaries. Results shows that this is a non-conservative assumption, and plates with a high frame span to load patch height ratio are implicitly allowed to develop more permanent deformations. The IACS URI design capacity is compared to the plate capacity formulation developed by Nyseth et. al.\cite{Nyseth}. Nyseth's formulation shows more consistent levels of permanent deformations for different plate and load patch geometries. The plate - load patch geometries within the validity of the assumed yield line pattern in Nyseth's formulation creates an upper bound of implicitly allowed permanent sets. Based on the results from analyses, this thesis recommends the Nyseth plate capacity formulation for utilisation in requirements.The requirements for transverse frames are derived from three different failure modes; a three hinge collapse mechanism, a shear hinge collapse mechanism and a symmetric shear failure. Results showed that there are some more conservatism in the shear hinge collapse mechanism than in the three hinge mechanism. In general, the frame design capacity corresponded well with the design limit of severe loss of stiffness.A Matlab program was developed to introduce moving pressure distributions into Abaqus. The pressure distribution is applied as a load patch to any plane plate in the Abaqus model. Assumed pressure distributions measured during deployments with USCGC Polar Sea were produced and implemented to a PC7 flat bar grillage. Static, quasi-static and dynamic analyses were performed.Static scaling of the worst pressure distribution, to the same strain level as with IACS URI design pressure, resulted in a similar pressure level. The nominal force from the worst pressure distribution was lower. This because, the smaller contact area distributed the force over fewer frames.Moving the pressure distribution quasi-statically showed indications of a shift in structural damage from the trailing side to the forward side of the load motion. This effect was amplified in dynamic analyses. The tangential velocity of the pressure corresponded with the eigenfrequency of a web-plate oscillating eigenmode. This decreased the structural strength of the frames and increased the magnitude of structural damage in the grillage. This lead to a further shift of structural damage towards the moving direction. Adding strain rate reduced this effect.