Icebreaker Grillage Structural Interaction and the Characteristic Stiffness Curve

摘要

The structural components of a typical icebreaker hull grillage section consist of hull plating, main frames, web frames and stringers. The grillage section is the main structure resisting local ice loads during icebreaking and maneuvering operations. As such, the structural integrity of the icebreaker is largely dependent on the design strength of the grillage sections along the length of the vessel. The latest release of the IACS Unified Requirements for Polar Ships specifically pertains to structural design of these local grillage sections. Within the IACS Unified Requirements, prescriptive formulas are used to define the hull plating and main frame strength requirements as a function of stiffening direction, longitudinal/vertical location and operational requirements. The stringer and web frame stability requirements however, are limited to meeting empirical criteria. Limited examples of stringer and web frame prescriptive design strength formulations are available in the literature. These formulations may lead to an overly conservative stringer or web frame section design due to the challenge of representing the grillage section structural component interaction. To properly understand the structural interaction of icebreaker grillage section components, LR has used nonlinear finite element methods to compute the characteristic stiffness curve well into plasticity. The characteristic stiffness curve is considered representative of the effective structural interaction of the section components and has been found to relate directly to the section design methodology (elastic or plastic). This paper presents the development of these stiffness curves; the relationship between stiffness curve characteristics and design methodology; and how stiffness curves may be used for structural design and verification.