Structural topology optimization of an active motion compensated gangway

Abstract

In the offshore industry, there is a growing demand for designing efficient, sustainable and competitive products. In order to fulfil the component requirements, a method named topology optimization can be applied. This is a mathematical design method which can be used in the early phases of the design process.At IHC there is an interest of applying topology optimization for their equipment development process. Therefore the possibilities and limitations of the method should be investigated thoroughly.

The research covers the optimization process of a motion compensated gangway. A motion compensated gangway is a walkway which can be used to provide access from the transport vessel to the offshore structure. Its function is to transport people and cargo safely from the ship to the offshore structure or vice
versa. The goal of this research was to determine to what extent topology optimization can be used in the design of a motion compensated gangway. Finding an optimized result in terms of weight and stiffness by using this mathematical method which satisfies all the requirements. The structural optimization is carried out with several commercial software packages which are compared by using a multi-criteria analysis.

During the optimization process it has been found that there are essentially two stages in the optimization process. In the first stage, the topology or beam orientation of the structure is defined by the topology optimization process. In this part the concept of the design is generated. Variation of the optimization parameters was used in order to develop an efficient structure. The objective for the optimizer was to minimize the compliance of the structure for a certain volume fraction.

In the second stage, the dimensions of all the beams and elements are defined by performing a size optimization. A line model is generated which represents the orientation of the members in the structure. During the size optimization the shape and the dimensions of the members are defined in order to fulfill the
objective. The objective is to minimize the mass of the structure while constraints are defined for the maximum allowable stresses in the members and the maximum vertical deflection of the structure. This post-processing step is required in order to obtain a feasible design. The structural stability of the gangway was improved by performing a linear buckling analysis and by adapting the structure in order to reduce the buckling behaviour.

In the final step of the optimization process, a CAD drawing is generated. This model is analysed by performing a finite element analysis. This showed that the new optimized design satisfies all the requirements which are stated by the DNV for designing a motion compensated gangway. The combination of the topology and size optimization resulted in a new design which yielded a weight reduction of 36,4% compared to the current design. The weight was reduced from 13,08 ton to 8,31 ton, while still satisfying all the constraints. Therefore it can be concluded that this methodology can be used in the optimization of a motion compensated gangway structure