RCID

Ship Ramp Design

Problem

MacGregor is a global supplier of ship’s equipment. They asked the RCID to carry out detailed calculations for new ro-ro ramp sections and associated operating gear for a ferry. The equipment was to replace existing components which have suffered severe fracturing due to operational loads.

The RCID worked in cooperation with the client’s engineers to develop the design, from initial concept to a complete set of design calculations. The work included finite element analysis of the ramps and pillars to be installed on the ship. Hand calculations were also carried out for the operating equipment which included hinges, hydraulic cylinders and safety linkages.

Modelling

Detailed 3D models of the ramp and pillar were created in Pro/Engineer, enabling easy modification of structural details and dimensions. Shell element models were then created using Pro/Mechanica software.

The models were analysed under loadings based on operational requirements. These were from in-service demands including tyre loads from 44 Tonne HGV’s, torsion loads caused by vessel heel and dynamic effects. It was also necessary to consider the passage of the ship in open water, where waves impact against the ramp’s underside.

Analysis

The RCID analyses showed that the highest stresses were concentrated at local areas in the primary stiffeners and tended to be dominated by twist rather than vehicle or wave loads. A fatigue calculation demonstrated adequate life at the stress hot-spots.

A frequency analysis of the ramp “cantilevered-out” showed that the natural frequency would be expected to be well above the vessel’s pitch frequency, indicating little likelihood of excessive vibration with the vessel underway.

The design was approved by the relevant ship’s classification bodies and the steelwork fitted to the vessel during winter 2004. One year later, the guarantee survey has shown the equipment to be robust in service, eliminating all the problems caused by the fractures in the previous structural design.