Ultimate and Residual Strength Assessment of Ship Structures

Licentiate thesis, 2020

The prevention of ship structural failures and reduction of accident consequences contribute to increased safety at sea and reduced environmental impact. A need for reliable and efficient ship designs facilitates knowledge accumulation and the development of tools for the assessment of hull structural responses to acting loads. Important in this regard, ship design criteria are the ultimate and residual strengths of a hull that determine whether a ship may be safely operated in intact and accidentally damaged conditions. Thus, an accurate and reliable procedure for estimating a ship’s strength accounting for all actual foreseeable scenarios and reasonably practicable conditions is necessary.

The main objective of this thesis is to develop a new precise and time-efficient methodology for the assessment of the ultimate and residual strength under vertical and biaxial loading conditions. A coastal oil tanker and a RoPax vessel were chosen for the parametric study of the ship’s structural arrangement, load type and corrosion effect on the crashworthiness and ultimate strength. Collision simulations under varying conditions were carried out by means of the finite element method. The assessment of the ultimate and residual strength was performed with the Smith method together with finite element analyses.

The thesis work contributed to a better understanding of the modelling and analysis setup for the realistic ultimate and residual strength estimates. A new approach for modelling corrosion in ship structures, which includes models for plate thickness reduction and corroded materials, was developed. Different collision damage modelling techniques were compared, and the importance of making full collision simulations, including plastic deformations and residual strains, was shown. The differences and limitations of the finite element and Smith methods were discussed, and improvements to the Smith method along with a new calibration procedure were introduced. The study resulted in a new unified methodology, combining the precision of the finite element method and the efficiency of the Smith method, for the assessment of the ultimate and residual strength of ships.