Influence of Ultimate Strength on Aged and Corroded ships Yikun Wang 1,2, Dr Julian Wharton 1 and Prof Ajit Shenoi 2 1 National Centre for Advanced Tribology; 2 Fluid Structure Interactions Research Group Faculty of Engineering and the Environment FSI & nCATS Acknowledgement: This project is supported by Dr Julian Wharton, Prof Ajit Shenoi and a University of Southampton PGR Scholarship. FSI Away Day 2012 Figure 2: Moisture effects on corrosion rate (Gardiner & Melchers, 2001) Background Over the past few decades, around 90% of aged ship losses have been attributed to structural degradation due to corrosion (Emi et al., 1991). A typical corrosion situation in ballast tank is shown in Fig 1 (Anderson, 2003). A Finite Element Analysis conducted by Sensharma et al. (2006) shows that the critical bulking load can be reduced by about 300 kN if corroded features are applied to a stiffened panel. Thus, to inform maintenance decisions, and to make structure life extension decisions economically, an investigation into the corrosion effects on the ultimate strength of such aged and corroded ship structures is therefore required. Aims This study will: Investigate the ultimate strength of ships in aged and corroded conditions by constructing a strength model. The strength modelling will be based on limit state, non-linear finite element method; geometric and material non- linearities will be considered. Also, numerical and experimental studies of the corrosion properties of shipbuilding steels will be included. References: Akpan, U.O., Koko, T.S., Ayyub, B. & Dunbar, T.E. (2003). "Reliability Assessment of Corroding Ship Hull Structure." Naval Engineers Journal, 115(4), Anderson C. (2003) "Protection against corrosion." University of Newcastle upon Tyne, Lecture Notes, ( Emi, H., Kumano, A., Baba, N., Ito, T. & Nakamura, Y. (1991). "A study on hull structures for ageing ships - A basic study on life assessment of ships and offshore structures. " Gardiner, C.P. & Melchers, R.E. (2001). "Bulk carrier corrosion modelling." Proceedings of the Eleventh International Offshore and Polar Engineering Conference, Stavanger, Norway. Guedes Soares, C. & Garbatov, Y. (1999). "Reliability of maintained, corrosion protected plates subjected to non- linear corrosion and compressive loads." Marine Structures 12(6): Huang, Y., Zhang, Y., Liu, G. & Zhang, Q. (2010). "Ultimate strength assessment of hull structural plate with pitting corrosion damnification under biaxial compression." Ocean Engineering, 37: Loseth, R., Sekkesater, G. & Valsgard, S. (1994). "Economics of high-tensile steel in ship hulls." Marine Structures 7: Paik, J.K. & Thayamballi, A.K. (2002). "Ultimate strength of ageing ships." Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment 216: Paik, J.K., Kim, S.K. & Lee, S.K. (1998). "Probabilistic corrosion rate estimation model for longitudinal strength members of bulk carriers." Ocean Engineering 25(10): Qin, S.P. & Cui, W.C. (2002). "A discussion of the ultimate strength of ageing ships, with particular reference to the corrosion model." Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment 216(2): Sensharma P.K., Willis, M., Dinovitzer, A. & Nappi, N. (2006). "Design Guidelines for Doubler Plate Repairs of Ship Structures." Journal of Ship Production, 22(4), Timoshenko, S.P. (1953). History of Strength of Materials, McGraw-Hill Book Company, New York. Conclusions and Further Work Corrosion mechanisms The corrosion process is time-variant and a corrosion rate with a unit of mm/year was introduced to define the amount of corrosion damage. Corrosion can be categorised into 3 types: General corrosion, Localised corrosion and Fatigue cracks due to localised corrosion. The main corrosion protection systems (CPSs) for ships are polymers coatings and cathodic protection (Paik and Thayamballi, 2002). The corrosion process is extremely comprehensive because it is affected by numerous factors, most of which cannot be controlled. Extensive work has been done to study the corrosion behaviour of shipping steels and develop models that simulate the corrosion rate more realistically. Apart from estimating the mean corrosion rate and its COV for different structural members and types of vessels (Loseth et al., 1994; Gardiner & Melchers, 2001; etc.), either Weibull function (Qin & Cui, 2002; Paik et al., 1998) or linear model (Gardiner & Melchers, 1999) is used as a corrosion model to fit the corrosion rate data obtained from real ships. Fig 2 and 3 are two examples of corrosion rate affected by moisture and locations (Gardiner & Melchers, 2001). Corrosion is often divided into 3 key stages: 1.The durability of coating; 2.The transition; 3.The progress of corrosion Almost all studies assumed the time of transition is zero. During stage 3, Qin & Cui (2002) believed that the corrosion rate is at its highest at the beginning, while Paik and Thayamballi (2002) suggested that the rate either accelerates or decelerates under different conditions. Fatigue cracks due to corrosion have not yet been considered. It is unclear what effect the orientation of each plate has on the corrosion rate. Overall, little or no maintenance has been assumed in the corrosion models. The quality of the corrosion modelling is largely dependent upon the quality of the actual corrosion data. A method still needs to be found for applying the time dependent corrosion model to the ultimate strength FEA model. Corrosion experiments and measurements will be performed to gain greater insight for the corrosion modelling. Figure 3: Relative depths effects on corrosion rate (Gardiner & Melchers, 2001) Ultimate strength The ultimate strength of structural members of ships has been investigated since 1953 (Timoshenko, 1953). A nonlinear finite element strength model will be constructed by considering material, geometric nonlinearities and initial imperfections. This model will be applied to simulate the progressive collapse behaviour of structural members of aged and corroded ships. Fig 4 gives an example of FEA modelling of pitting corrosion (Huang et al., 2010). Still water and wave induced loading will be applied. For simplicity, it can be assumed that the bending moments are independent of time (Akpan et al. 2003). Reference can be made to the midship section and the ship hull is considered to behave globally as a beam for both short-term and long-term conditions. By applying the corrosion process from the corrosion model to the strength model, the influence of ultimate strength can be obtained, and hence the structural performance. Figure 1: Typical corrosion in ballast tank (Anderson, 2003) Figure 4: A typical example of FEA model for pitting corrosion (Huang et al., 2010)