FSI & nCATS
Influence of Ultimate Strength on Aged and Corroded ships
Yikun Wang1,2, Dr Julian Wharton1 and Prof Ajit Shenoi2
1National Centre for Advanced Tribology; 2Fluid Structure Interactions Research Group
Faculty of Engineering and the Environment
yw5g10@soton.ac.uk
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 nonlinearities will be considered.
•Also, numerical and experimental studies of the corrosion properties of shipbuilding steels will be included.
Figure 1: Typical corrosion in ballast tank
(Anderson, 2003)
Corrosion mechanisms
Ultimate strength
•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 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 2:
Moisture effects
on corrosion rate
(Gardiner &
Melchers, 2001)
Figure 4:
A typical
example of
FEA model for
pitting
corrosion
(Huang et al.,
2010)
Conclusions and Further Work
• 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.
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), 37-48.
Anderson C. (2003) "Protection against corrosion." University of Newcastle upon Tyne, Lecture Notes,
(http://www.ncl.ac.uk/marine/assets/docs/NclUni_Lect1_1103.pdf).
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 nonlinear corrosion and compressive loads." Marine Structures 12(6): 425-445.
Figure 3:
Relative depths
effects on
corrosion rate
(Gardiner &
Melchers,
2001)
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: 1503-1512.
Loseth, R., Sekkesater, G. & Valsgard, S. (1994). "Economics of high-tensile steel in ship hulls." Marine Structures
7: 31-50.
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: 57-78.
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): 837-860.
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): 155-160.
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), 219-238.
Timoshenko, S.P. (1953). History of Strength of Materials, McGraw-Hill Book Company, New York.
Acknowledgement: This project is supported by
Dr Julian Wharton, Prof Ajit Shenoi and a University of Southampton PGR Scholarship.
FSI Away Day 2012