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Discussion on Are tankers strong enough for their lifetime mission? (CROSBI ID 493109)

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Parunov, Joško Discussion on Are tankers strong enough for their lifetime mission? // Transactions SNAME 2002 / SNAME (ur.). Singapur : London : München : Ženeva : Tokyo : Hong Kong : Taipei : Peking : Šangaj : Tianjin : Chennai: SNAME, 2002. str. 431-432-x

Podaci o odgovornosti

Parunov, Joško

engleski

Discussion on Are tankers strong enough for their lifetime mission?

I would like to congratulate the author for this very interesting paper that describes a number of most important items concerning the safety of tanker hull. The description of measures that IACS undertakes to improve ship safety including advanced calculation methods, enhanced survey program and hull condition assessment, are particularly interesting. A lot of important informations can be found in the introducing part of the paper giving a historical overview of methods for the strength analysis of tankers. It is well known that ships are designed according to the Rules of Classification Societies. The most important requirement for ship longitudinal strength is still IACS UR S11 &#8211 ; ; Longitudinal Strength Standard that is the basis for calculation of section modulus. Return from experience has shown that such Rules result in new ships with satisfactory structural safety. However, the safety margin that is inherent in such designs is generally unknown [Faulkner, 1998]. This is consequence of the fact that Rules for design of ships that have been used for years are deterministic Rules. Such Rules are based on single safety factor covering all uncertainties of loads and strength variables. Safety factor is calibrated mainly based on past experience. The main drawback of deterministic rules is that designs based on such rules show undesired spread of reliabilities [Mansour et al., 1996]. The situation is far more complicated when corroded ship hulls are analysed, since additional uncertainties due to corrosion, measurement methods and accumulated fatigue are involved. Ship reliability methods are used to quantify the safety margin by calculating failure probability. Reliability methods recognize uncertainties in strength analysis by modelling variables influencing the ship safety as random variables. Reliability analysis implies direct hydrodynamic and statistical analysis to calculate wave loads, statistical description of still water loads, statistical combination of extreme loads as well as definition of actual failure modes. However, it is well known that reliability analysis is not without difficulties. The main problem is the rational description of extreme wave loads, especially the vertical wave bending moment. Direct calculations of long-term distribution of vertical wave bending moments performed by various Classification Societies show important scatter [Nitta et al. 1992]. There are still unanswered questions regarding wave scatter diagram to be applied, various nonlinear effects and modelling uncertainties of methods for hydrodynamic analysis [Guedes Soares, 1996]. Comparisons of direct calculations with the vertical wave bending moment proposed in UR S11 shows that the calculated extreme wave bending moment might in some cases significantly overestimate the Rule value. This overestimation is often such important that it clearly can influence the ship safety. There are number of papers addressing this problem [Guedes Soares, 1996, Faulkner, 1995, 1997, 1998]. We may conclude that the safety of the corroded ship hull is a rather complex problem, since many variables may influence the safety in a quite unexpected way. Direct reliability methods may be used to cover this complex problem and to become some kind of additional safety standard for old tankers. As an illustrative example, we have calculated yearly reliability indices for ultimate longitudinal capacity of a hull-girder for three characteristic structures &#8211 ; ; FPSO unit operating in harsh environment , the new double-hull oil tanker and the old single-hull oil tanker. The last ship, i.e. the single-hull oil tanker in some aspects is similar to &#8220 ; ; Erika&#8221 ; ; . The reliability calculations are performed for new ships, i.e. for &#8220 ; ; as-built&#8221 ; ; scantlings, and for corroded hulls assuming &#8220 ; ; 20 years&#8221 ; ; corrosion according to Rules of Classification Societies. Furthermore, single-hull tanker is analysed also in &#8220 ; ; as-inspected&#8221 ; ; state, taking into account actual thickness measurements. Results are presented in the following table: Table 1 Yearly reliability indices for analysed ships &#8220 ; ; As-built&#8221 ; ; state &#8220 ; ; Corroded&#8221 ; ; state (after 20 years) &#8220 ; ; As-inspected&#8221 ; ; state FPSO 3.353 2.887 - Double-hull tanker 2.412 1.790 - Single-hull tanker 2.057 1.471 1.619 Details of the analysis are described in [Parunov, 2002]. Basically, the analysis includes direct hydrodynamic calculation of wave loads, statistical analysis of still water loads from loading manual, loads combination study, ultimate longitudinal strength calculation and reliability analysis using AFOSM method. Reliability calculation is preceeded by extensive uncertainty assessment. The whole procedure is very similar to analyses described in references [Casella et al., 1996, Casella and Rizzuto, 1998, Guedes Soares et al, 1996, Guedes Soares and Teixeira, 2000]. The results presented in Table 1 may be interpreted and used in different ways. The purpose of this discussion is to show that the single-hull oil tanker may have rather low safety against ultimate longitudinal failure. The safety index for &#8220 ; ; as-inspected&#8221 ; ; state, i.e. with measured thickness reads 1.619, that corresponds to yearly failure probability of more than 5%. Although these results refer to the North Atlantic environment, probability of failure is so high that clearly some actions need to be undertaken. The measures to improve safety are not necessarily replacements of the corroded material, but also the limitations on loading manual that may be imposed or the restriction to navigation areas where ship may operate safely. Since there is no mention of these methods in the paper, the author is kindly asked if he could comment very shortly what is the present status of reliability methods within the Classification Societies, in particular concerning safety evaluation of corroded ships. References: CASELLA, G. ; DOGLIANI, M. ; GUEDES SOARES, C. ; Reliability Based Design of the Primary Structure of Oil Tankers, OMAE, Vol.II, 217-224, 1996. CASELLA, G. ; RIZZUTO, E. ; Second-level reliability analysis of a double-hull oil tanker, Marine Structures, 11, 373-399, 1998. FAULKNER, D. ; On A. Nitta et al. &#8220 ; ; Basis of IACS Unified Longitudinal Strength Standard&#8221 ; ; , Marine Structures 8, 335-336, 1995. FAULKNER, D. ; Discussion on Environment, 13th International Ship and Offshore Structures Congress, Vol.3, 10-11, Trondheim, Norway, August 1997. FAULKNER, D. ; Discussion on Design Principles and Criteria, 13th International Ship and Offshore Structures Congress, Vol.3, 171-172, Trondheim, Norway, August 1997. FAULKNER, D. ; An Independent Assessment of the Sinking of the MV DERBYSHIRE, SNAME Transactions, Vol. 106, 59-103, 1998. GUEDES SOARES, C. ; DOGLIANI, M ; OSTERGAARD, C. ; PARMENTIER, G. ; PEDERSEN, P.T. ; , Reliability Based Ship Structural Design, Transactions SNAME, Vol. 104, 357-389, 1996. GUEDES SOARES, C. ; On the Definition of Rule Requirements for Wave Induced Vertical Bending Moments, Marine Structures 9, 409-425, 1996. GUEDES SOARES, C. ; TEIXEIRA, A.P. ; Structural Reliability of Two Bulk Carrier Designs, Marine Structures 13, 107-128, 2000. MANSOUR, A. ; WIRSCHING, P. ; WHITE, G. ; AYYUB, B. ; Probability Based Ship Design: Implementation of Design Guidelines, Ship Structure Committee, SSC-392, 1996. NITTA, A. et al. ; Basis of IACS Unified Longitudinal Strength Standard, Marine Structures 5, 1-21, 1992. PARUNOV, J. ; Contribution to mathematical modelling of extreme wave loads of ship structures, Ph.D. Thesis, University of Zagreb, 2002. (in Croatian)

oil tankers; safety; reliability

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Podaci o prilogu

431-432-x.

2002.

objavljeno

Podaci o matičnoj publikaciji

Transactions SNAME 2002

SNAME

Singapur : London : München : Ženeva : Tokyo : Hong Kong : Taipei : Peking : Šangaj : Tianjin : Chennai: SNAME

Podaci o skupu

2002 SNAME Annual Meeting

ostalo

01.01.2002-01.01.2002

Boston (MA), Sjedinjene Američke Države

Povezanost rada

Brodogradnja