Ge Wang

Behavior of a double hull in a variety of stranding or collision scenarios

A series of nine tests was conducted to investigate the behavior of a double hull in a variety of stranding or collision scenarios. Cones of five different nose radii were made to model accident scenarios ranging from grounding on a sharp rock to stranding on a relatively flat seabed or shoal, and collision with a sharp bulbous bow of a fast ship to collision with a large bow of a VLCC. Three sub-series were designed in which the cones pressed shell plating, main supporting members and intersections of main supporting members. The test results reveal that the nose radius and the location of penetration have a very strong influence on the behavior of a double hull. Therefore, careful definition of accident scenarios is of crucial importance to assess the strength of ship hulls in accidents, and it is necessary to base the assessment on probability of accidents. Characteristics of the response of structural members were identified and idealized as simple theoretical models. Analytical formulae were derived and discussed. Primary damage mechanisms include membrane stretching of shell panel, onset of rupture, crack propagation, folding of main supporting members, and crushing of intersections of main supporting members. The new plate punching model captures the phenomenon that the load-carrying capacity of a plate depends on the size of the striking object. The plate perforating model predicts the reduced strength of plates with cracks. It reflects the observed test phenomenon that loads do not drop to zero even after rupture occurs in shell plating. A simple analytical method was developed to calculate the global strength of a double hull. The method takes geometrical parameters of seabed rocks or bulbous bows into account, and can be used for a wide range of different accident scenarios. Calculations using this method compared satisfactorily with the test results. This method can be easily incorporated into a probability-based framework to properly assess structural performance for a variety of damage scenarios. Similar to the Wang et al. (J Ship Res 41 (1997) 241) paper on raking damage, which uses only four analytical models, this method also requires only a common calculator to carry out the calculations.

Nonlinear time dependent corrosion wastage of deck plates of ballast and cargo tanks of tankers

The corrosion wastage of deck plates of ballast and cargo tanks is analyzed based on a nonlinear corrosion model. This model is able to describe an initial period without corrosion due to the presence of a corrosion protection system, a transition period with a nonlinear increase of wastage up to a steady state of long-term corrosion wastage. This model is applied to corrosion wastage data of deck plates collected by the American Bureau of Shipping. The objective of this work is to fit this corrosion wastage model to the service measured data, determining the values of the model parameters that represent the best fit to the data so as to describe how corrosion wastage varies in time as a result of generalized corrosion.

Assessment of a ship's performance in accidents

Abstract This paper reviews the state-of-the-art research on collision and grounding. It focuses on the three issues that a standard for design against accidents needs to address: definition of accident scenarios, evaluation approaches, and acceptance criteria. Accident scenarios may be created through surveys of historical data, consulting experts’ opinions, or performing risk analysis. Structural crashworthiness of ships, oil outflow performance, and residual strength of damaged ships can be estimated using simple formulae, simplified analytical methods, or non-linear FEM techniques of different complexity. Performance of a ship in an accident can be measured by energy dissipation, penetration depth, quantity of oil outflow or residual hull girder strength. Acceptance criteria should be established on extensive comparative studies, and provide a means for balancing numerous variables in order to achieve an optimal solution.

Longitudinal strength of ships with accidental damages

This paper presents an investigation of the longitudinal strength of ships with damages due to grounding or collision accidents. Analytical equations are derived for the residual hull girder strength and verified with direct calculations of sample commercial ships for a broad spectrum of accidents. Hull girder ultimate strengths of these sample vessels under sagging and hogging conditions are also calculated, based on which correlation equations are proposed. To evaluate a grounded ship, using the section modulus to the deck would be optimistic, while using the section modulus to the bottom would be conservative. On the contrary, to evaluate a collided ship, using the section modulus to the deck would be conservative, while using the section modulus to the bottom would be optimistic. The derived analytical formulae are then applied to a fleet of 67 commercial ships, including 21 double hull tankers, 18 bulk carriers, 22 single hull tankers and six container carriers. The mean values, standard deviations and coefficients of variation for the coefficients in these new analytical formulae are obtained. The ship length exhibits little influence on these coefficients because they are close to the mean values although ship length spans from 150 to 400 m. The ship type shows some influence on the residual strength. Uniform equations are proposed for commercial ships which do not depend on a ships principal dimensions. These formulae provide very handy tools for predicting the residual strength in seconds, without performing step-by-step detailed calculations, an obvious advantage in cases of emergency or salvage operation.

Assessment of Corrosion Risks to Aging Ships Using an Experience Database

Damages to ships due to corrosion are very likely, and the likelihood increases with the aging of ships. Risk and reliability approaches are more and more frequently applied in design and maintenance planning. These advanced approaches require reliable data reflecting the structural condition of ships in service. Such data is scarce. This paper presents a database of corrosion wastage. It is based on over 110,00 thickness measurements recently collected from 140 trading tankers. This database is larger than most other corrosion databases in the public domain. Corrosion wastage exhibits a high level of variability. In addition to thickness measurements of individual structural members, this database also has information on hull girder’s geometrical properties and strength of ships in service. Corrosion wastage has an influence on the hull girder strength. Statistical interpretations of the database are used to represent corrosion wastage in oil tankers. The severity of corrosion is ranked by three levels: slight, moderate and severe levels corresponding respectively to 50, 75 and 95% cumulative probability on the database. The risks of corrosion wastage to aging ships’ structural integrity are assessed using the observations of the corrosion wastage database. The investigated risks are loss of local member’s strength, loss of global hull girder strength, and shortened inspection intervals. The experience database can be used in many aspects, such as design requirements for corrosion additions and wastage allowance for plate renewal, establishment of limits to hull girder strength of FPSOs, time variant reliability approach and risk based inspection schemes.

Non-Linear Time Dependent Corrosion Wastage of Deck Plates of Ballast and Cargo Tanks of Tankers

The corrosion wastage of deck plates of ballast and cargo tanks is analyzed based on a non-linear corrosion model. This model is able to describe an initial period without corrosion due to the presence of a corrosion protection system, a transition period with a nonlinear increase of wastage up to a steady state of long-term corrosion wastage. This model is applied to corrosion wastage data of deck plates collected by the American Bureau of Shipping. The objective of this work is to fit this corrosion wastage model to the service measured data, determining the values of the model parameters that represent the best fit to the data so as to describe how corrosion wastage varies in time as a result of generalized corrosion.Copyright

ESTIMATION OF CORROSION RATES OF STRUCTURAL MEMBERS IN OIL TANKERS

An accurate estimation of corrosion rates plays an important role in determining corrosion allowances for structural designs, planning for inspections, and scheduling for maintenance. This paper presents an estimation of corrosion rates of structural members in oil tankers based on a corrosion wastage database of over 110,000 thickness measurements from 140 single hull oil tankers. Corrosion rates may be described by a Weibull distribution function. Mean, standard deviation and maximum values of corrosion rates for structural members are obtained based on the entire population of the database. They are compared with the ranges of corrosion rate published by Tanker Structure Co-operative Forum (TSCF). The study aims to update knowledge on corrosion rates in steel ships, and to contribute to the efforts of mitigating the risks of corrosion.Copyright

Assessment of the Emerging Technologies for the Detection and Measurement of Corrosion Wastage of Coated Marine Structures

The application of a resistant coating on ships, offshore structures and pipelines is the primary prevention method of corrosion wastage in the marine industries. To guarantee coating integrity and to be able to thoroughly survey for corrosion wastage on marine structures, new advanced nondestructive methods are being sought. The requirements of convenient and rapid determination of corrosion wastage on coated structures, even in the difficult spatial positions of the structure, will require advanced technologies which are being developed for other industries that also require very high structural integrity. A review of the developments in electromagnetic wave (advanced microwave and eddy current practices) sensors, in elastic wave (advanced ultrasonic practices) sensors, in time-resolved thermography (thermal imaging), in electrochemical sensors and in other emerging analytical techniques is presented. Each of the identified emerging technologies was assessed based on a set of requirements necessary to be an acceptable marine corrosion wastage testing tool. This technology review and assessment include the use of both active and passive sensors and monitors, and the use of sensor arrays. The state of development and availability of the most promising technologies will be described.© 2003 ASME

Chapter 25 – Tanker corrosion

Publisher Summary nThis chapter focuses on corrosion of tankers. It presents an example of marine incident caused by corrosion. It categorizes the primary cost of corrosion protection in tankers into two types: the cost as part of the new construction of the ships and the cost of repairs and maintenance. The majority of the corrosion prevention costs on new ship construction results from the application of coatings to the hulls, decks, and the ballast and storage tanks. The actual cost of applying a coating is relatively more than the cost of the coating materials themselves. The chapter describes the structure of tankers, which is very complex. The hull structures are designed to have adequate strength against failures of yielding, buckling, and fatigue. Corrosion can cause wastage of the hull plates, reducing load-bearing capacity of structural members. Single-hull and double hull tanker structures are illustrated. The chapter discusses three types of corrosion that exist within ships: general, pitting, and grooving corrosion. Several ways to protect marine structures from corrosion damages, such as coatings, cathodic protection, or using corrosion resistant materials are also discussed.

A Statistical Analysis of Corrosion Wastage of Transverse Members in Single Hull Tankers

Corrosion wastage of structural members in oil tankers over the time causes variability in structural properties. Corrosion is a function of many variables and uncertainties, such as the types of corrosion protection systems used, types of cargo, temperature, humidity, etc. The traditional engineering and analysis, which uses simplified deterministic approaches to account for this time-dependent reliability or risk of aging ship structure, is not suitable. This investigation explores the use of statistical analysis approaches to predict corrosion wastage in oil carrying tankers. Corrosion wastage measurements from transverse members of 115 oil tankers were collected to add to an extensive corrosion wastage database. Lognormal function was found to best fit the collected data when compared to other methods, such as the Weibull function. Linear regression in the form of three mathematical models of corrosion wastage, developed by considering age of ship, coating life, and the location of the transverse members, is used to indicate the relationship between corrosion wastage and age of ship. Corrosion rates were determined by the derivatives of the corrosion wastage. The results show that corrosion wastage may accelerate after twenty years of service. It is expected that the study will help identify which structural members may be more susceptible to corrosion wastage and require more frequent inspection.© 2006 ASME