Anil Kumar Thayamballi

Residual strength assessment of ships after collision and grounding

Merchant marine vessels have provided invaluable information about weather and climate over the seas. In this note, it is shown how these vessels also contribute to ocean research through systematic surveys of upper ocean temperature, salinity and currents. By repeatedly sampling a particular route, one can obtain an accurate picture of the mean state of the ocean and where and how it varies. With a few examples drawn from our own work we show how commercial shipping and cruise vessels, with their unparalleled access to the oceans, could give society far more extensive and valuable information about upper ocean and atmospheric conditions on a regular basis. But for this to happen, a new generation of ocean instrumentation needs to be developed that is optimized for completely automatic and unattended operation on such vessels. It also means working with the merchant marine community to develop guidelines and procedures for future cooperative efforts.

Reliability with respect to ultimate strength of a corroding ship hull

A reliability assessment relative to the ultimate strength failure of a ship hull experiencing structural degradation due to corrosion is presented. The midship section modulus is considered a random process with a monotonically decreasing mean value. ‘Failure’ occurs when the maximum wave-induced bending moment associated with the extreme loading condition plus the stillwater bending moment exceeds the ultimate strength of the hull. Effects of uncertainties in the design variables (including wave and stillwater bending moments, yield strength, and section modulus) on structural performance can be quantified using statistical methods. A reliability analysis method for ultimate strength is developed and, as an example, the reliability assessment of a corroding tanker is illustrated. Reliability as a function of time (as predicted prior to service) is estimated. Also, conditional reliability, given that the ship has survived up to time τs, is computed as a function of t. The purpose of these reliability analyses is to provide information to ship designers and/or owners to be used for general risk assessment relative to decisions on corrosion margins and corrosion protection during construction, as well as decisions on steel replacement during the ships life.

An analytical method for the ultimate compressive strength and effective plating of stiffened panels

Abstract The aim of the present study is to develop a simple analytical method for calculating the ultimate strength of a stiffened panel subject to uniaxial compression. The behavior of the stiffened panel is analyzed by using a plate–stiffener combination model as representative of the stiffened panel. Three collapse modes, namely plate induced failure, stiffener induced failure and local buckling of stiffener web are considered. While several other approaches to the same problem exist, an analytical expression of ultimate strength is of value because it can explicitly show the influence of different parameters. The given formulations can be applied to stiffened plating employing all types of stiffener profiles. The Perry–Robertson approach is used to derive the criteria for plate induced and stiffener induced failures without rotation of stiffener web. The effect of possible stiffener web buckling is included through closed form expressions for predicting the elastic buckling strength of the stiffener web taking account of the influence of rotational restraints at the plate–stiffener and stiffener web–flange intersections. The web buckling strength expressions are used together with the Johnson–Ostenfeld formulation which provides a simple plasticity correction. The influence of initial imperfections (initial deflection and welding induced residual stresses) for both plating and stiffener are taken into account. As verification, the theoretical solutions from the study are compared with some existing experimental results for stiffened panel strength under uniaxial compression. As an interesting contribution, this study develops, presents, and uses a new refined procedure to reasonably accurately evaluate the effective cross sectional area of the representative plate stiffener combination in the stiffened panel. The effective width of plating between stiffeners is analytically formulated, accounting for applied compressive loads, initial deflections, and welding induced residual stresses.

A numerical investigation of tripping

The twin aims of the present study are to investigate numerically the characteristics of tripping failure of flat-bar stiffened panels subject to uniaxial compressive loads and also to study the accuracy of two available design formulations. A special-purpose nonlinear finite element method capable of efficiently analyzing the elasto-plastic large deflection behavior of stiffened panels is developed and used in the study. A benefit of the application of the nonlinear finite element method is that it makes possible a rigorous accounting of the interacting effects of stiffener tripping and plating collapse and also the inclusion of the influence of elasto-plastic rotational restraint at the plate-stiffener intersection prior to and during failure. A parametric series of elasto-plastic large deflection analyses for stiffened panels with flat-bar type of stiffeners under uniaxial compressive loads are carried out varying member proportions and structural parameters. Based on the computed results, a basic investigation of tripping behavior of flat-bars is made, and the accuracy of design formulations is studied. The calculations and comparisons of this paper are for flat-bar stiffened panels under uniaxial compression, but the special-purpose finite element method implemented is considerably more general.

Ship-Shaped Offshore Installations: Accidental Limit-State Design

Introduction As discussed in Chapters 3 and 5, limit states are classified into four categories: serviceability limit states (SLS), ultimate limit states (ULS), fatigue limit states (FLS), and accidental limit states (ALS). This chapter presents ALS design principles and criteria together with some related practices applicable for ship-shaped offshore units. ALS potentially leads to a threat of serious injury or loss of life, pollution, damage, and loss of property or significant financial expenditure. The intention of ALS design is to ensure that the structure is able to tolerate specified accidental events and, when accidents occur, subsequently maintains structural integrity for a sufficient period under specified (usually reduced) environmental conditions to enable the following risk mitigation and recovery measures to take place, as relevant: Evacuation of personnel from the structure Control of undesirable movement or motion of the structure Temporary repairs Safe refuge and firefighting in the case of fire and explosion Minimizing outflow of cargo or other hazardous material Different types of accidental events may require different methodologies or different levels of refinement of the same methodology to analyze structural resistance or capacity during and following such events (demands). The ALS design is then necessarily an important part of design and operation in terms of risk assessment and management that consists of hazard identification, structural evaluation, and mitigation measure development for specific types of accidents, as we describe in Chapter 13.

Ship-Shaped Offshore Installations: Risk Assessment and Management

Introduction Typically, the term “risk” is defined as either the product or a composite of (a) the probability or likelihood that any accident or limit state leading to severe consequences, such as human injuries, environmental damage, and loss of property or financial expenditure, occurs; and (b) the resulting consequences. In the design and operation of ship-shaped offshore units, as in many other types of structures, there are a number of hazards that must be dealt with in the process of risk assessment. Wherever there are potential hazards, a risk always exists. To minimize the risk, one may either attempt to reduce the likelihood of occurrence of the undesirable events or hazards concerned, or contain, reduce, or mitigate the consequences, or both. In the lifecycle of a ship-shaped offshore installation, assessing managing, and controlling the risk is required so that it remains under a tolerable level. The risk management and control should, in fact, be an ongoing process throughout the lifecycle of an installation – that is, involving feasibility study, concept, or front-end design, detailed design, operation, and decommissioning. The different stages of the lifecycle will offer different opportunities for risk management and control, as may be expected. Substantial efforts, such as the SAFEDOR project (http://www.safedor.org), are being directed by the maritime industry toward the application of the risk-assessment techniques together with risk-evaluation criteria to offshore design, operation, and human and environmental safety (e.g., Skjong et al. 2005).

Ship-Shaped Offshore Installations: Fatigue Limit-State Design

Introduction As we discussed in Chapters 3 and 5, limit states are classified into four categories: serviceability limit states (SLS), ultimate limit states (ULS), fatigue limit states (FLS), and accidental limit states (ALS). This chapter presents FLS design principles and criteria together with selected engineering practices applicable for the structure of ship-shaped offshore units. Under the action of repeated loading, fatigue cracks may in time be initiated in the stress concentration areas of ship-shaped offshore structures, and indeed have been reported by Hoogeland et al. (2003) and Newport et al. (2004), among others. In general, the fatigue damage at a crack initiation site is affected by many factors, such as material properties (e.g., elastic modulus, ultimate tensile stress); high local stresses (e.g., stress concentration, residual stresses); size of components; nature of stress variation (e.g., stress variation during the loading and off-take cycles, number of wave-induced stress range cycles); and environmental and operational factors including corrosion and performance of coatings. Potential flaws (e.g., poor materials, porosity, slag inclusions, undercuts, lack of fusion, incomplete weld root penetration) and misalignments can also significantly increase stress concentration and initial defects at welds. To achieve greater fatigue durability in a structure, therefore, stress concentrations, flaws, and structural degradation, including corrosion and fatigue effects, must either be avoided or minimized or, more commonly, their levels and effects either in design, construction, and/or service must be monitored and effectively controlled to acceptable levels.

Ship-Shaped Offshore Installations: Topsides, Mooring, and Export Facilities Design

Introduction As described in Chapter 1, the general arrangement and layout of ship-shaped offshore units designed for oil and gas operations may be grouped into several major parts: hull structures including storage tanks, topsides (processing facilities), export facilities, mooring facilities, accommodations, machinery space, subsea systems, and flowlines. All of these various parts are equally important to achieve successful operation, with due consideration of safety, health, environment, and costs versus benefits. This chapter focuses on topsides, moorings, and export facilities. The material presented herein is aimed at the nonspecialist introductory reader. It is consistent with the content of this book and is included, primarily, to complete the coverage of the various aspects relating to ship-shaped offshore units. Topsides consist of processing facilities that are typically located as elevated modules that are several meters (say, 3m or more) above the main deck of the vessel hull, but related piping systems may be located on the main deck of the vessel hull. Depending on the vessel size and topsides layout, the topsides modules may have multiple decks that contain the oil-, water-, and gas-processing facilities; utility systems; and similar functions. The preferred configuration, however, may be that to the extent possible, the topsides facilities would be incorporated as single-layer “pancake” units. The single-layer unit arrangement requires a larger main deck area for a given set of needs.

Ship-Shaped Offshore Installations: Design Principles, Criteria, and Regulations

Introduction Although substantial efforts are now being directed by the maritime industry toward the application of limit-state design approaches, the shipbuilding industry has traditionally used classification society rules for design of trading ships. On the other hand, the offshore industry has more extensively applied first-principles methods based on limit states. It may be said that the design approach for moored ship-shaped offshore structures, such as FPSOs, often takes a form that is a fusion of the two industry approaches. In a ship-shaped offshore installation, the structures of the vessel are of primary importance because they serve to house and support the systems and equipment needed for the overall success of the enterprise. The ability to correctly and consistently provide the necessary safety margins while meeting the twin requirements of structural safety and economy is key to the design of successful structures. This is where design principles, procedures, and criteria play an important part. Needless to say, successful structures during their life cycle also need to adequately meet the various requirements and regulations on health, safety, and the environment. This chapter presents principles and criteria for design and strength assessment of ship-shaped offshore structures with a focus on the limit-state approach. The importance of safety, health, and the environment is emphasized. The regulatory framework and international standards pertinent to design and operation are addressed. For additional information, see Barltrop (1998).

Ship-Shaped Offshore Installations: Environmental Phenomena and Application to Design

Introduction Actions arising from environmental phenomena on a ship-shaped offshore unit are different from those on a trading tanker. The nature of the offshore structures and their operation are such that winds, currents, and waves, among other factors, may induce significant actions and action effects on structures. Whereas waves are often the primary source of environmental actions on trading ships at sea, considerations related to specialized operations such as berthing are somewhat different. In the case of offshore structures, a good knowledge of the environmental conditions in the areas where the structures will be installed is necessary in order to design for and assure the required high-operational uptimes. Such information is also important for specialized weather-sensitive operations such as installation on site, the berthing of supply boats, and the design of mooring and station- keeping. This chapter presents environmental phenomena and discusses selected engineering practices helpful for the determination and treatment of environmental conditions for ship-shaped offshore units, considering design, transport, installation, and operations. Primary environmental phenomena that induce significant actions and action effects on offshore structures are presented. Although winds are typically regarded as a more elementary source of actions than waves because waves are caused by winds, this chapter starts its discussion with waves first. This is perhaps appropriate only because waves are a major source of actions on the particular types of offshore structures with which we are concerned.