Introduction

The performance of a structure and its components is described using limit-state functions that separate desired states from undesired states. The physical effects of exceedance of a limit state may be either reversible or irreversible. For the reversible case, removal of the cause of the exceedance allows the structure to return to a desired state. For the irreversible case, the same is not true and certain consequences, such as damage, may occur depending on the nature of the limit state. The consequences may, in turn, be either recoverable or unrecoverable from the deformed state. For example, if the damage is limited, say, in the form of a localized permanent set in a case where the same is not desired, the condition may be repairable, for example, by replacing the affected parts.

As discussed in Chapter 3, limit states are usually classified into four types:

1. Serviceability limit states (SLS) that represent criteria governing normal functional or operational use.

2. Ultimate limit states (ULS) that represent the failure of the structure and its components usually when subjected to extreme values of actions or action effects.

3. Fatigue limit states (FLS) that represent damage accumulation (leading to cracking when certain limits are exceeded) under repetitive actions.

4. Accidental limit states (ALS) that represent situations of accidental or abnormal events.

In limit-state assessment, such various limit states are considered against different target safety levels; the target to be attained for any particular type of limit state is a function of the consequences and ease of recovery from that state.

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.

Historical Overview of Offshore Structure Developments

Early History

One of the primary necessities in the progress of civilization has been energy. Industrial advances were first stoked by coal and then by oil and gas. Today, oil and gas are essential commodities in world trade. Exploration that initially started ashore has now moved well into offshore areas, initially in shallower waters and now into deeper waters because of the increasingly reduced possibilities of new fields in shallower waters.

The quest for offshore oil began, perhaps in California, in the late 1800s and early 1900s (Graff 1981). In the beginning, the techniques and facilities used for production of oil on land were applied to an offshore field by extending the field out over the water by jetty to distances of up to 150m off the coast. By the early 1930s, oil drilling was being undertaken by derrick systems located in waters more than a mile (1.6km) offshore, although the water depth at the drill sites was still limited to less than 5m. These derrick systems were constructed using timber. Barges transported supplies and produced oil, canals were dredged, and boats pulled the barges.

As the well sites moved farther away from shore and the water depths increased, it soon became evident that there were many challenges to overcome if efficient and safe offshore operations were to be possible.

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).

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.

The determination of wind effects, such as forces and heeling moments for the hull, topsides, accommodation areas, and helideck of a ship-shaped offshore unit, can be an essential task for the analysis of intact and damage stabilities and other strength aspects. Wind forces and wind moments should also be predicted for the analyses of mooring and thruster systems. Although theoretical and numerical simulations including computational fluid dynamics (CFD) methods may be employed, wind-tunnel tests are highly desirable to get more reliable estimates in this regard.

Wind-tunnel tests are also usually used to analyze smoke ingress and ventilation problems on board a ship-shaped offshore unit, aspects that are always involved in various environmental and safety risk assessments. Examples include assessment and optimization of the areas over the helideck, which are affected by disturbed flow and by temperature rises due to turbine exhaust emissions. To model emergency gas releases and fire scenarios and to identify the regions of poor ventilation, wind-tunnel tests may be required. The natural ventilation within the process areas of an FPSO can also be assessed by wind-tunnel testing.

For a detailed description of the wind-tunnel testing of ship-shaped offshore units involving test procedures, measurement techniques, and assessment criteria, refer to the UK HSE report titled Review of model-testing requirements for FPSOs, Offshore Technology Report, 2000/123, Health and Safety Executive, UK, 2000.

The vorticity equation

Irrotational flow can be established from a state of rest in an ideal incompressible fluid by the instantaneous transmission throughout the fluid of impulsive pressures from a moving boundary. If the boundary motion is subsequently arrested the motion everywhere ceases immediately. Kelvin's theorem (§2.10), that the kinetic energy of an irrotational flow is always smaller than that of any other flow consistent with the same boundary conditions, is a consequence of the fact that the number of degrees of freedom of irrotational motion is exactly the same as the number of degrees of freedom of the boundary itself. In a real fluid, however, there are typically an unlimited number of degrees of freedom, the flow is rotational, and the motion continues after the boundary stops moving. Kelvin (1867) therefore proposed the following definition of a vortex in a homogeneous incompressible fluid: ‘… a portion of fluid having any motion that it could not acquire by fluid pressure transmitted from its boundary’. Vorticity is actually a derived kinematic quantity, but its introduction greatly increases understanding of a complex flow and a knowledge of its distribution frequently permits the description of the fluid motion to be simplified.

When a small fluid particle is imagined to be suddenly solidified without change in its angular momentum, it continues to translate and rotate as a solid body. Its initial angular velocity of rotation is determined by its moment of inertia tensor, which depends on the particle shape.

Fluid mechanics impinges on practically all areas of human endeavour. But it is not easy to grasp its principles and ramifications in all of its diverse manifestations. Industrial applications usually require the numerical solution of the equations of motion of a fluid on a very large scale, perhaps coupled in a complicated manner to equations describing the response of solid structures in contact with the fluid. There has developed a tendency to regard the subject as defined solely by its governing equations whose treatment by numerical methods can furnish the solution of any problem.

There are actually many practical problems that are not yet amenable to full numerical evaluation in a reasonable time, even on the fastest of present-day computers. It is therefore important to have a proper theoretical understanding that will permit sensible simplifications to be made when formulating a problem. As in most technical subjects such understanding is acquired by detailed study of highly simplified ‘model problems’. Many of these problems fall within the realm of classical fluid mechanics, which is often criticised for its emphasis on ideal fluids and potential flow theory. The criticism is misplaced, however: For example, potential flow methods provide a good first approximation to airfoil theory, and ‘free-streamline’ theory (pioneered in its modern form by Chaplygin) permits the two-dimensional modelling of complex flows involving separation and jet formation.

Introduction

Steady free-streamline flows of water when gravitational forces can be neglected have been discussed in §3.7. Most unsteady free-streamline problems are intractable except by numerical means and generally become more so when gravitational forces are important. However, flows involving gravity where the unsteady motion is a ‘small’ perturbation of a relatively simple mean state occur frequently in the form of surface waves. In the absence of motion the free surface of a liquid in equilibrium under gravity is often ‘horizontal’. A disturbance applied locally that distorts the surface brings into play gravitational restoring forces that cause the disturbance to spread out over the surface in the form of ‘waves’. The waves carry energy away from the source region, propagating parallel to the mean free surface. The agitation produced by a passing wave and the energy flux is generally in the form of a transient disturbance of the fluid particles (around approximately closed particle paths), which are not in themselves transported to any great extent by the wave, and the influence of the wave on fluid at depths exceeding a characteristic wavelength tends to be negligible. In this section these general properties of surface gravity waves are discussed and illustrated by simple examples.

Conditions at the free surface

Consider the simplest case of water whose free surface in equilibrium can be regarded as horizontal and in the plane z = 0 of the coordinate axes (x, y, z), where z increases vertically upwards (Figure 5.1.1).

The fluid state

Consider a fluid that can be regarded as continuous and locally homogeneous at all levels of subdivision. At any time t and position x = (x1, x2, x3) the state of the fluid is defined when the velocity v and any two thermodynamic variables are specified. A fluid in unsteady motion, in which temperature and pressure vary with position and time, cannot strictly be in thermodynamic equilibrium, and it will be necessary to discuss how to define the thermodynamic properties of the small individual fluid particles of which the fluid may be supposed to consist.

The distinctive fluid property possessed by both liquids and gases is that these fluid particles can move freely relative to one another under the influence of applied forces or other externally imposed changes at the boundaries of the fluid. Five scalar partial differential equations are required for determining these motions. They are statements of conservation of mass, momentum, and energy, and they are to be solved subject to appropriate boundary and initial conditions, dependent on the problem at hand. This book is concerned with the use of these equations to formulate and analyse a wide range of model problems whose solutions will help the reader to understand the intricacies of fluid motion.