Offshore Structures

Design, Construction and Maintenance

Mohamed A. El-Reedy

• Publisher: Gulf Professional Publishing
• Year: 2012
• ISBN:
  9780123854766
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

With most of the easy gas and oil reserves discovered and prices rebounding, companies are now drilling far offshore in extreme weather condition environments. As deepwater wells are drilled to greater depths, engineers and designers are confronted with new problems such as water depth, weather conditions, ocean currents, equipment reliability, and well accessibility. Offshore Structure Design, Construction and Maintenance covers all types of offshore structures and platforms employed worldwide.

The ultimate reference for selecting, operating and maintaining offshore structures, this book provides a road map for designing structures which will stand up even in the harshest environments. The selection of the proper type of offshore structure is discussed from a technical and economic point of view. The design procedure for the fixed offshore structure will be presented and how to review the design to reach the optimum solution. Nonlinear analysis (Push over) analysis will be presented as a new technique to design and assess the existing structure. Pile design and tubular joint with the effect of fatigue loading will be presented also from a theoretical and a practical point of view.

With this book in hand, engineers receive the most up-to-date methods for performing a structural life cycle analysis; implement maintenance plans for topsides and jackets, using non destructive testing. Under water inspection is discussed for hundreds of platforms in detail. Advanced repair methodology for scour, marine growth and damaged or deteriorating members are discussed. Risk based under water inspection techniques are covered from a practical pint of view. In addition, the book will be supported by an online modeling and simulation program with will allow designers to save time and money by verifying assumptions online.

• One stop guide to offshore structure design and analysis
• Easy to understand methods for structural life cycle analysis
• Expert advice for designing offshore platforms for all types of environments
• Save time and money by verifying designs online

Chapter 1

Introduction to Offshore Structures

1.1 Introduction

Offshore structures have special economic and technical characteristics. Economically, offshore structures are dependent on oil and gas production, which is directly related to global investment, which is in turn affected by the price of oil. For example, in 2008 oil prices increased worldwide, and as a result many offshore structure projects were started during that time period.

Technically, offshore structure platform design and construction are a hybrid of steel structure design and harbor design and construction.

Only a limited number of faculty of engineering focus on offshore structural engineering, including the design of fixed offshore platforms, floating or other types, and, perhaps due to the limited number of offshore structural projects in comparison to the number of normal steel structural projects, such as residential facilities and factories. In addition, offshore steel structure construction depends on continuous research and study drawn from around the world.

All the major multinational companies that work in the oil and gas business are interested in offshore structures. These companies provide continuous support for research and development that will enhance the ability of their engineering firms and construction contractors to support their business needs.

1.2 History of Offshore Structures

As early as 1909–1910, wells were being drilled in Louisiana. Wooden derricks were erected on hastily built wooden platforms that had been constructed on top of timber piles.

Over the past 40 years, two major types of fixed platforms have been developed: the steel template, which was pioneered in the Gulf of Mexico (GoM), and the concrete gravity type, first developed in the North Sea. Recently, a third type, the tension-leg platform, has been used to drill wells and develop gas projects in deep water. In 1976, Exxon installed a platform in the Santa Barbara, CA, channel at a water depth of 259 m (850 ft). Approximately two decades earlier, around 1950, while the developments were taking place in the GoM and Santa Barbara channel, the BP (British Petroleum) company was engaged in a similar exploration off the coast of Abu Dhabi in the Persian Gulf. The water depth there is less than 30 m (100 ft) and the operation has grown steadily over the years.

The three basic design requirements for a fixed offshore platform are:

1. The ability to withstand all loads expected during fabrication, transportation, and installation.

2. The ability to withstand loads resulting from severe storms and earthquakes.

3. The ability to function safely as a combined drilling, production, and housing facility.

The importance of the second requirement, and the need to reevaluate platform design criteria, was highlighted in the 1960s, when hurricanes caused serious damage to platforms in the GoM. In 1964, hurricane Hilda, with wave heights of 13 m and wind gusts up to 89 m/s, destroyed 13 platforms. The next year, hurricane Betsy destroyed three platforms and damaged many others. Because Hilda and Betsy were “100-year hurricanes,” designers abandoned the use of “25- and 50-year storms” and began designing for the more destructive 100-year storms.

1.3 Overview of Field Development

Estimates of global oil reserves, based on geological and geophysical studies and oil and gas discoveries as of January 1996, indicate that about 53% of the reserves are in the Middle East, a politically troubled region. Overall, 60% of reserves are controlled by the Organization of Petroleum Exporting Countries (OPEC). Obviously, OPEC and the Middle East are very important for the world’s current energy needs.

Most researchers believe that the major land-based hydrocarbon reserves have already been discovered and that most significant future discoveries will be in offshore areas, the Arctic and other difficult-to-reach areas of the world.

Geological research indicates why North America, northwest Europe and the coastal areas of West Africa and eastern South America appear to have similar potential for deepwater production. During very early geological history, sediments were deposited in basins with restricted circulation and were later converted to the supersource rocks found in the coastal regions of these areas. The presence of these geological formations is the initial indication of the presence of hydrocarbons, but, before feasible alternatives for producing oil and gas from a field are identified and the most desirable production scheme is selected, exploratory work defining the reservoir characteristics has to be completed. First, geologists and geophysicists assess the location’s geological formations to determine if it has potential hydrocarbon reserves.

After the geologists and geophysicists decide that a field could be economically viable, further exploratory activities are undertaken to prepare cost, schedule and financial return estimates for selected exploration and production schemes. After that, the various alternative schemes are compared and the most beneficial one is identified.

During this phase, due to the absence of detailed information about reservoir characteristics, future market conditions and field-development alternatives, experts make judgments based on their past experience and on cost and schedule estimates based on data available from previous history. The success of oil and gas companies depends on this expertise, so most companies keep experts on hand and compete with each other to recruit them. Sometimes, the experiential data are not enough, so decisions are made as a result of brainstorming sessions attended by experts and management, and these are greatly affected by a company’s culture and past experiences.

The reservoir management plan is affected by the characteristics of the fluid the reservoir produces, the reservoir’s size and topography, regional politics, company and partner culture and the economics of the entire field-development scheme. Well system and completion design are affected by the same factors that affect the reservoir management plan, except perhaps the political factors. Platforms, facilities for processing and production, storage systems and export systems are affected by all these factors as well.

The field-development scheme has to take into account:

• Reservoir characteristics

• Production composition (e.g., oil, gas, water, H₂S)

• Reservoir uncertainty

• Environment (e.g., water depth)

• Regional development status

• Technologies available locally

• Politics

• Partners

• Company culture

• Schedule

• Equipment

• Construction facilities

• Market

• Economics

If the preliminary economic indicators in the feasibility study phase are positive, seismic data generation and evaluation, done by geophysicists, follow. These data comprise reasonable information about the reservoir’s characteristics, such as its depth, spread, faults, domes and other factors, and an approximate estimate of the recoverable reserves of hydrocarbons.

If the seismic indications are positive and the decision is to explore further, exploratory drilling commences. Depending on water depth, the environment and what’s available, an appropriate exploration scheme is selected. A jack-up exploratory unit is suitable for shallow water depths. In water depths exceeding 120 m (400 ft), ships or semisubmersible drilling units are utilized. At depths of 300 m (1000 ft), floating drilling units require special mooring arrangements or a dynamic positioning system. A floating semisubmersible drilling rig is capable of operating in water as deep as 900–1200 m (3000–4000 ft).

Exploratory drilling work follows the discovery well. This generally requires three to six wells drilled at selected points of a reservoir. These activities and production testing of the wells where oil and gas are encountered give reasonably detailed information about the size, depth, extent and topography of a reservoir, such as the fault lines, impermeable layers, etc., and its recoverable reserves, viscosity (API grade), liquid properties (e.g., the oil/water ratio), and impurities, such as sulfur or another critical component.

Reservoir information enables geologists and geophysicists to estimate the location and number of wells that will be required to produce a field and the volumes of oil, gas and water production. This information is used to determine the type of production equipment, facilities and the transport system needed to produce the field.

Obviously, the accuracy of reservoir data has a major effect on the selection of a field-development concept. In marginal or complex reservoirs, reliable reservoir data and the flexibility of the production system in accommodating changes from the reservoir appraisal are very desirable.

1.3.1 Field-Development Cost

Field development for a new project or for extending existing facilities is a multistep process. The first step is gathering input parameters, such as the reservoir and environmental data; the selection and design of major system components, such as the production drilling and the wells, facilities and offtake system; and the decision criteria, such as the economics. The next step is evaluating the different field-development options that satisfy the input requirements and establishing their relative merits with respect to the decision criteria. In this design process loop, not only alternatives for field-development systems, but also alternatives for each system, need to be taken into account.

At the next stage, a preliminary design for the selected system is started. In this phase, the selection activity is focused on the system components and detail elements. During this phase, design iterations are generated until all the members of all engineering and operation disciplines are satisfied from a technical point of view. All the system components and construction activities must be well defined. Once the design is complete, few changes to the system and its components can be made without suffering delays and cost overruns.

The operation phase includes maintenance, production, repair and reassessment and transportation activities. Viable field-development options are identified and developed and selection of the ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• The Author
• Chapter 1. Introduction to Offshore Structures
• Chapter 2. Offshore Structure Loads and Strength
• Chapter 3. Offshore Structure Platform Design
• Chapter 4. Geotechnical Data and Pile Design
• Chapter 5. Fabrication and Installation
• Chapter 6. Corrosion Protection
• Chapter 7. Assessment of Existing Structures and Repairs
• Chapter 8. Risk-Based Inspection Technique
• Index