Dynamics of Offshore Structures / Edition 2

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Overview

Dynamics of Offshore Structures Second Edition is a valuable text for students in civil and mechanical engineering programs and an indispensable resource for structural geotechnical and constructions engineers working with offshore projects.
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Product Details

  • ISBN-13: 9780471264675
  • Publisher: Wiley
  • Publication date: 10/9/2002
  • Edition description: REV
  • Edition number: 2
  • Pages: 344
  • Product dimensions: 6.20 (w) x 9.30 (h) x 1.00 (d)

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Dynamics of Offshore Structures


John Wiley & Sons

ISBN: 0-471-26467-9


Chapter One

Structures in the Offshore Environment

James F. Wilson

Offshore structures, constructed on or above the continental shelves and on the adjacent continental slopes, take many forms and serve a multitude of purposes: towers for microwave transmission, installations for power generation, portable pipeline systems for mining the ocean floor, and a few platforms and floating islands that serve as resort hotels. Most structures offshore, however, have been built to support the activities of petroleum industries-activities that include the exploration, drilling, production, storage, and transportation of oil. Exploratory drilling is done from mobile platforms or carefully positioned ships; production and storage operations involve more permanent structures; and pipelines, buoys, and mooring systems for floating structures and ships support all oil acquisition activities.

The design of marine structures compatible with the extreme offshore environmental conditions is a most challenging and creative task for the contemporary ocean engineer. The engineer involved in designing these marine structures must rely on the knowledge and experience of meteorologists, oceanographers, naval architects, geologists, and material scientists. The marine engineer's goal is to conceive and design a lasting structure that can withstand the adverse conditions of high winds and waves, earthquakes, and ice, remaining in harmony with itsenvironment. Mulcahy (1979) expressed this design philosophy as follows:

Offshore platforms are a bit like space capsules-for each pound of unnecessary deck space that can be trimmed from the structure, the magnitude of the structure needed to support it can be reduced. This is true for a guyed tower, a fixed platform, or a tension leg structure. Decreasing the wave load leads to lower overturning moments, a lesser requirement for pilings, and a smaller number of strength members in the structure. When this is accomplished, smaller launch barges can transport the structure to the work site.

In perspective, offshore structures include a great deal more than the towers and platforms. They include moored or mobile ships whose positions may be precisely controlled. They include the guy lines for compliant towers, the cables for buoys and for tension-leg platforms, and the associated pipelines without which the platforms and submerged oil production systems would be useless. Detailed descriptions of such installations may be found in the references at the end of this chapter. Of particular note is the review article on compliant offshore structures by Adrezin et al. (1996), with its 130 citations to the world literature on the subject up to the mid-1990s. For descriptions of current practice in all types of offshore installations, the reader is referred to the yearly conference proceedings such as found in the References at the end of this chapter.

This chapter begins with a short history of offshore structures, describes typical state-of-the-art installations, and concludes with a discussion of engineering challenges for future designs. Subsequent chapters address in some detail both the mathematical modeling and the environmental loading of offshore structures, together with ways to predict their dynamic responses and structural integrity, from both the deterministic and the statistical viewpoints.

1.1 HISTORICAL PERSPECTIVE

The earliest offshore structure for oil drilling was built about 1887 off the coast of southern California near Santa Barbara. This was simply a wooden wharf outfitted with a rig for drilling vertical wells into the sea floor. More elaborate platforms supported by timber piers were then built for oil drilling, including installations for the mile-deep well in Caddo Lake, Louisiana (1911) and the platform in Lake Maracaibo, Venezuela (1927). Soon after these early pier systems were built, it became apparent that the lifetime of timber structures erected in lakes or oceans is severely limited because of attacks by marine organisms. For this reason, reinforced concrete replaced timber as the supporting structure for many offshore platforms up to the late 1940s. Over the next 50 years about 12,000 platform structures were built offshore, usually of steel but more recently of precast concrete. The chief features of these structures, together with their supporting components such as mooring systems and pipelines, are discussed in this chapter. See also Gerwick (1999) and Will (1982).

Offshore mooring systems have a variety of configurations. All have anchors or groups of piles in the seabed with flexible lines (cables, ropes, chains) leading from them to buoys, ships, or platform structures. The function of a mooring system is to keep the buoy, ship, or platform structure at a relatively fixed location during engineering operations. Engineering efforts in mooring systems have focused in recent years on the development of new anchor configurations with higher pullout loads, larger capacity and lower cost of installation for deeper water applications.

When pipelines were first laid offshore, no extraordinary analyses or deployment techniques were needed since they were in shallow water and were of small diameter, somewhat flexible, and made of relatively ductile steel. As platforms were built in deeper and deeper water with multiple well slots, larger diameter pipelines of higher strength were required. During the 1960s, engineers met this challenge with new designs and with refined methods of analysis and deployment. Pipeline systems evolved into two main types: sea floor and vertical configurations. Both are used to transport gas and oil, but the vertical systems also include risers to carry drilling tools, electric power lines, dredge pipes for deep sea mining, and cold water pipes (CWP) for ocean thermal energy conversion (OTEC).

Throughout the world there are at present about 80,000 km of marine pipelines. Since 1986, the rate of building new marine pipelines has been about 1000 km per year. Individual pipelines on the sea floor vary in length from 1 to 1000 km and in diameter from 7 to 152 cm. For instance, a Norwegian project features a 1000 km line extending from the Troll field to Belgium, which was completed in 1992. At present, Kuwait has the loading line of largest diameter, 152 cm. The pipelines of smaller diameter are used to transport oil and gas from wellheads, and those of larger diameter are used to load and unload oil from tankers moored at offshore terminals. The deepest sea floor pipelines at present are the 46 cm diameter gas lines in the Gulf of Mexico, for which the maximum depth is 1400 m. Sea floor pipelines are often anchored to the seabed or buried in trenches for protection from erosion and the undermining effects of currents. Some seabed pipelines have a coating of concrete to add protection and to reduce buoyancy.

1.2 PLATFORMS

Six general types of offshore platforms are depicted in Figure 1.1. The first three are designed for depths up to about 500 m, and the last three are for depths to 2000 m. Not shown are subsea production platforms, which are presently rated for 3000 m depths.

Fixed-Bottom Platforms

A mobile structure often used for exploratory oil-drilling operations is the self-elevating platform commonly called a jackup or mat-supported rig. A constructed version of this platform, depicted schematically in Figure 1.1a, is shown in Figure 1.2. Typically, such a platform is supported by three to six legs that are attached to a steel mat resting on the sea floor. In soft soils, the legs pass through the mat and may penetrate the soil to depths of up to 70 m. To the bottom of each leg is attached a steel saucer or spud can to help stabilize the structure and to minimize leg penetration into the soil. The height of the platform above the seafloor, up to 100 m, may be adjusted by using motor drives attached to each leg.

A platform designed to be used in a fixed location as a production unit is shown in Figure 1.1b. Such a unit, called a gravity platform, consists of a cluster of concrete oil-storage tanks surrounding hollow, tapered concrete legs that extend above the water line to support a steel deck. See Graff and Chen (1981). A typical unit, of which there were 28 operating in the North Sea in 1999, has one to four legs and rests directly on a concrete mat on the sea floor. With ballast consisting of sand in the bottom of the tanks and seawater in the legs, these structures depend on self-weight alone to maintain an upright position when subjected to the highest waves that are expected to occur in a 100 year time period. A realistic 100 year wave that may occur in the northern North Sea is 27.8 m. At present, the largest concrete gravity platform is the Troll structure, and one of moderate size is the Statfjord-A Condeep structure, both located in the North Sea. The latter structure is 250 m high and has three legs. Located off the coast of Norway, the Statfjord-A Condeep unit has slots for 42 oil wells that reach to depths of 2800 m. When in operation, it accommodates a crew of 200 people who live and work on this structure.

Found more frequently among the permanent, fixed-bottom structures, however, is the steel truss or jacket template structure shown schematically in Figure 1.1c, where an installed structure is depicted in Figure 1.3. As for the gravity platform, each steel jacket unit is designed for a fixed location and a fixed water depth. The first such structure was operational in 1955 in water 30 m deep. By 1999 there were approximately 6500 jacket structures, the tallest of which was the Bullwinkle unit located in the Gulf of Mexico. The common characteristics of these jacket structures are their tubular legs, somewhat inclined to the vertical, and reinforced with tubular braces in K or X patterns. Piles driven through these legs into the sea floor and clusters of piles around some of the legs maintain structural stability in adverse weather. One of the largest jacket structures is the 380 m high Cognac unit, which has 10 legs with 24 piles extending 140 m into the soft clay of the Gulf of Mexico. As with all jacket template structures, its natural or fundamental bending frequency of 0.17 Hz is above the 0.11 Hz frequency of the highest energy sea waves in the Gulf of Mexico during storm conditions, as depicted in Figure 1.4.

Compliant Platforms

An alternative class of offshore structures meant for depths from 300 to 800 m is the compliant tower such as that shown in Figure 1.1d. Such a tower may or may not have mooring lines. It is a pile-supported steel truss structure designed to comply or flex with the waves and has considerably less structural material per unit height when compared with a common jacket template tower.

The first compliant tower was the Lena, which was installed in the early 1980s in the Gulf of Mexico. Including its three-level drilling and production deck and its drilling rigs, this tower reaches a total height of 400 m. Each of the 20 stabilizing cables, attached 25 m below the water line and arranged symmetrically about the structure, extends a horizontal distance of about 1000 m to a line of clumped weights that rest on the sea floor, to an anchor cable and an anchor pile. Under normal weather or small storm conditions, the cables act as hard springs, but with severe storms or hurricanes, the cable restraints become softer or compliant. That is, the amplitude of tower rotation increases at a rate greater than that of the loading, since the clumped weights lift off the sea floor to accommodate the increased storm loads on the tower. When storms or hurricane conditions are anticipated, operations on compliant towers cease and the crew is evacuated.

Installation of the Lena cables was more difficult and costly than anticipated. Subsequently, compliant towers without cables have been designed by Exxon, and two such designs were installed in 1999 in the Gulf of Mexico. Unlike the jacket-template structures, the compliant towers have natural frequencies in bending or sway near 0.03 Hz, or well below the 0.05 Hz frequency of the highest energy sea waves in the Gulf of Mexico during storm conditions. Thus, an important feature of such structures is that they are designed to have natural sway frequencies well removed from the frequency range of the highest energy waves for normal seas (0.1 to 0.15 Hz) and for storm seas (0.05 to 0.1 Hz). This frequency spread is necessary to avoid platform resonance, which can lead to failure. The sway frequencies of two platforms in comparison to the frequency range for the spectrum of the highest energy storm waves in the Gulf of Mexico are depicted graphically in Figure 1.4. The measurement and meaning of this wave height spectra, which is highly site-dependent, will be discussed in detail in subsequent chapters.

Buoyant Platforms

The tension leg platform (TLP) can be economically competitive with compliant towers for water depths between 300 m and 1200 m. The schematic design of the TLP is depicted in Figure 1.1e. In such designs, the total buoyant force of the submerged pontoons exceeds the structure's total gravity or deadweight loading. Taut, vertical tethers extending from the columns and moored to the foundation templates on the ocean floor keep the structure in position during all weather conditions. The heave, pitch, and roll motion are well restrained by the tethers; but the motions in the horizontal plane, or surge, sway, and yaw, are quite compliant with the motion of the waves. The first production TLP was built 150 km off the coast of Scotland in the mid-1980s. Conoco installed the Julliet in 1989, and Saga Petroleum installed the Snorre near Norway in 1991. The tethers for the Snorre are 137 cm in diameter. By the late 1990s, a total of eleven TLPs were installed, three in the North Sea and eight in the Gulf of Mexico.

For water depths of about 1500 m, a subsea production system provides an excellent alternative to a fixed surface facility. Much of a subsea system rests on the ocean floor, and its production of oil and gas is controlled by computer from a ship or other buoyant structure above the subsea unit. The buoyant structure and the subsea unit are often connected by a marine riser, which will be discussed presently.

A popular buoyant structure is the floating production system. Such a structure is practical for water depths up to 3000 m, and also at lesser depths where the field life of the structure is to be relatively short. An example of a buoyant structure is the semisubmersible with fully submerged hulls, shown schematically in Figure 1.1f , with an installed design shown in Figure 1.5.

Continues...


Excerpted from Dynamics of Offshore Structures Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents

Preface
Contributors
Acknowledgments
1 Structures in the Offshore Environment 1
2 Structure-Environmental Force Interactions 17
3 Deterministic Descriptions of Offshore Waves 61
4 Wave Forces on Structures 84
5 Deterministic Responses for Single Degree of Freedom Structures 100
6 Statistical Descriptions of Offshore Waves 144
7 Statistical Responses for Single Degree of Freedom Linear Structures 165
8 Multi-Degree of Freedom Linear Structures 195
9 Applications of Multi-Degree of Freedom Analysis 223
10 Continuous Systems 248
11 Behavior of Piles Supporting Offshore Structures 289
Conversion Table 321
Index 323
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