Johns Hopkins Whiting School of Engineering 1996 Annual Report [text version]

Whiting School of Engineering 1996 Annual Report

Civil Engineering
Waves and Weather Affect Offshore Structure Design Methods
Building on Uncertainty
When the Earth Moves
Department Facts

Waves and Weather Affect Offshore Structure Design Methods
When engineers build on something other than firm ground, the rules change. This is especially true of offshore structures in the ocean, where waves, winds, and/or earthquakes can make the job of extracting hydrocarbons a treacherous enterprise. Todd Ude, a new assistant professor, studies the behavior of such structures when subjected to random load effects caused by extreme environments. The motivation is two-fold: to confirm the safety of existing structures and to predict the behavior of novel, emerging platform concepts. “The oil industry would like to get more life out of existing structures, originally designed to last about 25 years,” according to Ude. Offshore oil rigs first made an appearance in the waters of southern California and the Gulf of Mexico in the 1940s. The fixed structures common in the Gulf can be built to depths of just over 1,000 feet. Beyond that depth, the economics of design favor more novel, floating concepts, such as the tension leg platform (TLP), a design that features four columns, pontoons, and vertical mooring lines.

Ude takes the tremendous amount of data collected by oil companies, including wave and weather information, and combines it with structural analyses and models of the uncertainties involved. “Many issues come together when examining the reliability of marine structures,” Ude says, “including economic factors, redundancy, and safety.” “Our analysis provides a broad view of structural safety and also identifies those areas that need to be investigated further.” His research can also be applied to the design of new types of oil rigs, such as the spar platform, a deep draft structure that features a vertical cylinder as its hull. The modern codes used in new designs are geared for a 100-year structure life.

Ude has visited Norway to examine offshore structures constructed for use in the North Sea, and he has also been a consultant to a 13-company consortium. In that role, he developed a software tool for statistical analysis of historical weather data and its impact on exposed structures. As oil companies explore new drilling sites in the South China Sea and off the west coast of Africa, it is clear that Ude’s efforts will be even more important to the industry’s success.

Building on Uncertainty
To Assistant Professor Roger Ghanem, uncertainty is all around us. “In everyday life, civil engineers deal with uncertain materials and loads in bricks, masonry, concrete, steel, and many other materials,” Ghanem says. “No two concrete blocks are the same, and their differences begin at the micro-scale.” Ghanem studies how uncertainty propagates from the micro- to macro-scale and how uncertainty at the engineering, or macro-, scale affects decision-making. “What I want to do is quantify the everyday uncertainty,” he continues.

One of Ghanem’s research projects involves geographic information systems (GIS), which are software packages for manipulating very large data sets that contain details on a certain geographic area. Ghanem has developed a way to integrate GIS with uncertainty models to answer questions on spatial location, pollution, and other concerns. For example, if you look at the land surrounding the Chesapeake Bay, several data sets are easy to identify, such as soil, topography, population, and industry. The information in these data sets can help to project the future state of the environment by using a combination of empirical and model-based techniques that provide for the uncertainty. In our example, the data set containing the most uncertainty is soil, since its texture and permeability can vary widely over the area of study. The relative efficacy of various decisions is then determined by analyzing their impact on the predicted state of the environment.

“Let’s consider the following scenario,” Ghanem proposes. “Someone wants to build an industry close to the Bay that produces wastes. The wastes are collected in a pit, but there is a chance that they might seep into the surrounding soil and ultimately reach the Bay. Combining the GIS data sets with analysis and decision modules can help determine if the industry should operate, and if so, what would the consequences be for the Bay water, for indigneous species, and for surrounding communities.” Ghanem hopes that his research in this area will mean improved analysis techniques that will lead to better-informed decisions.

When the Earth Moves
We like to believe that the earth underneath our feet is stable and rigid, yet under certain conditions even soil itself can behave like a liquid. This “liquefaction” of soil can be initiated by a form of instability that occurs in fine particulate materials such as loose, fine sands, silts, and snow. This type of soil behavior can result in disaster, such as debris flows or mud slides like those that occur in California and snow avalanches that can trap and bury skiers. These events are difficult to forecast, and they present a bit of a mystery because they do not conform to conventional methods of geotechnical engineering analyses for slope failures. Professor Poul Lade has been investigating the mechanisms involved in such catastrophic events for about ten years.

With the assistance of colleagues and students, such as postdoctoral fellow Jerry Yamamuro (now an assistant professor at Clarkson University) and undergraduate student Carl Liggio, Lade attempts to clarify the behavior and the conditions leading to instability and liquefaction of certain soils. In one project, he subjects several types of fine sands to different laboratory tests to determine which sand compositions are most susceptible to instability.

While experimentally studying several theoretical aspects essential to develop models of soil behavior, Lade virtually stumbled over what is believed to be the underlying mechanism for these instabilities. Loading a compressible, particulate material under decreasing stresses can lead to unstable behavior of water-saturated granular materials in undrained conditions. The soil will remain stable as long as it remains drained; that is, as long as the water can escape fast enough so that pressures do not increase. Loose, fine sands have sufficiently low permeabilities such that even small amounts of volumetric creep may temporarily produce undrained conditions in such soils, and soil instability results.

Lade continues to study the many factors—such as volume change tendencies, permeability, and creep—that play important roles in the stability of particulate materials.

Established 1919
Civil engineering was one of the original departments when engineering began at Hopkins. The department eventually merged with materials science and was re-established as a department in 1981.

Phone 410-516-8680

Email civil@jhu.edu

WWW http://www.ce.jhu.edu/

Students
1995-96 Academic Year
Graduate: 31
Undergraduate: 58

Faculty and Researchers
Bruce R. Ellingwood, Chair
Annalingam Anandarajah
James V. Cox
Roger Ghanem
Nicholas P. Jones
Poul V. Lade
Michael E. McCormick
Radoslaw Michalowski
Robert H. Scanlan
Emil Simiu
Todd C. Ude

Research Areas
Computational Mechanics
Foundation Engineering
Geoenvironmental Engineering
Geomechanics
Structural Dynamics
Structural Reliability
Wind, Earthquake, and Ocean Engineering

Civil Engineering Waves and Weather Affect Offshore Structure Design Methods Building on Uncertainty When the Earth Moves Department Facts

Civil Engineering
Waves and Weather Affect Offshore Structure Design Methods
Building on Uncertainty
When the Earth Moves
Department Facts