Wind And Wave Loads On A Tension Leg Platform: Theory And Experiment

This very low natural frequencies of compliant offshore structures have raised the concern about the significance of the dynamic action of wind in the response of such platforms. In this thesis the effect of wind loads on the response of a Tension Leg Platform is examined in detail.;The study examines the importance of wind loads through the use of two different numerical models and through an experimental study carried out in a wind-wave flume, where a dynamic model of a Tension Leg Platform was subjected to the action of properly scaled random wind and wave loads. The numerical models include a full diffraction analysis and the effect of second-order, non-linear wave drift forces. The study is the first, where a physical model of a Tension Leg Platform has been tested in a wind-wave facility, and is believed to be the first numerical study where the modeling of wind forces is accompanied by a complete model of the wave forces.;The agreement between the results obtained from the numerical simulations and the experiments was good, indicating that the numerical models used in this thesis can adequately predict the response of a Tension Leg Platform to the combined action of wind and wave loads.;The effect of the wind on the surge response of the platform was found to be strongly dependent on the length of the fetch of water over which the wind had been blowing and the duration of the wind storm. In cases where the fetch is short or the storm duration is short, so that large waves cannot develop, it was found that the wind dominated the surge response behaviour of the platform. In the cases where large waves were allowed to develop, the response was not governed by the action of the wind, but the dynamic action of the wind did contribute to the total surge response. In general, the wind induced surge response and the second-order wave induced surge response were found to be of similar importance.;The yaw response of the platform was found to be dominated by the action of wind

Conservation of grassland birds in North America: understanding ecological processes in different regions

Many species of birds that depend on grassland or savanna habitats have shown substantial overall population declines in North America. To understand the causes of these declines, we examined the habitat requirements of birds in six types of grassland in different regions of the continent. Open habitats were originally maintained by ecological drivers (continual and pervasive ecological processes) such as drought, grazing, and fire in tallgrass prairie, mixed-grass prairie, shortgrass prairie, desert grassland, and longleaf pine savanna. By contrast, grasslands were created by occasional disturbances (e.g., fires or beaver [Castor canadensis] activity) in much of northeastern North America. The relative importance of particular drivers or disturbances differed among regions. Keystone mammal species grazers such as prairie-dogs (Cynomys spp.) and bison (Bison bison) in western prairies, and dam-building beavers in eastern regions of the continent. Deciduous forests played a crucial, and frequently unappreciated, role in maintaining many grassland systems. Although fire was important in preventing invasion of woody plants in the tallgrass and moist mixed prairies, grazing played a more important role in maintaining the typical grassland vegetation of shortgrass prairies and desert grasslands. Heavy grazing by prairiedogs or bison created a low \u27grazing lawn\u27 that is the preferred habitat for many grassland bird species that are restricted to the shortgrass prairie and desert grasslands. Ultimately, many species of grassland birds are vulnerable because people destroyed their breeding, migratory, and wintering habitat, either directly by converting it to farmland and building lots, or indirectly by modifying grazing patterns, suppressing fires, or interfering with other ecological processes that originally sustained open grassland. Understanding the ecological processes that originally maintained grassland systems is critically important for efforts to improve, restore, or create habitat for grassland birds and other grassland organisms. Consequently, preservation of large areas of natural or seminatural grassland, where these processes can be studied and core populations of grassland birds can flourish, should be a high priority. However, some grassland birds now primarily depend on artificial habitats that are managed to maximize production of livestock, timber, or other products. With a sound understanding of the habitat requirements of grassland birds and the processes that originally shaped their habitats, it should be possible to manage populations sustainably on \u27working land\u27 such as cattle ranches, farms, and pine plantations. Proper management of private land will be critical for preserving adequate breeding, migratory, and winter habitat for grassland and savanna species

Genetic risk and a primary role for cell-mediated immune mechanisms in multiple sclerosis.

Multiple sclerosis is a common disease of the central nervous system in which the interplay between inflammatory and neurodegenerative processes typically results in intermittent neurological disturbance followed by progressive accumulation of disability. Epidemiological studies have shown that genetic factors are primarily responsible for the substantially increased frequency of the disease seen in the relatives of affected individuals, and systematic attempts to identify linkage in multiplex families have confirmed that variation within the major histocompatibility complex (MHC) exerts the greatest individual effect on risk. Modestly powered genome-wide association studies (GWAS) have enabled more than 20 additional risk loci to be identified and have shown that multiple variants exerting modest individual effects have a key role in disease susceptibility. Most of the genetic architecture underlying susceptibility to the disease remains to be defined and is anticipated to require the analysis of sample sizes that are beyond the numbers currently available to individual research groups. In a collaborative GWAS involving 9,772 cases of European descent collected by 23 research groups working in 15 different countries, we have replicated almost all of the previously suggested associations and identified at least a further 29 novel susceptibility loci. Within the MHC we have refined the identity of the HLA-DRB1 risk alleles and confirmed that variation in the HLA-A gene underlies the independent protective effect attributable to the class I region. Immunologically relevant genes are significantly overrepresented among those mapping close to the identified loci and particularly implicate T-helper-cell differentiation in the pathogenesis of multiple sclerosis