A set of expressions termed the generalized d’Alembert force is determined for application to two and three-dimensional dynamic analysis of discrete, nonlinear, multifreedom, constrained, mechanical dynamic systems. These expressions greatly simplify the task of developing a correct set of second order differential equations of motion for mechanical systems which are nonlinear because of large deflections or other geometric effects. They apply to both constrained and unconstrained mechanical systems via the method of Lagrange equations with constraint. The two-dimensional version of the expressions has been successfully applied in a type-varient computer program for the dynamic analysis of mechanical networks, and example problems simulated with this program are discussed.
As the offshore industry moves into deeper water, the dynamic behavior of structures becomes a very important parameter in the overall design procedures. In particular, the cyclic nature of wave loads has a significant effect on the fatigue life of the structure. This paper presents a procedure for fatigue analysis where the dynamic response of the structure is analyzed through a spectral approach. The sea waves which constitute the forcing function acting on the structure are represented as energy spectra; the response is obtained in spectral terms and is subsequently interpreted according to probabilistic concepts.As part of.this study we have considered the characteristics of the distribution of the response. In the usual approach to fatigue analysis one assumes that the peaks of the response follow a Rayleigh distribution. This assumption is valid if the Cartwright-Longuet-Higgins' measure of bandwidth (E) is equal to zero. However, in practical fatigue applications, this idealization is rarely, if ever, encountered. Thus, in order to better characterize narrow banded phenomena, we have investigated descriptions which do not assume E = o. An investigation of caisson designs indicates that fatigue results using the more accurate response distributions can differ appreciably from those obtained with the Rayleigh distribution.
This paper presents a procedure for fatigue analysis where the dynamic response of the structure is analyzed through a spectral approach. The sea waves, which constitute the forcing function acting on the structure, are represented as energy spectra; the response is obtained in spectral terms and subsequently is interpreted according to probabilistic concepts. Introduction The critical loading conditions for design of an offshore platform are primarily dependent on water depth and prevailing meteorological conditions such as waves, currents, and winds. If dynamic amplification is not a major consideration, the static applied force of the largest single wave traditionally is used as the critical or design load condition for fixed offshore structures. In the corresponding fatigue analysis the sea surface is represented by a discrete set of waves characterized by period, amplitude, and percentage of occurrence. The expected fatigue life then is obtained by applying each wave to the structure and totaling the individual effects.Due to the dynamic amplification of wave-induced response, this approach is not adequate for deepwater structures, where fatigue may be a controlling factor in design. In this case, distribution of the frequencies of wave loadings is needed over a much larger range to give a more realistic description of the sea surface. In practice, this is accomplished by statistically modeling the sea-surface elevation. It has been shown that the time history of sea-surface elevation over a period of several hours (i.e., a sea state) can be represented as a stationary random process with Gaussian probability distribution and process with Gaussian probability distribution and narrow-band power spectrum. This work discusses a fatigue analysis procedure based on these considerations.The first three sections of this paper describe a fatigue analysis procedure in which the spectral density of member end stresses is obtained by a frequency domain solution using existing dynamic analysis and space-frame static analysis programs. The fourth section discusses narrow-band approximations for the stress-range distribution and their effect on fatigue analysis. The fifth section presents fatigue results for a caisson structure. The presents fatigue results for a caisson structure. The sixth section gives conclusions and observations. Fatigue Analysis Procedure The first step in this fatigue analysis technique involves a static analysis. The structural model uses a space-frame, full-geometry simulation of the jacket with three-dimensional beam elements. Every member of the structure contributing to the overall stiffness is modeled by one or more elements. Unit loads are applied at selected levels of the three-dimensional model, and the resulting deflections for each unit load are noted. For each preselected member, the stresses at the member ends also are computed and saved for use later in the analysis.Next, the wave-induced dynamic response is calculated using a modified version of the TOWER spectral analysis program developed by Penzien et al. JPT P. 1614
This paper was selected for presentation by the OTC Program Commibe following revi_ of information contained in an abstract submitted by the authorIal. Conl8nts of the paper, as Pfesented, have not been reM~by the OIfshore Technology Conference and are sUbject to correction by the author(s). The material, as presented, does nol necessarily reIIect any position of the OIfshore Technology Conference or its officers. Electronic reproduction, distribution, or _98 of any part of this paper for commercial pufJ'0S8S without the writlen consent of the OIfshore Technoiogy Conference is prohibited. Permission to reproduce in print is restricted to an abstract of nol more than 300 words; illustrations may nol be copied. The abstract must contlin conspicuous acknowledgment of wtlere and by wtIom the paper was presented. AbstractProduction from Bay of Campeche constitutes a significant percentage of the wealth of Mexico. One of the primary objectives of PEMEX is to assure that this production is not interrupted.In 1995 hurricanes Opal and Roxanne passed through Bay of Campeche (BOC). Opal did not cause any noticeable damage. Roxanne on the other hand was the most severe hurricane recorded in the Bay of Campeche in this century. Its anomalous behavior and the resulting extreme environmental conditions did considerable damage. PEMEX implemented a rapid repair program and was able to bring the production back to its normal levels within two months. Subsequently, PEMEX jointly with Instituto Mexicano del Petr61eo (IMP), launched an extensive program to inspect, assess, and repair the facilities. Results of the initial assessments indicated that the assessment parameters as well as the metocean criteria utilized were not suitable for Bay of Campeche. A detailed risk study considering the site specific conditions and requirements was performed. Utilizing probability based risk assessment techniques a new criteria. known as the "transitol)' criteria" was developed. The transitol)' criteria was then used to re-assess the platforms and pipelines in the Southwest Marine Region of BOC.
Th~s paper was selected for presentation by the OTC Program Commtttee followmg revlew of ~nformat~on contained In an abstract submitted by the aulhor(s) Contents of the paper aspresented, have not been rev~ewed by the Offshore Technology Conference and are subject to mrrectlon by the author(s) The material, as presented does not necessarily reflect any postt~on of the Offshore Technology Conference or ~ts officers Electmnlc reproduct~on, dlstnbullon, or storage of any part of thls paper for commerc~al purposes w~thout the wrltten consent of the Offshore Technology Conference 1 s proh~b~ted Permlsslon lo reproduce In prlnt IS restraed to an abstract of not more than 300 words ~llustratlons may not be copled The abstract must contaln consplwous acknowledgment of where and by whom the paper was presented Abstract 'This paper summarizes the application of the new criteria for the assessment of fixed offshore platforms located in the Bay of Campeche, Mexico. The platform categorization process and the structural analyses performed are described. Design level analysis and ultimate strength analysis procedures and results are presented, which included the modeling of the damage, the application of the Load Reduction Factors (LRF's) and the calculation of the Reserve Strength Ratios (RSR's). For those platforms that exhibit inadequate deck height, calculation of the wave loading on the deck is described. Also, seismic assessment procedures and results are summarized. The results of these analyses were used to determine the fitness-for-purpose adequacy of seventeen PEMEX platforms located in the Southwest Marine Region of the Bay of Campeche. 2 L.E. REY, Y .O. BAYAZITOGLU, V.M. VALDES OTC 8693and load levels. Platform Assessment InformationA considerable amount of existing information was gathered in order to allow an engineering assessment of the overall structural integrity of the platforms. The information included: structural drawings, pile driving records, project design books, geotechnical studies and inspection reports before and after hurricane Roxanne. For some platforms, the design information was not available and certain assumptions had to be made. An essential part of the assessment process was to review the information collected for each platform. Platform damage prior and after the occurrence of the hurricane was identified. The actual location of the equipment and appurtenances, such as boat landings, riser guards, barge bumpers etc., was identified and compared with the location specified in the structural drawings. Pile driving records were used to compare the actual pile penetration with the corresponding design penetration. Actual marine growth measurements were compared to those used in the design of the platforms. Finally, all the information collected was used to properly model the platforms for structural assessment. Platform Special InspectionsAs part of the assessment process, special inspection programs were conducted to gather the most up-to-date information to be used in the structural assessm...
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