Conservative Design Approach Research Articles

A discrete post-processing method for structural optimization

This paper introduces a discrete variable post-processing method for structural design optimization. The motivation behind the method is to find a good discrete solution at manageable cost while the traditional discrete optimization algorithms are regarded as impractical for large-scale structural design problems. In this paper, the Design of Experiments (DOE) and Conservative Discrete Design (CDD) approaches have been proposed to deal with discrete variables with limited computational cost. Both methods work on the explicit approximate discreteproblem to explore the discrete design. These two approaches, together with engineering rounded-off methods, can be used to process discrete variables at any specified continuous design optimization cycle for structural design problems. Brief background and a theoretical discussion about these approaches are given in this paper. Finally, the methods that have been implemented in MSC.Nastran are demonstrated by academic and real engineering examples.

Notch geometry effects in fatigue: A conservative design approach

The understanding of stress concentrators (notches) is an important element in the prevention of failure in components and in the analysis of failures when they occur. This paper examines critically the methods currently used to predict the fatigue strength of components containing notches, with particular attention to the need for a conservative design approach. Current methods, if correctly applied, are shown to give conservative predictions of fatigue limit and high-cycle fatigue strength, and it is shown that the same philosophy can be extended to cover very small notches, including surface roughness. The presentation of these predictions in the form of a “mechanism map” for notch fatigue is advocated as a useful tool for designers. The problems of extending the approach to other types of stress concentrator, such as fillet radii, are discussed.

Groundwater as a feedwater source for membrane treatment plants: Hydrogeologic controls on water quality variation with time

A major misconception in the design of membrane water treatment facilities is that if a groundwater source is used to feed the plant, water quality will be relatively stable with time. There are wide variations in the rate of groundwater quality changes caused by pumping aquifers based on the local geology and hydrology. Brackish groundwater sources tend to show increases in the total dissolved solids concentration with time, while both brackish and seawater groundwater sources can have changes in the chemistry of the feedwater from basic dilute or standard seawater to water enriched in specific cation concentrations, particularly with iron or manganese. Modeling of these possible water quality changes should be accomplished prior to the final design of any membrane treatment facility.The primary geologic controls on groundwater quality within a subsurface aquifer system are the natural barriers which control the vertical inflow of water from adjacent aquifers containing water with different chemistries. Since the pumping rate of production wells is greater than the natural horizontal recharge rate in virtually all semi-confined aquifer systems, aquifer flow under an equilibrium condition is predominantly vertical through the confining beds. Therefore, the confining bed leakance coefficient along with other aquifer hydraulic characteristics, and the pattern and rate of pumping tend to control the rate of water quality change within any production aquifer. This water quality change is caused by mixing of the leaked water with the natural water contained in the production aquifer.Aquifer and confming bed materials tend to control the type of chemical composition changes in the water yielded to the wells with time. In limestone aquifers separated by compacted lime muds, such as in Florida, there are few changes in water chemistry outside of dilute seawater type compositions with the possible exception of sulfate and hardness. In clastic aquifers separated by clay mineral confining beds, there is a tendency for enrichment of the feedwater in dissolved iron or manganese. In fractured rock aquifers within igneous or metamorphic rocks, a wide range in water chemistry changes can occur.Most significant water quality changes can be predicted to some degree of accuracy based on the hydrogeologic test program conducted prior to wellfield design. If the geology is known, the relevant aquifer characteristics are measured, and the initial water quality in the aquifer above and below the production aquifer are measured, then changes in water quality with time can be modeled. Since the long-term water quality changes can be controlled to a large degree by the design of the feedwater wellfield, the economics of both water treatment and plant construction can also be significantly altered by well spacing and pumping rates. The accuracy of predictive solute transport modeling is directly related to the availability of data on the aquifer system modeled. Large errors in the modeled water quality changes can occur when data are insufficient. When significant restrictions on data occur, a very conservative wellfield design approach is prudent.

Design Aid-Anchor Bolt Interaction of Shear and Tension Loads

A common practice among structural engineers involved in industrial building design is to specify either ASTM A36 threaded rod bolts or A307 headed bolts for use as cast-inplace anchors to carry combined shear and tension forces to the foundation. It is also fairly common to specify ASTM A325 or equivalent strength material for anchor bolts which must carry higher forces than can be accommodated by ordinary carbon steel bolts. Several articles have been written in the past ten years which have addressed the problem of anchor bolt design for combined loadings. By consolidating and summarizing the available data, the problem can be simplified for most situations encountered in normal practice. A conservative design approach is warranted, as suggested by Marsh and Burdette since test data is limited and consequences of bolt failures are quite unacceptable for steel structures which must carry expensive industrial equipment.

Design Methodology for Offshore Platform Conductors

Summary The problem of analyzing and designing that segment of an offshore platform conductor located between the mudline and deck is examined. A new set of efficient and easy-to-use conductor design equations has been developed on the basis of the fundamental principle that internal casing string loads do not produce any buckling tendencies. An example conductor design has been included which illustrates that the method, when compared with the typical conductor design approach, is extremely efficient and cost effective, capable of producing several million dollars of cost savings for a major platform project. platform project. Introduction The offshore structural designer is occasionally confronted with a special problem for which an efficient and easy-to-use design solution is not ready problem for which an efficient and easy-to-use design solution is not ready available. The designer, often working under the pressures of time constraints, is usually hard-pressed to develop the special design solution needed. Consequently, when special design problems do arise, the resulting design solution may be extremely conservative. While such a design is undoubtedly safe from the standpoint of structural collapse, it can be wasteful of resources. Furthermore, the constraints imposed by an extremely conservative design approach may lead to decisions that complicate and hamper offshore drilling and producing operations. In the conductor design problem presented here, very large components of the axial load and bending moment acting on the conductor are produced by the casing and tubing strings located inside. The effect of these internal loads is quite different from that produced by loads resulting from sources external to the member. The familiar American Inst. of Steel Construction (AISC) specifications for steel construction, for example, are frequently used to check the structural adequacy of load-carrying members. Since the AISC interaction equations for strength and stability are intended for stress conditions produced by external loads, direct application of these equations to conductor design produces extremely conservative and unrealistic results, which not only require excessive amounts of steel but also can lead to increases in platform environmental loadings and attendant construction costs. Designers have at times concluded that reasonably sized conductors do not have sufficient strength and stability to allow hanging the casing and production tubing off at the top of the conductor, and that mudline suspension systems are therefore necessary. Use of mudline suspension systems, however, increases the cost of drilling and work over operations and adds to the complexity of a well. This paper develops a methodology for checking the strength and stability of conductors, using the well-known AISC criteria as a basis. The fundamental principle that axial loads and bending moments caused by internal casing strings do not produce any buckling tendencies is used to derive a modified design method that more accurately reflects the actual behavior of a conductor. Designs produced with the modified method should be more realistic and cost effective. An example problem is presented to demonstrate use of the modified design procedure and to estimate potential cost savings. JPT p. 1973

Design and Testing of 15kV to 35kV Porcelain Terminations Using New Connection Techniques

New techniques for conductor connection in underground cable terminations were investigated in a design of a new distribution class porcelain cable termination. Connections to the conductor were accomplished using set screws, building upon previous designs with additions to assure a conservative design approach. The connector design was tested according to applicable standards for load cycling of connections, and the result appears capable of conservative performance in the operating environment. Use of such screw connections in distribution primary circuits has been very limited. They are not suitable for insulated splices because of the bulk and sharp edges found in this type of connection. Use in terminations has also been limited and compression connectors usage is commonplace. The compression connector is not without its problems as stated earlier and includes the problem of selecting the proper die, maintaining the equipment in proper condition to give full pressure, bringing the equipment from ground to bucket during riser pole installation and in manipulating the large heavy compression head during installation. With these drawbacks in mind a termination design using set screw connectors conservatively designed to reduce field problems and make reliable connections was investigated.

Design and Testing of 15kV to 35kV Porcelain Terminations Using New Connection Techniques

New techniques for conductor connection in underground cable terminations were investigated in a design of a new distribution class porcelain cable termination. Connections to the conductor were accomplished using set screws, building upon previous designs with additions to assure a conservative design approach. The connector design was tested according to applicable standards for load cycling of connections, and the result appears capable of conservative performance in the operating environment.

Turbomachinery Design Considerations for the Nuclear HTGR-GT Power Plant

For several years, design studies have been underway in the U.S. on a nuclear closed-cycle gas turbine plant (HTGR-GT). This paper presents design aspects of the helium turbo-machine portion of these studies. Gas dynamic and mechanical design considerations are presented for helium turbomachines in the 400 MWe (non-intercooled) and 600 MWe (intercooled) power range. Design of the turbomachine is a key element in the overall power plant program effort, which is currently directed towards the selection of a reference HTGR-GT commercial plant configuration for the U.S. utility market. A conservative design approach has been emphasized to provide for maximum safety and durability. The studies presented for the integrated plant concept outline the necessary close working relationship between the reactor primary system and turbomachine designers. State-of-the-art technology from large industrial gas turbines developed in the U.S., considered directly applicable to the design of a helium turbomachine, particularly in the areas of design methodology, performance, materials, and fabrication methods, is emphasized.

Pressures on Silo Walls

Overpressures up to four times the static pressure have been measured during discharge of sand from a model silo. The overpressures can be associated with the fundamental flow patterns. Design and calibration of the pressure cells ensured that underegistration and arching did not occur. The work suggests that previous pressure measurements may be unreliable due to inadequate pressure measuring devices. The measured overpressures were generally greater than those recommended for use in design by the recently revised German and Russian codes. A more conservative design approach is suggested herein.