Optimal maintenance strategies for systems with partial repair options and without assuming bounded costs

We study a repairable system with Markovian deterioration and partial repair options, carried out at fixed times $n=1,2,\ldots$ and look for optimal strategies under certain conditions. Two optimality criteria are considered: expected discounted cost and long-run average cost. Douer and Yechiali found conditions under which a policy in the class of generalized control limit policies is optimal. In this paper conditions are found under which an optimal policy is a control-limit policy. We explicitly explain how to derive this optimal policy; numerical examples are given, too

A discounted model for a repairable system with continuous state space

We examine repairable systems with a continous state space and partial repair options, carried out at fixed times $n=1,2,...$. Every time interval $[n,n+1)$ there is a manufacturing cost and a repair cost. These cost functions are not restricted to the class of bounded functions in this study. Conditions are found under which a control-limit replacement policy minimizes the discounted cost. Hence these conditions guarantee that there is an optimal policy under the discounted cost criterion which does not use partial repairs. We explicitly explain how to derive this optimal policy

Sedimentology of the Triassic–Jurassic boundary beds in Pinhay Bay (Devon, SW England)

Sedimentology of the Triassic–Jurassic boundary beds in Pinhay Bay(Devon, SW England). Proceedings of the Geologists’ Association, 112. 349–360. New exposures in Pinhay Bay (SE Devon) of the White Lias (Langport Member of the Lilstock Formation)and basal Blue Lias reveal rapidly changing palaeoenvironments during the Triassic–Jurassic(T–J) boundary interval. During deposition of the topmost White Lias a soft seafloor of micritic mudstone was lithified and bored. The resultant hardground was locally eroded, probably in a shallow marine setting, to form a spectacular intraformational conglomerate that was itself lithified. Brief subaerial emergence then followed and produced a fissured and pitted top surface to the White Lias. The regression was short lived and rapid transgression at the base of the Blue Lias established organic-rich shale deposition with a small framboidal pyrite population and low Th/U ratios indicative of a stable, sulphidic lower water column (euxinic conditions). The White Lias/Blue Lias contact thus records a short duration, high amplitude relative sea-level change. This sea-level oscillation has also been postulated for other T–J boundary sections in Europe, although the failure to identify it in regional-scale sequence stratigraphic studies is probably due to its brief duration. Deposition of the basal beds of the Blue Lias was marked by a discrete phase of syn-sedimentary folding and small growth fault activity that may record a regional pulse of extensional tectonic activity

Water 'Scarcity' in Chennai, India: Institutions, Entitlements and Aspects of Inequality in Access

Utilities, Water supply, Entitlements, Regulation, Asia

The Western Irish Namurian Basin reassessed

Current basin models for the Western Irish Namurian Basin (WINB) envisage an elongate trough along the line of the present-day Shannon Estuary that was infilled with clastic sediments derived from a hinterland that lay to the W or NW. This paper argues for an alternative basin configuration with source areas to the SW supplying sediment to a basin where deepest water conditions were in northern County Clare. Rapid subsidence along the present-day Shannon Estuary ponded sediment in this area throughout the early Namurian and, only with the rapid increase of sedimentation rates within the mid-Namurian (Kinderscoutian Stage), were substantial amounts of sediment able to prograde to the NE of the basin. This alternative model better explains the overwhelming predominance of NE-directed palaeocurrents in the Namurian infill, but requires fundamental revisions to most aspects of current depositional models. Deep-water black shales (Clare Shale Formation) initially accumulated throughout the region and were progressively downlapped by an unconfined turbidite system (Ross Formation) prograding to the NE. This in turn was succeeded by an unstable, siltstone-dominated slope system (Gull Island Formation) characterized by large-scale soft-sediment deformation, which also prograded to the NE. In the northern-most basin outcrops, in northern County Clare, this early phase of basin infill was developed as a condensed succession of radiolarian-rich black shales, minor turbiditic sandstones and undisturbed siltstones. The new basin model envisages the northern exposures of County Clare to be a distal, basin floor succession whereas the traditional model considers it a relatively shallow, winnowed, basin margin succession. Later stages of basin infill consist of a series of deltaic cycles that culminate in major, erosive-based sandstone bodies (e.g. Tullig Sandstone) interpreted either as axial, deltaic feeder channels or incised valley fills genetically unrelated to the underlying deltaic facies. Within the context of the new basin model the former alternative is most likely and estimated channel depths within the Tullig Sandstone indicate that the basal erosive surface could have been generated by intrinsic fluvial scour without recourse to base-level fall. The northerly flowing Tullig channels pass down-dip into isolated channel sandbodies interbedded with wave-dominated strata that suggest the deltas of the WINB were considerably more wave-influenced than hitherto proposed. The retreat of the Tullig delta during sea-level rise saw the rapid southerly retrogradation of parasequences, as may be expected if the basin margin lay to the SW of the present-day outcrops

European Union environmental regulations and their potential impact on market access for Africa's exports

international trade;impact assessment;European Union;Africa;access to markets;environmental protection;exports;international agreements;environmental standards

Consumer Preferences for Water Supply? An Application of Choice Models to Urban India

Water supply, Consumer preferences, Discrete choice, Lexicographic preferences

Genetic diversity in wild stocks of the giant freshwater prawn (Macrobrachium rosenbergii): implications for aquaculture and conservation

The giant freshwater prawn (Macrobrachium rosenbergii) is cultured widely around the world but little is known about the levels and patterns of genetic diversity in either wild or cultured stocks. Studies have suggested that genetic diversity may be relatively low in some cultured stocks due to the history of how they were founded and subsequent exposure to repeated population bottlenecks in hatcheries. In contrast, wild stocks have an extensive distribution that extends from Southern Asia across Southeast (SE) Asia to the Pacific region. Therefore, wild stocks could be an important resource for genetic improvement of culture stocks in the future. Understanding the extent and patterns of genetic diversity in wild giant freshwater prawn stocks will assist decisions about the direction future breeding programs may take. Wild stock genetic diversity was examined using a 472 base-pair segment of the 16S rRNA gene in 18 wild populations collected from across the natural range of the species. Two major clades ("eastern" and "western") were identifi ed either side of Huxley’s line, with a minimum divergence of 6.2 per cent, which implies separation since the Miocene period (5-10 MYA). While divergence estimates within major clades was small (maximum 0.9 per cent), evidence was also found for population structuring at a lower spatial scale. This will be examined more intensively with a faster evolving mtDNA gene in the future

Optimal Taxation and Social Insurance in a Lifetime Perspective

Advances in information technology have improved the administrative feasibility of redis- tribution based on lifetime earnings recorded at the time of retirement. We study optimal lifetime income taxation and social insurance in an economy in which redistributive taxation and social insurance serve to insure (ex ante) against skill heterogeneity as well as disability risk. Optimal disability benefits rise with previous earnings so that public transfers depend not only on current earnings but also on earnings in the past. Hence, lifetime taxation rather than annual taxation is optimal. The optimal tax-transfer system does not provide full disability insurance. By offering imperfect insurance and structuring disability benefits so as to enable workers to insure against disability by working harder, social insurance is designed to offset the distortionary impact of the redistributive labor income tax on labor supply.Optimal lifetime income taxation;optimal social insurance

Irrigation investment trends in Sri Lanka: new construction and beyond

Investment, Water management, Rehabilitation, Rice, Crop Production/Industries, Farm Management,