Jack up reliability analysis: An overview

Jack up is a mobile unit used for oil and gas exploration and production in offshore fields. On demand, the unit is moved and installed in a given location and used for a period up to 12 months before being un-installed and moved to another location. Due to its mobility and re-usability, when the unit is offered for use in a given offshore location, its suitability in terms of safe operation is evaluated in accordance with the guidelines of Site Specific Assessment (SSA) of jack up. When the unit failed safety assessment criteria, the guideline recommended that it is re-assessed by increasing the complexity of the assumptions and methods used. Reliability analysis theories are one of the frameworks recommended for the safety assessment of the units.With recent developments in uncertainty and reliability analysis of structures subject to stochastic excitation, this study aims at providing a review on the past developments in jack up reliability analysis and to identify possible future directions. The results from literature reviewed shows that failure probabilities vary significantly with analysis method used. In addition, from the variants of reliability analysis approach, the method of time dependent reliability for dynamic structures subject to stochastic excitation have not been implemented on jack ups. Consequently, suggestions were made on the areas that need further examination for improvement of the efficiency in safety assessment of the units using reliability theories.     

Hard-rock LHD cost estimation using single and multiple regressions based on principal component analysis

In feasibility studies and mine planning, accurate and effective tools and methods facilitating cost estimation play an important role. Load-Haul-Dump (LHD) machines are a key loading and haulage equipment in most of the underground metal mines and hard rock tunnels. In this paper, a cost estimation model of these vehicles has been presented in the form of single and multivariable functions. These functions have been provided on the basis of costs types (i.e. capital and operating costs) and motor types (diesel and electric). Independent variables, in the single regression analysis is bucket capacity and in Multiple Linear Regression (MLR) analysis include bucket capacity, overall width, overall machine height and horse power (HP). The MLR is conducted in three steps. First, with the help of Principal Component Analysis (PCA), correlation between independent variables is omitted. Thereafter, significant PCs are selected and used as independent variables in the MLR functions. Finally, the cost relationships are established as functions of initial LHD variables. The mean absolute error rates are 11.59% and 6.87% for the single and multiple linear regression functions, respectively.     

Cement take estimation using neural networks and statistical analysis in Bakhtiari and Karun 4 dam sites,in south west of Iran

Bulletin of Engineering Geology and the Environment - Water seepage from dam foundations causes reservoir water loss and raises the risk of dam instability. One method of remediation for...     

Verification of unified effective stress theory based on the effect of moisture on mechanical properties of fiber reinforced unsaturated soil

The unified effective stress theory based on suction stress (SSCC theory) enables the characterization of soils under both saturated and unsaturated conditions with one closed-form relationship. This study provides experimental verification of this theory through the unconfined compressive strength test (UCS) and indirect tensile test strength (ITS) on silty clay soil stabilized with fiber. A series of matric suction, ITS, and UCS tests were conducted to validate the SSCC theory through the representation of the results of ITS and UCS tests in terms of mean total stress (p) versus deviatoric stress (q) and mean effective stress (p`) versus deviatoric stress (q). The results of the validation procedures showed that the SSCC theory is applicable and valid at a range of 6%–16% of water content on the silty clay and the silty clay fiber-reinforced soils. There is a small fluctuation in the increase of ITS and UCS values with increasing fiber content due to randomly oriented distribution of the fiber. The addition of glass fiber does not significantly affect the capacity of water retention of the soil. It improves the condition of the mechanical soil properties at the end of construction more than of the effective stress condition.     

Stabilization of Indian peat using alkali-activated ground granulated blast furnace slag

Bulletin of Engineering Geology and the Environment - Peat is an exceptionally problematic soil for construction purposes and is often stabilized by traditional stabilizers (like cement), which...     

Measurement of additional shear during sludge conditioning and dewatering

Optimum polymer dose is influenced both by the polymer demand of the sludge and the shear applied during conditioning. Sludge exposed to additional shear following conditioning will experience a decrease in cake solids concentration for the same polymer dose. Therefore, it is necessary to measure or quantify the additional shear in order to optimize the conditioning and dewatering. There is currently no direct or indirect method to achieve this. The main objective of this study was to develop a method based on torque rheology to measure the amount of shear that a sludge network experiences during conditioning and dewatering. Anaerobically digested sludge samples were exposed to increasing levels of mixing intensities and times, and rheological characteristics of samples were measured using a torque rheometer. Several rheological parameters were evaluated including the peak torque and totalized torque (area under the rheograms). The results of this study show that at the optimum polymer dose, a linear relationship exists between the applied shear and the area under the rheograms, and this relationship can be used to estimate an unknown amount of shear that the sludge was exposed to. The method is useful as a research tool to study the effect of shear on dewatering but also as an optimization tool in a dewatering automation system based on torque rheology.     

Damage avoidance design of special truss moment frames with energy dissipating devices

An innovative concept using energy dissipating devices, such as buckling restrained braces (BRB), is proposed for special truss moment frames (STMF). The configuration of the proposed system consists of pins introduced at the ends of the top and bottom chord elements of the special segments. Subsequently, energy dissipating devices are used in the form of diagonal braces inside the special segments. An energy-based design methodology is adopted such that the BRBs are designed to provide sufficient energy dissipation capacity with respect to seismic input energy demand on the structure. This energy-based methodology is demonstrated to be accurate by means of a series of nonlinear time-history analyses. The overall seismic response of the proposed system is contrasted with the conventional STMF in terms of story displacements, interstory drifts, story shears and overturning moments, as well as observed damage to structural elements. The proposed system leads to more predictable seismic response and would potentially allow lighter construction and significant cost savings, due to significantly reduced member forces (up to 50% compared with conventional design). Furthermore, damage to structural elements is largely mitigated, hence allowing damage avoidance design of STMFs.     

Experimental investigation on the performance of multi-tiered geogrid mechanically stabilized earth (MSE) walls with wrap-around facing subjected to earthquake loading

Construction of mechanically stabilized earth (MSE) walls in multi-tiered configurations is a promising solution for increasing the height of such walls. The good performance of this type of walls after recent major earthquakes was reported in a number of technical studies. In the present study, an experimental approach was adopted to compare the seismic performance of single-tiered and multi-tiered MSE walls using physical modeling and through conducting a series of uniaxial shaking table tests. To do so, several geogrid-reinforced soil walls with wrap-around facing (i.e., three-, two-, and single-tiered) with a total height of 10 m were designed in the form of prototypes of 1-m-height wall models. The step-wise intensified sinusoidal waves were applied to the models in 14 typical forms. Comparing the shaking table test results confirmed the post-earthquake advantages of multi-tiered MSE walls. The results revealed that tiered walls exhibited better behaviors under earthquake loading in terms of the seismic stability of the wall, displacement of the wall crest, horizontal displacement of the wall facing, deformation mode and failure mechanism of the wall, settlement of backfill surface, and seismic acceleration responses.     

Thermodynamical analysis of dry cooling tower and solar chimney power plant combination

Solar chimney power plant is one of the rather new technologies that can produce power from solar energy. Its high stack is one of the important parts of the system in which the differential density of air among its top and bottom sections causes air-flow. Dry cooling towers are used in industries for cooling condensing water by utilising this concept and generating air-flow. The structural and conceptual similarity between these two systems cause the feasibility of their combination and usage of waste energy in industries. In this article, a simple and useful analytical thermodynamic model is improved in order to estimate the thermodynamic flow properties for combining both systems. Results show that the stack height and diameter are effective parameters in recovery of power. Moreover, the results show that the collector area does not remarkably affect the system performance.     

Progressive failure analysis of notched composite plate by utilizing macro mechanics approach

In this study, gradual and sudden reduction methods were combined to simulate a progressive failure in notched composite plates using a macro mechanics approach. Using the presented method, a progressive failure is simulated based on a linear softening law prior to a catastrophic failure, and thereafter, sudden reduction methods are employed for modeling a progressive failure. This combination method significantly reduces the computational cost and is also capable of simultaneously predicting the first and last ply failures (LPFs) in composite plates. The proposed method is intended to predict the first ply failure (FPF), LPF, and dominant failure modes of carbon/epoxy and glass/epoxy notched composite plates. In addition, the effects of mechanical properties and different stacking sequences on the propagation of damage in notched composite plates were studied. The results of the presented method were compared with experimental data previously reported in the literature. By comparing the numerical and experimental data, it is revealed that the proposed method can accurately simulate the failure propagation in notched composite plates at a low computational cost.