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Showing 9 results for Offshore

Kimiaei M., Shayanfar M.a., Hesham Ei Naggar M., Agha Kouchak A.a.,
Volume 2, Issue 2 (6-2004)

The seismic response of pile-supported offshore structures is strongly affected by the nonlinear behavior of the supporting piles. Nonlinear response of piles is the most important source of potentially nonlinear dynamic response of offshore platforms due to earthquake excitations. It is often necessary to perform dynamic analysis of offshore platforms that accountsfor soil nonlinearity, discontinuity condition at pile soil interfaces, energy dissipation through soil radiation damping and structural non linear behaviors of piles.In this paper, an attempt is made to develop an inexpensive and practical procedure compatible with readily available structural analysis software for estimating the lateral response of flexible piles embedded in layered soil deposits subjected to seismic loading. In the proposed model a BNWF (Beam on Nonlinear Winkler Foundation) approach is used consisting of simple nonlinear springs, dash pots and contact elements. Gapping and caving-in conditions at the pile-soil interfaces are also considered using special interface elements. This model was incorporated into a Finite Element program (ANSYS), which was used to compute the response of laterally excited piles. A linear approach was used for seismic free field ground motion analysis. The computed responses compared well with the Centrifuge test results.This paper deals with the effects of free field ground motion analysis on seismic non linear behavior of embedded piles. Different parts of a BNWF (Beam on Nonlinear Winkler Foundation) model, together with quantitative and qualitative findings and conclusions for dynamic nonlinear response of offshore piles, are discussed and addressed in detail. The proposed BNWF model (only using the existing features of the available general finite element software) could easily be implemented in a more comprehensive model of nonlinear seismic response analysis of pile supported offshore platforms.
Golparvar Fard M., Yeganeh Bakhtiary A., Cheng L.,
Volume 3, Issue 1 (3-2005)

This paper presents a k- turbulence model for simulation of steady current and itsinduced vortex shedding caused by the presence of an offshore pipeline. Performance of the modelaround a circular cylinder above a wall with gap to diameter ratios of 0.1, 0.35 and 0.5 underdifferent flow regimes with Reynolds numbers of 1500, 2500 and 7000 is studied. The flow field iscomputed with solving the Reynolds Averaged Navier-Stokes equations (RANS) the seabed underpipeline is treated as a plane boundary with no-slip boundary condition on pipe surface. Thegoverning equations are solved using Finite Volume Method in a Cartesian coordinate system.Based on the numerical solutions, the flow field, vortex shedding and distribution of shear stressdue to the presence of the pipeline near seabed are studied. In addition the mechanism of vortexshedding with different gap to diameter ratios is examined with focusing on the effect of vortexshedding on bed shear stress. It is found that the k- turbulence model can well predict the flowfield and its induced vortex shedding around a pipeline hence it can be easily applied forsimulation of scour below an offshore pipeline.
Saleh Zadeh H., Procter D.c., Merrifield C.m.,
Volume 3, Issue 3 (9-2005)

The unique behaviour of carbonate materials under shear loading has stimulated in investigating of their geological and engineering properties.Carbonate soils composed of calcium or other carbonates and most abundant in tropical marine environments are of interest from geotechnical view, especially for offshore engineers engaged with Fossil-based fuel exploitation. This was initiated in the early 1960's, when the first offshore borings in the Persian Gulf identified layers of calcarenite and thick layers of sand containing visible shell fragments.For the purpose of exploiting gas and oil resources in hot and temperate climates (e.g. Persian Gulf) off-shore structures have been placed on carbonate soils. The carbonate sediments are high crushable compared with low crushable sediments such as quartzic soils.To examine the crushability of these problematic sediments a series of monotonic compression, extension and post-cyclic triaxial tests under different densities and confining pressures was carried out to study the crushing behaviour of "Rock" carbonate sand obtained from Cornwall, England.It was shown that crushing coefficient decreases with increasing in maximum principal effective stress ratio for both loose and dense states. It seems that for skeletal carbonate sand maximum and minimum dry densities will be changed during shearing loading. In other words, even though the sample has experienced an increase in density, it may also have experienced a reduction in relative density.
A. Yeganeh Bakhtiary, A. Ghaheri, R. Valipour,
Volume 5, Issue 1 (3-2007)

Determination of allowable free span length plays a crucial role in design of offshore pipelines. The seabed intervention cost and safety of an offshore pipelines project are largely influenced by pipelines free spanning during the project life time. Different criteria are proposed by both the current designing guidelines and researchers there is however lack of comprehensive assessment of independent parameters affects the design length of free span. In this note, it is intended to investigate the effects of seabed formation along with axial force on Natural Frequency of offshore pipelines. Based on this assessment a new simple formula is proposed. Finally, to evaluate the result of this study, the allowable free span length of Qeshem Island pipelines is calculated as a case study and compared with those of the DNV (1998) and ABS (2001) guidelines and the modal analysis.
M. Heidarzadeh, M. Dolatshahi Pirooz, N. Hadjizadeh Zaker, M. Mokhtari,
Volume 5, Issue 4 (12-2007)

Makran Subduction Zone (MSZ) offshore of Iran and Pakistan is one of the most tsunamigenic sources in the Indian Ocean. Historically, the MSZ has generated some tsunamigenic earthquakes like that of 28 November 1945 with the death tool of more than 4000 people along the coasts of Iran, Pakistan, India, and Oman. In this study, the tsunami hazard associated with the MSZ is investigated. At first, a review of historical tsunamis in the Indian Ocean basin was performed which reveals the Makran region has experienced al least 4 tsunamis including events of 326 BC, 1897, 1008, and 1945. Consequently, since the pattern and extent of vertical ground deformation from an earthquake determines whether or not a tsunami is formed, a computer program is developed to predict the seafloor deformation due to the earthquake occurrence in the MSZ. The model was verified through run of it on some actual tsunamis so far occurred. Then, using the data of the 1945 Makran tsunami, the seismic parameters of the MSZ were calibrated. Finally, we used the developed computer program to calculate seafloor deformation at the location of Makran subduction zone for several earthquake scenarios with moment magnitudes ranging between 6.5 and 8.5. The results of this research show that the risk of tsunami generation from MSZ can be classified into three main categories, as follows: (1) very little risk for tsunami generation in the case of the occurrence of an earthquake having magnitude up to 7 (2) little to medium risk for moment magnitudes ranging between 7 and 7.5 and (3) high risk for moment magnitude greater than 7.5. At the end of the paper, modeling of tsunami propagation is performed for an earthquake scenario with magnitude of 8 offshore Chabahar, in order to give preliminary information about tsunami behavior in this region.
H. Ghiassian, M. Jalili, I. Rahmani, Seyed M. M. Madani,
Volume 11, Issue 4 (12-2013)

The concept of Geosynthetic Cellular Systems (GCS) has recently emerged as a new method in construction of breakwaters and coastal protective structures. The method potentially has significant advantages compared to conventional systems from the standpoint of constructability, cost effectiveness, and environmental considerations. This paper presents the results of physical model testing on the hydraulic responses of GCS structures under wave action. A series of model tests were carried out in a wave flume on GCS models with different shapes and soil types, subjected to various wave characteristics. Horizontal wave forces acting on the models were measured at different elevations. The maximum horizontal force in each test was calculated and compared with conventional formula of predicting wave pressure on breakwaters. The results show that Goda’s equation overestimates the hydrodynamic water pressure on these structures. This can be attributed to the influence of seeping water through the GCS models because of relative permeability of the GCS.
N. Abedimahzoon, A. Lashteh Neshaei,
Volume 11, Issue 4 (12-2013)

In this paper, a new approach is presented for estimating the vertical and horizontal distribution of undertow in the surf zone for reflective beaches. The present model is a modification of the original model presented by Okayasu et al., (1990) for natural, non-reflective beaches to include the effect of partially reflected waves. The nonlinearity of waves, wave-current interaction and nonlinear mass drift of the incident wave are also included in the present model. The results of experimental investigation and model development show that existence of reflective conditions on beaches results in a reduction in the magnitude of undertow and modifies its distribution across the beach profile. Comparison of the results by those obtained from the experiments clearly indicates that by taking the nonlinearity and wave-current interaction, the predictions of undertow in the surf zone are much improved. In particular, due to the effect of turbulence induced by wave breaking for nonlinear waves, the predicted results show more consistence with the measurements.
A. Gholizad, P. Kamrani Moghaddam,
Volume 12, Issue 1 (3-2014)

High performance and reliability of refurbish able knee braced steel frames has been confirmed in previous researches trying to get an optimal design for its configuration. Buckling of diagonal member which affects the hysteretic behavior of KBF under cyclic loadings has not been foreseen in previous evaluations of this system. This deficiency can be improved by utilization of adjustable rotary friction damper device (FDD) as knee element. Diagonal element buckling can be prevented considering a suitable value for FDD sliding threshold moment Mf. Lower values of Mf Lower energy dissipation rate in FDD and this leads to an optimization problem. Nonlinear time history analyses have been performed in addition to lateral cyclic loading analyses to evaluate the response of single story KBF subjected to seismic excitation. Optimal Mf in FDD has been chosen according to these analyses results. Roof displacement and acceleration, base shear and diagonal element’s buckling status have been compared in optimally designed KBF and FDD utilized KBF (FKBF) with different configurations. Nonlinear dynamic analyses have been performed for one, four, eight and twelve story frames under different seismic records with several PGAs. More than 60% displacement response reduction has been earned for the FKBF without considerable increase in base shear.
A. Eslami, I. Tajvidi, M. Karimpour-Fard,
Volume 12, Issue 1 (1-2014)

Three common approaches to determine the axial pile capacity based on static analysis and in-situ tests are presented, compared and evaluated. The Unified Pile Design (UPD), American Petroleum Institute (API) and a SPT based methods were chosen to be validated. The API is a common method to estimate the axial bearing capacity of piles in marine environments, where as the others are currently used by geotechnical engineers. Seventy pile load test records performed in the northern bank of Persian Gulf with SPT profile have been compiled for methods evaluation. In all cases, pile capacities were measured using full scale static compression and/or pull out loading tests. As the loading tests in some cases were in the format of proof test without reaching the plunging or ultimate bearing capacity, for interpretation the results, offset limit load criteria was employed. Three statistical and probability based approaches in the form of a systematic ranking, called Rank Index, RI, were utilized to evaluate the performance of predictive methods. Wasted Capacity Index (WCI) concept was also applied to validate the efficiency of current methods. The evaluations revealed that among these three predictive methods, the UPD is more accurate and cost effective than the others.

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