International Journal of Coastal, Offshore and Environmental Engineering
Volume:4 Issue: 2, Spring 2019

A three-dimensional numerical simulation of regular waves passing over a monopile with square and circular cross-sectional shape was carried out to investigate flow field and vortex induced vibration. The rectangular wave flume and monopile are modeled with a solver; available in the open-source CFD toolkit OpenFOAM®. This solver applies the Reynolds-Averaged Navier-Stokes (RANS) equations with the volume of fluid technic (VOF) for tracking free surface. The motion equation together with mesh deformation was applied to capture monopile displacement. To validate the numerical model, results were compared to experimental data, and an admissible agreement was seen. Computations were conducted for four cases with two different wave characteristics and different Keulegan-Carpenter (KC) numbers for square and circular cross-sectional shape. Vorticity field and Q criterion around the square and circular pile were depicted. It was seen that when KC increased, the difference in vortices around the square and the circular pile was more distinct. Investigations continued on transverse force coefficient and its oscillations. It was seen that by increasing KC, this coefficient and its frequency increased. When KC=20, the lift coefficient is larger for square pile compared to the circular pile. For both square and circular cross-sectional shape, the number of pile oscillation increased by increasing KC number. Also, the Strouhal number and vortex shedding frequency were larger for the circular pile compared to that of the square pile in vortex shedding regime. However, cross-flow vibration frequencies of the square and circular pile were close together.

The objective of present research was the derivation of probability density functions (PDFs) for the degree of bending (DoB) in tubular KT-joints commonly found in jacket-type platforms. A total of 243 finite element (FE) analyses were carried out on 81 FE models of KT-joints subjected to three types of in-plane bending (IPB) moment loading. Generated FE models were validated using experimental data, previous FE results, and available parametric equations. Based on the results of parametric FE study, a sample database was prepared for the DoB values and density histograms were generated for respective samples based on the Freedman-Diaconis rule. Thirteen theoretical PDFs were fitted to the developed histograms and the maximum likelihood (ML) method was applied to evaluate the parameters of fitted PDFs. In each case, the Kolmogorov-Smirnov test was used to evaluate the goodness of fit. Finally, the Generalized Extreme Value model was proposed as the governing probability distribution function for the DoB. After substituting the values of estimated parameters, nine fully defined PDFs were presented for the DoB at the crown, toe, and heel positions of the central and outer braces in tubular KT-joints subjected to three types of IPB moment loading.

Assessing the impact of physical properties on active reaction changes in the coastal and offshore area is one the most important aspect of marine environment of the Caspian Sea. Therefore, updated techniques and modern instruments have been used in order to evaluating the coastal and offshore area conditions. In the current research, collected data with spatial and temporal variety have been evaluated for investigation on seawater characteristics in the shallow and intermediate layers over the southern Caspian Sea. Some phenomena such as mixing, turbulence, water column stability and stratification are the effective elements on variability of physical and natural structures of the sea. Active reaction and dissolved oxygen as two properties of seawater are very important items for coastal engineering, piping in seabed, breakwaters and port constructions. Thus, in the current study, the above mentioned parameters were assessed over the southern shelf of the Caspian Sea. Vertical and horizontal variations of chemical characteristics such as active reaction of seawater were observed in several stations between coastal and offshore stations across surface, intermediate and deep layers. Results of the field operations showed that the normal values of pH in the study almost varied around 7.9-8 while during some seasons increased ore decreased more than 0.5 units due to human activates.

Cyclones are as one of the most dangerous meteorological phenomena of the tropical region that generate strong winds and heavy rainfall, impacting coastal regions. Behavior of tropical cyclone trajectories needs to be better understood in order to find predictable aspects of landfall potentially. This research aims to analyze the cyclone tracks statistically and then study of the associated meteorological effects of Gonu cyclone in June 2007 as an example. Using the cluster analysis (K-mean method) 5 principal clusters have been derived and spatial – temporal studies including the monthly variation of cyclone trajectories and their intensity and frequencies are performed. The 4th cluster indicated more spatial variability and expansion (4º-31ºN and 48.5º-78ºE). The second cluster showed the highest frequency with 349 events as well as the highest maximum intensity and standard deviation of 235.2km/h and 47.96km/h respectively. During 1-7 June 2007, the Gonu super cyclone traversed the Arabian Sea and reached the Iranian southern regions affecting the coastal infrastructures and communities. In this 7-day period the daily composite maps of different atmospheric levels showed that by intensifying of Gonu cyclone the axis of subtropical high pressures in the lower levels moved to the east and in the 500hp level they moved to the northward over the study region.

Among the numerous problems that decrease the capability of a harbor in the country, seasonal sedimentation is identified as a major problem for many fishery harbors. In 2007, Zarabad Fishery Harbor conditions were also identified as critical due to the large volume of sand accumulation and consequent closure of its entrance.  Numerical modeling of coastal bed level change was implemented to provide insight into the typical response of the Zarabad beach to regular wave attacks, and to obtain an operational and validated model for the site. Advanced numerical models employed to predict coastal evolution at a variety of time and spatial scales usually include many free parameters that require calibration to the available field data. The XBeach numerical model was selected for its capacity to accurately model hydrodynamic and morphological processes over a two-dimensional domain.  It comprises about 250 model settings that approximately 150 of these settings relate to physical and numerical Behavior and the other 100 are case-specific parameters. In this research, 11 parameters are adopted to optimize the model prediction efficiency for Zarabad Fishery Harbor area. For calibration and validation stages, two cross-shore profiles and two medium-term time periods are selected. The model showed great promise in predicting the evolution of cross-shore profiles under water, but as expected, the dry part results showed major errors. XBeach proved to be an operational tool to predict cross-shore profiles in the area, in such timescales. Although, more tests are needed to utilize the model in longer time periods with regard to the duration of simulations.

The capacity curve obtained from the pushover analysis of jacket-type offshore platforms gives their structural performance levels, ultimate capacity and ductility. Accurate estimation of structural capacity curve is of great importance. Accurate modeling of the global and local buckling of compression tubular members in a correct form is an effective part of studying the behavior of offshore jackets under all various types of loading conditions at any given time of their life. Modeling of compressive braces by shell or solid elements when the imperfections are applied leads to deformations due to local buckling based on buckling modes. This paper aims to achieve more accurate compressive behavior of compression members. The ABAQUS finite element software has been used for this purpose. Regarding to the results achieved from investigation of buckling in tubular members proper elements have been introduced to investigate the global and local buckling phenomena. Then pushovers results of Ressalat jacket with conventional modeling versus more accurate modeling proposed in this paper for compressive members have been compared as a case study. According to the results applying improper mesh size for compressive members can under-predict the ductility by 33% and under-estimate the lateral loading capacity up to 8%. Finally, ISO equations and Marshall strut theory have been applied to investigate critical buckling load and post-buckling response of tubular braces. The innovation of this paper is investigating the interaction of global and local buckling in the braces of jacket with 1-Dimentional elements using ISO equations and buckling envelope derived from the solid element results, which results in low computational costs.