International Journal of Coastal, Offshore and Environmental Engineering
Volume:2 Issue: 4, Autumn 2017

Fully dynamic analysis of offshore structures under random wave loads in time domain is sometimes necessary for calculating structural responses in design issues. Such analyses are very time consuming and therefor simplified methods for estimation of acceptable response of these structures can be very useful in initial design. In this paper an innovative method to obtain response spectrum of fixed offshore structures caused by extreme waves is represent based on concept of impulse response spectrum. For this purpose the structural system is considered as a simple one degree of freedom structure and the different sea states are equalized to different half sinusoidal pulses. Response spectrum of structure is defined as a plot of structural response to these pulses for different periods. By using this method the cost of computations is decreased significantly while the accuracy of results was preserved.

Coastal erosion has always been a serious problem in shorelines, which causes heavy damage to coastlines and public properties around the world. Rapid and uncontrolled coastal developments were started to prevent the exacerbation of erosion hazards. However in turn they have had a damaging impact on coastal ecosystems. The current study presents a comparative environmental impact assessment of two protective alternatives in coastal and shoreline structures, including traditional rubble system and a more novel one called geotube system. A case study of two breakwater structures in Bushehr and Qeshm in the south of Iran was performed and beneficial aspects of geotube system were identified.Coastal erosion has always been a serious problem in shorelines, which causes heavy damage to coastlines and public properties around the world. Rapid and uncontrolled coastal developments were started to prevent the exacerbation of erosion hazards. However in turn they have had a damaging impact on coastal ecosystems. The current study presents a comparative environmental impact assessment of two protective alternatives in coastal and shoreline structures, including traditional rubble system and a more novel one called geotube system. A case study of two breakwater structures in Bushehr and Qeshm in the south of Iran was performed and beneficial aspects of geotube system were identified.

Structural integrity evaluation of an existing offshore platform typically is based on a combination of non-linear structural analysis together with risk and reliability analysis. Quantitative assessment of the probability of failure of a jacket platform under extreme metocean loads is a multi-disciplinary task, poses significant challenges and involves a large number of uncertainties regarding the metocaen hazards, structural system and modelling, loads, material behaviour and geotechnical information. The probability of failure is commonly estimated using a reliability analysis to account for uncertainties in derivation of both the loading and the strength.
Different sources of uncertainties contribute to the platform ultimate strength. Sensitivity analysis should be carried out to provide quantitative information necessary for classifying random variables according to their importance. These measures are essential for reliability-based service life prediction of deteriorating materials and structures. Accordingly, in this paper, Tornado approach has been used to identify those variables that affect the failure most so that more research can focus on those variables. To this end, six existing offshore platforms located in Persian Gulf are investigated. The results have been presented in the form of a Tornado diagram which will graphically show the sensitivity of the target function to each random variable.

Monopiles are common foundations for offshore structures such as wind turbines and they are commonly used as fender piles in port structures. In such structures, especially in OWTs, the ratio of pile length to diameter (L/D) is small which makes the pile behave as a rigid structure. However, the pile flexural (bending) stiffness still affects the pile load-displacement characteristics and maybe should not be ignored in the pile design. In this study, the effect of pile flexural stiffness on a short monopile subjected to static lateral load is investigated. The modeled pile has aspect ratio of 5 (L/D=5) which is driven into medium sand. The main characteristics of pile static lateral behavior including lateral resistance, stiffness, deflections, bending distribution and toe-kick are investigated and results are discussed. Results show that in the studied case, the pile behavior pattern is in the middle of flexible and completely rigid pile and therefore bending stiffness has meaningful effects on some important parameters related to monotonic performance.

Developing numerical tanks to study wave structure interaction drew engineers’ attention in last decade. Numerical wave tanks are absolutely essential for investigating wave-structure interaction. This paper presents two different numerical software capabilities to generate regular gravity waves in a wave tank. The wave generation was performed using the FLUENT package and Flow-3D. Both models are based on Navier-Stokes and VoF equations. The results of the mentioned models were compared with theoretical results. Free surface elevation and horizontal component of wave particle velocity were the two parameters which have been considered for comparison. Results indicate that Flow-3D in some cases is a bit more accurate than Fluent in capturing free surface elevation. In numerical models it is important to dissipate wave energy and prevent wave reflection. In this way four different slopes were evaluated to determine the minimum slope needed for wave energy dissipation. The results showed that a minimum slope of 1V:35H is needed to avoid wave reflection. The variation of streamlines and velocity potential are also studied. The pattern of horizontal and vertical velocity variation in the fluid domain is similar to stream function and potential velocity function variation, respectively.

Structural response of different types of conventional deep water riser affected by mooring systems in harsh environment is focused in this study. With increasing water depth, the mooring system design of a floating-type production platform becomes more important and complicated from cost as well as safe operation point of view. With the development of offshore oil and gas exploitation, FPSO with many advantages can better adapt to the complicated environment of deep sea. According to the current mooring system development, this study compares the effect of spread mooring system and turret mooring system on riser structural response. Both systems consists of 12 mooring lines and each line is made from two components. Three types of more conventionally used deep water risers have been selected and applied with the mooring systems, resulting in 6 different case studies. The case studies have been modeled in OrcaFlex software with same extreme environmental conditions. Comparing bending moment in the risers, as one of the main design parameter, shows the effect of proper selection of the riser and mooring system.