International Journal of Coastal, Offshore and Environmental Engineering
Volume:2 Issue: 3, Summer 2017

One of the crucial parameters in the fatigue reliability assessment of an offshore structure’s tubular joints is the stress concentration factor (SCF). Depending on the joint geometry and loading type, the SCF exhibits considerable scatter which emphasizes the significance of deriving its governing probability distribution function. In the present paper, results of 144 finite element (FE) stress analyses, verified against experimental measurements, were used to develop a set of probability density functions (PDFs) for the SCFs in uniplanar tubular KT-joints under four types of in-plane bending (IPB) moment load cases. Based on a parametric FE investigation, a sample database was created for the chord-side SCFs of central and outer braces; and density histograms were generated for respective samples. Nine theoretical PDFs were fitted to the developed histograms and the maximum likelihood method was applied to evaluate the parameters of fitted PDFs. The Kolmogorov−Smirnov test was applied to each case to assess the goodness of fit. Finally, the Inverse Gaussian and Gamma models were proposed as the governing probability distribution functions for the central- and outer-brace SCFs, respectively. After substituting the values of estimated parameters, 10 fully defined PDFs were presented for the chord-side SCFs of central and outer braces in uniplanar tubular KT-joints under four types of IPB loading.

Submerged breakwaters are a special type of breakwater associated with low wave reflection. They can also save large quantities of engineering resources from the view of economics. Although there have been previous investigations on the interaction between waves and rubble mound low-crested breakwaters, performance of impermeable submerged breakwater is somehow different from rubble mound structures. Thus, the accurate estimation of reflection and transmission coefficients is essential in designing of this kind of structures in near shore zone. In this paper, performance of impermeable trapezoidal submerged breakwater was investigated experimentally using regular waves. In the experimental plan, for three submergence depths of breakwaters different wave groups were used to measure transmission and reflection coefficients. Based on the test results, empirical expressions were formulated to describe the transmission and reflection coefficients for solid submerged breakwater under regular group waves.

Un-trenched submarine pipelines will experience the wave and current loads during their design lifetime which potentially tend to destabilize the pipeline both horizontally and vertically. These forces are resisted by the interaction of the pipe with the surrounding soil. Due to the uncertainties involved in wave, current and soil conditions, there will be a complex interaction between wave/current, pipeline and seabed that needs to be properly accounted for. The design of submarine pipelines against excessive displacement due to hydrodynamic loads (DNV-RP-F109) is defined as a Serviceability Limit State (SLS) with the target safety levels as given in DNV-OS-F101 (2013). In this paper uncertainties associated with on-bottom stability design of submarine pipelines are investigated. Monte Carlo Simulations (MCS) are performed as the basis for probabilistic assessment of the lateral stability of the pipeline located on the seabed. Application of the method is illustrated through case studies varying several design parameters to illustrate the importance of each design parameter for exceeding a given threshold of the SLS criterion. Uncertainties in significant wave height and spectral peak period are found to be important parameters in describing the UR distribution. Type of soil has also an impact on the distribution of UR, i.e. how the passive resistance in the pipe-soil interaction model is accounted for. Therefore, the definition of characteristic values of both loads and resistance variables are important for the Utilisation Ratio (UR).

The accidental release and ignition of flammable vapours in petrochemical facilities generate overpressure and drag load which can impact the safety of installation and people. The intensity of the blast loads depends on many influencing factors including congestion, geometry, type & amount of fuel, leak size, and points of ignition among others. Given the stochastic nature of these parameters, it is obvious that the design for accidental load must be determined using a probabilistic method. This paper discusses a methodology known as “explosion exceedance diagram” and draws on recent developments in vapour cloud explosion research to determine the design accidental load (DAL). A case study demonstrates the application of the method.

Construction quality plays an important role in the integrity of submarine oil and gas pipelines during their lifetime. Quality of material and quality of construction contractors are two major contributors to the durability of the pipelines. The risk regarding quality material and contractors creates major concerns in durability of pipelines and has a significant impact on the optimized balance between CAPEX and OPEX in Risk-based integrity management of pipeline. In this study, the impacts of construction quality and corresponding uncertainties on the probability of failure of submarine pipelines are investigated in a reliability analysis using Monte Carlo Simulation. A sensitivity analysis is also conducted to show the important parameters within the study. The results show that the 33 percent reduction in uncertainty of construction quality has led to a reduction of more than 99 percent of the probability of failure. This indicates a high sensitivity of the probability of failure to structural uncertainty.

Fixed offshore structures are considered as an important structure in shallow water. Iranian oil and gas offshore structures which have been located in Persian Gulf are mostly fixed jacket. So reliability assessment of these kind of marine structures seems to be very important and is an essential part of offshore structure design. Mont Carlo is a powerful method which is used broadly for prediction of structure failure. The most advantage of this method is simplicity of implementation, the main limitation of this method is about the computational time due to the huge number of structural analyses. Incorporating the artificial neural network for the reduction of the sample size is used to get rid of MCS‘s time bottleneck. An MCS based method is introduced to take advantage of precision in optimization part. To solve the scaling problem of a large reliability analysis, an artificial neural network is employed. In this paper, an almost new constructed fixed jacket platform in the South Pars is selected and modeled using SACS software. In this regard, the nonlinear static pushover analysis is performed by application of nonlinear soil-pile interaction. Analytical results show that the simultaneous use of these two techniques lead to more accurate and also faster reliability assessment. In MCS method probability of failure calculated using divide the number of failed sample by total samples which is concluded to the value of 9.4e05 for the test case structure in the current study. , and the reliability index is resulted to 3.73.