نشریه مهندسی دریا
پیاپی 16 (پاییز و زمستان 1391)

based assessment of offshore jacket type platforms against wave loading. This approach can be employed to obtain a more realistic estimate for the ultimate capacity of offshore platforms, in comparison to the commonly used push-over analysis. On the other hand, ocean and sea waves are both random; they are irregular in shape, height, speed, period and directions. Therefore, one of the most important uncertainties in the offshore field loading is the stochastic nature of the waves. In this paper a probabilistic method has been proposed in order to estimate the ultimate capacity of offshore platforms subjected to random waves using IWA. To this end, a sufficient number of long-term simulations (3hours) and constrained simulations (using Constrained NewWave methodology) have been carried out.

It is undeniable that instability plays a significant role in the design of pipelines. Local buckles induced by excessive force on specific part of pipeline can locally reduce the collapse pressure of a pipeline and cause local failure. Periodic placement of buckle arrestors along the pipeline can ensure that collapse only effect the length of pipe between two arrestors on either side of the initiation site. Arrestor spacing is usually derived by specific practical considerations on the particular line (e.g. installation and repair procedure) but typically spacing of 90-240m has been used. In the present study, the propagation and arresting of dynamic buckle are modeled using the finite element method. For the post buckling analysis, the material and geometrical nonlinearity are considered. The effects of arrestor spacing on cross over pressure have been evaluated for different diameter to thickness ratio of pipeline (D/t), arrestor’s thickness to pipe’s thickness ratio (h/t), and arrestor’s length to pipe’s diameter ratio (La/D).

Steel offshore platforms are some of those structures which are built to withstand environmental and accidental loads during oil exploitation operation. Particular attention is being paid to earthquake loads in seismic active areas because it directly influences the capacity of the offshore structure installations. The uncertainties available in earthquake loads as well as in strength of structural components and also the need to gain confidence about the performance of the structure, have convinced the engineers to use probabilistic methods for design purposes. To this end, Probabilistic Seismic Demand Analysis (PSDA) is used to compute the mean annual frequency of exceeding a particular structural response parameter. In this paper, the efficiency of advanced scalar ground motion intensity measures (IM), Inelastic Spectral Displacement (Sdi) which, was proposed by Thosong and Cornell (2007) in Stanford University, has been compared for PSDA of Jacket Type Offshore Platforms (JTOP) with conventional scalar intensity measure, Elastic Spectral Acceleration or correspondingly Elastic Spectral Displacement (Sde). An efficient Im is the one which, leads to lower dispersion in response parameter of structure in an arbitrary level of Im. In this paper, the numerical model of a Jacket Type Offshore Platform in accordance with its real behavior was simulated in OpenSees software. The case study platform has been modeled in OpenSees considering buckling and post buckling behavior of bracing members and material low cycle fatigue effects. In order to consider soil-pile-structure interaction effects, the near field soil was modeled using Beam on Nonlinear Winkler Foundation method including Lateral resistance, skin friction resistance and end bearing resistance. In the current research, Incremental Dynamic Analysis (IDA), which is among one of the most recent and capable approaches in evaluating ultimate capacity of structures, has been performed and a brief discussion has been presented about the efficiency of different Edps in indicating various structural response modes of platform. As a consequence, the same Edp as simple building structures – maximum interstory drift angle – has been proposed for case study platform. Furthermore, the multiple Ida curves in terms of two aforementioned Intensity Measures for 40 pulse-like near fault ground motion records have been depicted and the dispersion of Ida curves in dynamic instability range have been computed for these two Ims. Based on lower computed dispersion for inelastic spectral displacement intensity measure, this Im has been chosen as the most efficient intensity measure for case study platform.

In this paper, in order to understand the flow-pipe interaction more clearly, the variations on flow separation around offshore pipelines with different ratios of fitting distances due to the steady current have experimentally and numerically been investigated. For the experimental modeling, a number of experiments have been conducted in the Hydraulic Research Laboratory. The experiments have been carried out in a flume with 10 meters length, 0. 3 meters width and 0. 5 meters depth using P. V. C pipe with 6. 35 centimeters in diameter (for different distances to diameter ratios of fitting). To visualize the flow separation, the polystyrene particles having 1. 05gr/cm3 in density were used. Also, in order to understand the phenomenon as well as possible, the whole processes of tests have been recorded using a digital camera. For the numerical simulation section, the flow fields have been analyzed using a computational fluid dynamics software «FLUENT». Finally, the results of numerical and experimental models were compared with one to another and good agreements were achieved between them. The results showed that with increasing the fitting distance between pipes, the separation length at downstream of the second pipe was gradually decreased, however, after a given distance the two pipes behaved separately.

In canal and seaway the ship speed must be reduced and it has been a subject for many research studies. One of the most important phenomena connected to this issue is the increasing of ship draught when passing in the restricted water which is called squat. This study deals with the squat thorough systematic model testing. Two models from Series60 body form have been tested in a set of canal depth, width as well as different model speed. The change on their draught has been recorded at fore and aft of the model. The test results have been manipulated, validated and regression formula has been developed for prediction of squat at ship fore perpendicular. The formula has been compared with the other existing formula where a good agreement achieved. The advantage of the new formula is the coverage on wider range of parameters affecting the ship squat.

There are significant number of pile supported wharves around the world and this type of structures are commonly chosen for the construction of new waterfront port facilities. Batter piles have been used for a long time to resist large lateral loads from winds, water waves, impacts and earthquake. Their distinct advantage over vertical piles is that they transmit the applied lateral loads partly in axial compression and tension, rather than only through shear and bending. Thus, batter piles offer larger stiffness and bearing capacity compared to vertical piles with the same diameter and depth. However, batter piles have been extensively damaged at several ports worldwide during past earthquakes. This paper is focused on the application of seismic isolation devices for improving seismic performance of existing pile supported wharves with battered piles. To assess efficiency of this method, a structure with batter piles was selected to represent typical seismically vulnerable pile supported structures. The selected structure is analyzed using dynamic time history analysis taking into account the nonlinear behavior of structure components and soil–pile-structure interaction effects. 2D models of structure and soil are developed using OPENSEES platform. The soil is modeled using nonlinear elasto plastic elements and the soil pile structure interaction behavior is modeled using p-y, t-z and Q-z nonlinear spring elements. Models that include axial load effects in lead rubber bearing (LRB) isolator behavior are used. Time history analyses for 475 and 2475 years level have been carried out for existing and retrofitted structures. The structures retrofitted by seismic isolators had longer period of vibration and increased damping both of which reduced the seismic base shear, axial force of battered piles for tension and compression and also moment and shear action in deck.

In practice, steel tubulars lack a mathematically perfect cylindrical shape, due to non-uniformities introduced during the manufacturing process, construction methods, damages incurred in the transport. The imperfections might exist in the cross section and along the tubular member. Imperfections can greatly affect the behavior of tubular members. The work presented in this paper is perusing effect of geometrical imperfections on steel tubes under monotonic axial compressive loading. An experimental model testing along with a numerical simulation approach has been employed. The small scale experiments were conducted on X70 steel pipes with diameter 44 (mm) and D/t of 22. The results showed that under monotonic loading the presence of imperfection in the tubular member leads to local buckling of the tube and has a decreasing effect on the limit stress (σ_L) and on the strain corresponding to the ultimate load (ε_L). The geometry of the imperfections introduced to the numerical model is based on the scaled down buckling mode shapes of the tubular specimen. In general, a reasonable agreement has been noticed between the experimental and the numerical results. The numerical model has then been used to study the effect of geometry of imperfection and imperfection amplitude (A), D/t and (λ/L) on the response of steel tubes to monotonic axial compressive loading. The results showed that the monotonic response was more sensitive to higher mode shapes as compared to the lower modes and to symmetric modes as compare to non-symmetric modes. The limit stress and the strain corresponding to the ultimate load decrease as imperfection amplitude increases. The stress-strain path was also found to be affected by the geometry of imperfections, D/t and the imperfection wave length. The results of the current study also shows that the tube imperfection has a more profound effect on the limit strain (ε_L) in comparison to that on the limit stress (σ_L).