نشریه مهندسی دریا
پیاپی 18 (پاییز و زمستان 1392)

In this research، the response of fixed offshore platforms equipped with shape memory alloy elements at the different depths under extreme wave loading are taken into account. For this purpose platforms located in different water depths 70، 110 and 150 has been considered. At first for evaluation of the nonlinear behavior of three platforms، pushover analysis using SACS has been engaged. Then nonlinear time history analysis under storm environmental condition using ANSYS finite element program has been performed on platforms situated in each water depths for two cases of with and without SMA. Based on results of this study، platform located in 70 meter water depth under storm wave has linear behavior and use of SMA doesn’t have any effect on it، while with equipped SMAs on 110 and 150 meters water depth platforms، SMA can be used such as a damper in structure because they have re-centering and energy dissipation capabilities. Applying the SMA damper in the jacket bracing، topside displacement and energy dissipation is controllable via mechanical properties of SMA selected by designer judgment as well re-centering capability of related damper prevents to generate residual strain in structure joint cans and members.

A numerical model has been introduced to simulate wave interaction with oscillating water column (OWC) in two dimensional vertical plane. Two-dimensional Navier-stokes equations have been used. To track free surface، Volume of fluid method has been employed. Also a porous wall is situated at the beginning of the canal to absorb the reflected waves. It’s valuable to mention that the Poisson equation has been solved using an implicit non-iterative method. In order to calculate pressure and water elevation inside OWC، an iterative method has been used. After model verification، the interaction of wave and OWC has been simulated. The effect of different wave conditions and also the immersion depth of the OWC’s front wall have been investigated. The numerical results were consistent with available experimental results.

This paper evaluates the hydrodynamic performance of a damaged ISSC TLP which is caused by a tendon disconnection. Performance evaluation of a TLP with a disconnected tendon in a rough sea state is the major aim of this paper. First off، modeling of a platform in a proper sea state is carried out and then after disconnecting one of the tendons، the transient effect on the other tendons is assessed. Also، the steady state forces are validated using analytical results. Additionally، time history of heave and surge accelerations، TLP six degree of freedom motions and also tendon forces are determined in regular waves. In order to assess the hydrodynamic performance of the platform، a numerical simulation is conducted، using multipurpose boundary element software. At last the analysis shows that the modeling of a damaged platform is carried out properly and also the conclusion can be drawn that it is possible to evaluate the performance of a damaged platform in Regular waves.

Following the growing economic and industrial development، concrete is to find a special place as building materials، as optimization of materials and components of this structural material in order to increase its durability and long service life، significant economic and industrial benefits will follow. So Performance of Durability control tests، during and after the concrete construction، Approach to achieving durable concrete production in order to obtain sustainable development in the region is essential. In this paper، some of the concrete durability tests were performed on 216 samples of six concrete mixtures with different water-binder ratios، pozzolans (fly ash and silica fume). Based on the scattered results obtained from the experiments، depth of penetration of water under pressure test is not reliable in concrete quality evaluation. Also، lack of equipments and high expense of RCPT، despite its extensive use in many regions of the world، have led to limited use of this method in concrete projects in Iran. The capillary water absorption test for its quickness، convenience، low cost and good repeatability، seems to be able to serve as a measure of the concrete durability in marine areas، although it would require more research.

In this research، hydrodynamic distribution pressure in continuous times is done and stress and displacement of a high speed vessel are studied. Hydrodynamic distribution pressure is studied for different dead rise angle according to vertical and horizontal vessel velocity at dry chine and wet time steps and is applied to vessel at continues times of vessel variable position. Vessel body is divided to some parts according to different dead rise angles and loading is applied as incremental loading. Shell of composite vessel is modeled using ABAQUES commercial finite element software as layer to study the effect of ever layer in stacking sequences، then change of the core thickness and optimization of vessel structure is studied that causing safety factor is increased and weight is decreased. At the end، results of critical stress and maximum displacement of different parts of vessel is compared and evaluated in initial and optimized vessel.

The corrosion fracture life of marine riser is investigated. Riser will be modeled as a long beam and the wave and current forces is estimated by short wave theory and modified Morison’s equation. Dynamic equilibrium equations of riser will be obtained on based of Euler theory. After bending stress calculation along the riser that includes effect of riser weight and platform motion، loading history can be obtained in riser cross section. Since risers exposed to seawater and sour crude oil، an initial crack can have destructive effects and corrosion fracture can be occurred. So effect of initial crack in riser cross sections in different depths and at present of corrosive fluid is studied. At the end the effect of seawater and crude oil on crack propagation and corrosion fracture life is investigated. In order to solve riser dynamic equilibrium equation Range-Kutta and finite difference methods will be used.

In this paper، the modification of Endurance Wave Analysis (EWA) in assessment of offshore platforms based on the new-wave theory is introduced. EWA is a new method to estimate the response of offshore structures in different sea states by use of incremental records. This study is offered practical procedures to select time duration and intensifying trend of the records. In addition، a simplified multi-degree of freedom model of a jacket platform under extreme waves is assessed through current study. A comparative study between this analysis and 3-hour random wave simulation to investigate the accuracy has also been carried out. The results show that the proposed procedure is a convenient method in design and assessment of offshore platforms.

Abstract Prediction of maximum wave height and the relative period as the fundamental parameters for assessment and design of offshore structures are forced with uncertainties. This paper proposes two algorithms for calculation of the probable extreme wave height in the Persian Gulf with respect to existing statics data in the South Pars region. A long-term joint distribution model of the corresponding wave height and wave period is needed in first manner and in the second model، a joint density function is defined in terms of a marginal distribution and a series of conditional density functions. Furthermore a relation is proposed for calculating of the period based on monthly and yearly data of the region. Finally some curves will be derived which may be used for estimating annual exceedance probability of maximum wave height for life time of offshore structures.