International Journal of Coastal, Offshore and Environmental Engineering
Volume:2 Issue: 1, Winter 2017

The Steger and Warming flux vector splitting implicit scheme is used to numerically solve two dimensional Reynolds Averaged Navier–Stokes (RANS) equations governing the vortex induced vibration of a flexible riser laterally supported by a spring and a damper. The k–ε model is used as turbulence model to simulate the turbulent flow in the wake of the riser. To update the new position of the riser, the lift coefficient obtained from the previous RANS iteration is coupled by the body motion equation. The proposed numerical solution is able to provide fair results in terms of lift coefficient, amplitude of oscillation and the effect of reduced velocity on it. The numerical results are compared with the available experimental and computational data where fairly good agreement even at the lock-in regime has been obtained. Taking wider external boundary, using conservative form of the equations, applying k-ε turbulence model for the separated flow and finally using the variable time step as the lock-in region approaches, are main features of the proposed numerical model.

Many procedures suggest for reduction of responses of riser to Vortex Induced Vibrations (VIV). Natural frequencies of marine risers is an important parameter that can affect the responses of riser to VIV. Change of riser properties such as top tension and bending stiffness can alter natural frequencies. In this study effects of riser specifications on the responses and fatigue damage of marine risers were investigated analytically and numerically. For numerically analysis 2D wake-structure coupled model is used for modeling of VIV of riser in two directions of Cross Flow (CF) and In Line (IL). The wake dynamics, including IL and CF vibrations, is represented using a pair of non-linear Van der Pol equations that solved using modified Euler method. The Palmgren–Miner Rule is used for evaluation of fatigue damage. Riser of Amir-Kabir semisubmersible placed in Caspian sea is used for case study. Because VIV is self-limiting, it is showed that lower modes have lower curvature, that in some cases this is lead to lesser stress and also fatigue damage. The results show that for tension dominant modes of vibration, natural frequencies was increased with top tension and for a certain Strouhal frequency, dominant modes of vibration was reduced which leads to reduction of stress and fatigue damage. The results show that stress and fatigue damage increased with module of elasticity of riser and reduction of this leads to reducing of stress and fatigue damage. Therefore suitable procedure for reduction of VIV responses of riser should be selected based on the current velocity.

Drilling Operations are exposed to a variety of hazards, some of which may be location and activity dependent and each could pose different risk from different paths. Drilling operation may be vulnerable to hurricanes in one region and be exposed to Geohazards in another. However, there are other hazards, (e.g. corrosion, age degradation, poor maintenance), which equally affects every rig. Identifying what can go wrong and their likelihood and possible consequences provides insight into vulnerability of the operation and helps to generate mitigation options. Filtering and Ranking risk contributors enable to decide priorities and to focus on the most important risk contributors. This paper offers a framework to identify, assess, prioritize, and manage drilling risks, which includes: (1) a holistic approach to risk identification; (2) prioritization of a large number of risk influencing factors or risk scenarios; (3) structured elicitation of experts’ opinion and effective integration of experts judgment into qualitative and quantitative analyses to supplement limited data availability; (4) extreme and catastrophic event analysis; and (5) use of multi-objective framework to evaluate risk management priorities.

Subsea structures such as manifolds, line pipes and flow lines are important investments. Also because of the sensitivity of environmental issues, corrosion of these structures is of vital importance. Subsea corrosion management is different from on-shore and shallow water off-shore corrosion management in mainly three factors: materials, corrosion management practice and cathodic protection. There are important limitations in many aspects of these three factors that make them different from their “counterparts” in other industries. In this paper, some of these differences especially with regards to corrosion prediction softwares and associated design strategies are addressed and discussed.

In this paper the TELEMAC-3D model has been hired to simulate and study the high waves’ interaction with coastal structures. Therefore a special arrangement of TELEMAC-3D has been prepared in this study to simulate wave generation, coastal processes, wave set-up and overtopping over coastal structures. Experimental data has been used to verify this arrangement of the model. Thereafter, the model has used to simulate the interaction of waves induced by Cyclone Gonu and Ramin port breakwaters. Quality comparison between images provided by Iran Fisheries Organization during the Gonu event and the TELEMAC3D results concludes that the numerical model could simulate the incidence with a good accuracy. Different kind of results like inundation area, wave overtopping discharge over the breakwaters and wave penetration in port were obtained in this simulation.

The current manuscript presents the validation of Smoothed Particle Hydrodynamics (SPH) techniques for wave generation by underwater explosion, utilizing the so-called DualSPHysics numerical model. This numerical method is used to analyze generated waves which are initiated by man-made or natural explosions below free surface level of sea. In spite of the modeling limitations (e.g. absence of open boundary conditions), reasonable agreement is accomplished with predictions of the existing formula as well as experimental results. This proved that SPH techniques such as incorporated in DualSPHysics are becoming a suitable alternative to existing classical approaches to this particular water waves problem. It is also provided an inherently more accurate computational for the prediction of wave characteristics generated by underwater explosions.