نشریه مهندسی دریا
پیاپی 5 (زمستان 1385)

During the offshore drilling operations by drillships or semisubmersible vessels the drillstring in its first drilljoint to the Kelly suffers bending due to float vessel roll or pitch. The cyclic bending stress at any point along the drillstring, is a function of the roll or pitch angle, Kelly position in bushing, and hookload. The cumulative fatigue damage of the drillpipe due to cyclic stresses near the drillpipe connection to Kelly has been calculated in this paper. The effects of the cyclic stress amplitude variation are also taken into account. Then five multiaxial fatigue models are evaluated under variable amplitude loading (similar to drillpipe loading) conditions. Comparing the results of the five models predictions by the muliaxial fatigue tests experimental data under variable amplitude axial-torsional loading, show that the combined critical plane and energy models predict the fatigue life better than the others. It works much better for variable amplitude loading than the existing models.

In the design of marine pipelines, like other thin-walled structures, structural stability plays major role. Generally, two kinds of instabilities, namely global buckling and local buckling (collapse) may occur in marine pipelines. However, for deep water pipelines in addition to occurrence of collapse, another concern is the potential occurrence of propagating of this collapse along the pipe due to high external pressure. In the present study, details of 2-D and 3-D finite element modeling for collapse propagation simulation are outlined. In order to verify the accuracy and validity of the finite element modeling, the numerical results, obtained from nonlinear finite element analyses for several pipe samples have been compared with the experimental results. These proposed 2-D and 3-D modeling methods are easily applicable. Also, the comparison shows that the results of these methods have very close agreement with the experimental behaviour. Using nominal geometric properties, finding minimum required imperfections to eliminate bifurcation points and using corrected Ramberg-Osgood material behaviour for steel pipe are the main characteristics of the present 3-D method. The study shows that this method gives more appropriate results than the previous proposed method by Toscano et al. (2002).

In this paper, the local scour process around pipelines due to steady currents has been investigated from different points of view using the numerical and physical models. In order to calculate the maximum scour depth under the pipe, a mathematical model based on the two-dimensional Laplace equation has been developed and the scour process on the 2D x-z vertical plane has been simulated, with the finite volume method being used to discretize the governing equation. In the developed numerical model, an equivalent boundary calculated using the Newton-Raphson iteration method, has been used to determine the deformed bed profile due to scour resulting from the subjected forces on the sediment particles on sea bed. In the physical models, emphasis has been focused on the effects of different parameters such as pipe diameter, water depth, velocity of flow and the interaction of the parallel pipes on local scour process. Finally, the numerical model results have been compared with the results obtained from the physical model and the other experimental data.

Oil and Gas offshore pipelines, often pass through large geographical areas, from the supply point to the end-user, crossing seismic-active areas. For unburied pipelines, both seismic ground wave and permanent ground deformation can cause severe damage to pipelines, depending on the pipeline geometry and connected structures. Due to the largely non-linear nature of the problem, a finite element analysis (FEA) is the most general tool for sub sea pipeline design under seismic loading. Throughout this research, the responses of an offshore unburied pipeline resting on the seabed in shutdown and operation conditions will be investigated under various earthquake loading including various types of seismic faults (strike-slip, Normal, reverse and oblique) and also seismic ground wave propagation for Probable Persian Gulf earthquake magnitudes. The pipeline integrity regarding operation criteria in critical conditions will be assessed. In addition, effects of two different site soil properties and pipeline end restraint on pipeline behavior will be investigated.

Prediction of hydrodynamic loads during water impact is of great significance in the structural design of flying vehicles. Also the importance of the slam force of waves on the members of offshore structures cannot be overemphasized. No theoretical tool is available to handle this complicated phenomenon exactly and the experimental procedures in the laboratory are both time-consuming and expensive. In this paper, a computer program based on the VOF method is applied to evaluate the water impact outcome in a real situation. To asses the capabilities of the CFD code, two classical problems including water impact of a wedge and a circular cylinder traveling at constant speeds are studied. In a real situation the descend velocity is decreased by the impact loads known as “deceleration effect”. This change in speed is taken into account in an iterative procedure and the flow field around the base of a WIG craft is computed. All flow field computations in this study include the effects of viscosity, turbulence, gravity and surface tension. The comparison of results with experimental data shows the efficiency and correctness of this straightforward iterative method.

However it is possible to use of numerical methods such as beta-Newmark in order to investigate the structural response behavior of the dynamic systems under random sea wave loads but because of necessity to analysis the offshore systems for extensive time to fatigue study it is important to use of simple stable methods for numerical integration. The modified Euler method (MEM) is a simple numerical procedure which can be effectively used for the analysis of the dynamic response of structures in time domain. It is also very effective for response dependent systems in the field of offshore engineering. An important point is investigating the convergence and stability of the method for strongly nonlinear dynamic systems when high initial values for differential equation or large time steps are considered for numerical integrating especially when some frequencies of the system is very high. In this paper the stability of the method for solving differential equation of motion of a nonlinear offshore system (tension leg platform, TLP) under random wave excitation is presented. The key point of suitability of MEM for solving the TLP system is that the maximum frequency of the system is about 0.5 Hz. The stability criterion and the convergence of the numerical solution for critical time steps are numerically discussed.