International Journal of Coastal, Offshore and Environmental Engineering
Volume:1 Issue: 2, Spring 2016

Control of a group of autonomous surface vessels, called agents, with realistic dynamic for circling mission is addressed with the aid of Lyaponov and graph theory. In this brief, to obtain a cooperative controller in between agents, new coordination transfer are presented and graph theory is used to illustrate communication between the agents. With the aid of Lyaponov theory and graph theory application, decentralized and scalable controllers are designed for group of autonomous vessels to converge to a desired geometry for circling around a specific target point. Due to the realistic agent dynamics, non-holonmic dynamics and turning constrains of the vessels are considered in the design process. Advantage of the proposed controller is: it uses domestic information between agents and the controller is designed based on these information. The agents herein represent a large class of autonomous vessels with realistic limitation on vessel motion. Besides, in previous works inertia and damping matrix of the agents were assumed to be diagonal and constant, in this research work non-diagonal inertia matrix and variable damping matrix are under consideration. MATLAB and Simulink are used to represent the effectiveness of the proposed controllers. As the simulation results show, designed controllers perform well on the system and the objective duty is achieved appropriately.

This work introduces a structural integrity assessment strategy for Jacket structures based on the finite element model updating and a novel simplified method. Hereof, model reducing and model updating procedure is established based on a optimization technique. Since the number of measured degrees of freedom is most of the time restricted in practice, this paper represents a methodology using the cross model cross mode method (CMCM) in combination with an iterative procedure which uses limited, spatially incomplete modal information. This research is an empirical study on a laboratory model of a jacket structure with the aim of establishing Refined Simplified FE Model (RSM) to conduct damage detection. In addition to elimination of uncertainty effects in the damage detection results, RSM technique is employed because of practical considerations and also this technique provides a fast damage zone diagnosis procedure. Also, improved reduction scheme is utilized based on static reduction scheme to carry out damage detection in jacket structure.

To improve sea surface wind speed in coastal regions, we used nadir satellite altimeter measurements in the Persian Gulf and Arabian Sea. With combining normalized radar cross section for two bands of satellite altimeter measurements and significant wave height suggested the method to obtain “true” sea surface wind speed. In the coastal regions, we used a dimensionless significant wave height to gain empirical dependency to fetch-limited wind wave development. In this research, normalized radar cross section is simulated by using inverse wave age and fetch laws. As established this method helps to refine altimeter measurements of sea surface wind in the coastal regions.

Stability of Tension Leg Platforms (TLP) is sensitive to tendons situations. Therefore, behavior and exact calculation of stiffness matrix of TLP is important for dynamic analysis and investigation of TLP in intact and damaged tendon conditions is necessary to accurate and reliable design of TLP. In this paper deals with dynamic analysis of TLP when tendons are intact and one or three tendons are damaged. Static stability of TLP in damage condition has been studied and static offsets have been estimated, then stiffness of surge, heave and pitch has been derived. Finally, responses of heave and pitch for three load conditions have been predicted.

Offshore jacket structures have been used in petroleum industry for decades. Due to increasing the age of operating platforms, structural damages will be generated by corrosion, fatigue, ship impacts and other reasons. Improvements in the oil and gas recovery from several fields have raised the interest for using these platforms well beyond their intended design life. Life extension of an existing jacket platform needs proper reassessment of its structural members, such as piled foundations. This paper represents a case study of the existing fixed offshore platform located in Persian Gulf by in-place strength analysis. The objectives of this analysis are to verify whether the platform can meet the structural requirements, as per API RP 2A and AISC, for the In-place extreme met-ocean loading. The structural assessment is performed based on the best estimates of the existing conditions of the structure data on the future corrosion allowance. Since the response of the jacket platform to the environmental loads is intensely affected by the pile soil interaction, in current study the foundation is modelled using uncoupled non-linear soil springs acting along the piles length. The load cases, which include all situations relevant in the In-place analyses are taken into account. Results of the In-place analysis of the drilling platform indicate that the jacket structure does not assure the code provisions.

The internal waves complicate the propagation process of sound in the water. These waves are considered the main cause of disturbances in sound speed, and now it is known that the internal waves are the dominant parameter in the change process of sea frequency spectrum, as these changes range from many hourly cycles (floating frequency) to almost one daily cycle (inertia frequency).
The profile of mass sound speed in shallow waters depends on salinity and temperature gradients in turbulence internal waves. Here, the assumption is that the only probability function source is the turbulence internal waves in a water column. This investigation aims to use the mathematical models to study the internal wave effects on propagation of sound waves in shallow waters and that the waves how affect the sound propagation and depend on what parameters? We used the data gathered from Persian Gulf to calculate the parameters such as: sound speed, floating frequency, the ratio of resulted turbulences in sound propagation by vertical movement, phase functions and internal wave domain. Meantime, based on a given wave length (in the study area: 235AWT IMAGEm.), the shape of first mode has been compared to the other modes. The probability density functions have been calculated for two different modes.
Comparing the ratio of generated turbulences in sound propagation by vertical movement and horizontal speed of particle, showed the horizontal movement is considerably less than the vertical one and also by increasing the depth (consequently decreasing the floating frequency), vertical movement is raised highly. The highest floating frequency and turbulences generated in sound propagation by vertical movement are found on the places near the water level and this is due to thermocline existence and on the other hand in the same places we have the lowest range of vertical movement.