Jordan Journal of Civil Engineering, Volume 6, No. 4, 2012 - 418 - Dynamic Behavior of Jacket Type Offshore Structure Anis A. Mohamad Ali, Ahmed Al-Kadhimi and Majed Shaker Department of Civil Engineering, University of Basrah, IRAQ. E-Mail: anismohamadali53@yahoo.com ABSTRACT Unlike structures in the air, the vibration analysis of a submerged or floating structure such as offshore structures is possible only when the fluid-structures is understood, as the whole or part of the structure is in contact with water. Using the commercial F.E.A. program ANSYS (v.12.0) (to model the Winkler and Brick-full bond models) and program ABAQUS(v.6.9) (to model the Brick- interface model), the stress matrix considering a dynamic load was superposed on the stiffness matrix of the structure. A time domain solution is recommended, using the generalized Morison’s equation by FORTRAN90 program to construct a program to calculate the wave forces, and Airy's linear and Second Order Stoke's wave theories are employed to describe the flow characteristics by using MAPLE13 program (to solve and apply the boundary conditions of the problems on Laplace's equation), and the results are compared and discussed. Both free and forced vibration analyses are carried out for two case studies. KEYWORDS: Natural frequency, Fixed jacket offshore, Finite element method, Validity of wave theory, Dynamic analysis. INTRODUCTION The development of the offshore oil industry led to numerous installations of offshore platforms. The major use of these platforms is in the drilling for oil and gas beneath the seafloor. Other uses include, but are not limited to, military applications, navigational aid to ships and generating power from the sea. Modal test which is one of the examination assessments is the method to analyze the dynamic characteristics. It is a method to measure frequencies, finding the natural frequency of the fixed jacket platform and forecasting the vibration phenomenon for mode shape. In case of domestic applications, the study of fixed jacket platform system has been actively conducted in some big corporations, small and medium enterprises and national researches. But it was impossible to obtain systematic data. Based on this design, we calculated the complex load on the offshore (fixed jacket platform). Calculations in the offshore have to take into account wave load(caused by waves). However, since current load is insignificant compared to wave load, it can be ignored. LOAD CALCULATION IN OFFSHORE The loads that strongly affect offshore structures can be classified into the following categories; 1. Permanent (dead) loads. 2. Operating (live) loads. 3. Environmental loads. 4. Construction-installation loads. 5. Accidental loads. Whilst the design of buildings onshore is usually influenced mainly by the permanent and operating loads, the design of offshore structures is dominated by environmental loads, especially waves, the impact and loads of which arise in the various stages of constructional installation. Accepted for Publication on 14/6/2012. © 2012 JUST. All Rights Reserved.