Analysis of offshore jacket platform

The structural design requirements of an offshore platform subjected to wave induced forces and moments in the jacket can play a major role in the design of the offshore structures. For an economic and reliable design; good estimation of wave loadings are essential. A nonlinear response analysis of a fixed offshore platform under structural and wave loading is presented, the structure is discretized using the finite element method, wave plus current kinematics (velocity and acceleration fields) are generated using 5th order Stokes wave theory, the wave force acting on the member is calculated using Morison's equation. Hydrodynamic loading on horizontal and vertical tubular members and the dynamic response of fixed offshore structure together with the distribution of displacement, axial force and bending moment along the leg are investigated for regular and extreme conditions, where the structure should keep production capability in conditions of the 1-yr return period wave and must be able to survive the 100-yr return period storm conditions. The result of the study shows that the nonlinear response investigation is quite crucial for safe design and operation of offshore platform.

Ayush sharma (M.tech ,SMBS,VIT University) K.Muthumani (Professor ,SMBS, VIT university ) Index Terms-Jacket structure, airy wave theory, API code(American petroleum institute),SACS 5.3

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.

Journal of Marine Science and Application, 2019

API RP2A WSD is a design code in practice for design of jacket platforms in the Persian Gulf but is based on the Gulf of Mexico environmental condition. So for the sake of using this code for the Persian Gulf, it is better to perform a calibration based on this specific region. Analysis and design of jacket platforms based on API code are performed in a static manner and dynamic analysis is not recommended for such structures. Regarding the fact that the real behavior of the offshore jacket platforms is a dynamic behavior, so in this research, dynamic analysis for an offshore jacket platform in the Persian Gulf under extreme environmental condition is performed using random time domain method. Therefore, a new constructed offshore jacket platform in the Persian Gulf is selected and analyzed. Fifteen, 1-h storm, simulations for the water surface elevation is produced to capture the statistical properties of extreme sea condition. Time series of base shear and overturning moment are derived from both dynamic and static responses. By calculating the maximum dynamic amplification factor (DAF) from each simulation and fitting the collected data to Weibull distribution, the most probable maximum extreme (MPME) value for the DAF is achieved. Results show that a realistic value for DAF for this specific platform is 1.06, which is a notable value and is recommended to take into practice in design of fixed jacket platform in the Persian Gulf.

Offshore Construction is the installation of the structures and facilities on a marine environment, usually for the production and transmission of electricity, oil, gas and other resources. Offshore structures are being challenged to counteract the depletion of oil resources with the new set of discoveries. By 2010, the increase in drilling platforms induced the demand for offshore structures in deep sea. Hence, the quest on the research and development of the deep-water structures has resulted in the recent advancement and thrust in this area. In the design of buildings onshore, it is influenced mainly by the permanent and operating loads, whereas the design of offshore structures is dominated by environmental loads, especially waves and winds, and the loads arising in the various stages of construction and installation. In this paper, the focus is on wind load, introduction to wind load, its effect on offshore structures, analysis procedure and the design considerations of wind load for offshore structures. A case study of Hurricane Andrew has also been studied.

Revue Roumaine de Sciences Techniques - Mecanique Appliquee, No. 5/1999, 1999

The paper presents an approach to the problem of wind loads acting on the above-water structure of the fixed offshore drilling, extraction or storage platforms, using experimental and theoretical methods. The wind action is one of the important loads on offshore platforms. Usually, this action appears simultaneously with wave action. The offshore platform above-water superstructure is a complex structure, which generates many problems relative to estimation of wind loads. For the “template” type platforms, wind loads up to 10% of the whole external loads (waves, wind, currents and earthquakes). Wind loads on above-water offshore platform structure were computed using a mathematical model, based on semi-empirical formulas, which is presented in this paper. The paper presents also the experimental study results obtained on the wave and wind tunnel belonging to The Research and Engineering Institute for Environment from Bucharest. The tests were carried out for the offshore platform PFC-III, designed for the Black Sea Romanian zone of the continental shelf. Some results of tests are presented in graphical form. The paper stresses the need to carry out both theoretical and experimental studies, for the design, in safe and optimal conditions, of the offshore platforms.

ICOE 2009, IITM, 2009

This paper discusses the wind loading and associated nonlinear dynamic response of a compliant offshore platform. Special reference is made to articulated loading platforms (ALP). These platforms derive their stability against lateral movement from large buoyancy forces. As far as excitation of wave frequencies is concerned, the system opposes wave forces by inertial effects. In this study, the response of double hinged ALP subjected to random forces generated by gusting of wind and wave is investigated. The exposed portion of the tower is subjected to the action of turbulent wind, while the submerged portion is acted upon by wave forces. The fluctuating component of the wind is modeled by Emil Simiu's spectrum. The waves are characterized by P-M spectrum. The time histories of wind velocity fluctuations are simulated as single-point and multi-point Gaussian random processes using a Monte Carlo simulation technique. The equations of motion are derived by Lagrangian approach. The important nonlinear characteristics of the platform and loading are incorporated at each time step in the numerical scheme. Newmark's-β integration method is used to solve the equation of motion in iterative fashion and response time histories are obtained. A detailed parametric study is reported that includes maximum, minimum, mean and standard deviation of responses. Some typical power spectral density functions (PSDFs) of deck displacement, hinge shear and bending moment are shown to illustrate the effects of sub-and super harmonic resonances in the system.

29th International Conference on Ocean, Offshore and Arctic Engineering: Volume 1, 2010

Regular wave theories such as Airy and 5 th order Stokes are widely employed to compute wave loads on offshore platforms. These deterministic approaches are based on certain wave heights and frequencies, so, it does not suitably account for the randomness of the sea state. The most accurate methods for estimating extreme responses are based on time domain but they need long duration's storm inputs and are computationally very time consuming. NewWave theory offers a different timedomain approach for the extreme response evaluation. By constraining a NewWave with a predetermined height within a completely random background Constrained NewWave is generated. Constrained NewWave allows for the easy and efficient evaluation of extreme response statistics; achievable without the need to simulate many hours of real time random seas. In the present study the extreme response of a Jacket offshore platform has been evaluated using a 5 th order Stokes wave; long duration random reproduction of the sea surface; and a Constrained NewWave method. In general, it has been found that the Constrained NewWave method provides reasonably accurate results, as compared to those from long duration random reproduction of the sea surface, with significantly lower computational time.

Marine Structures, 2009

In this paper, the dynamic response of a scale model of a jacket offshore structure is investigated both theoretically and experimentally. The experiments were conducted both in air and in water. The in-water experiments were done in the towing tank of Memorial University to simulate the realistic operating conditions. The model was subjected to random wave loads. Froude's law of modeling was used to obtain the dimensions of the scale model based on the dimensions of an existing structure. The effects of varying the structure's weight, and the characteristics of the wave loading were investigated. The structure's weight was changed by adding weights to the structure's deck. A finite element model was designed to determine the dynamic response of the model. Excellent agreement between the experimental and theoretical results was obtained. The reaction force at the foundation was estimated from strain measurements and compared with the finite element calculations. Fair agreement was obtained. This work is the first stage of a project whose objective is to develop a method for structural damage detection using the free vibration response of the structure. The free vibration response will be obtained from the stationary random response of the structure using the random decrement method. Having an accurate model to describe the dynamic response of the structure is the first step in this study.