Jacket Type Offshore Platforms Research Articles

Friction damper for vibration control in offshore steel jacket platforms

The performance of friction dampers to mitigate the wave-induced vibrations in jacket-type offshore platforms has been investigated in this study. Due to the random nature of ocean waves, a full stochastic analysis method has been used to evaluate the response of the structures equipped with these devices. A stochastic linearization technique has also been used to take the nonlinear behavior of these hysteretic dampers into account. At last, the developed mathematical formulation has been applied to evaluate the response of realistic models, and to find out the optimal values for the adjustable parameters of the friction dampers to dissipate the wave induced vibrations of the platforms.

Cyclic inelastic behavior and analytical modelling of pile–leg interaction in jacket type offshore platforms

Structures in locations susceptible to severe seismic disturbances should be designed properly in order to resist lateral forces induced by earthquake motions. Steel offshore platforms are some of those structures which are built to withstand environmental and accidental loads during oil exploitation operation. Particular attention is being paid to earthquake loads in seismic active areas because it directly influences the capacity of the offshore installations. In this paper, a small-scaled planar platform has been modelled analytically using nonlinear finite element program, based on an experimental test, conducted simultaneously in order to assess the local and global behavior of pile–leg interaction in Jacket Type Offshore Platforms (JTOPs). A combination of nonlinear beam column elements and fatigue affected elements are used to capture the inelastic cyclic behavior of planar frame as accurately as possible. Results of analytical tests are to be compared with experiments and it is concluded that an analytical approach can be best used for modelling JTOPs with reasonable accuracy regardless of the type and scale of the structure. Moreover, a special study on joints has been carried out and the best model has been selected to simulate brittle behavior of joints resulting from heat affected zone.

Nonlinear stochastic optimal control of offshore platforms under wave loading

A nonlinear stochastic optimal (NSO) control strategy for wave-excited jacket-type offshore platforms is proposed. Wave force is determined according to linearized Morison equation. Spectral density functions of water particle velocity and acceleration are approximated by some rational forms, respectively. Wave force vector is then treated as output of a linear filter driven by white noise. A set of partially averaged Itô equations for controlled modal energies are derived by applying stochastic averaging method for quasi-integrable Hamiltonian systems. Optimal control law is then determined by using stochastic dynamical programming principle. To demonstrate the effectiveness and efficiency of the proposed control strategy, performances of uncontrolled, linear quadratic Gaussian (LQG)-controlled and NSO-controlled example platforms under different sea states are evaluated. Numerical results show that the NSO controller has better control effectiveness and efficiency than the LQG controller.

Feedforward and feedback optimal control for offshore structures subjected to irregular wave forces

This paper studies the vibration control of a jacket-type offshore platform with an active mass damper (AMD) and presents a feedforward and feedback optimal control (FFOC) law. The linearized Morison equation is employed to estimate the wave load. The offshore structure is simplified into a single degree of freedom (SDOF) system. The original vibration control is formulated as the optimal control for a linear discrete-time system affected by external disturbances with known dynamic characteristics but unknown initial conditions. We give the existence and uniqueness conditions of the FFOC law. Simulation results show that, compared with the classical state feedback optimal control (CFOC) law, the presented control scheme is more efficient in reducing the displacement and velocity of the offshore structure subjected to irregular wave forces.

H2 active vibration control for offshore platform subjected to wave loading

Offshore platforms are usually located in hostile environments. These platforms undergo excessive vibrations due to wave loads for both normal operating and extreme conditions. To ensure safety, the displacements of the platforms need to be limited, whereas for the comfort of people who work at the structures, accelerations also need to be restricted. This article is devoted to developing a proper procedure on applying H 2 control algorithm for controlling the lateral vibration of a jacket-type offshore platform by using an active mass damper. In comparison with earlier studies, a number of improvements in problem formulation, wave force filter design, and control algorithm implementation are made. The present paper also numerically demonstrated the effectiveness of H 2 active control. As expected, it significantly outperforms the corresponding passive control that uses a tuned mass damper.

Feedback-feedforward control of offshore platforms under random waves

This study investigates the control of jacket-type offshore platforms. The deck displacement of jacket-type offshore platforms can be controlled using both passive and active control mechanisms. Among the passive control mechanisms, a tuned mass damper concept is studied in this paper. Active control mechanisms considered here include the active mass damper, the active tendon mechanism and the propeller thruster. An optimal frequency domain approach to active control of wave-excited platforms is used in which the H2 norm of the transfer function from the external disturbance to the regulated output is minimized. In this study, the hydrodynamic drag force is evaluated using the JONSWAP wave spectrum. Unlike conventional linearization approaches, the influence of non-linearity in the drag force is retained in this scheme by expressing the non-linear force components in terms of higher-order convolutions of the water-particle velocities. To demonstrate the effectiveness of this scheme, the platform performance with and without control devices under different sea states is evaluated. It is demonstrated that the control devices are useful in reducing the displacement response of jacket-type offshore platforms, especially when the wave forces are concentrated at frequencies close to the natural frequencies of the platform. This becomes especially significant in deep waters because the natural frequencies of jacket-type platforms fall closer to the dominant wave frequencies in deep waters. Adding control devices to deep water platforms will ensure a reduction both in the global response of the platform and in localized effects, such as the fatigue of welded joints. Copyright © 2001 John Wiley & Sons, Ltd.

Three-dimensional stochastic response of offshore towers to random sea waves

In this paper, a three-dimensional model for dynamic response analysis of jacket-type offshore platforms under random sea waves is established. The sea waves are considered to be zero mean, stationary and ergodic Gaussian random processes specified by the P-M wave height spectrum. Linear wave theory and the generalized Morrison equation are used to calculate the wave forces on the structure. The fluid-structure interaction and the couplings of the motion in different directions at each node are considered. The fluid drag forces are linearized by stochastic linearization techniques. The mean-square responses are obtained by Gauss-Legendre numerical integration. Numerical results for a platform with a height of 65 m to symmetrical and unsymmetrical loads are presented and contrast with the corresponding results of one- and two-dimensional models. Some more detail and practical description, such as the response in the vertical direction and the unsymmetrically loaded behaviour, are given.

Management Errors and System Reliability: A Probabilistic Approach and Application to Offshore Platforms.

Probabilistic risk analysis, based on the identification of failure modes, points to technical malfunctions and operator errors that can be direct causes of system failure. Yet component failures and operator errors are often rooted in management decisions and organizational factors. Extending the analysis to identify these factors allows more effective risk management strategies. It also permits a more realistic assessment of the overall failure probability. An implicit assumption that is often made in PRA is that, on the whole, the system has been designed according to specified norms and constructed as designed. Such an analysis tends to overemphasize scenarios in which the system fails because it is subjected to a much higher load than those for which it was designed. In this article, we find that, for the case of jacket-type offshore platforms, this class of scenarios contributes only about 5% of the failure probability. We link the PRA inputs to decisions and errors during the three phases of design, construction, and operation of platforms, and we assess the contribution of different types of error scenarios to the overall probability of platform failure. We compute the benefits of improving the design review, and we find that, given the costs involved, improving the review process is a more efficient way to increase system safety than reinforcing the structure.

Floating Body Equilibrium by Potential-Energy Minimization

The paper presents a computer-based approach aiming to solve the equilibrium equations of a floating body by means of potential-energy minimization. After a brief and general discussion, the principle that the potential energy of the system must be at a minimum is adopted as the condition for identifying the stable equilibrium positions of a marine vehicle. A consistent mathematical formulation is then developed. The problem is solved, therefore, by searching for the minimum of a function of three variables using a simple and efficient iterative method. This makes it possible for the equilibrium positions to be determined directly, unlike the classic methods-that is, without any previously constructed table of hydrostatic data and regardless of the magnitude of waterplane rotation compared with the initial orientation. No restriction is stipulated on hull form. Some study cases relating to a prismatic barge and a jacket-type platform are presented and analyzed. Relevant numerical results allow the procedure to be optimized so as to improve convergence. Finally, peculiar features of the proposed method are discussed, with particular reference to jacket-type offshore platforms, and further valuable applications are indicated.

Organizational Aspects of Engineering System Safety: The Case of Offshore Platforms

Organizational errors are often at the root of failures of critical engineering systems. Yet, when searching for risk management strategies, engineers tend to focus on technical solutions, in part because of the way risks and failures are analyzed. Probabilistic risk analysis allows assessment of the safety of a complex system by relating its failure probability to the performance of its components and operators. In this article, some organizational aspects are introduced to this analysis in an effort to describe the link between the probability of component failures and relevant features of the organizaton. Probabilities are used to analyze occurrences of organizational errors and their effects on system safety. Coarse estimates of the benefits of certain organizational improvements can then be derived. For jacket-type offshore platforms, improving the design review can provide substantial reliability gains, and the corresponding expense is about two orders of magnitude below the cost of achieving the same result by adding steel to structures.

Probabilistic Collapse Analysis of Offshore Structure

This paper proposes a method for probabilistic collapse analysis of an offshore structure. Wave loads are estimated by using Stokes third-order theory and Morison’s formula. Plastic collapsing is evaluated by taking account of the combined load effect to generate the safety margins, using a matrix method. Probabilistically dominant collapse modes are selected through a branch-and-bound method. The proposed method is successfully applied to a jacket-type offshore platform.

Optimum Design of Offshore Platforms Minimizing Expected Total Cost

In this study, expected total cost, which is composed of structural cost and expected maintenance cost, is introduced as a reasonable utility function for the multi-objective optimization design problem such as maximum safety and minimum cost. The optimum design procedure is proposed for offshore platforms minimizing the expected total cost. The design variables are the geometrical dimensions of the structure while its configuration and the materials to be used are given. Partial failure due to formation of plastic hinges and/or fatigue cracks of the critical sections during design life is considered. The design is performed for the platforms subjected to wave loads. In addition, the effect of periodic inspection is investigated. The optimum design problem contains many parameters which are difficult to estimate exactly, and thus the optimum solution may not be absolute. However, the failure probability corresponding to the optimum solution is used as an index for specifying the allowable failure probability in probabilistic optimum design problems. Numerical examples for a jacket-type offshore platform are provided to illustrate the applicability of the proposed method.