Jacket Platform Research Articles

Hybrid-Driven-Based H∞ Control for Offshore Steel Jacket Platforms in Network Environments

This paper investigates the hybrid-driven-based networked control for an offshore steel jacket platform subject to external wave forces. A hybrid driven strategy is introduced to deal with the problem of networked control for offshore platforms. Then, the networked closed-loop system is modeled as a stochastic delay system. Based on this model, a stability criterion is derived using the stochastic control theory and the Lyapunov-Krasovskii functional method. Simulation results show that the hybrid-driven-based networked $H_{\infty }$ controller is effective to suppress the vibration of the platform and save the limited network resources as well. Moreover, the designed controller is flexible in terms of maintaining a balance between performance requirements of the offshore platform and the utilization of communication network bandwidth.

Dynamic analysis of an offshore jacket platform with a tuned mass damper under the seismic and ice loads

Herein, we present numerical simulation based model to study the use of a \'Tuned Mass Damper (TMD)\' - particularly spring mass systems - to control the displacements at the deck level under seismic and ice loads for an offshore jacket structure. Jacket is a fixed structure and seismic loads can cause it to vibrate in the horizontal directions. These motions can disintegrate the structure and lead to potential failures causing extensive damage including environmental hazards and risking the lives of workers on the jacket. Hence, it is important to control the motion of jacket because of earthquake and ice loads. We analyze an offshore jacket platform with a tuned mass damper under the earthquake and ice loads and explore different locations to place the TMD. Through, selected parametric variations a suitable location for the placement of TMD for the jacket structure is arrived and this implies the design applicability of the present research. The ANSYS*TM mechanical APDL software has been used for the numerical modeling and analysis of the jacket structure. The dynamic response is obtained under dynamic seismic and ice loadings, and the model is attached with a TMD. Parameters of the TMD are studied based on the \'Principle of Absorption (PoA)\' to reduce the displacement of the deck level in the jacket structure. Finally, in our results, the proper mass ratio and damping ratios are obtained for various earthquake and ice loads.

A Coupled Discrete–Finite Element Method for the Ice-Induced Vibrations of a Conical Jacket Platform with a GPU-Based Parallel Algorithm

Flowing ice draws special attention due to the dynamic response of jacket platforms. In this study, a coupled discrete element method (DEM) and finite element method (FEM) are developed to analyze the interaction between sea ice and conical jacket platforms to determine the ice-induced vibrations (IIVs) of the structure. To model the ice cover and to investigate ice loads, a DEM with bond-breaking spherical elements is adopted. Meanwhile, the FEM (with a beam element) is applied to model the IIVs of the jacket platform. An efficient transmission scheme between the bond-breaking spherical elements and the beam element is proposed. The graphics processing unit-based parallel algorithm is developed to improve the computational efficiency. The simulated ice loads are verified by comparing them with the full-scale data and different ice load functions. The simulated IIV accelerations of the JZ20-2 MUQ conical platform in the Bohai Sea (China) are consistent with the full-scale data under various ice conditions (e.g., velocity and thickness). The numerical results show that the IIV acceleration increases linearly with the ice velocity and the square of the ice thickness.

Effect of grooves on the static strength of tubular T joints of offshore jacket structures

The present research work deals with effect of grooves on static strength and stiffness of tubular T joints of offshore jacket structures. Tubular structures are commonly employed in offshore jacket platforms due to better mechanical properties and economic advantages. The tubular T model considered for this study has chord dimensions of 1000 mm length, diameter 102 mm and thickness 5 mm and brace dimensions of 388 mm length, diameter 55 mm and thickness 4 mm. Material used for present analysis is Steel with Young’s modulus of 210 GPa and the Poisson ratio of 0.3. Three-dimensional finite element analysis has been carried out to investigate the effect of grooves placed over thickened chord section on the static strength of tubular T joints subjected to axial compressive load. In the present study, unstiffened tubular T joint is analysed and compared with stiffened tubular T joints to observe the improvement in the static strength of the tubular T joints. The groove configurations considered are: 5 circumferential grooves at 16.5 mm spacing, 3 circumferential grooves at 16.5 mm spacing and 3 circumferential grooves at 33 mm spacing. The grooves were having the dimension of 1 × 1 mm, 2 × 2 mm were used for the analysis. The joint local stiffness of ungrooved tubular T joint is 116.43 N/mm. The grooved configuration increases the joint local stiffness by more than 250 percent.

Signal denoising method for modal analysis of an offshore platform

The modal analysis of offshore structures is a key element of structural health monitoring based on vibration. A difficulty encountered by practitioners and researchers is that an accurate modal analysis is challenging in noisy environments. The objective of this work is to develop and implement a signal denoising method based on solving the inverse singular value problem of a measured (noisy) data matrix with prescribed entries to reconstruct a filtered data matrix. The measured (noisy) impulse response function (IRF) is used to build a square or nearly square Hankel matrix. The normalized rank determination indicator (RDI) of the Hankel matrix is introduced to determine the number of prescribed non-zero singular values in the prescribed entries inverse singular value problem (PEISVP). The reconstructed (filtered) matrix must maintain the original Hankel structure, and preserve the number of non-zero singular values. Once the filtered IRF has been obtained, the complex exponential (CE) method is applied for the modal analysis. To validate the proposed method, hereafter referred to as PEISVP-CE, we undertake the numerical simulation of a five-story shear building. Once successfully validated, we apply the PEISVP-CE method to an offshore field experiment of a jacket platform under the step relaxation. We find that the PEISVP-CE method is effective at eliminating noise and, therefore, appropriate for the modal analysis of offshore structures.

A novel hybridized metaheuristic technique in enhancing the diagnosis of cross‐sectional dent damaged offshore platform members

AbstractOffshore jacket platforms are widely used for oil and gas extraction as well as transportation in shallow to moderate water depth. Tubular cross‐sectional elements are used to construct offshore platforms. Tubular cross sections impart higher resistance against hydrodynamic forces and have high torsional rigidity. During operation, the members can be partially or fully damaged due to lateral impacts. The lateral impacts can be due to ship collisions or through the impact of falling objects. The impact forces can weaken some members that influence the overall performance of the platform. This demonstrates an urgent need to develop a framework that can accurately forecast dent depth as well as dent angle of the affected members. This study investigates the use of an adaptive metaheuristics algorithm to provide automatic detection of denting damage in an offshore structure. The damage information includes dent depth and the dent angle. A model is developed in combination with the percentage of the dent depth of the damaged member and is used to assess the performance of the method. It demonstrates that small changes in stiffness of individual damaged bracing members are detectable from measurements of global structural motion.

Challenges in calculation of critical buckling load of tubular members of jacket platforms in finite element modeling

Accurate estimation of the capacity curve of offshore jacket structures to achieve performance levels and ductility is of great importance. Proper modeling of compressive members to correctly investigate global and local buckling is crucial in estimation of the capacity curve. Buckling modes and deformations due to local buckling can be considered, if the compressive braces are modeled by shell or solid elements. The purpose of this paper is to achieve the correct compressive behavior of braces with solid type elements and investigate the effects of five different parameters such as D/t, L/D, mesh size, mesh size ratio, and imperfections. ABAQUS FE software is used for this purpose. The percentage of difference between model results and ISO equation for critical buckling load and the best choice of the above variables has been presented to achieve the maximum precision. Also the effect of these parameters on buckling capacity curve of tubular members has been estimated. The percentage of the effect of mentioned parameters is shown as separate pie charts. Consequently, mesh size ratio has the greatest effect on elastic stiffness and post-buckling strength. On the other side, the imperfection value is the most effective parameter on critical buckling load.

Seismic Behaviour of Offshore Steel Jacket Platform Braced in Horizontal and Vertical Planes

The offshore jacket platforms are primarily installed in the large oceans mainly for drilling the crude oil, carbohydrates and production of electricity. The current studies emphasize on the structural performance of offshore deck jacket platform with different bracing systems. Earthquake analysis has been performed to calculate the seismic responses, with the help of bracings to control the seismic induced vibrations of the jacket platforms. For this study, a jacket platform made up of steel members has been modeled and then analyzed under earthquake and wave loadings. This paper mainly deals to compute and compare the seismic behavior of offshore steel deck platform using SAP 2000 v20 software with bracing in the horizontal plane and bracing in both horizontal and vertical planes. The total number of 8 models has been analyzed in the SAP2000 software with bracing i.e. X, V, Inverted V and K in the vertical plane and bracing i.e. X, V, Inverted V and K in both horizontal and vertical plane. A relative study has been carried out in Time period, deck displacement and base shear. Seismic analysis using linear static, i.e. Equivalent static method (ESA) and linear dynamic, i.e. Response spectrum method (RSA) has been performed. Further deck displacement, time period and base shear are determined by Equivalent static method and Response spectrum method for various types of bracing models in both horizontal and vertical planes. Among the all various types of bracing models, Inverted V bracing in the vertical plane is found to be the optimum model among all other models.

Structural control of a fixed offshore structure using a new developed tuned liquid column ball gas damper (TLCBGD)

Structural control of a system can be achieved by employing different control methods. The effectiveness of a control technique straightly depends on the performance of the control device. Among control devices, the tuned liquid column gas damper (TLCGD) has shown high vibration control capabilities. In this study, a newly developed damping system as a tuned liquid column ball gas damper (TLCBGD) system is introduced. The developed system is a combined version of a tuned liquid column ball damper (TLCBD) and a TLCGD system. The new damper is utilized on an offshore jacket platform and its qualification is studied under both regular and irregular wave incidences. The efficiency of the TLCBGD system is investigated numerically by implementing ball-tube ratio and gas pressure parameters. The effects of the ball-tube ratios and the gas pressure values on the vibration suppression capacity of the system are assessed through a parametric study. In comparison to a passive TLCGD system, the results revealed that the utilizing a TLCBGD system in the jacket platform improves the structural vibration suppression capability.

Vibration control of offshore jacket platforms through shape memory alloy pounding tuned mass damper (SMA-PTMD)

The production of intelligent materials in the design of vibration control devices have made significant progress in the past two decades. Since the shape memory alloy (SMA) is a superelastic intelligent material highly capable of dissipating energy, use has been made in this research of the SMA pounding tuned mass damper (SMA-PTMD) to control the wave-induced vibrations of the offshore jacket platforms. The SMA-PTMD dynamic vibration absorber (DVA) is a combination of SMA-TMD and PTMD connect the mass to the deck through a piston-based SMA dissipation device composed of varying-geometry SMA bars and gaps. Here, the vibration equations have been developed by simulating the platform and the DVA as a multi-degree of freedom system and solved by the direct integration method. To define the SMAs' hysteretic behavior, its idealized constitutive model has been implemented and the geometry of the SMA bars has been adjusted using the ideal gas molecular movement optimization algorithm. The time history responses have indicated significant reductions in the maximums and RMS of the deck displacements of the optimized SMA-PTMD-installed jackets vibrated under the action of irregular waves. Results have also shown high tuning and robustness of the SMA-PTMD under broadband frequency excitations and off-tuning conditions.

The ultimate strength analysis of jacket leg under deck load

Jacket platform is one of the offshore structure where the pile is fixed at the bottom sea level. Jacket leg has a significant influence on local and global structural strength under axial and lateral loads. Jacket leg is one of the most important elements for the fixed offshore structure. On the other hand, the jacket leg is also very critical to resists not only axial compression but also lateral pressure. The objective of the present study is to analyze the ultimate strength of a jacket leg under deck load. The numerical calculation is used to analyze jacket leg due to deck load. It is assumed that the support condition is fixed at bottom sea level. The result obtained by numerical analysis is therefore presented in this study. It found that the jacket leg obtained by numerical analysis can withstand the axial compression load.

Pile deflection analysis due to asymmetrically distribution loads

Jacket platform is one of the offshore structure where the pile is fixed at the bottom sea level. Pile and brace configuration has a significant influence on local and global structural strength under axial and lateral loads. Pile is one of the most important elements for the fixed offshore structure. On the other hand, the pile is also very critical to resists not only axial compression but also lateral pressure. The objective of the present study is to analyzed pile deflection due to asymmetrically distribution loads. The numerical calculation is used to analyze pile deflection due to asymmetrically distribution loads. In this case, large deflection analysis is separated into two asymmetrically loading conditions those are trapezoidal and triangular with different end support. The result obtained by numerical analysis is therefore compared with the analytical solution using direct integration. It found that the pile deflection obtained by numerical analysis is almost identic with direct integration. It is also observed that the end support condition gives significant influence to support reaction and deflection.

Decommissioning impacts on biotic assemblages associated with shell mounds beneath southern California offshore oil and gas platforms

The decommissioning of southern California offshore oil and gas platforms will create major economic, engineering, and environmental challenges in the next decade. Platform jackets, conductors, and shell mounds often host a diverse and productive marine community, and among the myriad considerations associated with decommissioning planning, platform operators and federal and state regulatory agencies will consider the ecological value of existing underwater structures as artificial reefs. In the event of partial removal of platform structure, fish assemblages on decommissioned platforms may remain unchanged in areas where structure is left intact. However, on the seafloor beneath the platforms, a mound of debris often called the shell mound will likely change over time if the supply of falling mussels and other organisms from the productive surface part of the structure is removed. In this study, we review shell mound research relevant to decommissioning, including mound formation, contaminant loads, associated biological communities, and transitions following the removal of platform structures at four sites. To address the gap in knowledge of shell mound fish community structure, we used manned submersible and remotely operated vehicle surveys from 1997 to 2013 to estimate the biomass, density, species composition and similarity between shell mounds at 22 southern California platforms. We found a wide range of variability in fish density and shell mound areal extent. Species composition also varied among sites, with three significant community clusters primarily distinguished by species depth preferences. These results will help inform a comprehensive net environmental benefit analysis of southern California offshore platform decommissioning alternatives.

Effects of sample size of ground motions on seismic fragility analysis of offshore jacket platforms using Genetic Algorithm

Intense seismic excitations might be considered as one of the most important loading cases imposed on offshore jacket platforms. To perform seismic assessments for these structures, appropriate suites of ground motions can be selected by different methodologies. Since time history analyses of fixed offshore platforms are truly time consuming, number of selected records can considerably change the computational expenses. This paper quantitatively investigates the effects of sample size of records on seismic performance of these structures. Thus, several record suites with different sample sizes are selected from a target population with 40 records so that the differences between the logarithmic mean and standard deviation of their response spectra become minimum using Genetic Algorithm. The results show that the fragility curves based on the suites with larger sample sizes are closer to the target fragility. Nevertheless, according to the results of the investigated case study, some suites with smaller sample sizes (e.g. 20 or 25) could lead to relatively acceptable accuracy at almost half the computational expenses. Also, the correlation coefficients between the collapse fragilities of the selected record sets and the target one are very close to unity, indicating that these collapse fragilities are linearly correlated with the target one.

Frequency Domain Response of Jacket Platforms under Random Wave Loads

This article presents a procedure that simplifies an offshore jacket platform as a non-uniform cantilever beam subjected to an axial force. A Ritz method combined with a pseudo-excitation method is then used to analyze the responses of the jacket platform under random wave loads with the associated power spectral densities, variances and higher spectral moments. The theoretical basis and pertinent governing equations are derived. The proposed procedure not only eases the process of determining the pseudo wave loads, but also requires only the rudimentary structural details that are typically available at the preliminary design stage. Additionally, the merit of the proposed procedure is that the process does not require one to compute the normal modes, which saves time and is particularly convenient for the dynamic-response analysis of a complex structure (such as an offshore platform). An illustrative example based on a three-deck jacket platform is presented to demonstrate the procedure used to obtain the power spectral densities, variances and second spectral moments of jacket-top displacement and the bending moment of the jacket at the mud line. The results obtained are compared with those obtained using a Finite Element Mothed (FEM) model. Based on the findings of the study and good agreement shown in the comparison of results, it is concluded that the proposed method is effective, simple and convenient, and can be a useful tool for the preliminary design analysis of offshore platforms.

Hybrid Control of Offshore Jacket Platforms Using Decentralized Sliding Mode Control and Shape Memory Alloy-Rubber Bearing Under Multiple Hazards

Abstract The motivation and objective of the present study are to propose a hybrid control system for offshore jacket platforms to mitigate the vibrations induced by multiple hazards, namely, the earthquakes and regular and irregular waves. State-of-the-art indicates that not much work is reported on hybrid control of offshore jacket platforms for multiple hazards using a control algorithm, which is robust against uncertainties. A decentralized sliding mode control algorithm using magneto-rheological (MR) dampers is employed for the semi-active controller because of its robustness against parametric uncertainties and reliability. Passive shape memory alloy rubber bearings (SMARBs) are selected as passive isolators because of their high damping capacities, high fatigue resistance, and super elastic behavior, which are highly desirable for offshore applications. The scope of the present study is to demonstrate the efficiency of the proposed controller and investigate the effects of different influencing parameters. A jacket platform, reported in the literature, is taken as an illustrative example. A significant reduction in the top deck displacement is observed. The position and number of MR dampers affect the performance of the controller significantly. Limitations of the controller imposed due to the greater weightage or penalty imposed on displacements by the semi-active control algorithm as well as due to the magnetic saturation of MR dampers are overcome by the high energy dissipation of the passive SMARBs, thus making the hybrid controller highly efficient. The effectiveness of the controller is more for the earthquakes and random waves than for the regular waves.

Time History Dynamic Analysis of a New Constructed Offshore Jacket Platform in Persian Gulf Due to Random Waves

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.

Wave-induced vibration control of offshore jacket platforms through SMA dampers

Since permanent wave-induced vibrations of offshore jacket platforms reduce the service life of the jacket structure and deck equipment and increase the fatigue failure of the welded connections, this research has used SMA (shape memory alloy) dampers to control the jacket platform oscillations. Superelasticity, high durability, and energy dissipation capability make SMA elements good nominees for the design of vibration control devices. In this research, to model the force-displacement hysteretic behavior of SMA elements their idealized multi-linear constitutive model has been implemented and the time history responses of vibration equations have been evaluated by direct integration method. To analyze the SMA damper effects on the vibration suppression of the jacket platforms, a 90 (m) high jacket located 80 (m) deep in water has been selected as a case study. Numerical results have shown that optimized SMA dampers with constant-geometry SMA bars will improve the dynamic behavior of the jacket platform under the action of an extreme regular wave. However, under the action of two irregular waves, SMA dampers with varying-geometry SMA bars will cause significant reduction in the dynamic responses of the jacket platform. The power spectral density function of the deck displacements have shown that the previously mentioned SMA dampers avoid resonance by shifting the natural frequencies of the jacket structure away from the excitation frequencies.

Delayed proportional-integral control for offshore steel jacket platforms

This paper is concerned with the problem of delayed proportional-integral control of an offshore platform subject to self-excited nonlinear hydrodynamic force. By using current and distributed delayed states, a delayed proportional-integral controller is designed to stabilize the offshore platform. Under such a controller, the closed-loop system of the offshore platform is modeled as a nonlinear system with discrete and distributed delays, which allows us to employ the Lyapnov–Krasovskii functional method to analyze its asymptotic stability. Since an affine Wirtinger-based inequality is exploited to estimate the derivative of the Lyapunov–Krasovskii functional, a new stability criterion for the closed-loop system is derived, based on which, suitable control gains can be designed provided that a set of linear matrix inequalities are feasible. It is found through simulation results that the proposed control scheme can improve the control performance remarkably. Moreover, (i) compared with the existing delay-free controllers, the proposed controller can reduce the required control force and the oscillation amplitudes of the platform significantly; and (ii) compared with several delayed controllers, the proposed controller requires less control cost.

Simulation of Wind, Waves, and Currents During Hurricane Sandy for Planned Assessment of Offshore Wind Turbines

The simulation of wind and wave fields for the evaluation of jacket-supported offshore wind turbines during Hurricane Sandy of 2012 is the focus of this study. For realistic load assessment of offshore wind turbines, it is important that coupled wind, wave, and current fields with appropriate spatial resolution are provided throughout the evolution of the hurricane. A numerical model describing the hurricane track and intensity variation with time is used to generate the coupled wind, wave, and current fields. Time series of turbulent wind fields and coupled wave kinematics are generated using the hurricane simulation output, and a detailed procedure is presented. These coupled wind, wave, and current fields form the inputs for the analysis of a 5-MW offshore wind turbine supported on a jacket platform sited in 50 m of water and assumed located in the path of Hurricane Sandy. The turbine’s and support structure’s performance under the simulated loading conditions is the subject of a separate study.