Applications of approximate optimal control based on sensitivity approach to jacket offshore platforms

This chapter presents an optimal tracking control methodology for an offshore steel jacket platform subject to external wave force. Based on a dynamic model of an offshore steel jacket platform with an AMD mechanism and a linear exogenous system model of the external wave force on the offshore platform, an optimal tracking control scheme with feedforward compensation is proposed to attenuate wave-induced vibration of the offshore platform. A feedforward and feedback optimal tracking controller (FFOTC) can be obtained by solving an algebraic Riccati equation and a Sylvester equation, respectively. It is demonstrated that the wave-induced vibration amplitudes of the offshore platform under the FFOTC are much smaller than the ones under the feedback optimal tracking controller (FOTC) and the feedforward and feedback optimal controller (FFOC). Furthermore, the required control force under the FFOTC is smaller than the ones under the FOTC and the FFOC.

Hybrid damping systems in offshore jacket platforms with float-over deck

Offshore drilling has become a key process for obtaining oil. Offshore platforms have many applications, including oil exploration and production, navigation, ship loading and unloading, and bridge and causeway support. However, vibration problems caused by severe environmental loads, such as ice, wave, wind, and seismic loads, threaten the functionality of platform facilities and the comfort of workers. These concerns may result in piping failures, unsatisfactory equipment reliability, and safety concerns. Therefore, the vibration control of offshore platforms is essential for assuring structural safety, equipment functionality, and human comfort. In this study, an optimal multiple tuned mass damper (MTMD) system was proposed to mitigate the excessive vibration of a three-dimensional offshore platform under ice and earthquake loadings. The MTMD system was designed to control the first few dominant coupled modes. The optimal placement and system parameters of the MTMD are determined based on controlled modal properties. Numerical simulation results show that the proposed MTMD system can effectively reduce the displacement and acceleration responses of the offshore platform, thus improving safety and serviceability. Moreover, this study proposes an optimal design procedure for the MTMD system to determine the optimal location, moving direction, and system parameters of each unit of the tuned mass damper.

This study addresses the dynamic response control of deep-water jacket offshore platforms under typhoon and misaligned wave loads by proposing a Tuned Mass Damper (TMD)-based vibration suppression strategy. Typhoon loading is predicted using the Weather Research and Forecasting (WRF) model to simulate maximum wind speed and direction, a customized exponential wind profile fitted to WRF results, and a spectral model calibrated with field-measured data. Correspondingly, typhoon wave loading is calculated using stochastic wave theory with the Joint North Sea Wave Project (JONSWAP) spectrum. A rigorous Finite Element Model (FEM) incorporating soil–structure interaction (SSI) and water-pile interaction is implemented in the Opensees platform. The SSI is modeled using nonlinear Beam on Nonlinear Winkler Foundation (BNWF) elements (PySimple1, TzSimple1, QzSimple1). Numerical simulations demonstrate that the TMD effectively mitigates dynamic platform responses under aligned typhoon and wave conditions. Specifically, the maximum deck acceleration in the X-direction is reduced by 26.19% and 31.58% under these aligned loads, with a 17.7% peak attenuation in base shear. For misaligned conditions, the TMD exhibits pronounced control over displacements in both X- and Y-directions, achieving reductions of up to 29.4%. Sensitivity studies indicated that the TMD’s effectiveness is more significantly impacted by stiffness detuning than mass detuning. It should be emphasized that the effectiveness verification of linear TMD is limited to the load levels within the design limits; for the load conditions that trigger extreme structural nonlinearity, its performance remains to be studied. This research provides theoretical and practical references for multi-directional coupled vibration control of deep-water jacket platforms in extreme marine environments.

In this paper, a method to optimize the location of tuned mass damper (TMD) for enhancing chatter stability limits of thin-walled workpiece by using numerical techniques is proposed with due consideration to mass effect of TMD. The dominant mode of workpiece is identified from the simulated and measured frequency response functions of the workpiece. Two TMDs each having single degree of freedom are designed and a finite element model to predict the response of the workpiece-damper system is developed. The effect of damper location on the chatter stability of the thin-walled workpiece is investigated. Further, the locations for TMDs are optimized for enhanced chatter stability of workpiece by using response surface methodology. Milling tests were performed on workpiece with and without TMDs. A three-fold improvement in minimum stable depth of cut is realized after incorporating the TMDs at their optimal locations on thin-walled workpiece.

Ambient and forced vibration tests were recently conducted on an offshore steel template platform in the Ekofisk complex of the North Sea. Valuable information about the dynamic characteristics of the structure was obtained. This paper focuses on the measurement and analysis of damping data for the platform. Modal damping values are very important in the dynamic analysis of offshore structures. Correct values of modal damping need to be utilized for the dynamic analysis to accurately predict the response of the structure. Little experimentally verified information of the modal damping characteristics of the fundamental and higher order modes of offshore structures is currently available. Damping values were calculated for the fundamental and higher order jacket modes which included natural frequencies up to approximately 5 Hz. Damping values are compared for forced and ambient vibration data, for bandwidth and response techniques, and for different excitation levels. The general results indicated that the fundamental modes important in fatigue had damping values of 1 to 3 percent of critical and the higher order modes had damping values of 2 to 3 percent of critical. Emphasis is placed on the apparent merits and limitations of the data presented. Response comparisons are also presented using an analytic model of the platform with various values of critical damping. INTRODUCTION Accurately predicting the dynamic behavior of offshore oil and gas production platforms to withstand the effects of earthquakes and environmental fatigue due to wind and wave loading is an essential part of the design of such structures. Research and development work currently being undertaken to apply integrity monitoring techniques to these platforms also requires an accurate interpretation of the structural dynamic behavior. These applications of vibration technology translate directly into cost effective design and increased reliability and safety of offshore structures. The complexity of these structures requires a high level of theoretical analysis in developing models which duplicate the dynamic behavior under design loading conditions. Current state-of-the-art modeling methods usually apply finite element techniques which discretize the structure into an assemblage of mass and stiffness matrices which mathematically define the structure. The complexity of these structures also implies the need for confirmation and improvement of the structural models by experimental techniques. Using system identification methods, both analytic and experimental results can be used to accurately define the dynamic behavior of structures. Structural damping is one of the least understood yet most vital design characteristics of dynamically loaded structures. An analytical model of a structure in which low critical damping values are used will respond much differently to environmental loads than one in which higher damping values are used. This paper presents values of critical damping obtained experimentally on an offshore platform and comparisons are made to an analytic model of the structure. The vibration tests were conducted on an offshore steel template platform in the Ekofisk complex of the North Sea. Both ambient and forced vibration test procedures were utilized.

1 A Survey on Computational Optimal Control.- Issues in the Direct Transcription of Optimal Control Problems to Sparse Nonlinear Programs.- Optimization in Control of Robots.- Large-scale SQP Methods and their Application in Trajectory Optimization.- Solving Optimal Control and Pursuit-Evasion Game Problems of High Complexity.- 2 Theoretical Aspects of Optimal Control and Nonlinear Programming.- Continuation Methods In Boundary Value Problems.- Second Order Optimality Conditions for Singular Extremals.- Synthesis of Adaptive Optimal Controls for Linear Dynamic Systems.- Control Applications of Reduced SQP Methods.- Time Optimal Control of Mechanical Systems.- 3 Algorithms for Optimal Control Calculations.- Second Order Algorithm for Time Optimal Control of a Linear System.- An SQP-type Solution Method for Constrained Discrete-Time Optimal Control Problems.- Numerical Methods for Solving Differential Games, Prospective Applications to Technical Problems.- Construction of the Optimal Feedback Controller for Constrained Optimal Control Problems with Unknown Disturbances.- Repetitive Optimization for Predictive Control of Dynamic Systems under Uncertainty.- Optimal Control of Multistage Systems Described by High-Index Differential-Algebraic Equations.- A New Class of a High Order Interior Point Method for the Solution of Convex Semiinfinite Optimization Problems.- A Structured Interior Point SQP Method for Nonlinear Optimal Control Problems.- 4 Software for Optimal Control Calculations.- Automated Approach for Optimizing Dynamic Systems.- ANDECS: A Computation Environment for Control Applications of Optimization.- Application of Automatic Differentiation to Optimal Control Problems.- OCCAL: A mixed symbolic-numeric Optimal Control CALculator.- 5 Applications of Optimal Control.- A Robotic Satellite with Simplified Design.- Nonlinear Control under Constraints of a Biological System.- An Object-Oriented Approach to Optimally Describe and Specify a SCADA System Applied to a Power Network.- Near-Optimal Flight Trajectories Generated by Neural Networks.- Performance of a Feedback Method with Respect to Changes in the Air-Density during the Ascent of a Two-Stage-To-Orbit Vehicle.- Linear Optimal Control for Reentry Flight.- Steady-State Modelling of Turbine Engine with Controllers.- Shortest Paths for Satellite Mounted Robot Manipulators.- Optimal Control of the Industrial Robot Manutec r3.

This paper is concerned with pure delayed non-fragile control for an offshore steel jacket platform subject to non-linear self-excited wave force. By purposefully introducing a proper time-delay into control channel, a pure delayed non-fragile controller (DNFC) is proposed to improve performance of the offshore steel jacket platform. The positive effects of time-delays on non-fragile stabilization control for the system are investigated. It is shown through simulation results that (i) the DNFC and the delay-free non-fragile controller are capable of attenuating the vibration of the offshore platform to almost the same level, while the required control force under the DNFC is less than that under the delay-free one; and (ii) both the oscillation amplitudes of the offshore platform and the ranges of the control force under the pure delayed state feedback controller (DSFC) are smaller than the ones under the non-linear controller and the dynamic output feedback controller; and (iii) the oscillation amplitudes of the offshore platform under the DSFC are almost the same as the ones under integral sliding mode controller and the delayed dynamic output feedback controller, while the control force required by the former is less than the one by the latter.

This study aimed to rigorously evaluate the impact of tuned mass dampers (TMDs) on structural response under seismic excitation. By strategically placing TMDs at various levels within the structures, the research sought to determine their effectiveness in mitigating structural movement. A single-degree-of-freedom (SDOF) system incorporating TMDs was utilized to model structures of 10, 13, and 16 stories, each configured with TMDs at different heights. The structures were subjected to near-fault earthquakes to assess the efficacy of TMDs in reducing structural response. The findings revealed significant enhancements in structural performance when TMDs were optimally positioned. Specifically, a 50% reduction in both acceleration and displacement, alongside a 65% decrease in maximum drift, underscored the effectiveness of TMD deployment. Furthermore, the study demonstrated that distributing multiple TMDs along the height of the structure provided superior drift control. Notably, positioning TMDs within the upper one-third of the structure yielded the most pronounced improvements in acceleration, displacement, and maximum drift. Finally, the research indicates that the strategic incorporation of TMDs can significantly enhance the seismic resilience of structures. The results highlight the substantial benefits of TMDs in optimizing acceleration, displacement, and drift, thereby affirming their critical role in seismic design and retrofitting strategies.

PurposeDuring its operation, vibrations in jacket platform due to high operational and environmental loads could reduce its productivity and endanger its safety. Tuned mass damper (TMD) is one of the vibration control devices commonly used in buildings to reduce their response. Basically, TMD is a device attached to a structure as a mass on properly tuned spring and damping elements. The purpose of this paper is to study the utilization of TMD to reduce the wave-induced vibration of platforms.Design/methodology/approachFirst, the study investigates the optimum TMD parameters to reduce the response of the platform. The effectiveness of the optimized TMD in reducing platform response due to wave load is then analyzed. Finally, the reduced response of the platform is attributed to an extension of platform service life. Cyclic load in the form of vibration is related to cyclic stress on the tubular connections on a jacket platform, which causes fatigue failure. Fatigue performance of a platform is quantified by its fatigue life, which should be at least twice the intended service life.FindingsIt is found that the optimized TMD cannot be achieved by simply adjusting the TMD damping as high as possible. The results show that the response of controlled platforms by properly tuned TMD could be reduced by about 20 percent from the uncontrolled case.Originality/valueAlthough vibration of a structure can be reduced by increasing its stiffness, this approach is considered not economical since it increases the cost for material construction. With the falling of oil prices, an optimization of jacket platform structure is considered necessary to reduce its construction cost. TMD can be used as an alternate solution, even though the only recorded utilization of TMD on a jacket platform is known in Sakhalin Platform in Russia. This fact gives a motivation to carry out more studies about TMD application on jacket platform, as this device is already used in numerous of inland structures.

Tuned mass dampers (TMD) are devices employed in vibration control since the beginning of the twentieth century. However, their implementation for controlling the seismic response in civil structures is more recent. While the efficiency of TMD on structures under far-field earthquakes has been demonstrated, the convenience of its employment against near-fault earthquakes is still under discussion. In this context, the study of this type of device is raised, not as an alternative to the seismic isolation, which is clearly a better choice for new buildings, but rather as an improvement in the structural safety of existing buildings. Seismic records with an impulsive character have been registered in the vicinity of faults that cause seismic events. In this paper, the ability of TMD to control the response of structures that experience inelastic deformations and eventually reach collapse subject to the action of such earthquakes is studied. The results of a series of nonlinear dynamic analyses are presented. These analyses are performed on a numerical model of a structure under the action of near-fault earthquakes. The structure analyzed in this study is a steel frame which behaves as a single degree of freedom (SDOF) system. TMD with different mass values are added on the numerical model of the structure, and the TMD performance is evaluated by comparing the response of the structure with and without the control device.

Walk-to-work (W2W) concepts are widely used in the offshore wind farms as means of access to offshore wind turbine platforms. This is typically achieved using a Service Operation Vessel (SOV) equipped with a suitable W2W gangway. This system has also been explored in the Oil and Gas industry for temporary access to offshore platforms. However, the use of this system as a primary means of access to operating offshore oil and gas platforms remains limited. When it comes to selecting height of a W2W landing, from basic Naval Architectural hydrodynamic principles one can expect height of a W2W landing from sea level to have an inverse relationship with operability of the SOV gangway [1]. As such, the lower the W2W landing, the higher the SOV gangway operability. However, lowering the W2W landing, potentially exposes the W2W landing structure to extreme design wave loads (e.g. 10,000-year return period design waves). Loss of W2W landing to an extreme wave event could jeopardize further accessibility of the W2W landing and expose any subsea infrastructure to risk of dropped objects. This paper is intended to summarise results of an assessment where height of the “Munin” fixed offshore platform W2W landings with regards to SOV gangway operability is optimised. In this study effect of W2W landing elevation from mean sea level in relation to SOV gangway operability is quantified for the Munin Platform site specific conditions. Further, a qualitative evaluation is presented to show how benefits of raising W2W landings outside of the extreme wave load envelop outweighs the marginal SOV operability improvement due to lowering W2W landings elevation.

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20208, "Sakhalin-1: Technology Development for Frontier Arctic Projects," by J.M. Hamilton, ExxonMobil Upstream Re - search, and J.E. Jones, ExxonMobil Development Company, originally prepared for the 2009 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. As the oil and gas industry pushes into frontier Arctic areas, new technologies and design criteria will be required to ensure safety and cost effectiveness of increasingly challenging developments. Sakhalin-1 provides a good example of a development in a frontier subarctic area where location-specific design criteria had to be developed and innovative-technology solutions were required to deal with demanding environmental conditions including sea ice, large breaking waves, and earthquakes. Introduction First production from the Exxon Neftegas Limited (ENL) Sakhalin-1 Phase-1 development of Chayvo field began in October 2005, marking the culmination of nearly 10 years of design-criteria and technology development. The development concept consists of an offshore drilling platform, an onshore drilling and treating facility at Chayvo, an oil-export pipeline that runs east/west across the island and Tatar Strait to mainland Russia, and a storage and tanker export terminal at DeKastri. The oil is exported by ice-strengthened tankers south through the Tatar Strait, which is partially covered by up to medium first-year ice (70 to 120 cm) in the winter. The offshore drilling platform, Orlan, is approximately 10 km offshore in approximately 15-m water depth. The platform annually is exposed to approximately 4000 km of drifting first-year pack ice up to 1.5-m (level ice) thickness, which originates in the Sea of Okhotsk north of the island and drifts south along the eastern coast. The controlling ice load features are pressure ridges, with keels extending to depths greater than 30 m (2000-year design value). The design ice load on the 80-m-wide, vertical-sided structure is approximately 385 MN. Design wave loads are generated by late-summer and fall storms that can produce breaking waves at the platform. Global wave loads are on the order of the ice loads, and wave run-up and locally high slamming pressures represent design challenges. Sakhalin Island seismicity is moderate to high, with the active Piltun fault system running north/south on the island as close as 10 km from onshore facilities and 20 km from the offshore platform. The 1995 Neftegorsk earthquake, which registered moment magnitude 7.1, had its epicenter approximately 50 km from the Sakhalin-1 facilities. All facilities must be designed for earthquakes, and the export pipeline, which crosses multiple faults, must be able to withstand several meters of fault displacement.

Dynamic monitoring of an offshore jacket platform based on RTK-GNSS measurement by CF-CEEMDAN method

Many developed and developing countries are investing a huge amount of resources to tap the potential of offshore wind energy. It has advantages over onshore wind farms in many aspects as it neither uses landmass nor creates noise problem to the human habitat. Continuous wind availability and better wind quality are some other advantages of offshore wind farms. With the advancement of offshore technology, deep draft floating platforms like spar have been developed recently. These deepwater floating-type platforms experience extreme marine environmental forces such as wind and waves. Floating offshore structures that are form-dominant types resist the dynamic wind and wave loads by means of displacement responses. Platform tower fore-aft, platform surge, and platform pitch movements/vibrations are the dominant responses among them. The dynamic tower fore-aft displacement reduces the power generation efficiency of the turbines and causes fatigue failure at the tower base. Hence, these movements need to be controlled to increase the performance of the wind turbines. Tuned mass damper is one of the commonly adopted damper systems to mitigate such unintended dynamic movements. Several studies with tuned mass damper on offshore structures reported that it can reduce the unintended dynamic structural responses positively. The present study intends to review the free vibration responses (to review the basic dynamic characteristics of the structure) of the spar platform with and without tuned mass damper (placed at the tower nacelle). The OC3 Hywind spar platform (benchmark) developed by National Renewable Energy Laboratory (NREL) is taken as the study structure. The benchmark study wind turbine structure incorporated OpenFAST software work frame is used for the evaluation. For the vibration response control, tuned mass damper properties (mass, stiffness, and damping) are evaluated corresponding to different mass ratios (tuned mass to tower mass). Initially, free vibration analyses are carried out in OpenFAST to find the natural frequencies in the respective degrees of freedom. Similarly, for free vibration control analysis, fore-aft initial displacement conditions are set, and corresponding free vibration tests are carried out with and without tuned mass dampers and the resulting responses are evaluated.KeywordsFloating offshore wind turbinesFore-aft displacementTuned mass damper