A new operability and predictability enhanced riser control system for deepwater marine operation: an integrated riser hybrid tensioning system

This dissertation presents a novel riser hybrid tensioning system by integrating an electrically powered riser tensioning system into existing hydro-pneumatic tensioners. Compared to current passive hydro-pneumatic tensioners, this new riser hybrid tensioning system provides the capability of dynamically controlling the tension in the riser string. This feature opens a wide horizon of different active riser control strategies to achieve the systematic riser control solution. The objective of this study is to increase the predictability and safety of the whole riser system, and to extend the operability of the riser tensioning system into other operations. An overall structure framework of this novel hybrid riser tensioning system is proposed, comprising a direct driven electrical tensioners, hydro-pneumatic tensioners, a super-capacitor based energy storage system, power dissipaters, an overall tension controller and a power management controller. Hardware configurations are suggested. A riser data logging system is introduced, providing more comprehensive riser status data. A power management control strategy and overall coordination architecture to integrate the whole system are proposed. As the main functionality of the riser tensioning system, a new active heave compensation control strategy is analyzed in detail, by using this new riser hybrid tensioning system. A LQG controller and a H [subscript infinity symbol] controller are designed. The position chasing technique produces predictive and accurate tension commands for the electrical tensioners. Both Matlab simulation and hardware implementation confirm the feasibility of this concept, and further verifies that a more accurate control performance could be achieved by the electrical tensioners 180° compensating the tension fluctuation caused by the hydro-pneumatic tensioners. A novel testability and predictability enhanced anti-recoil control algorithm is implemented in the electrical tensioners. A position control strategy is proposed with the objective of moving the riser body to a desired elevation height in a predictive manner. A system model and a Kalman estimator are built, and a LQG controller is designed. The simulation demonstrates that the riser lifting height can adjust to any reasonable value for different test environment. This anti-recoil control concept reduces the risk of catastrophic damage, and allows us to perform maintenance tests much more frequently to bring back operator’s confidence. During harsh sea state, the VIV can be suppressed by using the dynamic control of the hybrid tensioning system, at frequencies and magnitudes made available by the electrical tensioning system. The objective is to achieve the VIV suppression by avoiding the excitation of the oscillation locking into the resonance conditions, and by reducing oscillation energy to be built in riser. A modal analysis of a tensioned Euler-Bernoulli beam is studied. Two control methods are proposed. Simulations results demonstrate that the oscillation is effectively reduced at the dominant lock-in frequency. Finally, this riser hybrid tensioning system opens the possibility to extend the tensioning system operability into other drilling operations. A motion stabilizer supporting the heave compensation of the drill pipes and the DST tools can be eliminated by connecting the drill pipes onto the telescopic joint. Another application would be that the electrical tensioners can run under position control mode after the riser is recoiled and soft hang-off on tensioners. The riser string position with respect to the seabed can still be controlled, during the vessel moving among different well heads.Electrical and Computer Engineerin

Modelling and control of subsea installation

Ph.DDOCTOR OF PHILOSOPH

Reliability assessment of drag anchors and drill strings in floating offshore drilling units

As the oil and gas explorations move to deep and ultra-deep water, reliable and economical operation of floating offshore drilling units becomes significantly important. Despite significant improvements achieved in design of all of the components involved in drilling operation, there are still operation failure reports that threaten the vulnerable offshore environment. This, in turn, mandates the reliability assessment of the key elements of these floating drilling units. The station keeping of drilling platform in harsh environment and the structural integrity of the drilling system under both vibrations and environmental loads are the key areas of concern that affect the reliability of these systems. In this study, two crucial elements affecting the overall system reliability was investigated, including the reliability of drag embedment anchors, as a key element of station keeping, and the fatigue reliability of drill strings, as a key element of structural integrity. First, a comprehensive reliability analysis of drag embedment anchors was conducted through the probabilistic modelling of anchor capacity and incorporation of inherent uncertainties. A plastic yield loci was used to characterize the fluke-soil interaction and failure states. The embedded profile and the frictional capacity of the anchor chain at the seabed were also considered in the calculation of ultimate holding capacity. A 3D coupled finite element (FE) model was developed to obtain the characteristic mean and maximum dynamic line tensions for 100 years return period sea states, as well as the design line tension and corresponding line angle at mudline. Catenary mooring system was considered to maximize the vessel motions and approach the worst case scenarios. First order reliability method (FORM) was used through an iterative procedure to obtain the probabilistic failures. The study revealed the 3 sensitivity of the reliability to key components of anchor geometry, seabed soil properties, and the environmental loads. The study revealed that the reliability index depends on the fluke length and is largely irrelevant to the anchor weight. As well, the level of the reliability indices obtained for drag embedment anchors was found to be lower than the other anchoring solutions such as suction caissons. Second, the fatigue reliability assessment of the drill string under stick-slip vibration and first-order vessel motions was comprehensively investigated. An efficient approach for FE modeling of stick-slip vibrations of the full drill strings was developed, and a comprehensive analysis was conducted to observe the influence of the field operating parameters on the structural dynamic response of the full-scaled drill string under stick-slip vibration. The model was developed based on a rate-dependent formulation of bit-rock interaction, for which the cutting process is integrated through the frictional contact. The nonlinear effects of large rotations, the geometrically nonlinear axial-torsional coupling, and the effect of energy dissipation due to the presence of drill mud were taken into account. The performance of the developed numerical model was verified through comparisons with a lumped-parameter model and published field test results. Time-domain analyses were conducted by incorporation of both stick-slip vibration and vessel motion under the environment loads. Then the fatigue reliability assessment of drill string was conducted by damage calculation under different excitation scenarios using the deterministic S-N curve approach and defining the safe, low risk, and high risk damage zones. The points of most severe fatigue damage and the corresponding risk under simultaneous drilling vessel motions and mechanical vibrations were identified. The results showed the significant influence of the rotary table velocity on the stick-slip characteristics 4 of the drill string in comparison with other field operating parameters, i.e., weight-on-bit and damping ratio. It was found that the coexistence of stick-slip vibrations and horizontal vessel motions is detrimental to reliable performance of the drill string and can result in premature fatigue failure of the top-most drill pipe, the drill pipe passing through the BOP, and the lower drill pipe connected to drill collar. Overall, the study provided an in-depth insight into this challenging area of engineering and resulted in developing robust methodologies for reliability assessment of the key components of floating drill systems from station keeping to drill string

Bit-rock interaction in rotary drilling: numerical and experimental study

This investigation describes the dynamic model of a rotary drilling system equipped with a PDC bit. Torsional and axial dynamics are modeled separately with the bond graph technique and coupled is given by a bit-rock interaction model that considers cutting and friction components at the bit cutters. The cases for contact loss are analyzed and included as numerical functions to account for bit-bounce and stick-slip during drilling. Cosimulation of the drillstring and bit-rock models is proposed to simplify the numerical implementation. Verification confirmed that the model was captured with sufficient accuracy and yields predictable results for known inputs. A methodology for bit-rock parameter acquisition is suggested. Simulation of a real drilling setup was performed and validated against experimental tests. For the analyzed ranges, simulations were in agreement with experimental results. This shows that a close prediction of the drilling response of a PDC bit is possible with the considered model

The design of a deep water catenary riser

The overall aim of this study is to propose and develop a cost effective production design concept suitable for oil reservoirs situated in deep (1500 m) water which can be quickly and safely installed in areas with limited weather windows. The proposed design is based upon a steel catenary riser which will connect an FPSO directly into either a wellhead or seabed pipeline system thereby eliminating both the connection complexes and high cost associated with a central manifold. The catenary geometry will ensure that the structure is inherently compliant whilst a carrier pipe arrangement will provide structural protection and buoyancy to a flowline bundle contained within. The interface between the riser and the surface production vessel is a critical part of any riser system and so for the purposes of this study two design arrangements are considered. The first is based upon a direct connection between an FPSO turret and riser whereas the second is a hybrid design in which the riser is supported by a sub-surface buoy which is hydraulically connected to an FPSO using flexible flowlines. This hybrid connection has the advantage of decoupling FPSO and riser motions. Design development is carried out by examining a range of critical areas

Structural Response Evaluation Using Non-Uniform Sensor Arrays

Sensor arrays strategically deployed on various offshore structures may provide valuable information in addressing issues related to the complex dynamic response behavior due to varying environments, changing hydrodynamics and purposely attached engineering devices. The current work was devoted to developing techniques to (1) optimize the sensor array according to specific engineering goals, (2) use response data obtained from the sensors to evaluate structures‟ extreme responses, (3) extract modal parameters, and (4) analyze strength conditions. The computational tool developed in this study integrated genetic algorithms, modal recognition techniques, damage detection methods, time series and spectral analysis methods. Genetic algorithms, originally proposed for solving optimization problems based on natural selection, have demonstrated capabilities in obtaining the optimal sensor array configurations in extracting a single mode or two modes simultaneously. This finding laid the foundation for further modal recognition and damage analysis. The first application discussed herein focused on response evaluation of long and flexible subsea transmission lines; specifically, evaluating the performance of flow-induced vibration suppression devices and buoyancy elements. With laboratory data, the study demonstrated that airfoil fairings, ribbon fairings and helical strakes can all effectively suppress the undesired vibrations in a uniform current; however, the first two devices were not quite effective, especially airfoil fairings, when the structures were subjected to combined loads of current and waves (though all devices significantly increased the damping). In addition, the study showed modal parameters extracted with optimized sensor arrays can help detect, locate and size damages in a structure via numerical simulation (though the performance of the methodology may decrease with localized non-uniform strength profiles and excessive marine growth). The second application extended the methodologies from 1-D beam-like structures to 2-D plate-like structures. These studies focused on strength analyses of various ice sheet formations. The results illustrated, in spite of the exponentially increased computational volume, fine-tuned genetic algorithms can still locate near optimal sensor arrays regardless of boundary conditions and placement restrictions due to complicated Arctic environments. Furthermore, the damage detection methodology utilized herein proved to be able not only to detect weak regions but also to detect strengthened areas in ice sheets, for example an ice ridge, thus complete strength analyses of selected ice sheet formations can be conducted

Modeling and simulation of vibration in deviated wells

During the engineering of deviated well, drillstring is in the complicated moving state, strong vibration is the main reason that induces drillstring failure. Drillstring vibrations usually have axial vibration, lateral vibration, torsional vibration and the drillstring near the bottom of well usually coupled vibrates strongly. A dynamic model to predict the effect of drillstring parameters on the type and severity of vibration is desired by the oil industry, to understand and prevent conditions that lead to costly downhole tool failures and expensive tripping or removal of the string from the wellbore. High-fidelity prediction of lateral vibrations is required due to its coupling with potentially destructive axial and torsional vibration. This research work analyses the dynamics of a horizontal oilwell drillstring. In this dynamics, the friction forces between the drillstring and the borehole are relevant and uncertain. Drillstring contact with its borehole, which can occur continuously over a line of contact for horizontal shafts such as drillstrings, generates normal forces using a user-definable stiff spring constitutive law. Tangential contact forces due to friction between the drillstring and borehole must be generated in order for whirl to occur. The potential for backward whirl and stick-slip requires the transition between static and dynamic Coulomb friction. The proposed model computes the relative velocity between sliding surfaces when contact occurs, and enforces a rolling-without-slip constraint as the velocity approaches zero. When the surfaces become ‘stuck’, a force larger than the maximum possible static friction force is required to break the surfaces loose, allowing sliding to resume. The drillstring bottom-hole-assembly has been modeled using a three-dimensional multibody dynamics approach implemented in vector bond graphs. Rigid lumped segments with 6 degrees of freedom are connected by axial, torsional, shear, and bending springs to approximate continuous system response. Parasitic springs and dampers are used to enforce boundary conditions. A complete deviated drillstring has been simulated by combining the bottom-hole-assembly model with a model of drill pipe and collars. The pipe and collars are modeled using a lumped-segment approach that predict axial and torsional motions. The proposed dynamic model has been incorporated the lumped segment approach which has been validated with finite element representation of shafts. Finally, the proposed contact and friction model have been validated using finite element LS-DYNA® commercial software. The model can predict how axial and torsional bit-rock reactions are propagated to the surface, and the role that lateral vibrations near the bit plays in exciting those vibrations and stressing components in the bottom-hole-assembly. The proposed model includes the mutual dependence of these vibrations, which arises due to bit-rock interaction and friction dynamics between drillstring and wellbore wall. The model can simulate the downhole axial vibration tool (or Agitator®). Simulation results show a better weight transfer to the bit, with a low frequency and high amplitude force excitation giving best performance but can increase the severity of lateral shock. The uniqueness of this proposed work lies in developing an efficient yet predictive dynamic model for a deviated drillstring

STUDIES ON THE NONLINEAR INTERACTIONS ASSOCIATED WITH MOORED SEMI SUBMERSIBLE OFFSHORE PLATFORMS

The design of moored semi submersible systems constitutes a challenging engineering problem in which, the platform offset, stability, payload and system-optimized cost requirements are to be met simultaneously. This problem is complicated by the incomplete understanding of the nonlinearities associated with the multiple interactions such as wave to wave, wave to platform, platform to mooring, fluid to mooring and mooring to seabed. In this study, an attempt has been made to probe into these nonlinearities through numerical, experimental, and parametric studies. In the numerical study, moored semi submersibles were analyzed in the time domain. The dynamic equilibrium conditions were satisfied through a set of coupled nonlinear differential equations for the six DOF motions. For representing the platform to mooring nonlinear interactions, the 6x6 mooring stiffness matrix was derived based on the mooring stiffness and on the fairlead coordinates relative to the structure CG. For the evaluation of the slow frequency horizontal motions of the platform, the second order wave forces resulting from the second order temporal acceleration and the structural first order motions were formulated. For the assessment of the fluid to mooring and mooring to seabed nonlinear interactions, a deterministic approach for the dynamic analysis of a multi-component mooring line was formulated. The floater motion responses were considered as the mooring line upper boundary conditions. Lumped parameter approach was adopted for the mooring line modeling. Mooring to seabed nonlinear interactions were modeled assuming that the mooring line rested on an elastic dissipative foundation. A numerical dynamic analysis method in the time domain was developed and results for various mooring lines partially lying on different soils were validated by conducting a comparative study against published results. The contribution of the soil characteristics of the seabed to the dynamic behavior of mooring line was investigated for different types of soil. Two phases of experimental studies were conducted to provide benchmark data for validating the numerical methods. In the first phase, the seakeeping performance of a semi submersible with eight circular columns was studied. The model was built to scale of 1:100 using Froud’s law of similitude. The tests were conducted for head, beam and quartering seas. In the second phase, a semi submersible with six circular columns was modeled using the same scale as for the first semi submersible. Linear mass-spring system was arranged to facilitate measurements of the horizontal drift forces. The system natural periods, still water damping, nonlinear viscous damping, drag coefficient and inertia coefficient information were evaluated from the free decay tests. Seakeeping tests were conducted for head and beam model orientations. The measured drift forces were compared to available formulae in the literature to assess the available semi-empirical methods for evaluation these forces. In both experimental phases, twin-hulled conventional semi submersibles were considered. By comparing the results of the numerical and experimental models, the validity of the numerical method was established. Based on the validated numerical algorithm, a number of parametric studies were conducted for investigating the contributions of various design parameters on the dynamics of moored semi submersibles. The effects of pretension, mooring line configuration, clump weight, cable unit weight, elongation, breaking strength and pretension angle on the behavior of multi-component mooring line, were investigated by using an implicit iterative solution of the catenary equations. On the other hand, using linearized frequency domain analysis, the contributions of platform payload, platform dimensions, number of columns, number of mooring lines, the wave environment mathematical model, the wave characteristics and the operating (intact or damage) conditions to the responses of moored semi submersibles were investigated. The experimental and published results verified the efficiency of the developed numerical model for prediction of the wave frequency and low frequency motions and mooring dynamic tension responses of the semi submersible. Moreover, experimental results indicated that in addition to the modeling of the mooring system stiffness, typical or hybrid modeling of the mooring system and attachments are necessary for the critical assessment of the mooring system damaged conditions

Sizing the Actuators for a Dragon Fly Prototype

In order to improve the design of the actuators of a Dragon Fly prototype, we study the loads applied to the actuators in operation. Both external and inertial forces are taken into account, as well as internal loads, for the purposes of evaluating the influence of the compliance of the arms on that of the "end-effector". We have shown many inadequacies of the arms regarding the stiffness needed to meet the initial design requirements. In order to reduce these inadequacies, a careful structural analysis of the stiffness of the actuators is carried out with a FEM technique, aimed at identifying the design methodology necessary to identify the mechanical elements of the arms to be stiffened. As an example, the design of the actuators is presented, with the aim of proposing an indirect calibration strategy. We have shown that the performances of the Dragon Fly prototype can be improved by developing and including in the control system a suitable module to compensate the incoming errors. By implementing our model in some practical simulations, with a maximum load on the actuators, and internal stresses, we have shown the efficiency of our model by collected experimental data. A FEM analysis is carried out on each actuator to identify the critical elements to be stiffened, and a calibration strategy is used to evaluate and compensate the expected kinematic errors due to gravity and external loads. The obtained results are used to assess the size of the actuators. The sensitivity analysis on the effects of global compliance within the structure enables us to identify and stiffen the critical elements (typically the extremities of the actuators). The worst loading conditions have been evaluated, by considering the internal loads in the critical points of the machine structure results in enabling us the sizing of the actuators. So that the Dragon fly prototype project has been set up, and the first optimal design of the arms has been performed by means of FEM analysis