Optimal control of the heave motion of marine cable subsea-unit systems

One of the key problems associated with subsea operations involving tethered subsea units is the motions of support vessels on the ocean surface which can be transmitted to the subsea unit through the cable and increase the tension. In this paper, a theoretical approach for heave compensation is developed. After proper modelling of each element of the system, which includes the cable/subsea-unit, the onboard winch, control theory is applied to design an optimal control law. Numerical simulations are carried out, and it is found that the proposed active control scheme appears to be a promising solution to the problem of heave compensation

Simuleringsstudie ved bruk av "HeaveLock" for dempning av nedhullssvingninger som følge av hivbevegelser i rig

Lett anskaffet olje er i dag allerede produsert. Både nye og modne felt med muligheter finnes, men mange ligger i forhold som gjør dem vanskelig å bore. Det er også en lavere oljepris enn det har vært i lang tid, som resultat av den nåværende verdenssituasjonen, og dette også et insentiv til å øke lønnsomheten i felt. Ved boring i krevende forhold, kan Managed Pressure Drilling (MPD-teknikker) brukes for å unngå trykkrelaterte problemer. Et slikt problem er for eksempel når en flytende rigg må gjøre en forlengelse, altså å forlenge borestrengen. Da blir strengen festet til oljeriggen og eventuell hivbevegelse i riggen gjør at borestrengen fungerer som et stempel i brønnen, og skaper både høye og lave trykk. Riggbevegelser kan da ikke lenger frikoples matematisk fra strengens bevegelser, noe som gjør at konvensjonelle MPD-teknikker ikke kan brukes. Strategien i dag er å vente til at vær og vind er på et tilfredsstillende nivå, for og så gjenoppta driften. HeaveLockprosjektet er et forsøk på å løse dette problemet. HeaveLockaktuatoren er en ventil montert inne i borestrengen, over bitet, med formålet å regulere trykksvingninger i brønnen, spesielt nedihull. Denne avhandlingen er en simuleringstudie av HeaveLock i en oljebrønn. De viktigste parameterne i simulatoren, for funksjonen av HeaveLock når den skal gjøre en forlengelse, er identifisert. En prosedyre for fremgangsmåte er funnet, med en tilfredsstillende regulering av nedihulls trykksvingninger. Alle endringer av parametere som forbedrer prosedyren har derimot sin pris. En mindre nominell HeaveLock åpning krever et høyere pumpetrykk og skaper trykkforstyrrelse ved initialisering og terminering av HeaveLock. Tidsintervaller kan forlenges og skape bedre vilkår i forstyrrelser, men ettersom det er ønskelig å gjøre forlengelsen så effektiv som mulig, blir dette også sett som en ulempe. En forlengelsesprosedyre er imidlertid dokumentert, og viser best mulig konfigurasjon som er funnet i denne avhandlingen. For den aktive perioden av HeaveLock, så kan rundt 70% av nedihulls trykksvingninger dempes, med 3,2 bar og 3,7 bar i nedrampingsforstyrrelse av HeaveLock

Manipulation of the choke pressure during Managed Pressurized Drilling (MPD) operation

Master's thesis in Petroleum EngineeringIn the present investigation, the disturbance and its effect on the bottom hole pressure has been studied. After digging into the prior works, it’s figured out that the reason of their failure is oversimplification and ignoring some parameters that have considerable impact on the BHP like frictional forces resulting from fluid and drill string movements. Following the weakness of the last studies, the feasibility of applying new hydraulic model has been studying and new assumptions have been presented. The model is provided by a set of linked PDEs 1. the pressure dynamics of the well annulus during unsteady Couette flow with a pressure gradient; 2. The movement of the elastic drill string coupled with the pressure dynamic through viscous friction and displacement of drilling mud. It is shown how the model can be simplified to a linear system and under which assumption this simplification can be proceeded. By using the Laplace transformer and inserting appropriate boundary conditions, the transfer function is derived from the linear system. The resulting model uses heave disturbance and controlled flow into the wellbore as input, and the measured pressure at the top of the well, as well as the pressure at the bottom of the well as output. Based on the Hydraulic model developed, for the sake of simplification and faster running time, a lower order of the model with fewer control volumes have been introduced for the Model Predictive Controller (MPC). Two control algorithms for both normal and intervention operations have been evolved by application of PID and Model Predictive Controller (MPD). The Optimization of the PID controller has been done theoretically and validified by MATLAB

Waves and Ocean Structures

Ocean Structures subjected to actions of ocean waves require safety inspection as they protect human environment and everyday lives. Increasing uses of ocean environment have brought active research activities continuously. The newly developed technology of ocean energy even pushed the related needs forward one more step. This Special Issue focuses on Analysis of Interactions between wave structures and ocean waves. Although ocean structures may cover various practical and/or conceptual types, we hope in the years to come, the state-of-the-art applications in wave and structure interactions and/or progress review and future developments could be included. There are fifteen papers published in the Special issue. A brief description includes: Lee et al. [1] presented a concept of a water column type wave power converter. Li et al. [2] considered submerged breakwaters. Lin et al. [3] studied an ocean current turbine system. Thiagarajan and Moreno [4] investigated oscillating heave plates in wind turbines. Chiang et al. [5] proposed an actuator disk model. Tseng et al. [6] investigated Bragg reflections of periodic surface-piercing submerged breakwaters. Lee et al. [7] analyzed caisson structures with a wave power conversion system installed. Yeh et al. [8] reported motion reduction in offshore wind turbines. Wu and Hsiao [9] considered submerged slotted barriers. Tang et al. [10] studied floating platforms with fishnets. Chen et al. [11] calculated mooring drags of underwater floating structures with moorings. Jeong et al. [12] estimated the motion performance of light buoys using ecofriendly and lightweight materials. Zhang et al. [13] considered vibrations of deep-sea risers. On the other hand, Shugan et al. [14] studied the effects of plastic coating on sea surfaces

Aspects of Articulated Column Design Including Rigid Body and Elastic Vibration

The work contained in this thesis mainly relates to aspects of the dynamic behaviour of articulated column production platforms. However, the work can be applied in a more general sense to certain other compliant structures which share some of the dynamic characteristics of articulated columns. After an introductory chapter on the characteristics and uses of articulated columns, some time is devoted to the examination of fundamental aspects which will have an important bearing on the feasibility of the articulated column concept. Such features as the amount of buoyancy required in terms of payload and static heel considerations, space utilisation within the structure, installation procedure and human awareness to motion responses are, discussed. Data are provided which are intended to give general guidance to designers and also establish the inter-dependence of certain parameters. Chapters 4 and 5, are concerned with the rigid body motion response in the time domain. Computer programs have been developed to solve the equations of motion on a time incremental basis, using the modified Morison equation as the forcing function. Once developed, the programs have been applied to examine certain of the non-linear behaviour characteristics of articulated columns in regular waves. Chapter 5 is devoted to examining those aspects of dynamic instability which are readily examined in a time series analysis. Instability mechanisms examined are those due to regular waves and wave groups. Experimental results have been obtained and comparisons with theory are made. The slenderness of the construction of articulated columns gives rise to elastic vibration characteristics which may result in undesirable resonant vibrations. Chapter 6 is concerned with an examination of this aspect, in terms of free vibration analysis, and of those parameters which have the greatest influence on vibration. The finite element method has been used for the free vibration analysis. In Chapter 7, the development of programs to examine the full vibration analysis of articulated columns, in the time domain, is described. The programs have been used to examine, in the time domain, certain of those parameters which were examined in the free vibration analysis presented in Chapter 6. Some experimental results for a very flexible structure are presented and the programs have been adapted to simulate the model construction and test conditions. These results give credibility to the use of a full vibration analysis in the time domain and comparisons of observed non-linear behaviour and predicted non-linear behaviour are made. The non-linear behaviour of articulated columns is shown to play a major part in concept feasibility as are elastic vibration characteristics. However, the concept comprising a lower column of relatively straightforward structural section is shown to be feasible in water depths up to 300 metres. Thereafter, greater flexural rigidity is required

Monitoring of Deep Fluids in the Nankai Subduction Complex, SE offshore Japan

Fluids are of great importance for various geological processes. To improve our understanding of fluid generation and migration in the Nankai Trough subduction complex, SE offshore Japan, this PhD thesis uses different borehole monitoring techniques that were applied in the course of the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment. Long-term monitoring with a SmartPlug borehole observatory was conducted at a splay fault system cutting through the accretionary prism. Further landward, beneath the Kumano forearc basin, information about in situ composition of fluids was obtained by drilling mud gas monitoring and sampling during riser drilling with drilling vessel Chikyu. Long-term pressure data revealed that the fluid pressure data is affected by a wide range of natural signals. Tidal noise was used to estimate hydrogeological fault zone properties. Beneath the Kumano forearc basin, fluid migration and mixing in the accretionary prism is supported by analyses of drilling mud gas focusing on noble gas isotopes and hydrocarbon gas composition. Fluid pathways and mechanisms of fluid migration remain unclear

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

Evaluation of Ocean-Energy Conversion Based on Linear Generator Concepts

A turbine generator is a device that converts mechanical rotation into electrical energy. Unfortunately, the primary driving force that has been used to provide this rotation has, thus far, been fossil fuels. Although fossil fuels have proven to be a reliable resource to continually supply the growing demand for electrical power, they are not without their financial and environmental drawbacks. With the continuously increasing demand for energy, it is hypothesized that the world may, in time, exhaust this precious natural resource and/or inflict permanent environmental damage upon our planet. However, motions that occur in nature, such as ocean waves, can play a significant role in generating environmentally safe and economically viable energy for human utilization. As part of research at Old Dominion University, we propose to use a linear electrical generator (which uses a back-and-forth motion of a piston rather than a rotational movement) to probe the production of electrical energy simply from ocean waves. This would also be a less complex design compared to that of conventional rotational versions. Quantitative analysis for the voltage and power produced from the linear generator for a given set of ocean-wave characteristics will also be carried out, probed and discussed. Previous research into this topic has primarily relied upon modeling an ideal buoy (i.e., one that matches the waves\u27 height and motion at each instant in time) responding to the surface ocean waves under the regular wave regime. This work, however, more closely analyzes the physical properties of the buoy and predicts the electrical power generation capabilities from a seabed mounted linear generator Wave Energy Convertor (WEC) tethered to the floating buoy operating under the influence of a non-ideal buoy and the more realistic irregular wave regime. Several buoy sizes will be modeled to exploit the buoys\u27 natural heave frequency in an attempt to create a greater heave response for a given set of sea state conditions. It will be shown that a greater heave response from the buoy generally leads to an increase in the generated power from the linear generator

Control-Oriented Modeling for Managed Pressure Drilling Automation Using Model Order Reduction

Automation of Managed Pressure Drilling (MPD) enables fast and accurate pressure control in drilling operations. The performance that can be achieved by automated MPD is determined by, firstly, the controller design and, secondly, the hydraulics model that is used as a basis for controller design. On the one hand, such hydraulics model should be able to accurately capture essential flow dynamics, e.g., wave propagation effects, for which typically complex models are needed. On the other hand, a suitable model should be simple enough to allow for extensive simulation studies supporting well scenario analysis and high-performance controller design. In this paper, we develop a model order reduction approach for the derivation of such a control-oriented model for {single-phase flow} MPD {operations}. In particular, a nonlinear model order reduction procedure is presented that preserves key system properties such as stability and provides guaranteed (accuracy) bounds on the reduction error. To demonstrate the quality of the derived control-oriented model, {comparisons with field data and} both open-loop and closed-loop simulation-based case studies are presented