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Pressure Control for Offshore Managed Pressure Drilling (MPD): Analysis, Design, and Experimental Validation

Mahdianfar, Hessam
Doctoral thesis
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URI
http://hdl.handle.net/11250/2387222
Date
2016
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Abstract
Deepwater offshore drilling operations are among the most complex activities. For

example in regions such as the Gulf of Mexico, wells are drilled in water depths

of up to 3 kilometers, drilling depths can exceed 6 kilometers, and geologic formation

pressures can exceed 1400 bar. In offshore wells safe drilling and completion

operations can be difficult due to narrow margins between pore pressure and fracture

pressures. Moreover multiple zones of uncertain pore pressures and fracture

pressures in reservoir formation makes drilling operation even more challenging.

To avoid fracturing, collapse of the well, or influx of fluids from the reservoir surrounding

the well, it is crucial to control the pressure in the open part of the well

within a certain operating window. One method to regulate bottom-hole pressure

is Managed Pressure Drilling (MPD). In Constant Bottom-hole Pressure (CBHP)

MPD, the annulus is sealed and the drilling fluid exits through a controlled choke,

allowing for faster and more precise control of the annular pressure. In automatic

MPD systems, the choke is controlled by an automatic control system which manages

the annular well pressure to be within specified upper (fracture pressure) and

lower (pore pressure) bounds.

In the North Sea environment, a large number of subsea wells are drilled from

floating rigs. In this case, there would be the extra challenging factor of severe

vertical motion (heave) of the rig in harsh weather, typically more than 3m up

and down with a 10 - 20 seconds period. When drilling from a floating rig, the

rig moves vertically with the waves. While drilling with weight on bit (WOB), a

heave compensation system is in operation that isolates the drill string from the

heave motion of the rig. As drilling proceeds, the drill string needs to be extended

with new sections, referred to as pipe connection. Thus, every couple of hours or

so, drilling is stopped to add a new segment of about 27 meters to the drill string.

During connections, the pump is stopped and the drill-string is disconnected from

the heave compensation mechanism and put into slips rigidly connected to the rig.

The drill string then moves vertically with the heave motion of the floating rig, and

acts like a piston on the drilling fluid in the well. The heave motion in this case

can result in severe pressure fluctuations (surge and swab pressures) in the bottom

of the well. Pressure changes have been observed to be beyond the standard limits

for pressure regulation accuracy in MPD.

The goal of this thesis is mainly to develop control design methodologies for

regulating bottom-hole pressure (BHP) in offshore drilling operations. In essence

extending control over BHP to the pipe connections scenario when the rig pumps

are off and the drilling string is moving vertically. Two different lines of research are presented, which will be discussed in the sequel. This thesis comprises two parts.

First part gives an introduction to MPD, and discusses briefly variants of MPD.

Next common equipment and technology components in MPD are discussed. Then

different estimation and control techniques proposed in the literature are reviewed

thoroughly. Variety of estimation and control techniques have been applied in different

scenarios in drilling operations. The majority of the control methods are

based on PID and Model Predictive Control (MPC). While estimation techniques

are mostly based on Kalman filters and Lyapunov techniques. Then an introduction

to the connection scenario and down-hole pressure fluctuations (surge and swab)

caused by the heave motion of floating drilling rig is given. Methods for modeling

and prediction of surge and swab pressures are reviewed and different techniques

proposed for compensating their effect in MPD and Under-Balanced Drilling operations

(UBD) are discussed. An introduction to MPD-Heave lab at NTNU and a

comparison with industrial scale MPD systems are given. Contributions are stated

in detail.

In the second part selected conference and journal papers are presented. In the

first paper a simplified nonlinear dynamic model based on mass and momentum

balances for managed pressure drilling (MPD) is presented. The sources of uncertainty

in drilling operations is discussed and two parameters for calibrating the

hydraulic model against uncertainties in the viscosity of mud, temperature distribution

in the well, frictional pressure losses, the geometry of the well, and bulk

modulus are considered. The key uncertain model parameters and the bottom-hole

pressure are simultaneously estimated using joint unscented Kalman filter based on

only available top-side pump and choke pressure measurements. The performance

of the algorithm is tested for the case of normal drilling operations and connection

operations, in which there is no flow through the drill-string and borehole

pressure reduces significantly. The results of simulations show accurate estimation

of the bottom-hole pressure and uncertain parameters during both transient and

steady-state drilling operations.

In the second paper a model describing the flow and pressure fluctuations in the

bore-hole due to drill-string movement has been presented. It consists of a pair of

coupled nonlinear partial differential equations modelling the distributed pressure

and flow in the well, and a simple oscillator for the disturbance. Considering only

top-side flow and pressure as measurements, it is shown that the model can be

represented by a linear time invariant finite-dimensional system with output delay.

This result is achieved by linearization and de-coupling using Riemann invariants.

An infinite-dimensional observer is designed that estimates the disturbance, and

the estimate is used in a controller that rejects the effect of the disturbance on

the down-hole pressure. A model reduction technique based on the Laguerre series

representation of the transfer function is used to derive a simple, rational, transfer

function for the controller. The performance of the full-order and reduced-order

controllers are compared in simulations, which show satisfactory attenuation of

the heave disturbance for both controllers. In the third paper the results of the

previous paper are extended by incorporating friction partially into the model and

considering the heave disturbance to be a superposition of multiple sine waves.

In the fourth paper, the output regulation problem for an offshore deep-water

managed pressure drilling system subject to periodic disturbances is addressed.

The disturbance is caused by the heave motion of the floating drilling rig during

pipe connections and the objective is to maintain a constant pressure at the bottom

of the well. The controller for the heave disturbance attenuation consists of three

cascaded parts: First, a nonlinear inversion element is applied to invert nonlinearity

of choke. Second, an adaptive compensator is designed based on internal model,

and certainty equivalence principles for asymptotic rejection of time-varying heave

disturbance. Third, an output feedback controller is synthesized using LMIs for

providing stability and improving transient performance of closed-loop system.

Robust stability of closed-loop time-varying system is analyzed using edge theorem.

Simulation results of the combined adaptive output regulator are presented, which

show satisfactory set-point tracking and attenuation of the heave disturbance.

In the fifth paper, a modified L1 adaptive controller is applied to handling heave

disturbances in MPD. It presents a modification to L1 adaptive control that allows

for disturbances entering at the plant output. By incorporating the disturbance

at the output into the reference model, it is shown that the L1 adaptive control

structure can be left unchanged while the original transient performance bounds

are preserved. It is further shown that rejection of the output disturbance can be

taken care of entirely in the filter design step of L1 adaptive control using the

internal model principle. A systematic filter design procedure based on LMIs is

provided, that requires only one tuning parameter to be adjusted by the designer.

While satisfying asymptotic robust regulation constraints, it guarantees a desired

level of performance measured in terms of peak value of output signal subject to

a peak value constraint on the control signal. The control design is applied for

disturbance attenuation and set-point tracking in the so-called heave problem in

oil well drilling, and successfully tested in a medium size experimental test facility

containing a 900m long well. The results demonstrate that the proposed regulator

efficiently regulates the down-hole pressure to the desired set-point, with significant

attenuation of unmatched periodic disturbances.

Finally in the last chapter, the conclusions and future work directions are offered.
Publisher
NTNU
Series
Doctoral thesis at NTNU;2016:96

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