In this section, a general comparison of the controllability of the industrial riser system using the u and the ug is presented. Firstly, it can be observed that the riser-pipeline system can be controlled by using either u or ug as the manipu-lated variable. Generally, the controllability analysis showed that both u and ug
have the ability to stabilise the system at large valve opening with the riser base pressure and the gas volumetric flow rate as the controlled variables. For the PRT, the controllability analysis showed its limited ability to stabilise the system at a large valve opening for the two manipulated variables.
From the simulation result analysis, the control with ug showed the ability to stabilise the system at a slightly higher valve opening than control with the u.
However, due to the differences in the individual valve characteristics in the industrial riser system, it is observed that although the control with ug achieved stability at a slightly higher valve opening, the control with u achieved lower PRB, when compared to an equivalent valve opening with the ug. This is shown in Table 5.16, which shows the maximum stable valve opening (ums), the PRBmin, and the production obtained through simulation for the PRB and the PRT.
Table 5.16: Controllability with u and ug comparison table ums (%) PRBmin (barg) Production
(m3/day)
Variable u ug u ug u ug
PRB 49 50 34.54 35.18 348 339.2 PRT 33 33 35.58 35.85 334.5 331.5
As will be discussed in Chapter 6 and 7, in order to maximise oil production it
is necessary to operate the system such that the flowline pressure is reduced under stable operating condition. Although it is desired to stabilise the system at a large valve opening in order to maximise oil production, this controllability analysis also reveals that when analysing the controllability of the riser-pipeline system for different valves as the manipulated variable, the minimum PRB ob-tained, which can be affected by the individual valve characteristics must be evaluated. This has been achieved through simulation analysis. It is shown that for the industrial riser system used in this work, closed-loop control with u will give a lower PRB than with ug for the same value of the valve opening.
5.7 Conclusions
In this chapter, the controllability analysis of the unstable riser-pipeline system for stability and production has been presented. The control objectives are defined to reflect the core operational targets of the riser-pipeline production system, which are the ability to ensure system stability and achieve maximum oil production. The interdependency between a stable valve opening and the accumulated production was explored as the fundamental basis for the control-lability analysis. It is shown that, the larger the stable valve opening achieved in the system, the higher the ability to maximise oil production.
The ability of a slug control system to achieve these desired control objectives are evaluated with focus on the choice of the controlled variables, using two manipulated variables, which include the riser top valve opening and the top-side separator gas valve opening. The controllability analysis was focused on applying the Hankel singular value analysis of the system linear model to
eval-uate the minimum control input magnitude required to stabilise the system at each open-loop unstable valve opening.
The controllability analysis of the industrial riser system using u as the ma-nipulated variable showed that theoretically, all the three controlled variables considered, namely: the PRB, the PRT and the QT, has the ability to stabilise the system at some open-loop unstable operating points without input satura-tion. Also, the controllability analysis of the same industrial riser system using the ug as the manipulated variable, showed that all the four controlled variables considered namely: the PRB, the Ps, the PRT and the QGesp, has the ability to stabilise the system at some open-loop unstable operating point without input saturation. Interestingly, this controllability analysis also revealed the varying ability of each controlled variable to stabilise the system at a large valve open-ing.
Generally, using u as the manipulated variable, it was observed that the PRB and the QT are able to stabilise the system at a larger valve opening than the PRT. Also, by using ug as the manipulated variable, it was observed that the PRB and the QGouts are able to stabilise the system at a larger valve opening than thePs and the PRT. These results are important as they reflect the pro-duction that is achievable with each controlled variable, under stable operating condition.
A more suitable slug control strategy in which the unstable riser-pipeline system is stabilised at a reference valve opening using a derivative controller action is implemented to perform closed-loop simulation for each controlled variable. In this controlled strategy, perfect tracking of the controlled variable set point was neglected in the system such that the controlled variable set point is set equal to zero, and the derivative controller input is the measured value of the controlled
variable, which is not necessarily zero. The derivative controller parameters are obtained using the Routh stability criterion.
The closed-loop simulations in OLGA confirmed the general predictions of this controllability analysis. In the simulation using u as the manipulated variable, it was observed that the PRB and the QT are able to stabilise the system at a larger valve opening than the PRT, as was predicted. However, it was also observed that the PRB achieved stability at a slightly larger valve opening than the QT. Also, in the simulation using ug as the manipulated variable, it was confirmed that the PRB and the QGouts are able to stabilise the system at a larger valve opening than thePs and the PRT. Simulation results also showed that accumulated production increased with the ability to stabilise the system at a large valve opening. In the simulation with the u, the maximum accumulated production was obtained with the PRB and the minimum with the PRT. Also, in the simulation with the ug, the maximum accumulated production was obtained with the PRB and the minimum with the Ps.
Interestingly, this controllability analyses has shown that most controlled vari-ables including the PRT which was considered to be unsuitable for slug control, and the QT which was considered to be suitable if it is used in the inner feed-back loop in a cascade control, can be used for slug control if an appropriate slug control strategy, such that presented in this chapter is implemented.
control system
6.1 Introduction
The primary objective of a slug control system which is to eliminate slugging and ensure stable system operation has guided the common approach to slug control systems design and implementation. One of the proven solutions to slug control is the choking of the riser top valve. Choking transforms the unsta-ble flow in the riser to staunsta-ble flow. However, due to the additional pressure drop across the valve, it induces extra back pressure on the pipeline. Active feed-back, feed forward and cascade control systems have been applied to dynamic choking for slug control [23, 32, 42, 51, 71, 75, 76, 101, 99].
Although the implementation of a slug controller in the active choking solution has shown its potential to successfully eliminate severe slugging with some benefits, it can also adversely affect the overall production of the system if it is implemented inappropriately. As a result of this, the emphasis on the perfor-mance of slug control systems has recently shifted from just achieving a stable
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system condition to also maximizing production [76].
However, the method for analysing the potential of a slug control system to maximise production and how this potential can be achieved have remained unclear. Most slug control systems are implemented without proper systematic assessment of its potential to maximise production in the system. In this work, a systematic method based on the pressure bifurcation map of the riser system is proposed to analyse the production and pressure loss relationship, and to reveal the potential of a slug control system to maximise production.
It is shown that for an unstable riser-pipeline system with known inlet and out-let boundary conditions, production loss or gain due to operation in stable or unstable operating conditions could be predicted using a pressure dependent dimensionless variable known as the Production Gain Index (PGI). The chapter starts with the description of the pressure and production dependency followed by production estimation using the PGI and finally a case study.