The Low Riser Return System (LRRS) is a method used for managing wellbore pressure during offshore drilling operations by adjusting the mud level in the marine riser by returning mud and cuttings to surface via a subsea pump in a separate conduit. It is a single mud gradient, open MPD system particularly designed for subsea drilling.The LRRS can be used in two application modes. The first has a full riser during static conditions using conventional mud weights and the capability to lower the fluid level to compensate for the ECD effect during circulation and drilling as needed. With this technique, conventional well control procedures are used. The second technique involves using higher than conventional mud weights and lower fluid levels for both static and dynamic (circulating) operations. With this technique, modified well control procedures must be used. Both of these methods improve safety margins, allow for better pressure control, and increase efficiency for most well operations.This paper focuses on the well control issues of drilling with a partially evacuated marine drilling riser. Two cases using data from deepwater US GoM wells illustrate how the LRRS gradient fits well inside the drilling window. Kick margins and kick detection are improved compared to conventional drilling operations, and riser margin can be achieved even for relatively deep water, which allows for safer disconnections. Improved cementing by compensating for density and ECD effects is another important result of applying this system.We conclude that this technology can greatly improve safety in drilling of deepwater exploration and production wells as well as for infill drilling in depleted fields; enabling safe access to more reserves and improving recovery.
This paper was presented as part of the student paper contest associated with the European Petroleum Conference. Abstract A new computer model named MultiPress can predict rapid pressure transients in multiphase pipelines and wells. The model shows that pressure pulses behave similar in horizontal multiphase flow as in single-phase flow. In vertical wells there are strong non-linear effects because gas content, density and speed of sound vary down into the well. Introduction Rapid pressure transients in wells and pipelines are created if a valve is quickly closed or opened, and if a burst disk breaks. Leaks, ruptures, and blowouts can also induce such pressure waves. It is very useful to know how pressure transients propagate in the pipelines. Important applications are design of production system, leak detection and flow control, ref. 1-3. In single-phase flow there are robust theories concerning pressure transients ref.4, but for multiphase flow the problem is more complex. Traditional two-fluid programs do not model pressure pulses in an adequate manner. There are two main problems. Firstly the traditional two-fluid models do not give a correct description of the pressure propagation velocity. Secondly the numerical methods are usually of first order which means that they smear out the solution and loose the rapid pressure variations. This work presents a new computer program, MultiPress that predicts rapid pressure transients in multiphase wells and pipelines. The program is based on advanced numerical methods and knowledge about pressure pulse propagation in gas-liquid flow. Rapid Pressure Transients The traditional two-fluid models ref. 5, give a good description of the flow variables, but the rapid pressure pulses are not correctly treated. The problem is the interaction between the phases. Drift-flux models ref. 6, on the other hand, give a correct description of the propagation phenomena, ref. 7. The homogeneous approach ref. 6, is a simple drift-flux model assuming that the gas and liquid move with the same velocity.
Offshore tests were performed by quickly closing a valve by the wellhead. The rapid pressure signals, created by this valve closure, were registered at three different locations upstream of the valve. The effective actuation time of the valve was about 0.5 second. The tests concluded that the velocity of sound in the multiphase mixture could be predicted using a homogeneous assumption. The pressure waves were attenuated because of friction, but also chokes and bends were important attenuation mechanisms for the rapid pressure waves in multiphase flow.
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