Harsh weather conditions result in severe heave motion on floating drilling rigs. The drill string is heave-compensated during drilling ahead, however, when slips are set during drill pipe connections, the topside heave compensation system is disabled, and the drill string is then moving up and down together with the rig. Pressure oscillations below and around the drill bit, induced by such movement, are known as "surge & swab". These oscillations can be in the order of 20 bar or more during drill pipe connections in the North Sea and pose a serious challenge to drilling of wells with narrow pressure margins from floating rigs in harsh weather environment. A managed pressure drilling (MPD) choke at the surface cannot be used to control rig heave-induced surge & swab due to the fast nature of the pressure oscillations in question, stochastic character of the sea waves that cause them and long time-delay between topside choking and bottomhole response. Continuous Circulation System (CCS) might be able to reduce the pressure oscillations somewhat by maintaining constant mud flow during connections.
Computer simulations and laboratory experiments were previously used to investigate a novel method for attenuating surge & swab by utilizing an autonomous choke to be installed in the bottom-hole assembly (BHA). The bottomhole pressure oscillations can be effectively reduced through dynamic in-situ control of the mud flow through BHA by a downhole choke. This paper presents the downhole choke system and the results from the first pilot trial conducted in a mud flow loop at Ullrigg test rig in Stavanger utilizing a full-scale version of the choke. The prototype was subjected to drilling mud with flow rates up to 2500 lpm and differential pressure up to 250 bar to investigate its ability to accurately control the flow while at the same time withstanding the demanding conditions.
Satisfactory choke valve characteristics were obtained, indicating ability to control the flow with sufficient precision. Flow testing resulted in severe erosion of carbon steel components while wolfram carbide components were able to withstand the erosive nature of the flow. The test also uncovered challenges related to operation of the choke with high differential pressure and flow rates which could later be related to the motor, used to control the downhole choke assembly.
The next phase of the project is to design a downhole prototype and test it in an onshore test well to achieve Technology Readyness Level (TRL) 4 qualification ("ready for first use offshore"). The final goal is to qualify the downhole choke together with MPD and a Continuous Circulation System (CCS) for use on floating drilling rigs in harsh weather environment.