Comparison of Analysis and Full-Scale Testing of Anti-Recoil System for Emergency Disconnect of Deepwater Riser

Abstract

ABSTRACT This paper describes the verification of a procedure for analyzing the emergency disconnect of deepwater drilling risers. The procedure was used to design an anti-recoil system for Sonat's Discoverer 534 drillship. The purpose of this system is to provide control of the riser's ascent velocity and hence to limit its impact with the diverter housing. The system was tested during an actual disconnect in 3700 feet of water. This test was full scale and instrumented to provide riser displacement and pressure drop across the anti-recoil valves in some of the tensioners. The data was used to verify the analysis and design procedure. This procedure is briefly discussed herein, but a full discussion is available in a companion paper (1). INTRODUCTION In late 1989, BP Exploration initiated plans for upgrading Sonat's Discoverer 534 Drillship to a 6000 ft operating water depth (2). Several primary systems were installed or modified. One of these was the riser tensioner system to which a riser anti-recoil system was added. The purpose of this system is to avoid accelerating the riser into the diverter and substructure following an emergency disconnect. The anti-recoil system and associated procedures were designed by computer analysis of the riser/tensioner system (1,3). The computer program used is called RISTEN and is owned by Shell Development Company. THE FULL SCALE TESTS Since the anti-recoil system was designed by computer simulation {jJ everyone agreed that a full scale disconnect test was needed to verify the computer analysis. The first opportunity for testing turned out to be the first well after the D534 upgrade was complete. This was in 3700 feet of water in the Gulf of Mexico. The tests consisted of a disconnect test and two bump tests. The two bump tests consisted of pulling the riser up from its post-disconnect resting position of 4-5 feet short of full TJ (telescoping joint) collapse. Refer to "Bump Tests" portion of this paper for more on these. Test instrumentation included the following:Video cameras located at strategic locations,Pressure transducers located on each side of the anti-recoil valves (see Fig. 1) of 3 selected tensioners,The rig's magnetic slider/tensioner position indicator located on one of the three selected tensioners, andTwo accelerometers located near the top of the TJ's outer barrel. The pressure transducers were installed on each side of anti-recoil valves (see Fig. 1) for tensioners 2B (#2 Bow), 4A (#4 Aft), and 5B (#5 Bow). These were Precise Model 555 strain gage pressure transducers. The magnetic slider/tensioner position indicator was pre-existing equipment on the D534. The electrical signal sees a unit step change each time the slider reaches a different monitored point. The monitored points are spaced 1 inch apart, which causes a unit stepped voltage output signal at slow speeds and high frequency pulses at higher speeds.