Analysis and Design of Anti-Recoil System for Emergency Disconnect of a Deepwater Riser: Case Study

ABSTRACT Emergency disconnect of the drilling riser is required when a dynamically positioned deepwater floater has either a driftoff or a driveoff. This cannot be anticipated; therefore, the riser tension setting cannot be adjusted to a preferred value. Hence, following emergency disconnect, highly tensioned risers have been known to accelerate into the diverter housing and lift the rotary table several feet. This paper describes an analysis method for quantifying this problem and how the method was applied to design an anti-recoil system for Sonat's Discoverer 534 Drillship. The ship was being upgraded to operate in 6000 feet of water. The analysis method described herein was compared to results of a full scale disconnect test in 3700 ft of water. As described in a companion paper (1), the comparison of analysis to test results was very good. INTRODUCTION In late 1989, BP Exploration initiated plans for upgrading Sonat's Discoverer 534 (0534) Drillship to a 6000 ft operating water depth (1). 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. Fig. 1 shows a sketch of upper riser arrangement for the 0534. Except for the latch mechanism, this configuration is typical of most floating drilling rigs. Also typical is the pneumatic/hydraulic system (as shown in Fig. 2) for providing tension to the drilling riser. These tensioner systems are designed to provide near constant tension whether the riser is connected or not. Therefore, if the tension was high and the situation called for prompt disconnect, then the riser could obviously be accelerated into the diverter/substructure. This accelerated riser motion is termed "riser recoil". The release of strain energy stored in a long riser also contributes to the recoil action. Designing an "anti-recoil system" requires some way of accurately predicting how the riser/tensioner system will respond to proposed system changes. The riser/tensioner system is too complex for hand calculation and too large for practical experimental study. Therefore, computer simulation is the most efficient tool for designing an anti-recoil system. This was done for the Discoverer 534. THE EMERGENCY DISCONNECT PROBLEM Drilling in water depths exceeding 3500 ft is generally performed from a DP (dynamically positioned) floater. Sometimes, these vessels experience driftoffs or driveoffs. When such happens there is no time for reducing top tension prior to disconnecting the riser. The top tension would be high if heavy mud was in use. Furthermore, it was determined that keeping the mud in the riser is not desirable for emergency disconnect. Thisreduces the riser's hanging weight and mass, thus increasing the riser's acceleration,