Development Of An Instrumented Riser Joint

Abstract

ABSTRACT For drilling operations in deep water, it is important to have a good insight into the stresses that occur in a riser during normal operation and under extreme weather conditions. It was therefore decided to develop, in conjunction with TNO-IWECO*, an instrumented riser joint that would be placed in the riser to record continuously stresses, mud pressures, and mud temperatures. This paper describes this instrumented riser joint (IRJ), designed for a 24-in. riser and capable of operating 500-m water depth. The unit is installed on the Sedco-700 drilling vessel operating in the North Sea area. The results obtained are discussed. INTRODUCTION When drilling in deep water, it is of importance to know what stresses caused by wave action and vessel motion occur in the riser. Repeated fatigue failures of marine riser couplings in the top part of the riser in North Sea operations1 demonstrated the obvious need for accurate measurement of the bending stresses in these couplings. These data are obtained best by the introduction of a special section (or joint) in the riser string, which is instrumented to make the necessary measurements. In addition, placing such an IRJ in the bottom of the riser gives immediate information on the status of the buoyancy modules and the ball joint at the BOP stack. After reviewing the developments of instrumented riser joints by various manufacturers, Shell and TNO-IWECO agreed in mid-1974 to cooperate in the development of an improved unit. In May 1975, the final KSEPL/TNO-IWECO design proposal was completed. The instrumentation involved has been patented In Britain under the claim K 2820 GBR (Aug. 30, 1976). At the end of 1975, finite-element calculations were carried out to establish the expected stress levels in the joint. A 24-in. full-scale IRJ, suitable for installation at any level down to 450-m water depth in a 24-in. × 5/8-in. riser, was fabricated and made ready for testing in Nov. 1976. A laboratory test program consisting of static hydrostatic and dynamic tests under simulated operational North Sea conditions then was carried out. Following the satisfactory performance of the IRJ in the laboratory, the IRJ was installed in cooperation with Shell Expro on the Sedco 700 in 1977. This floater operated in the North Sea on the Auk field during April and May 1977 and moved to the Brent area later in that year. OBJECTIVE OF PROJECT The objective of this project was to develop an IRJ capable of measuring stress, mud pressures, and mud temperature. These measurements were intended to be used for (1) establishment of a correlation between wind, waves, current and riser stresses in the most critical areas, and determination of the optimal riser tension to maximize the fatigue life of the riser connectors [if the joint is installed in the upper part of the riser (Fig. 1, position 2), it can be used to predict the life of the connectors and ensure timely replacement]; (2) verification of the computer programs used for the dynamic analysis of risers; (3) monitoring of the bottom ball joint and BOP-stack forces.