Marine Riser Monitoring With The Acoustic Ball Joint Angle-Azimuth Indicator

Abstract

ABSTRACT A marine riser ball joint angle-azimuth indicator with acoustic telemetry has been designed, manufactured, field tested, and is now considered an operational tool. The most positive, practical, and economical means to assure the structural integrity of a marine riser in floating drilling operations is to monitor the angle from vertical of the lower ball joint. If a marine riser equipped with a lower ball joint with a maximum rotation of 10 degrees from vertical is in acceptable mechanical condition, the lower ball joint angle should not exceed approximately 5 degrees in shallow water (400 ft. and under) or reach its mechanical stop (10 degrees) in deep water (600 ft. and over) to prevent overstressing. With knowledge of the lower of all joint angle and azimuth, adjustment of vessel position and applied tension will result in optimum operation of the marine riser. A number of very useful additional benefits can be obtained through monitoring the lower ball joint angle. The maintenance of a small ball joint angle will minimize the following: drill pipe fatigue and/or failure; wear in the blowout preventor area due to longitudinal movement and rotation of drilling assemblies; difficulties in running tools and drilling assemblies through the ball joint and blowout preventers; damage to the Hydril packing unit from drilling bits and stabilizers; and damage to wellhead seal assemblies during running. At locations where it is common to release the marine riser from the blowout preventers because of adverse weather, knowledge of the angle at the. ball joint can allow the operator to delay or avoid disconnecting the riser with confidence that the riser will not be overstressed. INTRODUCTION The two prime objectives of the marine riser are to serve as a return path for drilling fluids and to guide the drill string in and out of the wellhead. Because of its important role in maintaining well control and its high initial cost, strong efforts are made. to prevent overstress, excessive fatigue and excessive internal wear. In the brief l6-year history of floating drilling, marine risers of various types have been used with the most common and successful being the single wall tensioned riser. Through improvements in design, construction, and operating technique, the single wall tensioned riser system has now been used successfully and with confidence in a number of excellent treatments of the theory for predicting the behavior of marine risers and criteria'; for successful marine riser design have been presented. l, 2,3 These papers all show that the forces resulting from wave action, current action, riser weight, drilling fluid density, and the horizontal offset of the top. of the riser from the wellhead tend to increase riser stress and to deform the riser. These forces are resisted by applied tension atthe upper end of the riser. In general, available calculational methods are used to determine the tension that should be used with a given set of maximum conditions. If the actual conditions are less stringent than the assumed conditions, the resulting tension causes unnecessary wear on the tensioning system.