An Effective Tool for Monitoring Marine Risers
Authors: Childers Mark A. , Hazlewood Dale , Ilfrey W.T. 
Published: December 2007

This acoustic ball-joint angle and azimuth indicator has proved, in deep water and in severe weather, that it can monitor reliably and directly the structural integrity of the riser. The knowledge it provides of the ball-joint angle can give the operator the confidence to delay or avoid disconnecting the riser without fear of adverse consequences.

Introduction

The two prime objectives of the riser are to provide a return path for drilling fluids and to guide tool assemblies 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 16-year history of floating drilling, 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 been used successfully and with confidence in water depths up to 1,500 ft. In general, the riser system (Fig. 1) is composed of a hydraulic riser connector that fastens the riser to the blowout preventers (BOP's), a lower pivot point (ball joint) next to the riser hydraulic connector, a number of joints made of pipe with end connectors, a slip joint that allows vertical motion between the drilling vessel and the riser, a gimballed connection of the inner barrel of the slip joint and the vessel, and a means of applying tension to the riser. In deep water the addition of a ball joint below the slip joint can reduce riser stress and fatigue.

There are a number of excellent papers on the prediction of the behavior of risers and on criteria prediction of the behavior of risers and on criteria for successful riser design. These papers all show that the forces resulting from wave action, current action, riser weight, drilling fluid density, horizontal offset of the fluid density, and 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 tension applied at the upper end of the riser and in some cases by the addition of buoyant material to the riser. In general, available methods are used to calculate the tension that should be used with a given set of assumed maximum conditions. If the actual conditions are less stringent than the assumed conditions, the resulting tension causes unnecessary wear and reduces the life of the tensioning system. If actual conditions are more stringent than assumed conditions and the tension is not increased accordingly, overstress, excessive fatigue, or excessive internal wear may result. Riser operations are particularly vulnerable to unfavorable changes in ocean particularly vulnerable to unfavorable changes in ocean current profiles, which are difficult to measure, especially during critical times.

Studies based on the calculational methods described by Tidwell and Ilfrey showed that the angle of the riser with the vertical at the lower ball joint is directly related to the maximum stress in the riser. In essence, the angle of the ball joint shows the net effect of all the forces acting on the riser. For a given riser operating under a given set of conditions the angle of the lower ball joint is related to vessel offset and applied tension. Therefore, if that angle and its azimuth could be continuously monitored, the angle could be controlled by manipulating vessel offset and applied riser tension so as to minimize riser stress, fatigue, and internal wear.

JPT

P. 337