Riser Collapse Pressures Resulting from Release of Deepwater Mud Columns
Authors: Miller Jack E. , Stahl Matthew J. , Matice Christopher J. 
Published: October 2008

Emergency disconnect of a deepwater drilling riser filled with a heavy mud column creates the potential for collapse of the drilling riser. The concern for collapse is traditionally focused on the idea that the pressure equalization process at the bottom of the riser causes the mud column to "U-Tube" such that the heavy mud column dropping will create a large region of evacuated riser. External seawater pressure applied to evacuated riser defines this collapse condition. The maximum water depth for the evacuated region, and thus the maximum collapse-pressure, will occur about 30 to 90 seconds depending on mud weight and water depth. "Riser Fill Valves" are sometimes used to prevent evacuation of the riser. The authors have found that the "U-Tube" effect is not the only source for creation of a high pressure collapse condition. At the instant the mud column is released, a high differential pressure exists at the bottom of the riser. This differential could be as high as 3,000 to 4,000 psi for a 10,000 foot riser. The differential pressure will tend to rapidly equilibrate across the opening thus creating a negative pressure wave which will propagate up the riser at sonic speed. The phenomena is a classical "fast-opening" waterhammer. At some intermediate depth, the outer and inner pressures combine to create a maximum magnitude collapse pressure. This condition develops in a matter of seconds and it is developed whether or not a riser fill valve is in the system. We show that this collapse pressure is nearly equal to the collapse pressure caused by the "U-Tube" effect. We conclude that the riser fill valve is not sufficient to prevent riser collapse pressure resulting from the "fast-opening" waterhammer phenomenon.

Introduction

Emergency disconnect of the drilling riser may occur because of drift-off, drive-off or weather. Since the well must be cleared quickly, the emergency disconnect does not provide sufficient time to circulate out the mud and fill the riser with seawater. Dumping the mud from the deepwater riser during emergency disconnect is in many cases standard operating procedure because it has been shown that if the contents are retained, the dynamic riser loads may be significantly magnified while trying to suspend the deepwater riser1. It has long been recognized that following emergency disconnect of a deepwater riser and subsequent mud column release, a heavyweight mud may evacuate the riser to a sufficient depth that a riser pipe collapse condition can be created by the external sea pressure applied to the evaluated portion of the riser. The physics that control the draining of the riser and the depth at which the evacuation occurs is normally called the "U-Tube" effect. This is a simple matter of equilibrating the static head of the riser contents with the ea water at the bottom of the riser and calculation of the maximum external pressure is similarly simple. The way industry has dealt with the possibility of collapsing the riser due to this effect is to include in the riser a component which is called the Riser Fill Valve (RFV).