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 phenomenon 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.
Miller, J.E., Stahl, M.J., Matice, C.J., “Riser Collapse Pressures Resulting from Release of Deepwater Mud Columns”, Paper No. OTC-8853-MS, Offshore Technology Conference, May 4-7, 1998, Houston, Texas.