Riser Collapse A Unique Problem in Deep-Water Drilling

Abstract

Abstract While drilling an exploratory well in 1750 feet of water, a large kick emptied the marine riser. The two lowest joints collapsed due to the external pressure. A diverter failure also occurred. The equipment damage caused by this incident is described, and collapse calculations for the riser are presented. It is concluded that, if a kick is taken in deepwater after the subsea blowout preventers (BOPS) have been run, the subsea BOPs should be closed instead of activating the diverter, even if only shallow conductor casing is set. Diverter systems remain a must for shallow water operations, but their strength should be reevaluated and perhaps upgraded. Introduction The oil industry has had great success in pushing back the water depth frontiers for pushing back the water depth frontiers for exploratory drilling offshore. Many technical challenges have been successfully met in the course of this effort. Two areas that are particularly note worthy in this respect are the particularly note worthy in this respect are the extension of the capability of marine risers and the development of new well control techniques. However, important points remain to be considered in both these areas. This paper describes a well control problem that developed during an exploratory drilling operation in 1,750 feet of water. Further study of this difficult situation is important for two reasons:The problem caused major damage to the marine riser through hydrostatic collapse.The diverter performed its intended function for only a short time before failing. The hydrostatic collapse of the riser is discussed below. An explanation of the mechanism is offered, and it is shown that the use of the diverter instead of the subsea blowout preventers (BOPs) played a key role in the collapse. Both of the events noted above have important implications for deepwater well control procedures. These implications are highlighted in the closing discussion. SEQUENCE OF EVENTS During the spring of 1982, an exploratory well was being drilled from a semisubmersible unit in 1,750 feet of water in the Gulf of Mexico. The well had reached a depth of about 5,600 feet (below the rotary table). Twenty-inch casing was set to 2,652 feet (about 850 feet below the seabed). Sixteen-inch liner was set with the top at 2,322 feet and the shoe at 3,669 feet. While drilling 12 ¼-inch hole at 5,607 feet, a kick was taken. Mud weight was 11.0 pounds per gallon. The first indication of the kick was a 6-barrel pit gain. pit gain. The initial action taken was to increase the mud pump speed and switch to premixed 12.5 pounds per gallon kill mud. Next the diverter was per gallon kill mud. Next the diverter was activated to direct the flow overboard. The subsea BOPs were not shut-in at this point due to concern about formation integrity at the casing shoe. The diverter functioned properly for about three minutes. It then started leaking badly past the element. This lead to a situation where mud and gas were blowing through the rotary table about fifteen feet in the air. The mud pumps were shutdown, and the subsea BOPs were then closed. A casing pressure of 825 psi was measured. The gas flow through (and past) the diverter had essentially emptied the mud column from the riser. After an unsuccessful attempt to refill it, a TV camera was run to survey the situation. P. 307