ABSTRACT Deepwater drilling risers are equipped with flotation to reduce the shipboard tensioning requirements. When such a riser must be disconnected and hung off due to severe weather, certain steps should be taken to protect it and the drilling vessel against damage. This problem arises because the buoyed riser has low hanging weight compared to its mass. In a high sea, it will occasionally tend to fall more slowly than the heaving vessel. This leads to compression in the riser which increases its vulnerability to the hydrodyn9mic forces. Computer results for several combinations of parameters show that under periodic compression and wave forces, the alternating bending stress and the dynamic riser deflection grow rapidly. Several options for maintaining the tension in the riser required to limit the alternating bending stress are evaluated. INTRODUCTION Although offshore drilling operations have been conducted in water depths greater than 4,000 feet, and in stormy conditions, to date there has been no drilling in sites combining both very deep water and severe weather. Starting in the spring of 1979, two such areas will be the sites of multiple drilling operations. Several operators will be drilling off the east coast of Canada and also on the Exmouth Plateau off northwest Australia. Some of the latest deepwater drilling rigs will be involved. Their drilling risers, equipped with buoyancy material, are nearly neutrally buoyant. Protection of these risers in the presence of rapidly developing severe weather may present these operators with a problem which has not previously been confronted in offshore operations. Both east Canada and northwest Australia are subject to hurricane force storms, sometimes without much warning. For example, in the fall of 1975 the dynamically positioned drillships HAVDRILL, PELICAN and SEDCO 445 were all damaged by a hurricane off east Canada. Drilling in water depths less'than 1,500 feet, they were caught in various stages of riser and BOP stack pulling. All suffered varying degrees of damage to their BOP stacks, risers and moonpool structures. In deeper water the problem of storm survival is greater for two reasons. The more obvious one is that it simply takes longer to pull and stow the riser. This necessitates longer range storm forecasting. With more than 24 hours required to pull and secure a 4,000 foot riser, accurate forecasting for such a long period may not always be possible. The second problem is somewhat more subtle and is the topic of this paper. The situation arises when an attempt is made to survive a storm with the riser hung off, i.e., suspended from the moonpool area of the vessel. This may occur by choice, if the storm is anticipated to be not too severe, or may result from the crew's inability to get the riser onboard before vessel motions preclude further riser handling. The difficulty stems from the difference between the mass of the hanging riser and its suspended weight. Each joint of riser, equipped with buoyancy material, may weigh only a few hundred pounds in water.