Safety Analysis of the Hang-off of Deepwater Drilling Risers during a Typhoon Emergency Period

For semi-submersible platforms, proper response procedures for a typhoon include the recovery of all drilling risers and travel away from typhoon paths. But it takes a long time to recover risers resulting that sometimes there is no guarantee that the entire riser system can be recovered. Under such circumstances, suspensions of risers are inevitable. This study took the suspended risers as the research objects, considering dynamic tension, bending stress, flex joint angle of risers and the space interference with the moon pool. With the analysis method of finite element, the axial dynamic model of risers in suspension mode was established. with a super deep water well in the South China Sea (with water depth of 2460 m) being taken as an example, analysis models for soft and hard riser suspension models were set up,a deep water typhoon evacuation strategy was studied, the influence of different suspension models were reviewed, and drilling ship evacuation speed and sailing azimuths on riser safety were analyzed. Results showed that in the hard suspension model, when the sailing speed was 2 kn, safe evacuation azimuth was 0° to 45°, while for soft hang-off mode, at the same sailing speed 2 kn, the sailing azimuth was from 0° to 90°. Under the same ocean conditions, the sailing speed of the drilling ship allowed by soft hang-off model was higher than that by hard hang-off. Therefore, the adaptive capacity of the soft model to the wave load was greater than that of the hard model, so the soft model was recommended to use preferentially during evacuations from typhoons.