Experience on Deepwater Drilling Riser about Hang-off Modes in Typhoon Condition of South China Sea

Abstract Typhoon occurs frequently from July to November every year in South China Sea. It develops fast, moves in high speed, and changes trajectory frequently, especially the typhoon, which is formed in local sea area near the drilling rig. It always threatens the safety of deepwater drilling operation. Typhoon response plan is therefore required to avoid damage to the deepwater drilling operation. According to traditional response plan, all the riser joints should be retrieved onto the rig floor and then the drilling rig should be steered away from the typhoon developing trajectory. However, it is a huge challenge to retrieve all the riser joints in deepwater due to limited response time. For example, the related T-time (typhoon prepare time) of L-1, with water depth over 2500m, is more than 6 days, but the forecast precision of typhoon is only 4-5 days. This indicates that it is not practical to retrieve all riser joints onto the rig in a limited response time, thus contingency plan of hanging off riser system to resist typhoon is more practical for deepwater drilling operations. A case study example is presented in this paper to discuss soft hang-off and hard hang-off mode. Different hang-off lengths are analyzed in the case study. Also, combination effect of driving speed and drilling vessel heading is considered in the case study. This paper summarises the related experience of CNOOC about how to response to typhoon in hang-off mode.