One of the factors affecting offshore drilling uptime is the rig's ability to disconnect the riser in a timely manner in case of a drift-off scenario. A drift-off may occur as a result of complete or partial failure of the Dynamic Positioning (DP) system and the severity of the environmental conditions affect the allowable time to emergency disconnect the drilling riser. The conventional assessment to determine point of disconnect (POD) is typically based on a simplified approach that does not accurately capture partial DP loss and/or dynamic effects and may result in unnecessary downtime.
The most common methodology is to assume a full DP loss, even though the likelihood of this is much smaller than the likelihood of partial DP loss, i.e. single or multiple thruster failure or generator failure that results in reduced capacity on several thrusters. In this paper, partial DP loss cases are investigated, by assuming some thruster capacity is available to counter-act the environmental drift loading. In addition, this paper illustrates the benefits of including the transient effects during drift including vessel and riser coupling.
The paper contains comparisons between results for a DP semisubmersible subjected to typical Gulf of Mexico environmental conditions. A base case assuming full DP system failure and conventional riser offset limits is presented for the purpose of comparison. Multiple cases with partial DP system failure, coupled vessel/riser response and/or with dynamic drift-off riser limits are presented to illustrate how the vessel may be allowed to stay connected in harsher environment than when using the conventional approach.