Well Intervention in Shallow Waters with Dynamically Positioned Vessels: A Study Case for Southeastern Brazil

Abstract

This paper presents the technical analysis, simulation and work carried out in planning and executing an open sea workover riser operation for decommissioning a well from a dynamically positioned (DP) vessel in shallow waters, on southeastern Brazil. The main challenge in using a DP rig in this situation is the risk of thruster blackout, resuling in a driftoff scenario. In shallow waters, the system safe allowable vessel driftoff is usually strict, resulting in tight Red Alarm Offsets or even making the operation unfeasible. Real time monitoring and analysis may be used to obtain dynamic optimized Red Alarm Offsets during an operation, but simulations must be performed for operational planning. Statistical treatment of a database of measured and simulated metocean conditions led to reduced environmental loadcases than usually considered in standard industry analysis, while retaining an acceptable safety level. Modification of the riser control system also resulted in reduced Emergency Disconnection Sequence (EDS) time, resulting in broader Red Alarm Offset results. The methodology herein presented may be refined even further, in order to make possible the use of DP vessels in even shallower water scenarios.