A Success Story of Using Deepwater Semisubmersible Drilling Rig for Workover Operation in 400 Meters Water Depth without Utilization of Anchoring System

Abstract

Abstract This paper will be focused on the engineering process for first-time experience workover operation in shallow water depth with a 6th generation semisubmersible drilling rig. The 6th generation semisubmersible drilling rig is designed for operations in water depths of up to 3000 meters, with a typical minimum water depth requirement of 600 meters. However, the operation described in this paper was conducted at a depth of 400 meters solely relying on the dynamic positioning (DP) stability system without any additional mooring and anchoring system. The paper will cover the environmental conditions, engineering processes for drilling rig qualifications, operations envelope, mitigation plan, summary of the drilling rig performance, and lesson learned during the campaign. The well was initially completed in 2019 by generation 4 semisubmersible drilling rig with mooring assistance. However, indications of decreased production performance prompted the need for a workover intervention to restore the original production indicators. The workover operation was planned after development wells at nearby field with water depth ranging between 1300-1500 m. The well test was requested after the completion phase, which includes utilizations of Through BOP Intervention Riser System (TBIRS) inside the drilling riser. Due to the presence of subsea facilities surrounding the well, it was not advisable to use an anchoring system to maintain stability, as it could pose hazards to the flow line or umbilical in close proximity to the well. Engineering preparation was made to qualify the capability of the rig to operate in the shallow water depth. Preliminary review showed technical concerns of deep-water drilling rig to operate below its minimum water depth. The key areas where then studied and assessed further to prepare for shallow operations include riser analysis and fatigue management (subsea Christmas tree, wellhead, and landing string component). Operation mitigation is then implemented thru dynamic positioning preparations, ESD management, rig heading, power generators reliability, riser angle and weather monitoring. Based on assessment of actual data, the rig was successful in maintaining the operational window in terms of static load and time series load management. Stable station keeping and desired heading was achieved for most of the time which has resulted on ensuring structural load and fatigue of subsea equipment within the safety margin. Key success factors were the engineering assessment, early stage of preparation, real time monitoring of manual and electrical parameters, DP3 reliability, electro-hydraulic control system and great teamwork while performing the job.