The drill-through completion concept revolves around the principle that substantial cost savings can be realized by installing the subsea horizontal tree prior to running the BOP (Blow Out Preventer) stack thus eliminating one round trip of the BOP stack. The drilling operations are performed through the subsea horizontal tree. Additional cost savings can be achieved when a batch drilling and completion program of more than one well is performed by significantly reducing the number of rig moves and BOP stack trips required. This paper describes three drill-through completion system approaches that have been devised and the economic benefits of using them.


Interest in drill-through systems has increased in recent years as oil companies search for ways to reduce installation and workover costs associated with deepwater drilling and completions. As the water depth increases, both the time required to deploy equipment to the seabed and the day rate of the rig needed to operate in deeper waters increases. As a result, the value of reducing deployment trips and improving the reliability of the operations becomes greater as the water depth increases.

The horizontal subsea tree has had a large impact on the way completion and workover operations have been conducted. The horizontal design involves running the completion through the tree itself. Drill-through technology allows some or all of the drilling operations to be conducted through the tree - providing numerous opportunities to save time and money, particularly in deepwater operations.


During the drilling and completion of a typical non-drill-through well, a BOP stack is installed on top of the wellhead high-pressure housing just prior to drilling out for the first casing string. The casing strings are then installed, cemented, and tested. The lower completion is then drilled out, run and tested. The well is then secured and the BOP stack removed. A subsea horizontal tree is then installed on the top of the wellhead and the BOP stack is redeployed to finalize the completions work. This conventional approach to drilling and completing wells involves two round trips of the BOP stack.

The drill-through approach to drilling allows the horizontal tree to be installed on top the wellhead high pressure housing at an early stage prior to running the BOP stack so that all drilling operations are performed through the tree. The tree is capable of drifting all of the required drill bits, casing hangers, seal assemblies, and associated running/test tools during the drilling and wellhead installation process.

This method of drilling and completing a well requires only a single trip of the BOP stack. By eliminating one of the round-trip deployments of the BOP stack, a substantial cost savings can be achieved.

The deployment of the BOP stack and drilling riser system for a rig operating in 6000 to 8000 feet of water is estimated to require four to five rig days. Another 8–12 hours is required to test the BOP stack connections, functions and control system once it is landed on the wellhead or tree.

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