Conductor Drilling System Addresses Stability, Cost, And Health & Safety Issues In The North Sea

Drilling top holes offshore in soft soil and relatively shallow water can present a multitude of challenges. Most notably, soft soil can render traditional drilling techniques ineffective, as the newly drilled holes collapse before casing can be installed. The soil conditions in combination with a lack of weight above the drill bit can also mean that there is little to dampen drilling vibration. These undampened vibrations can shake drilling equipment to the point where dropped objects occur with serious health and safety implications.

When Hess Corporation approached Claxton Engineering for support with a conductor drilling system, it was exactly these challenges that it was hoping to overcome.

Hess was scheduled to install a 40″ conductor for Phase 3 of the South Arne development in the Danish sector of the North Sea. During previous drilling operations using 26″conductors, the company had encountered drill string vibration issues when drilling the top hole. Hess proposed an alternative conductor drilling system that effectively runs the conductor and the bottomhole assembly in tandem.

Replacing the relatively light drill string with heavier conductor strings helps to reduce vibrations and prevent hole collapse. Hess asked Claxton to develop a suitable design for a very large system that could be delivered quickly.

Claxton’s in-house project and design team put together a functional design specification for the conductor drilling system within days of the kick-off meeting, which Hess signed off with minimal alterations. The design work then ramped up and the whole system delivered in about three months.

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Picture: Conductor Running and Installation Tool (foreground). Dual Bore Centralizer (background)

“Claxton is not just an engineering company that makes one or two products,” explains Greg Brown, senior design engineer, Claxton Engineering. “We have expertise across many parts of the oil and gas industry and can provide solutions quickly. We are very responsive and often work to very tight schedules. The conductor drilling system was based on existing technology. The main challenge was its scale – we had never been asked to apply the technology to a 40″conductor. The sheer size was a test in itself; handling the system, getting it onto machines, even finding machines large enough to do the work.”

The System

The conductor drilling system comprises a hanger and landing ring which supports the weight of the bottomhole assembly, transfers the mud pressure load to the conductor as well as resisting the torsion generated by the mud motor, and providing an effective seal against the mud pressure. A centralizer was also incorporated lower in the conductor drilling system to reduce deviation of the bottomhole assembly during drilling.

An upper and lower stem act as interfaces between the hanger and the bottomhole assembly/coarse thread safety joint, and a 7 5⁄8″ regular lift tool provides the interface with the upper stem for handling the conductor drilling system–bottomhole assembly.

The conductor drilling system also uses a casing running tool screwed directly into the top of the casing string that forms a seal above the hanger. Mud is then pumped through the casing running tool and forced through the mud motor, thereby generating torque at the drill bit.

As the conductor drilling system bottom hole assembly penetrates the soil, a new joint of conductor is added to the top by removing the casing running tool and torqueing in the new joint. Once the system is at total depth, the coarse thread safety joint is engaged the connection between the landing ring and the hanger is sheared, and the bottom hole assembly retrieved.

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Picture: Conductor Running and Installation Tool during loading

Time and Cost Savings

By adding weight above the drill bit, the new system reduced vibrations, enabling Hess to successfully drill to depth (350 m) in one day without incident. Before using this system, drilling speeds had to be limited to minimise vibration and ensure the wellbore was properly cleaned. Charles Youell, senior engineer at Claxton Engineering said:

Rig costs are very high, so every hour spent not penetrating is very costly. The system also enables Hess to combine two processes by running the casing while drilling. As well as saving time, the system instantly stabilises the top hole and mitigates the risk of it collapsing.

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Picture: LEFT – Upper section of the conductor drilling assembly during interface testing, RIGHT – Dual Bore Centralizer during testing phase

Follow-On Work

Following the successful delivery of the conductor drilling system, which included design support during the trial and installation phases and supply of tertiary equipment, Hess asked Claxton for additional support. This included providing a dual-bore centralizer, J tool, swage, drilling and running tool interface, installation tool, false rotary table and brushing tool.

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