Top-Hole Technology Overcomes Challenging Sand-Based Seabed Conditions and Enables Record Drilling Performance in an Offshore Exploration Well

This summary is based on the paper "Top-Hole Technology Overcomes Challenging Sand-Based Seabed Conditions and Enables Record Drilling Performance in an Offshore Exploration Well," presented by Camilo Cardenas, Hans Erik Kolstrup Hansen, Sigvald Hanssen, and Harald Blikra (Repsol Norge AS), Wolfgang Mathis and Ole Kristian Holen (NeoDrill A/S), and Arjen Kort and Youhu Zhang (Norwegian Geotechnical Institute) at the SPE/IADC International Drilling Conference and Exhibition, 2021. The full paper can be accessed through the Society of Petroleum Engineers.

Essential Highlights

• CAN Technology Overview:

  • The Conductor Anchor Node (CAN) was used to support well construction on the Kathryn exploration well, marking the first application in sand-based seabed conditions.

  • The CAN integrates a conductor to form a stable foundation, using suction pressure to penetrate the seabed.

• Challenges and Solutions:

  • The seabed at Kathryn included dense sand and boulders, making conventional methods risky. A CAN with an integrated conductor was chosen for its lower risk, reduced environmental impact, and cost savings.

  • Contingencies like high-capacity pumps, water injection, and cyclic penetration were implemented to address potential installation challenges.

• Installation Success:

  • The CAN was installed efficiently in 65 hours, faster than the planned 84 hours, reaching the required penetration depth with minimal issues.

  • Real-time geotechnical monitoring was crucial for successful installation, confirming the CAN's load capacity on-site.

• Drilling Performance and Cost Savings:

  • The project achieved record drilling time, completing the well in 13.54 days, saving 2.8 rig days compared to traditional methods.

  • Top-hole construction costs were reduced by 51%, attributed to fewer materials, faster installation, and a simplified cementing process.

• Environmental and Safety Improvements:

  • The CAN technology reduced CO2 emissions by 31%, minimized the need for cement and steel, and decreased the risk of handling large components on the rig.

  • The pre-installed conductor allowed safer operations by avoiding manual handling of large tubulars.

Abstract

The successful implementation of CAN® technology in sand-based seabed conditions for this offshore exploration well in the North Sea demonstrated its effectiveness in overcoming geological challenges, such as boulders and dense sand layers. By providing a strong, pre-installed well foundation, the CAN reduced rig time, minimized uncertainties in cementing, load capacity, and fatigue, and significantly cut top-hole construction costs. Furthermore, the use of CAN technology enhanced operational safety and reduced the environmental footprint, making it a valuable solution for achieving high drilling performance in complex offshore conditions.

Introduction

Top-hole construction for subsea wells requires selecting the right solution based on seabed properties, expected loads, and well design. The CAN technology, introduced in 2006, reduces risks in top-hole construction by providing a verifiable load capacity through its suction anchor design. Initially used with conventional conductor installation methods, the CAN has evolved to integrate the conductor, simplifying installation and improving well stability. For the Kathryn exploration well in the North Sea, hard sand and high-strength sandy-clay conditions posed challenges for CAN deployment. This paper outlines the successful modifications made to ensure the CAN's installation, emphasizing its positive impact on safety, environmental footprint, cost, and drilling performance.

Concept selection

For the Kathryn project, three top-hole construction options were evaluated: drill and cement, CAN combined with drill and cement, and CAN with integrated conductor. Conductor jetting and hammering were discarded due to the presence of boulders. A risk-based comparison methodology was used to assess the main risks, including cement job problems, verticality issues, and accidental loads. CAN with integrated conductor had the lowest risk, particularly in mitigating cement and landing issues, and showed the potential to reduce environmental impact and cost by around 37%. Despite medium risks related to installation in hard sand, strategic mitigations were implemented, making the CAN with integrated conductor the preferred solution for its safety, environmental, and cost advantages.

CAN implementation

CAN Installation Principle

The CAN installation method uses suction pressure to drive the structure into the seabed, a technique proven in various offshore applications like jacket foundations and FPSO anchors. The CAN consists of a large steel cylinder (the skirt) and a concentric conductor guide pipe. Initially, the CAN sinks into the seabed by its own weight, and then water inside the cylinder is pumped out to create a pressure differential, generating a downward force that pushes the CAN further into the soil. One of the key advantages of this method is that the load capacity can be verified in real-time during installation, providing more certainty than conventional methods. However, the risk of piping—water leakage between the suction compartment and open water—can occur in more permeable soils, potentially limiting penetration.

Site-Specific Geotechnical Engineering Aspects at Kathryn

At the Kathryn site, geotechnical assessments showed a mix of clean sand interlayered with clay and silt layers, presenting challenges for CAN installation. Four soil profiles were developed to capture these variations, and penetration analyses indicated that a minimum depth of 5.8 meters was needed for the required holding capacity. However, intermediate clay layers could lead to suction pressures as high as 1000 kPa, posing the risk of penetration refusal. Contingency measures were designed, including a high-capacity pump to increase suction, water injection to reduce resistance at the skirt tip, and cyclic penetration to deal with dense layers. A key modification involved closing the conductor pipe to create communication between the conductor and the suction chamber, increasing penetration force in case of refusal.

Results

CAN Operations

CAN Installation Phase

The CAN installation phase for the Kathryn well, conducted in July 2019, was completed efficiently, with all operations performed faster than planned, taking 65 hours instead of the expected 84. Key tasks included installing the low-pressure wellhead housing (LPWHH), testing equipment, and setting up a water injection system. The installation process was successful, with the CAN achieving a penetration depth of 5.8 meters within 14.5 hours of net installation time. Real-time geotechnical calculations were used to monitor and guide the operation, ensuring the penetration record matched the expected design. Despite minor challenges, such as a 0.2-meter shortfall in penetration depth, contingencies like activating the conductor's internal volume helped achieve the target depth without needing the water injection system or cycling.

Retrieval of CAN unit

For the Kathryn well, personnel from the CAN provider were actively involved in planning the drilling phase, focusing on preventing washouts and broaching during the spud phase. Operational parameters from previous CAN installations were used to guide the drilling program, including spud circulation rates, BHA diameter, conductor ID, and cementing details. This knowledge transfer ensured proper preparation for a successful drilling operation.

Drilling performance

The impact of the CAN technology with integrated conductor in the drilling performance of the Kathryn exploration well is analyzed in four different factors: drilling time, cost, environment, and safety. For each factor, it is discussed how this solution contributed to achieve the different project goals.

Drilling time

The Kathryn well was drilled in 13.54 days, making it the fastest well drilled by the operator in Norway and one of the quickest offshore exploration wells globally. The CAN with pre-installed conductor played a key role in this performance, saving 2.8 rig days compared to the conventional drill and cement method. Additionally, the P&A operation was streamlined by cutting the surface casing inside the CAN, eliminating the need for rig retrieval. The well achieved only 1.3% non-productive time (NPT), as top-hole risks were mitigated before rig arrival. Furthermore, using the CAN reduced the need for cementing the entire surface casing, minimizing the risk of losses and avoiding the use of more expensive cement slurries.

Cost savings

The use of CAN with a pre-installed conductor saved 2.8 rig days and additional costs associated with the 36" conductor casing, rental of 42" BHA, and 36" casing running equipment. Cement volume was significantly reduced from 241 m³ to 53 m³. Overall, the technology reduced top-hole construction costs by 51%, equivalent to saving two days of drilling operations, making it a key factor in achieving cost-related goals for the project.

Environmental impact

The CAN with integrated conductor significantly reduced the environmental impact compared to conventional drill and cement methods, cutting greenhouse gas emissions by 31% and reducing other pollutants such as NOx and particulate matter. This reduction, quantified by an LCA study from Asplan Viak (2020), was primarily due to the savings in rig time, cement, and steel. Overall, the CAN helped lower CO2 emissions by 432 tons and decreased the environmental footprint across all impact categories evaluated.

Safety

The CAN technology with a pre-installed conductor enhances safety by eliminating the need for rig crews to handle large diameter tubulars, as the structure is pre-installed by a vessel in a "hands-free" operation. This significantly reduces the risk of incidents during drilling. Additionally, the technology allowed the operator to reduce the size of the bottom hole assembly (BHA) from 42" to 17 1/2", further minimizing operational risks.

Conclusion

The CAN with integrated conductor was successfully installed on the Kathryn well, marking the first use of this technology in sand-based seabed conditions. Four installation profiles were evaluated, and measures were implemented to ensure success, resulting in a safe, efficient installation completed in 65 hours (vs. 84 planned). Continuous monitoring and real-time evaluation against geotechnical models were key to achieving the target depth and confirming the CAN's load capacity. This project demonstrated the versatility of the CAN technology to handle challenging conditions, cutting top-hole construction costs by 50%, reducing environmental impact by over 30%, and enhancing drilling safety.

For more detailed insights, the full paper can be accessed through the Society of Petroleum Engineers.

References

Cardenas, C., Hansen, H.E.K., Hanssen, S., Blikra, H., Mathis, W., Holen, O.K., Kort, A., Zhang, Y. 2021. Top-Hole Technology Overcomes Challenging Sand-Based Seabed Conditions and Enables Record Drilling Performance in an Offshore Exploration Well. SPE/IADC Drilling Conference and Exhibition. SPE-204096-MS. Retrieved from Society of Petroleum Engineers.