Slot Doubler Pile Installation
A Novel Approach Without LBL Positioning

Complete paper presented at the ADIPEC, Abu Dhabi, United Arab Emirates, November 2025.
C. F. Amaechi; P. Halim
Paper Number: SPE-229791-MS
https://doi.org/10.2118/229791-MS
Published: November 03 2025

Read Full Technical Paper (SPE - OnePetro)

Summary

The below provides a condensed overview of a larger technical case study developed by Shell Nigeria Exploration and Production Company (SNEPCo), supported by Zupt LLC and other project partners, for installing a suction pile foundation for a slot doubler in the deepwater Bonga field in offshore Nigeria. Due to the unavailability of a qualified long baseline (LBL) acoustic positioning contractor, Shell adopted an inertial navigation system (INS) mounted on a remotely operated vehicle (ROV) as the primary positioning method. This approach was designed to meet strict deepwater tolerance requirements while maintaining schedule, reducing operational complexity, and ensuring reliable installation performance.

Challenge

The Bonga field, located approximately 120 kilometers offshore Nigeria in 1000 meters of water, relies on a network of subsea manifolds tied back to an FPSO. With all existing manifold slots fully utilized, Shell deploys compact three-slot "slot doublers" to bring additional wells online. These structures require a suction pile installed within a one-meter horizontal tolerance to avoid stressing subsea jumpers and ensure accurate well tie-ins.

Although INS had been used previously in Bonga for well metrology, it had never been applied to subsea foundation installation at this scale. Shell faced several constraints:

  • No experienced LBL contractor available during the required installation window.
  • INS does not provide real-time tracking of the pile.
  • No prior deepwater manifold foundation analogues existed for INS positioning.
  • A tight schedule and production impact risk required a dependable alternative.

These factors demanded a new installation methodology that could deliver the precision of LBL without the logistical burden of deploying an acoustic array.

Solution

To overcome these challenges, Shell and Zupt developed an INS-enabled procedure centered on the precise placement of seabed marker buoys. The process began with INS alignment at a nearby wellhead of known coordinates, followed by deployment of six marker buoys positioned with centimeter-level accuracy to establish a controlled installation grid.

Two ROVs were used simultaneously:

  • One ROV docked into a custom key on the suction pile to lock in heading during deployment.
  • A second ROV provided continuous visual monitoring of verticality, grid clearance, and touchdown alignment.


With the grid established, the suction pile was lowered using crane control supported by live ROV feedback. After installation, as-built coordinates collected with INS were cross-checked against ROV survey data to confirm positional accuracy.

This combined approach of INS buoy grid, ROV heading capture, and dual-vehicle verification provided the stability and precision required for deepwater manifold foundation installation.

Results

The INS-based installation delivered performance comparable to an LBL operation:

  • 0.36 meter horizontal deviation (well within the 1.0 meter allowable tolerance)
  • Minimal inclination error, confirmed by subsequent slot doubler installation
  • 25 percent reduction in offshore execution time compared to the 2021 LBL-based campaign
  • Elimination of more than 100 hours of quayside calibration and vessel transit
  • Lower vessel emissions and fewer subsea exposure hours
  • Decreased lifting and survey spread complexity, improving HSE performance


These results validated the INS-enabled approach as a precise, efficient, and repeatable alternative to LBL positioning.

Conclusion

This study demonstrates that an INS-based subsea installation method, when implemented with well-defined procedures and multi-ROV execution, provides a reliable and accurate alternative to LBL for deepwater suction pile placement. The strategy offers significant operational advantages in brownfield environments, especially where reference structures are available or where LBL contractor capacity is limited.

Shell's successful deployment in the Bonga field, supported by Zupt's INS capabilities and the coordinated offshore execution of the installation vessel and Shell's subsea and brownfield teams, highlights a leaner, faster, and technically robust approach for future subsea foundation installations. This method provides a new pathway for operators seeking high-precision placement with reduced cost, schedule impact, and operational risk.