A subsea conductor is the initially set large diameter (usually 30”) casing that is connected to the BOP stack and drilling riser while the rest of the well is being drilled. Conductors can either be driven to depth as on most offshore platforms or drilled and grouted with cement in subsea developments.

Drilled conductors can be supported by a subsea template for production wells or unsupported in the case of exploration wells and some production wells. As most unsupported conductors in the North Sea are set less than 120m below the sea bed, vertical loadings on the upper part of the conductor are transferred into unconsolidated sediment through the cement sheath around the conductor. Good soil contact around the cement is critical to support the weight of the BOP stack, conductor and other casing strings installed in the well as it is deepened. Any failure to support the conductor jeopardises the integrity of the entire well, while its being drilled and during its production life.

Due to the use of seawater at high circulation rates as a drilling fluid, wash out is confirmed in most conductor wellbores by the excessive volumes of cement required to fill the annulus void to surface around the conductor. Typically up to 200% over the calculated annulus volume using the bit diameter is pumped and in some cases, the annulus is still not filled to the seabed. Washout with seawater can affect the cement bond by softening the soil and reducing the probability of a good bond when the cement is essentially gravity filled into a wide borehole. Sandy formations (as found in the Southern North Sea) are even more prone to washout and sediment stress failure. As well as the vertical loads from the weight of BOP risers, conductors are subject to axial loads from rig movement and wave motion drilling  subsea wells to total depth. A poor cement bond can then become a contributing factor to separation of the cement sheath from the surrounding soil as the well progresses. Some unsupported conductors monitored by ROV during the drilling phase, develop funnel shaped voids and cracked cement sheaths at the seabed around conductors. From a group of unsupported conductors in the North Sea monitored by the 4Subsea SWIMTM system, 19% had major instability issues, of which one in four were evident while landing the BOP stack. These  were suspected to have either lower than expected soil strength or a failed cement job on the conductor.*

Responsibilities for well construction integrity have been stated by the UK Health and Safety Executive since 1992 in their guidance: “The effect on a foundation of conductor installation procedures should be reviewed with particular attention given to the possibility of wash-out, hydraulic soil fracture or other drilling problems endangering the foundations”. Perhaps it’s time to recognise the deficiency of seawater with bentonite sweeps to deliver stable, near-gauge bore holes for cementing conductors? Offshore of the US Gulf Coast, top hole drilling uses a continuous stream of bentonite drilling fluid. However, the logistical support required from supply boats and bulk plants makes this impractical for semi submersibles and jack-ups operating remotely from port infrastructure or in regions with severe weather conditions. An alternative approach is needed to apply environmentally acceptable polymers that can stabilise sands and prevent clay hydration and to modify the drilling hydraulics to reduce the contact time of seawater. Such an approach was successful on a North Sea exploration well using Pure-Bore as drilling fluid component, after failed attempts to set a conductor due to sandy soil and cement integrity issues.

*SPE 0923-0048 JPT Sept 2023 Case Study – Conductor-Integrity Monitoring in Subsea Wells in Harsh Environments, Harald Holden, 4Subsea.