Case Study: Conductor-Integrity Monitoring in Subsea Wells in Harsh Environments

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A decade-long project to monitor drilling operations on subsea wells in harsh environments has delivered interesting results.

Subsea production and exploration wells are subjected to loads from the drilling riser and subsea blowout preventors (BOP). The latter have grown in size and weight over time, resulting in larger wellhead loads.

The main task of a subsea conductor is to transfer these loads into the seabed. The bending load applied on the wellhead is transferred into the conductor, which again transfers the loads through the cement and into the soil.

In cases where the load exerted on the soil and cement exceeds the capacity, the soil around the conductor might fail. The cement filling the cavity between the conductor and the soil can also crack and consequently lose its intended function. This support degradation reduces the conductor’s ability to support the well.

4Subsea’s Subsea Wellhead Integrity Monitoring (SWIM) service for operations on subsea wells has now been in action for more than a decade. Sensors monitoring the motion and loads near the subsea wellhead have been mounted on more than 300 drilling campaigns (Fig. 1). In recent years, more than 70% of the drilling operations with semisubmersible drilling rigs on the Norwegian Continental Shelf (NCS) have been monitored using SWIM.

As a part of the service, integrity parameters showing whether the conductor gives the intended amount of support are calculated. A major finding is that loss of conductor support during drilling operations has been observed in several of these campaigns, with different integrity issues detected.

In at the Deep End

Wells in the North Sea, the Norwegian Sea, and the Barents Sea are often developed as subsea fields. The typical setup while drilling these wells utilizes a mobile drilling unit (semisubmersible or drillship) with a marine riser and subsea BOP. The well foundation must be designed to withstand the imposed loads.

In accordance with Norwegian Safety Authority facility regulations, wells and their foundations must follow strict regulations. These include that the conductor shall provide structural integrity of the well during its service life, and that it shall be evaluated with respect to structural loads and fatigue.

Subsea wells can be divided into two categories based on the type of support of the respective conductor.

- Supported conductors are the most-common configuration for subsea production wells in harsh environments, with a template structure supporting the conductor.

- Unsupported conductors are used for exploration and production wells in less-harsh environments.

During well foundation design, there are typically three different failure modes to consider.

- Extreme load failure (ULS, ALS) of the wellhead system and supporting structures

- Fatigue failure (FLS) of the wellhead system and supporting structure

- Failure of the soil support and cement around the conductor

SWIM covers both fatigue monitoring and detection of issues with the soil and cement support.