The Statpipe system, established in 1983, connects the gas fields of the North Sea to the Kårstø facilities. The system is owned by Gassled and operated by Gassco with Equinor as Technical Service Provider.
The landfall of the two pipelines (28” & 30”) was installed inside a 590m long immersed concrete culvert that has exceeded its design life. A cost and risk analysis concluded replacing the landfall due to the harsh shallow water environment.
Given the site conditions and routing of the existing pipelines, a new, complex tunnel construction was necessary. The landfall is a new “drill & blast” tunnel with a length of approximately 1100m parallel to and north of the existing culvert. It was constructed, open to sea and waterfilled prior to installation, with the pipelines being laid from a vessel stationary outside the tunnel and pulled through the tunnel by a cable connected to an onshore winch.
Tunnel configuration was dictated by geometrical constraints, notably entry/exit angle and location, and by the requirements of guaranteeing a minimum rock overburden to remain underneath the fractured seabed layers. This resulted in a peculiar S-shaped curvature featuring a transition of gradient from downslope to upslope in the central part of the tunnel, hence generating the need for purposely designed mitigations to control each individual phase of the cable and pipeline pull-in. Structural devices have been installed inside tunnel e.g. to avoid contact between pull-in cable and tunnel ceiling and to avoid contact between the pipelines.
In this paper, pipeline design is described, with focus on the challenges to define robust requirements for installation, to perform safe pull-in operations and to ensure the operational requirements throughout the pipeline design life.