While subsea production template and manifold designs have come to be dominated by standardized solutions tailored for specific hardware, the design of Pipeline End Manifolds (PLEM) remains largely project-specific. Nevertheless, some trends in PLEM design for large-diameter pipelines in moderate water depths have emerged in the past years in the North Sea and elsewhere; namely, large stand-alone structures on suction pile foundations with diverless spoolpiece tie-ins. This arrangement has proven successful on numerous projects; however, the move to remote arctic fields of significant production capacity and long design life introduces new design drivers that warrant a “fresh approach” to PLEM design.
The developments currently being considered for the arctic will have to deal with:
- Remote location making mobilization of installation assets a significant cost driver such that separate installation of pipeline and PLEM is relatively unattractive
- Harsh conditions and short weather windows for installation favoring designs that reduce the number of separate installation steps and vessels
- Poorer access for maintenance and repair during the operating life favoring designs that are modular and that allow recovery of critical components using the smallest possible intervention vessels.
When combined with envisioned field production lives of 40 to 50 years, this means a very different set of design drivers will apply to the PLEM design. This paper presents an alternative PLEM design developed to overcome these challenges by:
- Integrating of the PLEM with the pipeline to work around current industry limitations for large diameter diverless tie-in connector systems and to minimize ROV rotated sealing surfaces subsea in normal operation,
- Introducing plug technology to remove the critical dependence on long-term trouble-free performance of large bore valves,
- Introducing driven pile foundations to reduce structure size, prevent long-term settlements and eliminate the need for separate pipeline support frames by maintaining the pipe centerline close to the mudline,
- Modularizing the system such that key components (all remaining valves) can be retrieved without complete shutdown of flow and such that installation / intervention can be performed using a wide range of vessels, and
- Incorporating lessons learned from the successful design of a North Sea vertical diverless pig launcher unit.
This paper presents an overview of the alternative PLEM design and discusses the status of the technologies required.