This paper presents a novel, flexible, and economic solution used to mitigate anticipated pipeline walking of a flowline on a deepwater development, offshore West Africa, in about 1000 m of water depth. Aspects from conceptual, design, fabrication and installation are presented for this recently used anchoring solution.
The most common way to mitigate pipeline walking in the Deepwater offshore industry has been holding the flowline from the end opposite to the displacement, which, given the magnitude of the forces involved, can lead to unpractical and noneconomic anchoring solutions.
An alternative, developed for this Project, has been to identify midline positions to anchor the flowline and let the ends free to expand. This has the beneficial effect of considerably reducing the forces involved and thus allows the use of a more efficient anchoring structure with a Hybrid Foundation.
A Hybrid Foundation is the combination of a skirted shallow foundation (so called mudmat) and pin-piles added at each corner of the mudmat. The adjunction of pin-piles increases the holding capacity of the single skirted mudmat by a factor of about 3, which falls then within the range of the anchoring forces required at midline positions along the flowline.
These anchoring structures are installed simultaneously in-line by the pipelay vessel. Pin-piles can be separately installed by a light construction vessel with the assistance of simple clump-weights. This has a major benefit on the installation sequence and the overall schedule.
The present paper focuses on the selection of the mitigation concept, on geotechnical and structural analyses of the Hybrid Subsea Foundation (HSF), and provides feedback from the recent installation campaign.