The simplest and most cost effective foundation to support deepwater subsea structures is a mudmat. As loads and weights increase, the attractiveness of a mudmat foundation disappears and the typical solution has been a costly suction caisson. A hybrid subsea foundation (HSF) was found to be a cost effective and robust alternative. A HSF is a mudmat with four corner piles that provide additional bearing capacity in an effective and cost competitive way. This paper presents the design process, fabrication, and installation of a HSF for two flowline end terminations (FLETs) for a recent deepwater project.
Deepwater development projects require the design, fabrication and installation of a variety of subsea facilities or structures. Shallow foundations (or mudmats) have been used extensively for temporary and permanent subsea structures when the expected loads are relatively moderate.
Oftentimes, subsea structures especially in deepwater projects must be designed to sustain substantial lateral loads and overturning moments coming from product line expansion thrust, jumper loads or other external sources. In recent subsea applications, external loads imposed on a subsea foundation far exceed the bearing capacity and sliding resistance of a typical size shallow (mudmat) foundation.
Owing to the very soft clay conditions that typically exist in deep waters worldwide, the sizes of mudmats have increased, while foldable side-wings and skirts along the outside perimeter have been added to increase the vertical, horizontal and rocking resistance of the mudmat. However, the need to increase the mudmat size to meet the magnitude and complexity of the imposed loads has been challenged by the dimensions and geometry constraints from pipelay installation vessels. Typically, a deep foundation, such as suction pile or a driven pile (within the depth limits of operability of hydraulic hammers) has been the next solution to meet the need for high bearing and overturning moment capacities. However, the deep foundation option is quite costly compared to a typical mudmat foundation. When the loads are exceeding the bearing capacity of a typical mudmat but are not high enough to justify a costly deep foundation, a hybrid subsea foundation (HSF) has become an attractive solution.