Anchor's vertical holding capacity suited for taut and semi-taut mooring systems

ABB Anchor Contracting has further developed its range of anchors and mooring solutions with a high vertical loading (HiVel) anchor.

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Lower fabrication, transport costs

ABB Anchor Contracting has further developed its range of anchors and mooring solutions with a high vertical loading (HiVel) anchor. HiVel is intended for deepwater and ultra-deepwater applications where the loads in semi taut-leg and taut-leg mooring systems, for example, tend toward the vertical compared with shallow-water mooring systems.

The HiVel anchor is based on ABB's high-capacity HiCap Anchor, explains Marine Manager Johan Lassen-Urdahl. Unlike the conventional bucket anchor or suction pile, which consists of a steel cylinder with a closed top, the HiCap Anchor is a two-part system, comprising the anchor part, a steel cylinder open at both ends, and an installation lid, which is mounted on top of the anchor during installation but removed afterwards. The lid, which is fitted with equipment for pumping out the water inside the anchor, can perform multiple installations.

The HiCap Anchor system was supplied to Norsk Hydro for the mooring system for the Visund floating production unit in the North Sea. The system consists of 16 anchors installed in a water depth of 335 m.

Like a suction pile, the HiCap Anchor and HiVel anchor are installed in two phases. Initial penetration is achieved through self-weight. Water trapped inside the cylinder is then pumped out, creating suction within the cylinder. Vertical loads on the HiVel anchor are transferred into the adjacent, mostly undisturbed soil and are borne by its shear strength. No skin friction between the soil and the anchor surfaces is required to resist the loads, which means that the anchor, in contrast with other types, has full capacity shortly after installation. Removing the installation lid does not significantly affect its holding capacity.

In the HiVel system, the anchor cylinder is fitted with internal flaps. As the anchor penetrates into the seabed, the flaps are pushed upward and open. When penetration is complete, the anchor is retracted a short distance, pulling the flaps down into a horizontal position and creating, in effect, a closed bottom. The HiVel anchor has typical dimensions of 5 m in height for both the top and bottom sections, is 5 m in diameter, and has an embedment depth of 12 m.

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The internal flaps on the HiVel anchor increase its holding capacity.
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Compared with the HiCap Anchor, HiVel has increased vertical holding capacity, which makes it more suitable for semi-taut and taut mooring systems. It provides the same holding capacity as a suction pile at a quarter of the weight, thus saving on the volume of steel required and the fabrication and transport costs. The anchor weight is typically 40-50 metric tons, compared with 150-200 metric tons for a suction pile.

In turn it requires a lower crane capacity during deployment, so the savings are greater the deeper the water. The weight savings could be enough to make it possible for the installation operations to be performed by a dynamically positioned anchor-handling vessel rather than a crane-barge, according to Lassen-Urdahl. The anchors are designed to be lowered separately to the seabed, with the installation lid then deployed to install all of them.

There may also be savings on decommissioning, as the buried anchors can be left in place, while the conventional suction pile, which protrudes above the seabed, would in many cases have to be retrieved.

ABB Anchor Contracting plans to carry out a trial of the HiVel system in 100 m water depth later this year.

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