Tandem Anchor

Piggyback

Primary anchor

Piggyback

Primary anchor

Anchor Navmoor

Separate anchor by at least two anchor lengths

Primary anchor should be stable, i.e., Bruce or Navmoor

Separate anchor by at least two anchor lengths

Primary anchor should be stable, i.e., Bruce or Navmoor

Side-by-Side (staggered)

Side-by-Side (staggered)

Navmoor Anchor
Figure 2-42. Use of multiple anchors to improve anchoring.
Anchor Navmoor

Simple Deadweight

Deadweights with Skirts

Simple Deadweight

Deadweights with Skirts

Deadweight Anchor

Mushroom Wedge

Figure 2-43. Deadweight anchors.

Mushroom Wedge

Figure 2-43. Deadweight anchors.

2.8.2.1 Simple Deadweights. The capacity of a simple deadweight anchor in mud will increase with time as it becomes silted in and as the soil strengthens due to the anchor weight; eventually achieving a capacity of about 50 percent of the anchor's net submerged weight. The initial horizontal capacity for deadweights in mud or clay can be as little as 20 percent of their net weight, and they have been known to slide on slopes of as little as 5 degrees.

Holding capacity of deadweights in mud can be increased by the addition of skirts at the base of the anchor. The skirts should be vented to allow the water and soil to escape during embedment. The anchor's concrete or steel body embeds roughly the same for plain and skirted anchors, but the skirts are sized to penetrate to deeper, stronger soil. This results in a significant increase in anchor capacity. The horizontal holding capacity of a simple deadweight on sand is roughly 70 percent of the net submerged weight. Adding skirts for sand is useful in preventing scour at the base but has litUe effect on anchor capacity. Procedures for optimizing deadweight anchor design are given in the NCEL Geotechnical Engineering Handbook (Rocker, 1985) or from the NFESC Seafloor Engineering Division.

The idealized simple deadweight resists horizontal load by static friction and the vertical load by a portion of the submerged anchor weight. Any vertical load reduces the anchor's net weight and, thus, the capability of the anchor to resist horizontal loads. Simple deadweight anchor capacity can be estimated by the following equation:

F^ = horizontal load K = 0.5 (mud); 0.7 (sand) W = net weight of anchor Fv = vertical load

2.8.2.2 Mushroom and Wedge Anchors.

These anchors are more efficient forms of deadweight anchors when they are not subjected to significant vertical loading. For horizontal loading, these anchors will develop a holding capacity in mud equal to their weight in air; and a holding capacity in sand equal to 2-1/2 times their weight in air. The wedge anchor can be used on hard clays, coral, and rock seafloors with the addition of a steel digging plate at the front face.

Methods for improving the capacity of wedge anchors are being developed at NFESC.

2.8.3 Direct Embedment Anchors

Direct embedment anchors installable with equipment available to the UCTs are shown in Figure 2-44. They include driven, jetted, and screw anchors procured commercially or sized and built according to simple guidance provided in this manual. These anchors can be used alone for low capacity applications or used in multiples to satisfy high holding capacity requirements.

2.8.3.1 Driven Anchors. This type of anchor is installed by using a small hydraulic or pneumatic driver. The drive rod should have sufficient length to achieve a depth of embedment after anchor setting in sand and clay soils of at least five times the anchor's minimum dimension. For soft seafloors or for higher capacity, multiple extension drive rods are used to achieve deeper embedment. The user should be aware that extraction of deeply embedded drive rods can be difficult, particularly in sand. After driving, the drive rod is extracted and the anchor is available for setting if verification of final capacity is needed. For example, anchors used to restrain a pipeline must be set to prevent slack from occurring while the anchors are in use. During setting, the anchor rotates or keys to a position perpendicular to the direction of loading. A typical setting load is 50 percent of the anchor's rated capacity.

NFESC testing showed that simple plates or Manta Ray anchors can be installed on the beach to depths of 6 to 10 feet in a few minutes with a lightweight powered driver. Poinjar and Cobra pneumatic and Stanley hydraulic drivers were equally effective on the beach. The NFESC hydraulic rock drill was an effective driver dining offshore testing.

The Arrowhead (triangular) anchor is available in sizes of 2 to 17 inches; installers report difficulty in driving anchors larger than 10 inches in harder soils. In harder soils, the Arrowhead frequendy pulls out without setting. To avoid this, the hole should be backfilled and tamped prior to setting. This also is a manufacturer-recommended procedure for Manta Ray Duckbill anchors in hard soils.

The plate anchor shown in Figure 2-45 can be built from mild steel and sized to the application (NFESC can provide design guidance). The plate anchor sets (keys) more rapidly than the other anchors because of the eccentric wire connection, and can be fabricated from readily available steel sections. Small plate anchors (about 30-in.2 area) driven to depths of about 10 feet at the shoreline have shown extraordinary capacities of over 20,000 pounds. This simply suggests the potential of small driven anchors for satisfying many UCT needs.

2.8.3.2 Jetted Anchors. Piles, deadweights, mushroom, and simplified cone anchors can be jetted into the seafloor. Jetting is applicable to sand and mud soils, but is not suitable for clays. Tests at NFESC of the simplified cone anchor show that these diver-emplaced anchors are suitable for low to moderate uplift loads (2 to 10 kips). The following applies to jetting of simple pipe and cone anchors that can be accomplished with equipment available to the UCTs.

Drive head"

Drive head"

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  • Michael
    Can wedge anchors be installed underwater?
    4 months ago

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