Frequently Asked Questions
Q. What seabed soils are suitable for the Dennla?
A. The Dennla was specifically designed for taut-leg mooring in the soft clay seabeds found in deep and ultra deep water. For seabeds consisting of sand or firm clay, the Dennla Mk4 has a lock down mechanism which fixes its fluke centroid angle at 36° and allows the anchor to perform like a conventional MODU anchor. In such seabeds this enables the 12sq.m and 14sq.m Dennla Mk4 anchors to equal respectively the performance of a 12,000kg and a 15,000kg Bruce FFTS Mk4 anchor whose holding capacities in sand and firm clay are given by the formula HC = 46.86w0.94 tonnes.
Q. What is the ultimate holding capacity (UHC) of a Dennla?
A. A drag embedment anchor follows a curved embedment path in the seabed soil. Soil resistance on the anchor cable induces a moment on the anchor fluke, via the anchor shank, in proportion to the length of the shank that acts as a lever. This rotates the fluke progressively as cable is pulled below the seabed surface until the curved path becomes horizontal to give the Ultimate Holding Capacity (UHC) of the anchor and cable system when the anchor cable is pulled horizontally at the seabed. The length of the shank determines the depth at which the curved path becomes horizontal and so determines the UHC.

In conventional drag embedment anchors, the length of the shank is determined by the conflicting requirement for the shank to be sufficiently long to promote penetration of the seabed surface while at the same time being sufficiently short to facilitate deeper embedment. The result is a compromise in shank length that limits the depth that the anchor can reach and hence limits its UHC.

The Dennla avoids this problem by having a variable shank length that enables it to reach embedment depths unattainable by conventional anchors. To achieve this, the load application point on the Dennla is transferred from its shank shackle pin to the shank pivot pin when a shear pin is parted following penetration beneath the seabed surface exceeding two fluke lengths. This reduces the effective length of the shank by a factor of 5. Consequently, in a deeply penetrable seabed, the embedment path may not become horizontal until a depth is reached that would produce a UHC in excess of the structural strength of the anchor. Accordingly, for a Dennla with a wire forerunner for minimising penetration resistance, the UHC can be regarded as the load at the anchor shackle which induces first onset of yielding in the structure of the anchor. For a Dennla with a fluke area, A, in the range 10m² to 20m², this gives the approximate relationship: effective UHC = 81A tonnes
Q. How does the Dennla handle out-of-plane loading?
A. The Dennla, like all anchors with a shank, simply self-corrects for out-of-plane loading by veering into alignment with the direction of loading until the load is back in-plane.
Q. To avoid on-boarding synthetic rope in shallow water, can we recover Dennlas at a lower uplift angle than shown in 'Step 2' on your website?
A. Yes you can. The 45° uplift angle shown for Dennla recovery is to minimise the amount of line out and save time in anchor recovery in deepwater. The Dennla can be recovered backwards at 45° uplift, or at any lower angle.
Q. How can I make a case for my company to acquire Dennlas?
A. The Dennla is a low profile, near normal load anchor that is compact and easy to handle on deck. Because it is compact, an anchor handling vessel can carry a full mooring spread to site in one trip. The anchor doesn't need a ROV, pendant lines, nor sub-sea connect/disconnect, all of which increase installation and recovery time and add substantially to overall mooring costs. The Dennla's shank slides to the rear of the anchor for easy recovery at uplift angles of up to 45° and at loads typically about half of the installation load. For these reasons, a mooring spread of Dennlas can be installed, recovered, and reinstalled at a new location in significantly less time than VLAs and in considerably less time than suction piles, suction embedded plate anchors, and free-fall anchors. This translates into a saving of two to three days or more per rig-move - in addition to very large cost-savings from avoiding the need for a ROV