THE BRUCE® DENNLA Mk 4(Patented)

Uniquely Versatile

With Near Normal and Conventional
Drag embedment Modes and Bolster Bar Stowage Capability

Near Normal Load Mode

High capacity at high uplift for deepwater applications
Not subject to Vertical Load Anchor (VLA) pullout failure
Proven track record for deepwater applications

Conventional Drag Embedment Mode

Shank lock down gives high performance in sand and stiff clay

Either Mode

 Recovery at low load by mooring line Recovery does not need a ROV, pendant line, or sub-sea connector Rapid deck turnaround Single load path shank gives determinable fatigue life




Click to enlarge image.

Near Normal Load Mode

 The 'vertical' load anchor or VLA, which preceded the Dennla, was a special design of drag-embedment anchor that could be 'triggered' by parting an installation shear pin so that the load line through the centroid of its fluke increased to a final angle of 90º (i.e. normal) to its fluke. When triggered, the VLA provided a holding capacity exceeding twice the pull-in load but further loading could cause it to pull out of the seabed, a problem that was mitigated but not solved by imposing a higher safety factor on the VLA than on a conventional drag-embedment anchor. The Dennla Mk4 solves this problem by having a final centroid angle reduced from 90º to 78º ('near normal'), a modification that enables the anchor to continue penetrating after triggering and to do so when pulled at angles at the mud line as high as 45º, a feature important for deepwater applications. At the same depth of embedment as a triggered VLA of equal fluke area, the triggered Dennla has about 90% of the holding capacity of the VLA. However, on loading further, the VLA will pull out whereas the Dennla will continue to embed and generate an increasingly greater holding capacity than that at which the VLA would have failed. This is a crucial advantage of the Dennla.

Conventional Drag Embedment Mode

The Dennla Mk4 has the added feature of a shank that can be locked down to fix the fluke centroid angle at 36º. This enables the 12m2 and 14m2 Dennla Mk4 anchors to equal respectively the performance of a 12,000kg and a 15,000kg Bruce FFTS Mk4 anchor in sand and stiff clay.

Deployment

 For soft clay seabeds, the Dennla is pulled in until a chosen line tension is reached at a mooring line scope giving a final uplift angle of 15° to 25º at the mud line. Scope is then shortened, increasing the uplift angle to 35° to 45º. This produces leverage that parts the shear pin and allows the shank to rotate to establish a new centroid angle of 78º. On lowering the uplift angle at the mud line, the fluke inclination and anchor trajectory become steeper and the anchor penetrates deeper than would otherwise have been possible For sand and stiff clay, the Dennla Mk4 is installed conventionally with its shank locked down.

Recovery

Near Normal Load Mode

 To avoid the problem of high recovery loads, inherent with VLAs, the Dennla's shank can rotate and slide to the rear of the fluke. This enables an AHV to pull the anchor out backwards at well below the installation bollard pull. Typically, recovery loads are about half of the installation load.

Conventional Drag Embedment Mode

 Recovery is similar to that of a conventional high holding power drag embedment anchor, e.g. Bruce FFTS Mk4. In either mode the Dennla is recoverable without the need for a ROV, pendant line, or sub-sea connector.

The Dennla negotiates stern rollers smoothly and is easy to handle on deck, features contributing to its track record of high performance and low turnaround time for presets in deep and ultra deep water. The lock down feature of the Dennla Mk4 adds high performance in sand and stiff clay. The anchor can also be racked sideways on bolster bars ready for deployment from a MODU as a near normal load anchor for soft clay, or, by locking down its shank, as a conventional anchor for sand and stiff clay.

The Bruce Dennla Mk4: uniquely versatile