Alternative Anchor Options
In addition to the uplift resisting anchors described in the previous sub-section, a number of alternative options are also available:
"Anchors in the form of grouted in bars, tubes or cables are the most economical means of providing the required uplift resistance for piles taken down to a shallow rock layer." (Tomlinson, 1980)
High capacity load resisting foundations may be required to withstand large uplifting forces imposed upon the anchorage. For foundations of marine structures preformed or partially preformed piles lend themselves to ease of installation. Piles are driven to the design depth using a percussion hammer and grouted into position. They are suitable for use in very stiff, compacted clays and in rock. A tubular steel pile with a cutting edge is driven and down-hole equipment will ream and remove the rock surface until the required depth is achieved. Piling operations in the marine environment are very expensive and logistically difficult. While it may be necessary to utilise piling in certain circumstances, for the purposes of this design piling would only be considered as a last resort due to the prohibitive costs involved.
Suction Caissons (Piles)
These can be deployed on a large or small scale (suction anchor). At the larger end of the scale they are essentially a pile with the top end sealed. Once pushed into the seafloor an internal vacuum is created using suction until the desired holding capacity is achieved. Again, these are expensive to construct and install.
Vertical Load Anchors (VLA)
Vertical load anchors are a relatively modern development coming to the offshore industry in the last fifteen years or so. As the title suggests VLAs can resist vertical load and horizontal loads, while conventional drag embedment anchors are for horizontal loading applications only.
VLAs are installed in the same manner as drag embedment anchors with a horizontal load being applied to embed the anchor as deeply as possible into the seabed. Once installed the angle of the pulling point on the anchor can be changed so that the VLA can accept loads perpendicular to the fluke. The VLA is ideal for taut leg mooring systems. As the anchor is deeply embedded and always loaded normal to the fluke, the VLA can accept loads from any direction.
The image shows a Stevmanta VLA, which is "installed just like a conventional drag embedment anchor. During installation (pull-in mode) the load arrives at an angle of approximately 45 to 50 degrees to the fluke. After triggering the anchor to the normal load position, the load always arrives perpendicular to the fluke. This change in load direction generates 2.5 to 3 times more holding capacity in relation to the installation load. This means that once the required UPC of the VLA is known, the required installation load for the VLA is also known, being 33% to 40% of the required UPC." (Vryhof, 2010)
The anchorage type chosen will be completely dependent on the site-specific seabed conditions. In general however, the following conclusions can be drawn:
Deadweight anchors are the simplest and cheapest way to connect to the seafloor. However their use is limited due to their low load to weight ratio and poor performance in resisting horizontal loads.
In sands, sediments and clays VLAs are very suitable. They can resist both vertical and horizontal loads and can be quickly deployed (typically a one day operation).
If the seabed is purely rock, options are very much limited to drilled anchors or piled solutions.