Pursuit of improved anchor technology began more than 5,000 years ago when an early anchor craftsman bored a hole into a rock to increase the performance and reliability of a stone anchor. With each evolving step in anchoring technology there are always five aspects that must be addressed: anchor size, performance, ease of fabrication, ability to install and cost. The art is then obtaining the optimum balance among these five aspects.

In several places around the world, 4,000 years ago anchor technology took its first major step. The anchor fluke and shank were born. Construction consisted of mostly wood for shape and stone for weight. The concept of the anchor fluke and shank would remain current to this day.
Eventually the aspect of fabrication and cost drove the Greeks to develop the classic Greek anchor in about 800 BC. The Greek anchor was extensively utilized for more than 2,000 years and still represents the common symbol for anchor.
In the last 200 years anchor design has seen the most drastic evolution. Anchors began to be patented, like the Stockless Anchor in the year 1821. Design of these foundations became profitable and, in turn, fueled innovation and research.
The Danforth, LWT and Moorfast, along with the Stockless Anchor, all utilized pivoting flukes o a straight shank. The beauty of these anchors was their ability to fall easily to the seafloor and be drug into place. This greatly improved reliability, and with the design of the anchor flukes, efficiencies in performance or holding capacity increased noticeably from earlier anchor designs.
As the oil and gas industry began to build larger offshore facilities that required larger mooring foundations, it became clear that anchors once again had to provide more capacity with less weight. In the early 1970s, Peter Bruce developed the first Bruce Anchor. This anchor would prove to be the answer to the increasing needs of the offshore oil and gas industry and the birth of "high holding capacity anchors." The Bruce Anchor had a cast "L" shank with a cast three-bladed fluke that allowed for adjustment of the angle between the shank and fluke. This adjustment allowed the anchor to perform at its best in a variety of soils, from sand to soft clay.
The 1980s saw diversity in the supply of high holding capacity anchors, and the anchors themselves underwent several iterations to increase the efficiency while reducing fabrication costs. As these anchors matured into the '90s, the performance of the high holding capacity anchors proved to be reliable with 10° angle of uplift or more.

Moving into the deep
The mid-1990s approached, and the offshore oil and gas industry was moving into deeper and deeper water depths. It became apparent that anchors would need to be able to withstand uplift angles of greater than 10° to allow for semi-taut and taut-leg moorings, which would reduce the amount of vertical load on the offshore facility and reduce the loads on the vessels required to install these moorings. This trend brought with it an increased demand in the Gulf of Mexico for larger anchor handling vessels (AHVs) to facilitate efficient presetting of these semi-taut and taut-leg mooring systems in deep water.
Mooring designers began requiring uplift performance requirements on new anchor foundations from 20° up to 45° for both mobile offshore drilling units (MODUs) and permanent facilities. The answer that flourished first was the suction anchor.
The suction anchor concept had been around for many years; however, the application for deepwater anchoring was relatively immature. Research and field-testing in the mid-1990s led to significant improvements in suction anchor technology for both MODU and permanent facility moorings. The comfort level with suction anchors increased rapidly for permanent facilities. MODU mooring utilization, however, was hampered by the cost required to install these anchors since deployment required two AHVs, one to over-board and lower the pile, the other to deploy the connected mooring wire.
As soon as the first subsea mooring connector became available, suction anchors became a common foundation for deepwater MODU moorings since they could then be installed efficiently with a single AHV. Suction anchors allowed MODUs to break world water depth records one after another, currently at almost 10,000 ft (3,050 m).
Suction anchors have several key advantages. First, deployment is purely vertical to the seafloor and, once penetrated under its self-weight, installation to depth is achieved through remotely operated vehicle (ROV) - powered dewatering pumps. This means there is only minimal AHV bollard pull required for installation. Second, suction anchor position and capacity is known as soon as installation is complete. Position is apparent since a portion of the anchor protrudes
from the seafloor and can be surveyed. Capacity can then be back-calculated based on the insertion and pump pressure logs.
Suction anchors also come with inherent disadvantages. Their size requires more deck space than other anchors, which typically requires more than one AHV trip to preset a complete set of anchors. Suction anchor sizing needs to be adequate to allow for holding capacity factors of safety in excess of 1.0 since suction anchor holding capacity failure is catastrophic (leaving no residual holding capacity).
The disadvantages of suction anchors opened the door to another breed of anchor, the Vertically Loaded Anchor (VLA). The two leading embodiments of VLA technology are Bruce's DENNLA and Vryhof's Stevmanta. These VLAs have several key advantages. First, they are small in size relative to suction piles, requiring approximately one third as much AHV deck space as a comparable suction anchor. Second, they are installed similar to high holding capacity drag anchors in that they can be rig deployed. This feature is possibly a VLA's greatest asset to the MODU market in that they can noticeably increase the water depth and performance capability of a given MODU's mooring system.
For example, if a MODU has 10,000 ft of wire rope and 3,000 ft (915 m) of chain with a conventional high holding capacity anchor, it could operate with a conventional catenary mooring system in water depths of up to 5,000 ft (1,525 m). In that water depth, however, a catenary mooring system is relatively "soft" and may exhibit excessive offsets that limit the operability of the mooring in instances of high current. In comparison, if the rig utilized VLAs, it could eliminate the need for the rig chain and moor on strictly wire and a VLA. This would allow the rig to work in water depths of around 6,000 ft to 7,000 ft (1,830 m and 2,135 m) with a taut mooring system that exhibits favorable working offsets.
Once again, this anchor technology has its disadvantages. VLAs require an installation load of approximately one third the design holding capacity to set them in the soil. This means that a typical MODU capacity requirement of 1,200 kips would require an associated installation load at the anchor of 400 kips. A MODU has the capability to utilize its onboard winch equipment to achieve this load; however, if presetting, there is only one AHV in the Gulf of Mexico capable of the bollard pull required to set a VLA. Another challenge with VLAs is that unless a tracking device is installed with the anchor, the user is unable to verify installation depth or current capacity once installed.
Both VLAs and suction anchors are being actively utilized and specified for permanent moorings around the world. Both VLAs and suction anchors are being installed for MODU moorings. Currently, MODU deployment needs are being covered by VLAs, and preset needs are being covered by suction anchors. These activities are becoming more and more common as the industry moves both into deeper water and among more congested fields. The cost of AHV and other installation platforms is creating increased pressure for anchor choices that limit installation vessel requirements and time to complete installation.

The path forward
MODU moorings should continue taking advantage of VLA technology and the benefits they bring to increased performance and water depth capability. New-build MODUs may be designed for longer wire lengths and no chain to allow for taut-leg MODU deployed moorings in excess of 7,000 ft.
Preset anchors pose a more dynamic shift. The preset anchor foundation challenge is requiring anchor technology to move in a direction where installation time is reduced and installation vessel or platform requirements are further limited.
Installation time will be driven by the industry's current speed in the installation of suction piles, which are currently the time leader. Vertical installation will be the key for future anchor technology with the hope of eliminating the required time for ROV intervention to fully install the foundation. Installation vessel requirement will call for anchors that require similar or less space than that of VLAs and require very limited bollard pull or installation loads.
The apparent direction of future anchor technology will be in the area of gravity-installed foundations. This concept allows for subsea "free-fall" of the anchor from a specified distance above the seafloor, creating enough force to achieve a specified level of soil penetration.
Gravity-installed anchors will take advantage of the stronger soil strengths typically found at greater penetration depths, which in turn will allow for smaller sized anchors. Gravity-installed anchors should require limited bollard pull, if any, for installation.
Overall, preset anchor technology of the future will move in the direction of gravity-installed foundations. Creative design should allow for diversity in installation platforms and further increase the reliability of anchors near the ever more-prevalent array of subsea assets.

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