User:AlbertTong/sandbox

Embedded anchors are anchors that derive their capacity from the frictional and/or bearing resistance of the soil surrounding them. This is converse to gravity anchors that derive their capacity from their weight. As offshore developments move into deeper waters these gravity based structures become more costly and difficult to transport. This proves opportune for the employment of embedded anchors.

There are multiple types of embedded anchors that may be adopted to moor offshore oil and gas or renewable energy facilities. Often constructed using steel and/or concrete, these anchors may be adopted for either short-term or long-term applications. Examples of facilities that may need mooring are FPSOs (floating production storage offloading units), mobile offshore drilling units, offshore production platforms, renewable wave energy convertors, floating liquefied natural gas facilities etc. The figure below outlines selected deep water embedded anchors available to moor offshore floating structures.

Each of these anchors presents their own set of advantages for anchoring offshore structures. The choice of anchoring solution depends on multiple factors some of which are the type of offshore facility that requires mooring, the location of the development, economic viability, the lifetime of its use, soil conditions present and resources available.

Drag Embedment Anchors
Drag embedment anchors (DEA) are embedment anchors that derive their capacity from deep within the seabed. Thus their capacity is directly related to their embedment depth. They are installed by means of dragging using a mooring chain or wire. This relatively simplistic means of installation presents the DEA as a cost efficient option for anchoring offshore structures. DEAs are commonly adopted in industry for temporary moorings of offshore oil and gas structures e.g. mobile offshore drilling units. Their adoption in temporary mooring systems over permanent mooring systems may be largely attributed to uncertainty involving the anchors trajectory in soil. Uncertainty in the vertical position of the anchor results in uncertainty surrounding the final holding capacity (Aubeny 2017.

Under ideal conditions, DEAs are one of the most efficient types of anchors, recording efficiency level ranging from 33 - 50+ times their weight (Diaz et al. 2016). In regards to anchor efficiency, DEAs possess an inherent advantage over other anchoring solutions such as caissons and piles since their mass is concentrated deep within the seabed where soil resistance and hence capacity is greatest (Aubeny 2017). Anchor efficiency is commonly defined as a ratio between the ultimate capacity and dry weight of the anchor. Since DEAs generate their capacity from their embedment they often possess significantly higher efficiency ratios compared to other anchoring solutions.

Vertically Loaded Anchors
Vertically loaded anchors are essentially DEAs that are free to rotate about the fluke-shank connection. This pin connection allows the anchor to withstand both vertical and horizontal loading and thus, unlike DEAs, may be adopted in either a catenary or taut moored configuration. VLAs are initially embedded as DEAs for a specified drag interval. As a result, much of the design considerations required for DEAs is applicable to VLAs. Following this drag interval, the fluke is “released” and allowed to rotate freely about its connection with the shank. Coinciding with the release of the shank is VLA behaviour. This new anchor configuration results in the mooring line load acting essentially normal to the fluke of the VLA (Aubeny 2017). An example of this behaviour is seen in the figure below.