Offshore embedded anchors

Offshore embedded anchors are anchors intended for offshore use that derive their holding capacity from the frictional, or bearing, resistance of the surrounding soil, as opposed to gravity anchors, which derive their holding capacity largely from their weight. As offshore developments move into deeper waters, gravity-based structures become less economical due to the large size needed and the consequent cost of transportation. Each of several embedded-anchor types presents its own advantages for anchoring offshore structures. The choice of anchoring solution depends on multiple factors, such as the type of offshore facility that requires mooring, its location, economic viability, the lifetime of its use, soil conditions, and resources available. Examples of facilities that may need mooring offshore are floating production storage and offloading (FPSO) units, mobile offshore drilling units, offshore oil production platforms, wave power and other renewable energy converters, and floating liquefied natural gas facilities.

Drag-embedment anchors Drag-embedment anchors (DEA) derive their holding capacity from being buried, or embedded, deep within the seabed with their anchoring capacity being directly related to embedment depth. DEAs are installed by means of dragging, using a mooring chain or wire, this relatively simple means of installation making the DEA a cost-effective option for anchoring offshore structures. DEAs are commonly used for temporary moorings of offshore oil and gas structures, e.g. mobile offshore drilling units. Their use in only temporary mooring situations may be largely attributed to uncertainty involving the anchor's embedding trajectory and placement in the soil, which results in uncertainty with regard to the anchor's holding capacity. Under ideal conditions, DEAs are one of the most efficient types of anchors, with holding capacities ranging from 33 to greater than 50 times their weight; and such efficiency gives DEAs an inherent advantage over other anchoring solutions such as caissons and piles, since the mass of a DEA is concentrated deep within the seabed where soil resistance and, hence, holding capacity, is greatest. Anchor efficiency is defined as the ratio between the ultimate holding capacity and the dry weight of the anchor, with DEAs often possessing significantly higher efficiency ratios compared to other anchoring solutions.

A catenary configuration consists of "slack" mooring lines that form a catenary shape under their own weight. Since the catenary mooring lines lie flat along the seabed, they exert only horizontal forces on their anchors. Taut mooring lines arriving at an angle to the seabed exert both horizontal and vertical forces on their anchors. Since DEAs are designed to resist horizontal forces only, these anchors should only be used in a catenary-moored configuration. Applying a significant vertical load to a DEA will result in its failure, as the vertical force applied to the padeye will result in anchor retrieval. However, this does facilitate anchor retrieval, which contributes to the cost effectiveness of this anchoring solution.

Design

The three main components of a DEA are the fluke, shank, and padeye. For a DEA, the angle between the fluke and the shank is fixed at approximately 30 degrees for stiff clays and sand, and 50 degrees for soft clays.

Fluke The fluke of a plate anchor is a bearing plate that provides the large majority of the anchors holding capacity at its ultimate embedment depth. As well as contributing to anchor capacity, the fluke may contribute to anchor stability during embedment. Adopting a wider fluke can help in providing rolling stability which allows for deeper embedment and better holding capacity. There are industry guidelines pertaining to both the appropriate width, length, and thickness of anchor flukes, where width refers to the dimension perpendicular to the direction of embedment. Commercial anchors typically have a fluke width-to-length ratio of 2:1 and a fluke length-to- thickness ratio between 5 and 30.

Shank

Since DEAs derive their strength from their embedment depth, the shank should be designed such that soil resistance perpendicular to the anchor's embedment trajectory should be minimised. Frictional soil resistance against the parallel component of the shank, however, is less significant. Thus, the area of the shank in line with the direction of the embedment trajectory is often relatively large to provide anchor stability against rolling during embedment.

Padeye The padeye is the connection between the anchor and mooring line. Padeye eccentricity, often measured as the padeye offset ratio, is the relationship between the horizontal and vertical distance of the padeye position in relation to the fluke–shank connection of an anchor. The evaluation of the optimal padeye eccentricity for DEAs and vertically loaded anchors (VLAs) is limited to the appropriate choice of shank length given a fixed fluke–shank angle during embedment. A study conducted to investigate appropriate shank lengths considered a range of shank-length to fluke-length ratios between 1 and 2. It was determined that the shorter shank lengths (closer to ratios of 1) produced deeper anchor embedment.

Mooring Line Although the mooring line is not an anchor component unique to the DEA, its design significantly influences the behaviour of the anchor. A thicker mooring line makes for more resistance to anchor embedment. The properties of chain, versus wire, mooring lines have been investigated, with chain mooring lines causing reductions in anchor capacity of up to 70%. Thus, where appropriate and cost-efficient, wire mooring lines should be used. The embedded section of a mooring line contributes to the anchor's holding capacity against horizontal movement. It is, therefore, appropriate to analyse the contribution of the anchor's mooring line with respect to both the embedment process of the anchor and its contribution to the final anchor holding capacity.

Vertically-loaded anchors

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