Anchoring Point Types
Drag embedment anchor
This is the most popular type of anchoring point available today. The drag embedment anchor has been designed to penetrate into the seabed, either partly or fully. The holding capacity of the drag embedment anchor is gernerated by the resistance of the soil in front of the anchor. The drag embedment anchor is very well suited for resisting large horizontal loads, but not for large vertical loads although ther are some drag embedment anchors available on the market today that can resist significant vertical loads.
Dead weight
The dead weight is probably the oldest anchor in existance. The holding capacity is generated by the weight of the material used and partly by the friction between the dead weight and the seabed. Common materials in use today for dead weights are steel and concrete.
Pile
The pile is a hollow steel pipe that is installed into the seabed by means of a puling hammer of vibrator. The bolding capacity of the pile is generated by the friction of the soil along the pile and lateral soil resitance. Generally the pile has to be installed at great depth below seabed to obtain the required holding capacity. The pile is capable of resisting both horizontal and vertical loads.
Suction anchor
Like the pile, the suction anchor is a hollow steel pipe, although the diameter of the pipe is much larger than that of the pile. The suction anchor is forced into the seabed by means of a pump connected to the top of the pipe, ceating a pressure difference. When pressure inside the pipe is lower than outside, the pipe is sucked into the seabed. After installation the pump is removed. The holding capacity of the suction anchor is generated by the friction of the soil along the suction anchor and lateral soil resistance. The suction anchor is capable of withstanding both horizontal and vertical loads.
Vertical load anchor
A new development is the vertical load anchor (VLA). The vertical load anchor is installed like a conventional drag embedment anchor, byt penetrates much deeper. When the anchor mode is changed from the installation mode to the vertical (normal) loading mode, the anchor can withstand both horizontal and vertical loads.
Anchor Design
For anchor design and installation, the availability of good soil data is of utmost importance as the soil is of great influence on anchor behaviour. The following are influenced by the soil conditions encountered:
Anchor type – some anchors are more suited for soft soil conditions (soft clay), while others are more suited for hard soils (sand and hard clays), although there are a number of anchor types on the market that are suited for most soil conditions encountered.
Holding capacity – in hard soils, the maximum attainable ultimate holding capacity with a certain anchor type and size is higher than the attainable ultimate holding capacity in very soft clay.
Penetration and drag – in very soft clay the anchor will penetrate deeper than in hoarder soil like sand. As a consequence, the drag length of the anchor will also be longer in very soft clay than in hard soil.
Retrieval forces – when an anchor is installed in very soft clay, the required retrieval forces will be higher than in hard soil like sand. For example, in very soft clay the required force of an anchor can be equal to 80%-90% of the installation load while in hard soil (sand) the retrieval force might only be 20%-30% of the installation force.I'm a paragraph. Click here to add your own text and edit me. I’m a great place for you to tell a story and let your users know a little more about you.
Criteria for anchor design
There are several attributes of an anchor which are crucial in assuring its effective performance:
-
The anchor must offer a high holding capacity; a result of the fluke area and shank design in combination with penetration and soil type.
-
The design of the anchor should be such that the anchor is capable of being used successfully in practically all soil conditions encountered over the world, ranging from very soft clay to sand, corals and calcarenites.
-
The fluke/shank angle of the anchor should be easily adjustable, allowing the anchor to be quickly deployed in different soil conditions.
-
The design must be so conceived an produced that the high loads common in practice can be resisted and that the anchor can be easily handled, installed, retrieved and stored.
The penetration of the anchor depends upon its shape and design. Obstructing parts on the anchor should be avoided as much as possible.
-
The stability of an anchor encourages its penetration and, consequently, its holding capacity. Efficient stabilisers are an integral part of a good anchor design.
-
The shank must permit passage of the soil.
-
The surface area of an anchor fluke is limited by the required structural strength of the anchor.
-
The anchor design must have optimal mechanical strength to fulfil requirements and stipulations of the classification societies.
-
The anchor should be designed to ensure an optimum between structural strength of the anchor and holding capacity.
-
The anchor should be streamlined for low penetration resistance.
Holding capacity
The holding capacity of an anchor is governed by the following parameters:
-
The fluke area, which is limited by the strength of the anchor design.The penetration of the anchor.
-
The penetration of the anchor is governed by the soil type (deep penetration in very soft clay and shallow penetration in sand), the anchor type (design), the type of mooring line that is used (chain or wire rope) and the applied load.
An increase in fluke area or an increase in penetration depth of the anchor results in a higher holding capacity. An anchor connected to a wire rope mooring line will penetrate deeper than the same anchor connected to a chain mooring line. The is caused by the higher lateral resistance along the chain mooring line. This effect is noticeable in all soil conditions, but especially in very soft clay where very deep penetration can be obtained. The holding capacity of a chain mooring line due to friction in and on the seabed, is larger than the holding capacity of a wire rope mooring line.
-
A : Weight of the anchor
-
B : Weight of soil in the failure wedge
-
C : The friction of the soil in the failure wedge along fracture lines.
-
D : Friction between fluke surface and soil (fluke area)
-
E : The bearing capacity of the shank and mooring line.
-
E : The friction of the mooring line in and on the soil.
The holding capacity (P) of the party of the mooring line that is laying on the seabed, can be estimated with the following equation:
P=f x l x w
f : friction coefficient
l : length of mooring line laying on the seabed
w : unit weight of the mooring line in the water
Fluke / Shank angle
The penetration of an anchor into a certain soil type is greatly influenced by the selected fluke/shank angle. For hinging anchor types the fluke/shank angle is the angle between the anchor shackle, the hinge and the fluke tip. The method for measuring the fluke/shank angle for fixed shank anchors is not well defined. Often it is the angle between the anchor shackle, the rear of the fluke and the fluke tip, but not all anchor manufacturers use the same definition.
Typical Ultimate Pull-out Capacity (UPC)
D= 1.5 x k^(0.6) x d^(-0.7) x A^(0.3) x tan (α)^(1.7)
D : Penetration Depth [m]
K : quotient Undrained Shear Strength clay [kPa] and depth [m]
d : mooring line or installation line diameter [m]
A : Fluke area [m2]
α : Fluke/ shank angle [deg]
UPC=Nc x Su x A
UPC : Ultimate Pull-out Capacity
Nc : Bearing Capacity Factor
Su : (k*D), Undrained Shear Strength clay [kPa]
A : Fluke Area [m2]
In the graph can the influence of the diameter of the mooring line of installation line clearly be seen, whether a six strand of a spiral strand is used. The typical installation load to obtain a specified UPC is presented on the right vertical axis of the graph.
(the graph incorporates a Nc value of 10, an α value of 50 degrees and a k value of 2) In the graph below a comparison of different mooring lines is shown.
The top graph below shows the weight of various mooring line types and the graph below that shows the minimum breaking load in different lines.
Soil classification
Soil strength is generally expressed in terms of shear strength parameters of the soil. The soil type is classified mainly by grain size distribution.
Grain size Soil description
-
<- 2 um Clay
-
2 – 6 um Fine silt
-
6 – 20 um Medium silt
-
20 – 60 um Coarse silt
-
60 – 200 um Fine sand
-
200 – 600 um Medium sand
-
0.6 – 2 mm Coarse sand
-
2 – 6 mm Fine gravel
-
6 – 20 mm Medium gravel
-
20 – 60 mm Coarse gravel
-
60 – 200 mm Cobbles
-
>- 200 mm Boulders
In general, the soil types encountered in anchor design are sand and clay (grain diameter from 0.1 um to 2 mm). However, mooring locations consisting of soils with grain sizes above 2 mm, such as gravel cobbles, boulders, rock and such, also occur. Clay type soils are generally characterised by the undrained shear strength, the submerged unit weight, the water content and plasticity parameters. The consistency of clays is related to the undrained shear strength. However, American (ASTM) and British (BS) standards do not use identical values.
The undrained shear strength values Su can be derived in the laboratory from unconfined unconsolidated tests (UU). On site the values can be estimated from the results of the Standard Penetration Test (SPT) or Cone Penetrometer Test (CPT).
The mechanical resistance of sandy soils is predominantly characterised by the submerged unit weight and the angle of internal friction. These parameters are established in the laboratory. An approximate correlation between the angle and the relative density of fine to medium sand is given in the table.
The undrained shear strength of clayey soil can also be estimated based on manual tests.
-
In soft clay the thumb will easily penetrate several inches, indicating an undrained shear strength smaller than 25 kPa.
-
In firm (medium) clay the thumb will penetrate several inches with moderate effort, indicating an undrained shear strength between 25 kPa and 50 kPa.
-
Stiff clay will easily indedted with the thumb but penetration will require great effort, indicatin an undrained shear strength between 50 kPa and 100 kPa.
-
Very stiff clay can still be indedted with the thumbnail, but penetration is not possible, indicating an undrained shear strength between 100 kPa and 200 kPa.
-
Hard clay is indented with difficulty with the thumbnail, indicating an undrained shear strength larger than 200 kPa
The rock strength can generally be described by its compressive strength. See table.