Anchoring systems
All over the world their exist difference in bottom soil conditions. These soil conditions have a big influence on the load capacity of any anchor system. A greater load capacity of any anchor can be achieved by a deeper embedding, also the greater quantity of affected soil. Therefore permanent anchors can be designed for different types of soils. These will further be discussed in this chapter.
The most effective resisting force is by applying the force parallel to the bottom without deep embedment. As the resisting forces increases, it will dig deeper into the seabed. Catenary mooring system is using this principal advantage. Heavy chains is being used that forms a catenary shape. An average horizontal force of the platform is reacted from the point of attachment to the sea floor. Catenary moored anchors are less expensive, because the mooring forces are mainly horizontal, which requires less accurate anchor drop. Unfortunately a greater motion is experienced compared to other mooring systems, like taut-leg or vertical tension-leg systems.
Here we can conclude that a deeper the embedment, the greater the pull-out load, which is the key to maximize the load. Since horizontal loads can embed themselves deeper and deeper, vertical loads will take more effort to get deeper. Vertical loaded anchors are therefore more expensive to install. Finding a relatively cheap anchoring system with a high vertical load capacity that is easy to install will be very challenging.
Elements that define the cost of an anchoring system depend on the material cost, the type of installation and mooring lines of chain, cable, or pipe. In further chapter different types of seafloor anchors are discussed, which is listed below.
The most effective resisting force is by applying the force parallel to the bottom without deep embedment. As the resisting forces increases, it will dig deeper into the seabed. Catenary mooring system is using this principal advantage. Heavy chains is being used that forms a catenary shape. An average horizontal force of the platform is reacted from the point of attachment to the sea floor. Catenary moored anchors are less expensive, because the mooring forces are mainly horizontal, which requires less accurate anchor drop. Unfortunately a greater motion is experienced compared to other mooring systems, like taut-leg or vertical tension-leg systems.
Here we can conclude that a deeper the embedment, the greater the pull-out load, which is the key to maximize the load. Since horizontal loads can embed themselves deeper and deeper, vertical loads will take more effort to get deeper. Vertical loaded anchors are therefore more expensive to install. Finding a relatively cheap anchoring system with a high vertical load capacity that is easy to install will be very challenging.
Elements that define the cost of an anchoring system depend on the material cost, the type of installation and mooring lines of chain, cable, or pipe. In further chapter different types of seafloor anchors are discussed, which is listed below.
- Gravity-base anchor
- Drag-embedded anchor
- Driven pile anchor
- Suction anchor
- Driven anchor plate
- Torpedo embedded anchor
- Drilled and grouted pile
Gravity-Base anchor
Gravity-base anchors are mainly used in TLPs. The heavy dead weight ensures a safe capacity force in the vertical or horizontal direction. The material of the anchor is cheap, but a large amount of material is needed to achieve the demanded capacity. The difference between its weight and its buoyancy defines the load carrying capacity.
Drag-Embedded Anchor
This anchor type are dropped to the seabed and wired or dragged to achieve a deeper embedment. Movement over time may not be a high risk or critical. The weight of the mooring lines will cause a line tension to drive the anchor deeper. The advantages of this anchor are:
- Low anchor type cost
- Suitable for catenary moored systems because precise placement is not needed.
- Horizontal mooring forces exist
Driven Pile Anchor
Drivenpile anchor has been proven as very reliable and can achieve a very high load capacity. This anchor has been developed over the years of experience in the oil and gas industry. Because the reliable is very high, these anchors are the most commonly used for offshore oil production. The advantages are:
- They are permanent
- Precisely located
- Piles will not creep
- Well suited to take vertical loading
Suction Anchor
Suction anchors have a physical look of a long pipe, which is open at the bottom end and closed at the top. These are a good alternative for the drivenpile anchor. In order to evacuate the water and sucking the pipe into the bottom soil, the closed end is outfitted with pump fittings. A transverse tension direction is achieved on the pipe by attaching an anchor line to a pad eye near the midpoint of the pipe. In this way the tension line are placed well down in the deeper soil allowing a large wedge of soil to support the line load. A suction anchor is the most effective for vertical loading compared to drag embedded anchors.
Driven Anchor Plate
A driven anchor plate uses the same principle as the suction anchor, but uses less material, which lowers the costs. It can resist a large wedge of soil when tension loads are applied to the plate, in this way it will rotate in the soil. The advantages are:
- It can be precisely located
- Can sustain high vertical loads
- Not likely to creep
Torpedo Embedded Anchor
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This type
of anchor is dropped to the seabed with a great force by its own weight drives
it into the bottom. The least expensive approach for offshore wind turbines
using vertical mooring systems is a combination of torpedo with a driven plate,
which can rotate when tension is applied. Over the years, great research and
development has been achieved for anchoring floating oil platforms with this
kind of anchor type.
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Drilled and Grouted Pile
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The seabed
does not always have a permeable soil conditions where an anchor can be
embedded in. If the seabed contains a rocky surface, an anchor can be attached
by drilling into the rock and grout a pile into the hole. The used pile will
contain of similar in size and shape as the driven pile. The advantages of
drilled and grouted pile compared to driven piles are:
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