Hybrid Anchor

Field

The present application relates to a hybrid anchor and method for installation of a hybrid anchor, which is particularly suitable for conditions where lateral stiffness and precise positioning of the anchor are required within tight tolerance ranges.

Background Of The Invention

The concept of micropiles is well established in the art of foundation technology. Micropiles have seen strong development and use in a variety of applications since they were first used commercially by Fernando Lizzi in the 1950s. Micropiles are small diameter (<300mm) drilled and grouted friction piles. Each pile includes steel elements that are bonded into the bearing soil or rock - usually with cement grout which is poured during installation. Micropiles comprise distinct advantages over various other type of piling and anchoring due to their small size, light structure and low level of vibration during installation and are particularly used in situations where access is restricted or where there is low headroom for driving piles (for example underpinning structures affected by settlement). Micropiles do however also have their limitations. Disadvantages of micropiles include low degree of lateral stiffness, whilst micropile anchors are capable of resisting significant horizontal loads, they are an inherently compliant solution which allows some degree of elastic deformation under load. This elastic deformation can be a positive attribute for some applications; however, other application may require a higher degree of stiffness in order to maintain the position of anchors or foundations within very tight tolerances.

In addition, grouted micropiles typically require a period for the grout to cure before loads can be applied, which may result in some uncertainty in regard to exact positioning (including levelling) of anchor templates and contact with the soil prior to loading.

EP 2576330 discloses an undersea anchor that includes a frame which is fixed to the seafloor by one or more pile anchors, for example a grouted pile anchor. The frame is configured to permit use of or more seabed drills to allow the pile anchors to be installed through the frame at an angle relative to the horizontal axis of the frame so the pile anchors can take horizontal and vertical loads.

Separately, suction caissons (also known as suction piles or suction anchors) are another well-known anchoring technology, which has been used extensively in the North Sea since the 1990s and which is particularly suited for offshore and subsea use. Suction caissons are a fixed platform anchor comprising at least one sidewall (known as a skirt), preferably in the shape of a cylinder, which encloses a space therein and two ends wherein a first end is open and a second end is closed. The sidewall is preferably a high strength metal such as steel. The open end is positioned directly on the substrate on which the caisson will be installed. The open end of the caisson experiences a certain level of natural drawdown due to gravity and self-weight, which effectively seals the open end of the caisson to the substrate on which it is placed. The internal space within the caisson may be filled with air, water or any other form of gas or liquid which is already present in the installation environment. Once the caisson is in place and sealed, the contents of the space of the caisson are then evacuated through a valve which is located in the closed end (also known as lid or cap) of the caisson. Further drawdown is induced due to the suction force which is generated by evacuating the internal space of the caisson. This induced drawdown causes the sidewalls of the caisson to sink further into the foundation upon which the caisson has been positioned. It is thus not necessary to physically drive the caisson into the foundation. Once the caisson is at the desired penetration depth, the valve in the lid or cap of the caisson is usually plugged in order to avoid further movement. Suctions caissons are particularly advantageous in comparison to conventional offshore foundations as they are quicker to install than traditional deep foundation piles and, as they are essentially an empty shell, they are also considerably easier to remove once they have been installed in comparison to traditional deep foundation piles. Suction caissons do however also have their limitations. Disadvantages of suction caissons include the requirement for a relatively large size of caisson as suction caissons normally require large diameters and long cylinder height to achieve the required load capacity which drives up both weight and cost of the anchor as well as the vessel spread (i.e. boat, crew, ancillary marine equipment etc.) to support installation.

In addition, suction caissons also require specific soil conditions with no obstructions to allow for the required penetration of the anchor. These soil / foundation requirements thus limit the range of soil conditions in which suction anchors are suitable and gives rise to expensive project delays where obstructions are discovered during installation.

US 6122847 discloses an apparatus for anchor installation of a plate anchor mounted at the bottom of a suction follower. Upon engagement of the suction follower and anchor secured thereto with the seafloor, the suction follower is used to pump water out of the suction follower causing the anchor to penetrate into the sea floor. The anchor is them disengaged from the suction follower and the suction follower is retrieved. The plate anchor may comprise first and second plate members, the second plate member pivoting relative to the first plate member to prevent upward movement of the anchor.

It is an object of the present invention to mitigate the limitations of such anchoring technologies.

Summary According to the present invention, there is provided a hybrid anchor according to claim 1. The application also provides a method as detailed in claim 5. Advantageous embodiments are provided in the dependent claims.

Brief Description Of The Drawings

The present application will now be described, by way of example, with reference to the accompanying drawing in which:

Figure 1 illustrates an elevation view of a hybrid anchor in accordance with an embodiment of the present invention.

Detailed Description Of The Drawings

Referring now to Figure 1 , there is illustrated an elevation view of a hybrid anchor, generally indicated as (10) according to an embodiment of the present invention.

The hybrid anchor is shown in situ as it is partially submerged in a substrate 20 upon which it is sitting.

The hybrid anchor comprises a traditional suction caisson (also known as suction anchor or suction bucket) type anchor 30 which is described in the background section above, wherein the suction caisson is employed, in conjunction with a drilling template (not shown) which is fixed on the lid 35 of the caisson. The drilling template is located on the caisson prior to loading it onto the vessel for transport to the installation site. In an alternative embodiment, the drilling template and the caisson may be loaded onto the vessel separately and assembled on the vessel, platform or other suitable structure prior to deployment to the seabed. Initially, the drilling template holds a plurality of micropiles 40 (only 3 shown) in position on top of the lid 35 of the caisson. Once the caisson is located in place in the substrate 20, the piles are drilled through the caisson lid 35 and into the substrate 20. In the example, each pile 40 is drilled into the substrate at an angle of 90 degrees to the plane of the lid of the caisson. However, in variations of the embodiment, the piles 40 may be oriented at an angle up to the point where the piles would intersect with the caisson skirt 37, so avoiding interference between any pile 40 and the caisson skirt 37.

As detailed above in the background section, micropiles are small diameter (<300mm) drilled and grouted friction piles. Each pile includes steel reinforcement elements which are bonded into the bearing soil or rock - usually with cement grout.

The hybrid anchor combines elements of both suction caisson and micropile installation in order to function as seabed anchor or foundation solution. Suctions caissons traditionally require large sizing in order to benefit from self weight as an initial driving force for the caisson into the substrate. In combining the teachings of these two considerably different anchor technologies, it is possible to reduce the traditional size of the caisson elements to a significantly smaller size than that of traditional suction caissons for an equivalent load requirement.

The hybrid anchor additionally comprises a drilling template, which enables at least one micropile to be driven through the caisson element at one of number of predetermined positions once the caisson 30 has been deployed to its optimal anchored position within the substrate 20.

The hybrid anchor provides a solution which enhances both lateral stiffness of the anchor and ability to precisely position the anchor during installation according to project requirements.

In the illustrated example a single hybrid anchor 20 is shown, however, it will be appreciated that multiple hybrid anchors can deployed as a subsea foundation system in order to support a large structure. In some cases, one hybrid anchor 20 can be located at the foot of each leg of the structure, directly connecting the structure to the substrate 20.

Advantages of the hybrid anchor 20 include:

• Reduced size, weight and cost of anchor/foundation solution compared with alternative technologies and traditional methods.

• Ability to install anchors in a wide range of soil conditions as hybrid anchor caissons 30 do not need to penetrate deep into the soil (thus avoiding certain obstructions), whereas micropiles 40 can be drilled through obstructions and into rock formations.

• Retain ability for levelling foundations and platforms by adjusting differential pressure on each caisson until the desired position is achieved.

• Combined structures are able to withstand substantial horizontal and vertical load resistance with minimal deflection.

• After positioning, any remaining internal space within the hybrid anchor bases can optionally be filled with grout to enhance bonding between the soil and structure.

• Less costly installation equipment - in subsea applications as each micropile of the hybrid anchor is remotely installed with a seabed drill, the need for an expensive deep water drilling rig on the sea surface is eliminated.

• As the hybrid anchor employs a suction functionality in order to position the caisson, hybrid anchors are classified as direct embedment anchors. This means the precise location and depth of the anchor is known and they may be deployed in areas where a clear area to drag an anchor to the desired location may not be possible.