NO346861B1

NO346861B1 - A suction anchor system

Info

Publication number: NO346861B1
Application number: NO20210937A
Authority: NO
Inventors: Frode Arnesen
Original assignee: Offshore Wind Design As
Priority date: 2021-07-23
Filing date: 2021-07-23
Publication date: 2023-02-06
Also published as: WO2023003476A1; NO20210937A1

Description

Field of the Invention

The present invention relates to a suction anchor system that use outside hatches to increase the load capacity and can be used as a foundation for bottom fixed subsea structures or as anchoring and mooring for any floating structures. All suction installed subsea foundations, such as suction anchors, suction caissons, suction piles, suction buckets and suction cans are in this application referred to as suction anchors.

Background

The offshore wind market is rapidly increasing and thereby a need for more costeffective subsea foundations, anchors and mooring to provide seabed support for either bottom fixed or floating offshore wind turbines.

Today’s foundation systems are a significant cost element of the offshore wind project development total cost. Larger offshore wind turbines require higher static and dynamic load capacities than subsea oil and gas foundation systems made for supporting self-weight, drilling and trawling loads can cost-efficiently provide today. The requirement for offshore wind mooring load capacity is expected to further increase as the developers want larger wind turbines producing more power per unit, a trend that is expected to continue for many decades.

For deep water areas where floating offshore wind is the only viable solution, the softer soil conditions often make a suction anchor the preferred type of foundation, anchoring and mooring solution. Suction anchors have been used in many applications for more than 25 years in the oil and gas industry.

Today’s field proven and standard suction anchor concepts, has the advantages of fast and easy installation and retrieval, silent installation to not disturb marine life, but has its disadvantage with uncertain soil strength properties as there may be local variations and unpredictable layered soils. This results in low and uncertain vertical, inclined, and horizontal design load capacity, especially for dynamic cyclic loads which is the dominant load in offshore wind mooring and anchoring applications.

Suction anchors are used as seabed support for a wide range of subsea foundation, anchoring and mooring applications. They are mostly designed as a simple caisson which may also include some internal stiffener plates. The top plate has openings for air and water expelling that would be closed later and facilities for the connection of suction pumps. These pumps basically apply suction or under pressure to reduce the pressure inside the suction anchor such that the hydrostatic pressure outside creates a downward force that penetrate the suction anchor into the soil. The pumps can also create overpressure to force it out again, often in combination with lift wire tension.

The standard suction anchor geotechnical design principles are to design for the required vertical, inclined or horizontal load capacity by sizing the caisson diameter and skirt depth, then estimate the penetration resistance and find required and maximum allowable suction to penetrate to target depth and at the same time ensure sufficient skirt wall thickness to avoid buckling.

Even though the design principles may seem simple, the geotechnical engineering of the suction anchor is complex and very specific to the soil conditions where it shall be installed. There are local variation and unpredictable soil layering. There are also often significant uncertainties with the soil strength parameters (remoulded shear strength and friction angle) even if extensive soil investigation has been done and both upper and lower bound soil parameters are used together with materials and safety factors.

In addition, there are sensitivity and uncertainty to dynamic/cyclic load capacity that will increase pore pressure and degrade, called cyclic degradation, the remoulded shear strength of the soil. The design load will hence be based on very low soil strength estimates and high safety factor will need to be applied.

This often leads to a very high design penetration resistance and a very low vertical, inclined and horizontal design load capacity when using conservative soil strength parameters. High theoretical design penetration resistance then requires a high design suction (under pressure) to ensure penetration to target depth which again limit the size of the suction anchors due to buckling effects. The size limitations in turn limit the design load capacity as it is directly based on using only skirt friction area for vertical drained or undrained load capacity.

CN 108423122 A describes a suction penetrating umbrella anchoring foundation which comprises a supporting plate, a geomembrane, a barrel, a middle steel plate and a support rod.

CN 103132521 A describes an undersea suction anchor which comprises a main drum, a top face of the main barrel is sealed, a lower end of the main barrel is opened, and a main barrel drainage hole is arranged on the top face of the main drum.

NO 305871 B1 describes a suction anchor for floating vessels.

Summary of the Invention

It is an object of the present invention to provide a suction anchor system with an increased vertical, inclined and horizontal load capacity by using ribs and/or a hatch mechanism that can be unfolded after installation and penetration to target depth.

The new suction anchor design intends to use the advantages of traditional suction anchors, but reduce the disadvantages by increasing the load capacity using the present invention.

The new suction anchor invention allows for including vertical bearing in the geotechnical design load calculations. The new design methodology is to design the skirt friction and tip areas to give a penetration resistance that is just lower than the maximum pumping capacity, allowable suction and buckling limits, meaning forcing as much suction anchor skirt depth as far down into the soil as possible to significantly increase the load capacity.

During operation, it can be characterized as a hybrid between a suction anchor and a plate anchor. This will form a ring of multiple hatches around the lower part of the suction caisson to provide bearing capacity in all load directions. The ribs and/or hatches are in vertical position during installation to minimize penetration resistance and the hatches in horizontal position during operation to provide bearing capacity and maximize the load capacity.

The subject invention uses the principle of bearing capacity to obtain significantly higher and more reliable load capacity. The submerged density of the soil is often more accurately defined and is used to calculate the bearing capacity and pull-out capacity. The invention could hence reduce cost by not require detailed and costly soil investigation for each suction anchor location which may be significant on a large wind farm over a large area.

Other means to further lower the penetration resistance, such as apply low friction paint, can also increase the dimensions and load capacity of the suction anchor.

These objects are achieved in a suction anchor system as defined in the appended claims.

Brief Description of the Figures

The invention will now be described in detail with reference to the figures included, wherein:

Fig. 1 is an illustration of the suction anchor with the support structure and the hatches in open position during installation,

Fig. 2 and 3 are illustrations of the suction anchor with the support structure and hatched in closed position and support structure without hatches and with ribs only,

Fig. 4 is an illustration of the lower part of the suction anchor with the support structure and closed hatches left in the soil with lift lines to the seabed while the upper part of the suction anchor has been retrieved,

Fig. 5 is an illustration of the geotechnical principles for vertical load capacity for standard suction anchors,

Fig. 6 is an illustration of the geotechnical principles for vertical load capacity for the invention,

Fig. 7 is an illustration of the geotechnical principle of bearing capacity versus skirt friction,

Fig. 8 is an illustration of the geotechnical principle of lateral capacity for standard suction anchor versus new suction anchor invention,

Fig. 9 is an illustration shows the suction anchor hatch principle applied using more than one (2 and 4 off illustrated) suction anchor and wide areas of hatches,

Fig. 10 is an illustration of the rib and hatch mechanism design principle where the hatches are closed and opened from the top of the suction anchor,

Fig. 11 is an illustration of the same mechanism where the hatch cuts through the soil to minimize the force required to close/open.

Detailed Description

The present invention relates to an application as a subsea foundation, anchor or mooring or more specific a suction anchor that uses multiple hatches/plates or similar supported on an outside support structure made of ribs which is fixed (welded or bolted) to the suction anchor caisson to give bearing capacity in addition to skirt friction and hence increase the load capacity for both upward and downward vertical, inclined and horizontal loads.

The ribs and hatches are vertical during installation to minimize the penetration resistance. The ribs can have smaller wall thickness at the bottom than at the top. This serves two purposes, the first is to hide the hatch above the rib to avoid additional penetration resistance. The second is for the rib to better cut through the soil, the knife principle. The tip must however be thick enough to withstand the possible impact of boulders during penetration. The ribs are also used to transfer the bearing loads from the hatches into the caisson.

The side of the ribs can also be painted with low friction paint to further minimize the penetration resistance. To obtain maximum skirt friction after installation, the highest possible suction, limited by pumping equipment, allowable suction or caisson buckling, can be used to penetrate the suction anchor as far into the soil as possible before closing the hatches.

The rib design as described above will increase the load capacity also without the hatches or the hatches can be small and provide local bearing capacity on each rib or larger and fully joined together to provide a more global bearing capacity.

The hatches, if used, are unfolded, can be self-locked or forced into horizontal position in any direction during operation by a mechanism using either suction, overpressure or/and lift line, mechanically using push-pull or torque or by using hydraulic cylinders. The mechanism can also be a combination of the abovementioned methods. The operation of such mechanisms can be done or controlled by ROV from the top of the suction anchor or by a lift line to a vessel.

The hatches, can be made in any suitable material, unfold to jointly create a large ring or square around the caisson that will provide the additional bearing capacity when loads are applied.

The support structure with ribs and hatches can be attached to the suction caisson with a mechanism such that it can be disconnected from the caisson after the hatches are closed. In this case, the suction caisson is used for driving the support structure into the soil and are then recovered. The same caisson can hence be reused to drive multiple support structures into the soil.

The vertical pull-out capacity is significantly increased by applying additional bearing capacity upwards compared to conventionally suction anchors only using skirt friction and/or weight of soil plug inside the anchor. To pull out the suction anchor with hatches closed, a massive amount of soil needs to be lifted up as long as the local bearing capacity limitation are met at the hatch soil contact areas.

For rapid loads, such as accidental drift off or large dynamic environmental loads, the soil will be undrained, and the underside of the hatches will also provide massive, inverted bearing capacity which is also proportional to the closed hatch area.

The suction anchor invention using the hatch mechanism also increases the stiffness of the suction anchor which can be used for reduction of motion and adjustment of the natural frequency of floating structures, such as floating offshore wind foundations. This stiffness also reduced settlements and hence gives a more predictable long-term behavior of the suction anchor.

The suction anchor invention also minimized the failure displacement, which is the displacement needed for the suction anchor to mobilize its full pull-out capacity. In addition, creep from sustained loads is also minimized.

As the suction anchor invention rely on bearing capacity and not skirt friction only, it will also reduce the risk of increased pore pressure build up and thereby soil strength degradation due to cyclic loads. In addition, it reduces the risk of strength degradation due to seismic liquification of the soil.

The optimal tradeoff between caisson diameter, depth and bearing hatch area, required suction and penetration resistance of the arrangement can be calculated based on soil parameters for each suction anchor. The invention makes is possible to standardize design over larger areas in a wind farm as the design is less sensitive to local soil parameter variations. This option allows for further load capacity and cost optimization of the design for each project.

As for standard suction anchors, there is a wide range of applications for this invention, but it offers most commercial benefits as load capacity improvement of conventional suction anchors or plate anchors in soft soil (very soft to hard clay) where there is need for a high tension/buoyant capacity where there are also cyclic loads involved over many years, such as foundations for deep water floating offshore wind turbines.

It can however also be used for bottom fixed offshore wind foundation such as suction buckets, monopiles, tripods and jackets. For floating wind, it can be used for anchoring and mooring of tension leg platforms (TLP), semi submersibles and SPARs.

The invention can also be retrofitted to existing suction anchors and be used in a combination with more than one suction anchors. The ribs will then be fixed to several anchors and with hatches closed provide a massive floor and bearing capacity.

A new suction anchor design shown in Fig. 1 consisting of a top plate (1) which has one or more openings for suction ventilation hatches which are open during installation to evacuate air and water and a system for connection of the suction pump system, beams and stiffeners for structural strength and a connection system for the tension, anchoring or mooring line. A caisson (2) and a support structure made of ribs (4) and other support and stiffeners plates (5) that support multiple hatches (3). The ribs (4) can be massive, or hollow made of plates and have a V shape to protect the hatches from penetration resistance during installation. The support structure (4) and (5) is attached to the caisson (2) closed to the skirt tip (6).

The hatches (3), shown as simple plates for illustration purposes in Fig 1-4, can be vertical during installation and soil penetration to minimize penetration resistance and become horizontal prior to operation, see principle in Fig. 2, they can cover the bearing structure fully Fig. 2 as illustrated or partly Fig. 3. During operation, the closed hatches provide additional bearing capacity and hence significantly increase the vertical and horizontal load capacity of the suction anchor.

Fig. 4 shows the lower part of the suction anchors with hatches closed if it has been disconnected and the upper part has been retrieved. Tether lines (7) are left to the seabed floor to be connected to mooring or anchoring lines.

Fig. 5 shows the principle of skin friction in standard suction anchors when subjected to long term sustained loads (drained soil). It also shows the increased inverted bearing capacity when the suction anchors are subjected to rapid (undrained) loads. The undrained vertical pull-out capacity is calculated by adding submerged weight to outside skirt friction and the inverted bearing capacity of the bottom area. The drained vertical pull-out capacity is calculated by adding submerged weight and both outside and inside skirt friction. An alternative is to add outside skirt friction and the weight of the soil plug (normally less).

Fig. 6 shows the principle of skin friction and bearing capacity in the new suction anchors when subjected to long term sustained loads (drained soil) versus skin friction plus vertical bearing capacity from the hatches when in horizontal position. It also shows the increased inverted bearing capacity when the suction anchors are subjected to rapid (undrained) loads. The undrained vertical pull-out capacity is calculated by adding submerged weight of subsea anchor and soil to outside failure plan shear and the inverted bearing capacity of the bottom area. The drained vertical pull-out capacity is calculated by adding submerged weight of suction anchor and soil to outside failure plane. An alternative is to replace inside skirt friction with the weight of the soil plug.

Fig. 7 shows the principle of skin friction in conventional suction anchors versus skin friction plus vertical bearing capacity from the hatches for vertical loading. The doted lines are indicating the soils friction and shear failure planes.

Fig. 8 shows the principle of horizontal and inclined bearing capacity in standard suction anchors versus additional bearing capacity from the hatches when in closed position on the lower level. A second support structure can also be included to increase the diameter, and there by the projected area, at the top to add more horizontal and inclined bearing capacity to the suction anchor. The tension can be applied horizontally on the top or at an angle with the tension line connected to a padeye typical located 2/3 down on the caisson (2).

Fig. 9 shows the application of the suction anchor hatch principles using two, three or four anchors connected with a support structure above the seabed (8) and with the hatch system with ribs (10) and hatches (9) at the bottom with support systems between the suction anchors.

Fig. 10 shows a more detailed illustration of one feasible mechanism that can be used to achieve the load increasing hatch mechanism principles of the invention. The hatches can be hinged on the support structure and be protected or covered by the support ribs. They are open during installation as illustrated and closed by rods from the top of the suction anchor using ROV operated torque tools and/or push/pull tools after the suction anchor has penetrated to final depth. Each hatch is designed with sector-shaped end plates (11) joined by a circular curved endplate (12). The hatches are hinged to the support structure at the tips (13) of the side plates (11). When operated, the hatches may cut through without moving the soil as the shape of the hatches make the hatch edge follow the path of a circle. The ribs are designed with an upper curved part (14) corresponding to the shape of the end plates (12). The lower ends (15) of the ribs are tapered to minimize penetration resistance. The hatches are operated by torquing/pulling/pushing on the rod (16).

Fig. 11 shows details of the same mechanism and how it cuts through the soil without the need to move mass. By this principle the resistance against closing the hatches are only tip resistance and skin friction. The hatch is moved until it is closed and rest on supports that can transfer the load from the soil into the support structure when loads are applied.

Claims

1. A suction anchor system including a caisson (2) with a top plate (1) and an open lower end,

characterized in a support structure fastened to the lower end of the caisson (2), the support structure including a number of ribs (4) hingedly supporting a number of movable hatches (3), wherein the movable hatches (3) are adapted to take a vertical orientation during installation and soil penetration, and may be moved to a horizontal position to increase the load capacity of the suction anchor system.

2. The suction anchor system according to claim 1, wherein the orientation of the movable hatches (3) may be reversed for retrieval.

3. The suction anchor system according to claim 1, wherein the caisson (2) is detachable from the support structure by a mechanism.

4. The suction anchor system according to claim 1, further including a second support structure without hatches located near the top of the caisson (2).

5. The suction anchor system according to claim 1, where each hatch (3) includes two sector shaped side plates joined by a curved endplate, the side plates being hinged to the support structure.

6. The suction anchor system according to claim 5, where each rib has a V-shape with smaller wall thickness at the bottom than on the top to minimize penetration resistance.

7. The suction anchor system according to claim 5, wherein the ribs (4) are designed with curved upper surfaces facing the endplates of the hatches (3).

8. The suction anchor system according to claim 5, where the hatches are hinged in such way and has a shape that make the hatch edge follow the path of a circle during opening and closing for minimum soil resistance.

9. The suction anchor system according to any of the previous claims, wherein 2, 3 or 4 suction anchors are jointed together creating larger areas of closed hatches for increased load capacity.