Suction Piles

OE 4624 - Offshore Soil Mechanics

7

Suction Foundations

Suction Foundations

7.1

Suction foundations in general

Suction foundations are cylindrical structures, closed on one end and open on the other. For installation the open end of the cylinder is placed on the seabed and the water contained within the cylinder and the seabed is pumped out. This causes a vertical load on the structure, penetrating it into the ground. Furthermore, the reduced water pressure of the contained water induces groundwater seepage; upward seepage within the penetrated soil volume within the cylinder and downward seepage in the penetrated ground around the cylinder. Depending on the ratio of the length of the cylinder and the average radius of the cross-sectional area of the cylinder a suction foundation may resemble a shallow foundation if this ratio is small and a pile foundation if this ratio is large. Thus the difference depends solely on the shape of the struct ure.

Advantages of suction suction foundation over other types:     



 

usable in (very) deep water easy to handle simple installation equipment load can be instantly applied tremendous holding power in all directions better to predict failure loads than with anchors retrievable large range of possible use

Dependent on the depth of the water different types of foundation methods are used. In shallow waters jackets are very common, but in deeper waters moored system are used. In order to connect the mooring lines, foundations or piles have to be used. Jackets are fixed to the seabed during installation by driving piles into the ground. At deeper waters this is also a possibility but there is a good alternative in the form of suction foundations. Driven piles require pile-driving engines and at greater depths this could proof to be very difficult. Suction foundations only need a relatively simple pumping device that pumps the water out of the structure. An additional advantage of suction foundations is that they are retrievable in most cases. (>4m00s, Luger 03)

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Suction Foundations

The naming of the suction foundation types is basically a matter of shape. One speaks of a suction can when the penetration is not too deep, typically L/D < 2. In the case that the factor L/D > 2 it is called a suction pile. Typical for both suction cans and piles is, that they have a mooring attachment point. This attachment point is not placed on top of the can, but a bit lower, like in figure x.x. If it is placed on top the element tends to rotate towards the load. With an optimal attachment point level, or lug level, the foundation is not likely to rotate and can therefore bear more load. Figure x.x, mooring attachment system (>5m35s, Luger 03) The bucket foundation is another sort of suction foundation. It is one of the applications used for the foundation of jacket structures. A lot of jacket platforms are placed on the seafloor with mud mats installed in the corners. Right after placing the platform is just standing on the seabed. Later piles are driven through sleeves on the side of the platform legs to ensure the stability of the platform. Another foundation system for a jacket structure is the use of a so-called bucket foundation. A bucket foundation is a sort of suction can but it differs from suction cans because it has no mooring attachment.

7.1.1

Installation

The installation method is similar to that of the skirted shallow foundations. The suction can is placed on the seafloor and the enclosed water is pumped out of the can. The larger pressure on the outside of the can pushes the can into the soil. (>8m40s, Luger 03) Suction foundation can be applied in sands as well as in clays, but especially in softer clays they work very well. Of course the installation of suction foundations can fail.

Installation in clay One of the failure mechanisms is that when there is a lot of friction on the outside of the can, plastic failure of the soil can occur. In that case it can happen that instead of a penetration of the can the soil can be sucked up into the can. This phenomenon is called reversed end bearing failure. Normal end bearing failure occurs when after installation too large a load is placed on the soil inside and right underneath the can the soil will move out of th e can. The reversed can happen if the suction applied is too large or if the pull force on the can is too large.

Installation in sand

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Suction Foundations

In sands another failure mechanism can occur which is related to the inflow of water. In figure x.x the flow net around the suction can has been sketched. The inflow will always be present because of the pressure differential between the sea and the inside of the can and the fact that sands are relatively permeable. If the suction force is increased the flow will be faster and because the can has only a limited size large flow gradients can occur. It is known that in sand with an up-flow of water the effective stresses decrease. So if the up-flow is too large effective stresses can diminish resulting in quick sand. (>12m00s, Luger 03)

At the start of the installation process in sand the initial stress distribution on the inside and outside of the can is equal. The water pressure in this example is 120 kPa at the top (12.0 mwc). The effective stresses right beneath the surface are quite low. When the pumping starts the water pressure inside the can decreases. But because of the up-flow of water the effective stresses decrease more strongly. Increasing the suction pressure further and further the sand inside the can gets close to liquefaction because there are hardly any effective stresses left in the soil. The water flow starts outside the can and flows right beneath the tip of the suction can, so the tip resistance decreases as well which makes penetration easier. Medium dense sand without pore water flow can be very strong. This implies that when a suction pile is installed in sand with one or more thin sealing clay layers, the tip resistance at greater depth in the sand could prove to be difficult to penetrate. (>14m00s, Luger 03) During installation there is a transition from a st ate without any liquefaction of the sand at all to a state with full liquefaction. Initially the sand is strong on the inside as well as on the outside of the can. The tip resistance determines the penetration force only. Consequently not a lot of suction pressure is required. The further the can penetrates in the ground the more shear resistance builds up. To assure a continuous penetration the suction pressure is increased. The

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Suction Foundations

upward water flow increases accordingly reducing the effective stress inside the can. The suction pressure can be increased until the soil liquefies completely. In the graph in figure x.x two lines have been plotted. The continuous line shows the relation between suction pressure and penetration without liquefaction effects, while the dashed shows the relation for fully liquefied sand. In reality the transition line goes from the first to the second situation.

7.1.2 Boundary conditions for the use of suction piles (>15m40s, Luger 03) The use of suction piles is not unlimited. First of all there needs to be sufficient water depth to create enough suction pressure. The theoretical upper bound for the length of the pile is the water depth plus the underpressure in the water measured in meters. To prevent cavitation in the pumps at least 20 to 30 kPa absolute pressure needs be available. This means that the pile cannot be larger than the water depth + 7 m. This is the upper bound to ensure that the water pressure inside the can is lower than outside the can. But if it’s too close to this bound little suction pressure is generated. An under-pressure of 100 kPa is very normal. In that case the length of the pile cannot be larger than the water depth itself. If the soil resistance is high a lot of suction pressure might be needed and therefore more water depth. (>18m35s, Luger 03) Piles with low L/D ratio are easier to install. The tip resistance and the friction on the inside and outside of the suction pile determine the required penetration force. However, only the pile diameter determines the maximum possible penetration force, suction pressure times inner area. So the longer the pile the more friction will be generated using the same penetration force.

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Suction Foundations

(>19m25s, Luger 03) Suction piles are generally feasible in clay, sand and sand-over-clay soil profiles. Clay-oversand profiles may be problematic. In sand the resistance decreases due to partial liquefaction, but if a clay layer lies on top of the sand the water flow is interrupted. (>20m20s, Luger 03) Suction piles are not feasible in cemented sand and rock. In these soils one can only use systems that use pile driving engines or drilling techniques. (>20m50s, Luger 03) A recap of the advantages of suction piles 









7.1.3

They can easily be used in deep waters for there is no need for heavy driving pile engines. A simple pumping device will do. They are relatively easy to handle. Suction piles, although not small, are a lot lighter than normal anchors. Normal mooring anchors can easily weigh up to 50 tons. Load can be instantly applied. After installation there is little time needed to reach full capacity unlike in-situ made concrete piles, that need time to harden, or piles that create large excess pore pressures, which need to consolidate. Suction piles also need time to reach full capacity, but relatively little because of the thin walls. Better prediction of failure loads is possible than with traditional anchors. Traditional anchors are well designed to dig into the soil upon loading, but it is tremendously difficult to determine the failure load exactly under varying soil conditions. Both in an analytical analysis and in a numerical program, cylindrical shapes are easy to model. Suction foundations are retrievable. By applying an overpressure inside the suction pile, the pile will push itself out of the ground.

Range of possible use (>25m35s, Luger 03)

[NEEDS TO BE MENTIONED AT START OF CHAPTER]

Typical examples of the use of suction foundations   

Bucket foundations under jackets Bucket foundations under very deep platforms or subsea templates Mooring systems, most common application

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7.1.4

Suction Foundations

Design of a suction anchor (>26m20s, Luger 03)

A suction pile design starts with the type and magnitude of the load as determined in an earlier phase of the design process.

7.1.5

Determination of lug level (>26m55s, Luger 03)

The overall required capacity of the pile is more or less known at the start of the design of the suction pile. The diameter, length and the number of piles however, are not known. Also the lug level is unknown. It is most common to position it about 2/3 down the pile Determination of lug level (>4m40s, Luger 05) If a suction pile is loaded at the top at an angle of 45 degrees, the pile tends to rotate, while if the load is attached at a deeper level the pile tends to translate horizontally through the soil. The latter generates much more soil resistance and is therefore the better and more optimal design. (end intermezzo) But the optimal position also depends on the soil conditions and size of the pile.

If the strength profile of the soil is constant over the depth the position would be just below the middle of the pile. But if the strength profile increase linearly with depth, which is the case in a homogeneous layer, it is obvious to position the point lower than the middle. In this way force of the pile acting on the soil is concentrated in the stronger part of the soil profile. Figure x.x constant and linearly increasing strength profile

The depth at which the pile is placed can also depend on the retrievability of the pile after use. [tip: more on the retrievability would be nice to stress this advantage of suction foundations] (>29m15s, Luger 03) For the design of the pile itself again attention to the soil is essential for a sound solution. A suction pile is comparable with a beer can as one looks at the diameter-wall thickness ratio. Without reinforcement on the inside of the pile it would fold very easily under loading. So simply attaching a pad eye for connecting the mooring line would be disastrous for the pile. In reality the pile is stiffened on the inside by a sub-structure t o avoid buckling. In figure x.x an example of a stiffened pile is given. In order to design this sub-structure the soil conditions need to be known pretty well. Especially the soil stiffness is important in this case. (>30m40s, Luger 03)

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Suction Foundations

Three basically different pile types can be distinguished: Closed pile with permanent top Open pile with retrievable top Open pile with follower All three piles are closed on the top during installation. The difference between the three only   

becomes clear after installation. The first pile with a permanent top is the more traditional one. It has an application point for a pump unit on the lid. The open pile with a retrievable top is installed similarly to the closed pile, but after installation the top is being retrieved. The advantage is that the top can be re-used for other piles, which may be an advantage when installing more than one pile in a small area. The open pile without the top does lose some bearing capacity by removing the top. One other disadvantage is that the pile itself cannot be retrieved after use. Then there is the open pile with follower. This pile is used when there is a very weak top layer. From this top layer no contribution to the bearing capacity may be expected and the top of the pile is actually not needed. By using a follower the pile is installed and the top is retrieved from the weak layer. This top can be re-used to install other piles in the same area. If a large number of piles are being installed this can save a lot of space on the vessel so a smaller vessel might be needed or less uploading of the vessel with new suction piles.

7.1.6

Mooring line system

As is mentioned before, it is most common to position the pad eye about 2/3 down the pile. If it is attached higher than 2/3 the pile tends to tilt. But the optimal position also depends on the soil conditions and size of the pile. The mooring line is already attached when the pile is being installed. So if the penetrates the mooring line goes down with it into the soil. Without tension in the line during installation, the line stays close to the pile. During its lifetime the line will be loaded and is being dragged through the soil. In order to prevent too large a time lag between the loading of the line and the actual loading of the pile, the line is being pre-stressed by a tugboat.

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Suction Foundations

Of course, a vessel can never generate as much force as a platform during storm conditions. So the mooring line will always be dragged through the soil a bit more. This means that after a heavy storm the mooring line will be slacker. In the design phase the depth of the lug level is determined and the position of the mooring line in the soil. When this has been designed it can be checked how much force is required to reach the favoured position of the line.

Calculation example: Soil type:

clay

Strength:

Cu= 10 kPa

Diameter of mooring

dml= 0.2

line:

m

Figure x.x, sketch of suction pile and mooring line

Lug level:

zd= 10 m

Bearing capacity factor:

Nc= 10 14

The force acting on the soil from the mooring line when it is dragged through the soil equals the maximum bearing capacity of the soil surrounding the mooring line. In this case this is t he undrained shear strength in the homogeneous clay, Cu= 10 kPa. The tension force in the mooring line is equal to the tension in a spherical body, Ft



Fn z d ,

(7.2)

in which F n is the normal force acting on the mooring line (soil resistance) and z d is the radius of the spherical body, in this case equal to the depth of the attachment point. The soil resistance can be written in the form, Fn



d ml  N c  C u ,

(7.3)

in which the bearing capacity factor, N c , has a typical value between 10 and 14. The total horizontal force required by a tug, Fn Ft

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

d ml  N c  Cu Fn z d

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

1

0.2m  12 10kPa  24kN m , 1

24 kN m



10m



240kN .

(7.4) (7.5)

The smaller the angle between the mooring line and the horizontal, the larger the tension build up will become to pull it through the soil, because the radius of the circle increases.

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7.1.7

Suction Foundations

Bucket Foundations

Bucket foundations can be found underneath large platforms and jackets. The diameter of the cans easily exceeds 5 meter. Some can be 10 or 20 meters in diameter. The jackets, to which the bucket foundations are attached, have much l arger dimensions. So the installation of th ese structures can proof to be quite difficult to perform with the use of a crane. Then pontoons are used from where the structure is launched into the water. To the side of the structure containers assure sufficient buoyancy. By slowly filling the containers with water and aligning with the help of a crane, the structure is placed on the seabed. There is always enough water depth available. Why?

7.1.8

Failure modes (>10m05s, Luger 05)

A structure fails in the weakest direction. Design is partly based on finding different failure modes and the respective failure load or safety factor. For the structural side of the design it is not sufficient to know the failure load and mode. Here it is important to know the loads and deformations. >17m00s <20m30s Explanation of stiffness matrix. Suction piles are extensively tested in centrifuges.

7.2

Suction foundations in clay (>27m00s, Luger 05)

Different types of anchors:     

drag anchor vertical loaded anchor (VLA) driven pile anchor suction pile anchor (SPA) gravity block

The main advantage of suction pile anchor over a drag anchor or a VLA is the fact that the positioning is much more precise. The first mentioned first need to be pre loaded in order to slowly generate sufficient holding capacity while with the suction pile anchor only the slack needs to be taken out of the mooring line. During the pre-loading of a drag anchor the anchor is pulled over a certain distance horizontally. Therefore the positioning is less precise but it also means that enough space needs to be available. Sometimes the fields are just too tight and this space is simply not available.

One of the main advantages of closed suction pile anchors is that under loading the soil might become plastic inside the pile. At that point there is st ill some extra capacity available in the form

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Suction Foundations

of a suction force close to the top. If the pile is lifted the void needs to increase. The only way in which this is possible is by pore water from the soil inside the pile. The pore water pressure will decrease so the water pressure just outside the pile is larger than on the inside. This generates an extra downward force.

7.2.1

Foundation Safety and Loads (>32m15s, Luger 05)

Offshore foundations have to comply with a certain set of safety standards, which are made up by governing bodies. In the field of suction pile anchors, such standards do not exist. There are, however, some recommendations for the design of suction piles. API – American Petroleum Institute The API was the first to write codes and RP’s (Recommended practices). Their codes vary from the design of steel tubular structures to piping in topsides and offshore platforms. DNV – Det Norske Veritas DNV is involved in certifying designs for insurance purposes NPD – Norwegian Petroleum Directorate Codes mainly applicable for Norwegian cir cumstances.

Due to a lack in international standards these rules are often applied even when they are outside their application area.

ISO – International Standard Organisation – has started writing standards for design of offshore structures. In practice it is common that t he parties involved, usually including the client too, set their own safety standard they wish to achieve. Of course they take the guidelines into consideration but they are not binding in any way.

Safety matters – should be described elsewhere.

The type of loading has a large influence on the bearing capacity. For example, if during storm conditions an FPSO loads its anchor in a repetitive way close to the limit load, the clay tends to soften and decrease the capacity.

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7.2.2

Suction Foundations

In-Place resistance (>43m30s, Luger 05)

Loads are always combined. Uncoupled behaviour gives a maximum resistance for the horizontal and vertical loads. For a combined load the total resistance is smaller than in the uncoupled situation. In figure x.x different yield points have been found corresponding to the load angle. Connecting the various yield points gives the failure envelope for combined loading.

figure x.x

failure envelope for combined loading

The approximate equation is only valid in the following conditions:     

7.2.3

Typical SPA d/D ratio of 3 to 6 Failure definition: displacement of 10% Outer Diameter Normally Consolidated Clay SPA loaded in line with lug (no twist/torsion) Vertical SPA (no tilt) loaded at OLL

Soil parameters and Soil behaviour (>50m30s, Luger 05)

As has been mentioned before, it is important to have a good knowledge about the soil parameters and soil behaviour. One of the aspects of geotechnical testing is the determination of the unit volume weight of the soil. This measurement will always be a slight underestimate since a sample subjected to high pressures in-situ will expand if brought to sea level.

Typical shear strength (cu) measurements:        

Unconsolidated undrained triaxial tests (UU) Consolidated undrained triaxial compression and extension tests (CAUC and CAUE) Consolidated undrained direct shear tests (DSS) Laboratory vane tests (LV) Laboratory fall-cone tests (FC) Laboratory tor-vane and penetrometer tests (TV and PP) In-situ vane tests (VST) (CPT correlation, Nk inferred)

A structure like a suction pile anchor is loaded at lug level at an inclined angle. The surrounding soil behaves different and has different str ength properties depending on the location

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Suction Foundations

of the soil. If the soil is in the passive wedge slip plain the volume is experiencing triaxial extension. The soil is compressed and therefore the horizontal load will increase while the vertical load will hardly change. On the active side there is horizontal relaxation and triaxial compression occurs. On the bottom the pile will s hear creating direct simple shear. This simple example shows that for loading of a suction pile, three different geotechnical tests are needed to describe the soil behaviour.

figure x.x, Example of different geotechnical tests needed for one type of structure.

In clay they all depend on the undrained shear strength. But be careful! There is no such thing as one undrained shear strength. This strength depends on the failure mode. To indicate what type of failure mode the measured shear strength belongs to, often an extension of the parameter is chosen: C u

DSS

direct simple shear

C u

C

triaxial compression

E

C u

triaxial extension

The geotechnical tests are very useful in determining the type of soil and its properties, but again, it should be noted that the results of geotechnical tests show quite a scatter. It is very difficult to compare the results with known strength tests because the variation in strength is much larger in soil than in any other man-made material.

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Suction Foundations

Cyclic loading can soften the clay and decrease the bearing capacity. The figures below show that even when the loading is between 50% and 75% of the maximum load after a few cycles the soil can fail. To prevent the soil from failure the magnitude of a single load should be decreased (extra mooring facilities) or the cycles should be interrupted (wave reduction).

The installation of the suction pile disturbs the soil structure. The remoulded soil properties will lead to a decrease in vertical capacity. The horizontal capacity will not be affected much. After a day V max has increased significantly already. Slowly the strength is returning to its original state.

7.2.4

Installation and Retrieval (>1h05m40s, Luger 05)

The installation of a suction pile is driven by the self-weight of the pile and the suction applied. The mechanism is described before in paragraph 7.1. However, there is a limit to the suction pressure that can be applied. A large suction pressure can lead to base failure where the soil is sucked up into the pile leaving room for soil from outside the pile to follow into the pile. A larger diameter generates more penetration force to overcome the sidewall friction and can decrease the required suction pressure.

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Suction Foundations

For retrieval, instead of suction an excess pressure is applied to equal the self-weight and side friction. The tip friction has no significance in retrieving the pile.

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Suction Piles

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