Suction Installed Units

The installation process of a suction foundation typically involves lowering the unit to the seabed, allowing self-weight penetration, and then using pumps to evacuate water from the unit. The pressure differential created by water removal causes the unit to penetrate into the seabed.

Self-weight penetration refers to the initial penetration of the suction foundation into the seabed due to its own weight. It is important because it facilitates the subsequent suction-assisted penetration.

Suction assists in penetration by creating a pressure differential when water is pumped out of the unit. This pressure difference helps overcome soil resistance, allowing the unit to penetrate the seabed.

Evacuating free water during installation is crucial to ensure that the unit's weight is equivalent to the buoyant weight of the caisson and the enclosed material. It also provides undrained tensile capacity.

Safe reduction of the overburden term is achieved by using an appropriate value for the overburden capacity factor (Nq). In the equation, Nq=1 signifies that there is no active resistance from soil flow under the rim.

Reverse bearing capacity refers to the potential plastic failure within the caisson due to a significant difference between inner and outer vertical stresses. It could lead to instability if not considered.

Suction foundations are commonly used in clay or layered soil conditions, where soil strength and bearing capacity generally increase with depth. The depth of penetration required for stability depends on the specific soil characteristics.

The potential effects of steady-state seepage during skirt penetration in sandy soils include higher normal stresses and friction outside the skirts, lower stresses inside, reduced skirt tip resistance, the risk of soil plug liquefaction, and localized soil flow outside the caisson.

Seepage affects the in-situ effective stress state through hydraulic head changes, which modify the soil's effective stress. Darcy's law is a fundamental equation used to describe seepage and its impact on soil.

Factors essential for self-weight penetration in sandy soils include diameter, soil-wall friction angle (δ), earth pressure coefficient (K), and capacity factors (Nγ and Nq).

Suction-assisted penetration may lead to the inner soil plug becoming looser during upward seepage. Factors influencing this process include the current penetration depth and the permeability inside and outside the caisson.

Limits to suction-assisted penetration in sandy soils include the possibility of piping when the seepage force equals the buoyant soil weight. Factors like hydraulic gradient, V', and stress enhancement play roles in determining these limits.

When clay overlies a sandy layer, suction foundations need seepage to assist in penetrating the sand layer, while the clay layer acts as a hurdle. Seepage is critical in reducing the clay's resistance and facilitating penetration into the sand layer.

In the case of stiff or fissured clays, penetration and seal formation around the caisson rim can be challenging due to the higher soil resistance. Specialized techniques, such as pre-drilling or jetting, may be used to overcome these difficulties.

Materials like glacial tills or silts can lead to high flow during pumping. The determination of whether the behavior is drained or undrained can be challenging in silts, making accurate predictions more difficult in such conditions.

The role of pump capacity in the installation of suction foundations in sandy soils with seepage is to manage the flow of water and seepage within and around the foundation. Pumping is essential not only to evacuate water from the caisson but also to control and enhance seepage, which influences the penetration process.

The three components are the weight of the caisson, frictional resistance, and reverse end bearing.

Cyclic loading negatively impacts the pull-out capacity, causing a reduction of approximately 20-25% over one year.

The reverse end bearing contributes to the overall pull-out capacity and is related to base suction if the cap is sealed. It is a crucial factor for such anchors.

For unsealed-cap anchors, the reverse end bearing becomes negligible, and the pull-out capacity primarily relies on external shaft friction and, if applicable, inner friction and soil plug weight.

Factors influencing optimal pad-eye depth include the depth distribution of undrained shear strength (su) and the length-to-diameter (L/D) ratio of the anchor.

Misalignment, or horizontal rotation during installation, can lead to combined horizontal-vertical (HV) loading. This can have a significant impact on the anchor's performance.

The angle of torsion (β) influences the interaction coefficients a and b in the HV capacity. At small β values, torsion has minimal effect, while at high β values, it becomes dominant, reducing Hult.

Optimizing the pad-eye location helps maintain the desired mooring configuration and minimizes the risk of anchor rotation under H-loading, ensuring the anchor's ability to withstand horizontal loads.