Suction Anchors and Piles for Offshore Foundations The first application of suction piles for offshore engineering was in 1982, and in 1989, engineers successfully installed Gullfaks C concrete platform in a soft clay using suction piles for a gravity-based structure. Sixteen circular concrete cells with a diameter of 28 m (92 ft) and thickness of 0.4 m (1.3 ft) provided the structure’s foundation. The concrete skirt penetrated 22 m (72 ft) into the seafloor. Since then, engineers have used this new foundation system successfully for a variety of offshore structures in a range of environments. The advantages include its low cost, simplicity and efficiency. Engineers can install suction piles by reducing the water pressure inside the pile. This acts as an equivalent external surcharge to push the pile into the seafloor. These piles, sometimes called suction caissons, have a completely sealed top but the bottom is open (see Figure 1). Typically, they have a large diameter with a relatively small length to diameter ratio. By applying a higher water pressure inside the pile than the outside ambient water pressure, engineers can retrieve the pile later if necessary. The most common length to diameter ratios in practice are 1:1 to 2:1 for a sandy seafloor and 5:1 to 20:1 for a cohesive seafloor. The largest suction pile dimensions ever installed in the field are approximately 32 m (105 ft) in diameter and 36 m (118 ft) in length (Troll Field in the North Sea). Suction piles have several technical advantages over conventional piles. These include fast and easy installation at any water depth, extremely large resistance due to its huge dimensions and easy retrieval. Engineers and contractors expect that suction piles can provide a viable alternative to the conventional underwater foundation systems that use plate anchors, drag embedment anchors, or drilled and driven piles. Engineers mostly used suction piles in the past as anchors for floating structures and foundations of marine structures in deep-water locations. However, as they better understand the mechanism and behaviors of suction piles, they have 72 • DEEP FOUNDATIONS • MAR/APR 2012 Suction Submersible and Removable Pump Sealed Pile Top Outside Water Pressure Inside Water Pressure Length Pile Weight Skin Friction Diameter SUCTION PILE Figure 1. Schematic diagram of a typical suction pile Figure 2. Free body diagram of a suction pile, including the forces acting on the suction pile, during its installation process extended the applications into shallower waters in recent years such as the founda- tions for breakwaters and mooring systems. Mechanism A suction pump attached at the top of the pile provides the necessary water pressure reduction inside the pile and therefore accomplishes the entire pile driving operation (see Figure 2). Depending on the design, the suction piles can be subjected to lateral, vertical and/or inclined loads. The vertical load may be in tension or in compression. Examples of suction piles under tension load include foundations for tension leg platforms, floating structures, fleet mooring, etc. Installation. During installation, pumping water out reduces the water pressure inside the pile and consequently creates a driving force that pushes the pile into the seafloor. The suction pump capacity must be greater than the amount of seepage water flow from outside the pile to inside. If the pushing force is large enough to overcome the soil resistance, the pile starts to penetrate into the seafloor. This pile penetration stops at the point of AUTHOR: Sangchul Bang, Ph.D., P.E., Professor Civil and Environmental Engineering South Dakota School of Mines and Technology, S.D. equilibrium when the pushing force equals to the soil resistance. When the suction pressure increases, the pushing force increases and the pile begins to penetrate into the soil until the next equilibrium is reached. This procedure repeats until the pile penetrates its full design depth. The designer must determine the correct pile length to diameter ratio to insure full and complete pile penetration. Engineers then must carefully control the suction pressure so that complete installation of a suction pile is possible. The soil resistance corresponding to the pile penetration dictates the lower limit of the necessary suction pressure. That is, if the applied suction pressure is smaller than this value, the vertical pushing load is smaller than the soil resistance and therefore the pile cannot penetrate any further. However, if the suction pressure is too high, the soil inside the pile becomes unstable, and consequently soil fills the pile inside and the pile installation becomes incomplete. This defines the upper limit of the suction pressure. Loading Capacity. To determine the loading capacity of a suction pile, one must consider three separate conditions, e.g., when the suction pile is under horizontal loading, vertical loading and inclined loading. The determination of the hori- zontal loading capacity of a suction pile typically requires three-dimensional consideration of a soil failure wedge, which Tip Bearing Seepage
