Suction Caisson Foundations
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SUCTION CAISSON ANCHORS - A BETTER OPTION FOR DEEP WATER APPLICATIONS B. Sukumaran, Member, SWE Rowan University ABSTRACT As offshore exploration and development of oil fields reach water depths of 1,000 to 3,000 m, novel methods of anchoring production platforms become attractive due to cost savings associated with installation. Surface production systems that are viable in these water depths include Tension Leg Platforms (TLP), spar platforms, and laterally moored ship-shaped and semi- submersible vessels. Possible anchor systems for TLP and spar platforms include the traditional driven piles, drag anchors and suction caissons. Suction caissons become better alternatives to driven piles in deepwater because of technical challenges and costs associated with the installation equipment. In addition, suction caissons also provide a greater resistance to lateral loads than driven piles because of the larger diameters typically used. Initial penetration of the suction caisson into the seabed occurs due to the self weight; subsequent penetration is by the “suction” created by pumping water out from the inside of the caisson. This paper presents a brief overview of what the current state of knowledge is regarding the design of suction caissons. In addition, the paper also documents some finite element analyses results that were conducted to determine the capacity of suction caisson anchors founded in soft clays, typical of the Gulf of Mexico. INTRODUCTION Exploration and production of oil fields in deep waters has become a necessity as we exhaust oil resources near shore and on land. In the past few years, gas and petroleum reserves under deep water (1,000 to 3,000 m) on the continental slopes of the Gulf of Mexico have been demonstrated to be of enormous economic and strategic significance to the United States and other oil producing countries. In such deep waters, floating production systems such as Tension Leg Platforms (TLP), spar platforms, and laterally moored ship-shaped and semi-submersible vessels are economically viable. TLPs consists of four vertical cylinders or columns connected by the platform above water and pontoons underneath it. These buoyant structures are arranged in a rectangle. Vertical mooring lines called tendons or tethers attach the rig to the seafloor. On the seafloor, there are two basic components, the foundation and the TLP template. There are generally four massive foundations installed on the seabed and each has flexible joints on to which the tendons are attached. The TLP template is located directly above the predrilled oil wells and contains slots for each well and any number of pipelines. Tendons or tethers, the anchoring system for tension leg platforms, are essentially vertical mooring systems that connect the platform to the seafloor. Because of the excess buoyancy of the platform, each tendon is pretensioned. For Gulf of Mexico TLPs, driven piles have been the preferred method of anchoring tendons to the seafloor. Suction caissons or piles are gaining acceptance. Spars are buoyant single-column hull structure similar to a buoy. The hull remains submerged and tethered to the seafloor. In water depths less than approximately 1200 m (4000 ft), TLP and spar systems are competitive economically. For greater water depths, the spar platform offers some performance and economic advantages. Possible foundation systems for spars include the traditional driven piles, drag anchors and suction caissons or piles. Suction caissons (also called "buckets", "skirted foundations", and "suction anchors") were first introduced by Senpere and Auvergne (1982) as mooring anchors for a storage tanker in the Gorm field, which is located offshore Denmark. The offshore industry has recently developed a renewed interest in these foundations because of their simplicity and reliability. Suction caissons are a better alternative to driven piles in deepwater because of technical challenges and costs associated with the installation equipment for driven piles. Heavy lift vessels can be avoided, simplifying and shortening the installation procedure. Another advantage is that there is more control over the installation process. Therefore, the location of the anchors on the seabed is fixed and known with accuracy. Suction caissons also provide a greater resistance to vertical and lateral loads than driven piles and drag anchors because of the larger diameters typically used (Colliat et al., 1995). Initial penetration of the suction caisson into the seabed occurs due to the self-weight. Field observations have shown that the initial penetration of the pile into ocean sediments under self-weight is substantial enough to develop an adequate seal to facilitate suction installation (Cuckson, 1981 and Senpere and Auvergne, 1982). Subsequent penetration is by the “suction” created by pumping water out from the inside of the caisson (Figure 1). A submersible pump attached to the top of the sealed caisson applies suction pressure. By evacuating water from the inside, a pressure differential is created. The limiting value of this pressure differential, such that (a) (b) (c) Water pumped out creating suction pressure Seabed Suction penetration Figure 1 Installation sequence of suction caissons (a) Touchdown phase (b) Penetration due to self weight/ballast (c) Water pumped out to create suction penetration cavitation does not occur, is the sum of the atmospheric pressure and hydrostatic pressure outside the caisson. In very deep waters, large penetration or suction pressures can be created, which is only limited by the capacity of the pump. Once the required depths are reached, the pumps can be disconnected and retrieved. The feasibility of suction caissons have been demonstrated in the North Sea foundations for the Snorre and Heidrun TLPs (Christophersen et al, 1992), the Tordis wellhead protection silo (Guttormsen and Wikdal, 1994), Europipe 16/11-E and Sleipner (Tjelta, 1994). At present, the uses of suction caissons are being extended to the Gulf of Mexico. DESIGN CONSIDERATIONS The resistance to pullout of the suction caisson is derived from the following components (Albert et al., 1988): 1) Submerged weight of the caisson and ballast if applied. 2) Suction pressure that is created across the caisson under tensile loading. 3) Weight of the soil plug inside the caisson. 4) Skin friction. 5) Soil tensile strength at the caisson base. The lateral capacity is provided due to the active and passive pressures mobilized due to the horizontal translation of the caisson. In the deeper sections, lateral resistance will be afforded by soil flow around the caisson. The holding capacity obtained therefore depends on the suction pressure applied, soil conditions at the site, load attachment point and the anchor geometry used. The latter two points will be addressed here, namely the load attachment point and the anchor geometry suitable for the various soil types. Caisson geometry In sands, the pressure differential that is applied causes a hydraulic gradient in and around the caisson. Water flows into the caisson due to the suction pressure that exists inside the caisson. If the hydraulic gradient is increased sufficiently, a limiting condition will be reached when the effective stress of the soil inside and below the caisson approaches zero. The limiting condition is commonly referred to as the critical gradient. When the critical gradient is achieved, piping of soil inside and below the caisson occurs. A soil plug is formed inside the caisson due to this effect. A larger diameter caisson configuration has been found to minimize the formation of the soil plug as the gradients are more concentrated along the caisson wall (Sparrevik, 1995). In both stiff clays and sands, problems arise during installation due to the resistance offered by these stiff soils. To obtain sufficient suction forces to overcome this resistance, larger diameter caissons are used. Shorter caissons with larger diameters are therefore preferred for stiff clays and sands that usually provide sufficient holding capacity. Typically, caissons constructed in these materials have penetration to diameter ratios less than 2. In soft clay deposits, the shearing resistance of the soil usually improves with depth below seabed. Larger penetration depths will be required to mobilize sufficient holding capacity due to negligible side friction along the wall of the caisson. The suction forces that are required to drive the caisson in soft clays are not very high. Therefore, in normally consolidated clay deposits, large penetration depth to diameter caissons are typically used. Load attachment point The load attachment point is a very important factor influencing the holding capacity of the suction caisson. Finite element studies conducted by the author will be used to show this. The caisson analyzed has dimensions of 6.1 m diameter and 12.2 m depth. The soil surrounding the caisson is a normally consolidated clay. The shear strengths are assumed to be zero at the seabed and increasing linearly with depth as given below: DSS su = 1.41z (kPa) DSS where z is the depth below seabed in meters and su is the undrained static direct simple shear strength. The submerged unit weight of the soil is 6.3 kN/m3. The finite element analyses were conducted using ABAQUS (HKS 1996). A von Mises shear strength idealization was used to model the clay. The von Mises model implies a purely cohesive (pressure independent) soil strength definition. The elastic moduli used for the soil are 2394 kPa at the seabed increasing linearly to 25956 kPa at 36.6 m below the seabed. The yield strength was also assumed to increase linearly from 4.79 kPa at seabed to 89.5 kPa at 36.6 m below the seabed. The caisson is modeled as a linear elastic material. The inclination of the load was assumed to be 32° with the horizontal, measured counterclockwise. Several points of attachment for the mooring line were considered to study the effect of the attachment point on the load capacity. The optimal load attachment point is that which produces maximum capacity.