The use of suction caissons (suction piles) in marine environments has been increasing in the last decade. A suction caisson is a steel pipe with an open bottom and a closed top that is inserted into the ground by pumping water out of it. Pumping creates a differential pressure across the caisson's top that pushes it into place, thus eliminating the need for pile driving. There are a number of uncertainties in the design of suction caissons. First, the state of stress and soil conditions adjacent to a suction caisson differs from those around typical driven piles or drilled shafts. Second, the axial load capacity of suction caissons depends on the rate of loading, hydraulic conductivity, drainage length, as well as the shearing strength properties of the foundation material. Finally, during pullout, volume change characteristics of the surrounding soils may change the theoretical suction pressures. A review of the existing knowledge relating to the design and construction of suction caissons is presented in this paper along with the results of a laboratory study on model caissons in sand and clay. Test results indicate that the use of suction pressure for installation of caissons is a viable alternative to conventional methods. Suction was also shown to resist some axial tensile loads.Key words: suction, pile, bucket, foundation, anchor, capacity.

A cost-effective solution to mooring problems in the deep and ultra-deep waters of the Gulf of Mexico (GoM) is the suction anchor system. Suction caisson foundations, also known as skirt piles and bucket foundations, have been used with success in the North Sea for major structures such as the Gullfaks C gravity platform (1989), the Europipe fixed steel jacket (1994), and the Snorre tension leg platform (1991). In the normally consolidated clays of the deep GoM, the new foundation concept is implemented with mooring applications where the holding capacity derives from increased resistance with depth to pullout or lateral bearing capacity failure mechanisms. Knowledge of the geotechnical engineering properties of the deep GoM clays is key to accurate and efficient foundation design. An effective way to determining these properties is by way of correlation with actual caisson behavior during installation. The results of a laboratory modeling program of suction caisson installations are used in conjunction with observations from about half a dozen actual deployments to back-figure the geotechnical properties of the foundation materials. The model caissons were fabricated with length-to-diameter ratios ranging between 2 and 12 to investigate the effect of increasing caisson aspect ratio on the feasibility of using suction as the method of installation. In the field, the installed caissons had a diameter of 12 ft (3.7 m) and an aspect ratio of 5. The caissons were installed in water depths ranging between 4000 to 10,000 ft (1200 to 3000 m). In general, self-weight penetration accounted for approximately one half the installation depth and the suction penetration data were utilized to sketch profiles of undrained shear strengths with depth at the installation locations. Introduction One of the objectives of this project has been to investigate the installability, using suction, of caissons with increased length-to-diameter (aspect) ratios over those used in the past. Suction caissons with aspect ratios ranging between 0.35 and 2.0 had been used successfully in the North Sea, where the foundation material was mainly stiff clays and dense sands. In the Gulf of Mexico, however, in water depths of 4000 to 10,000 ft (1200 to 3000 m), the soil is composed primarily of soft, normally consolidated clays. To make use of the greater shear strength characteristics of the soil at increased depths, caissons with greater aspect ratios were introduced. To examine the potential of the research program, a 4-in. (100-mm)-diameter model caisson with an aspect ratio of 8 was fabricated and successfully installed using suction in a laboratory-prepared soil sample. From the results of this test, the components of the resistance to penetration were identified and incorporated into a limit equilibrium model. This model was utilized in selecting the dimensions of model caissons to be used in the testing program (El-Gharbawy, 1998a). The successful installation and testing of the model caissons with greater aspect ratios than those typically used for North Sea applications has paved the way for the inception of the Suction Anchor System for the mooring of mobile drilling units in the GoM (El-Gharbawy, 1998b). In this paper, the driving and resistance forces acting on the caisson during installation are identified. Plots of caisson penetration versus time for the different caissons are presented to substantiate the limit equilibrium model. Soil shearing resistance profiles are estimated on the basis of installation observations and thence utilized in design.

In the last decade, use of suction caissons (suction piles) has increased in offshore arena. Suction caissons have the appearance of inverted buckets with sealed tops and are installed by pumping water out of them. Pumping creates a differential pressure across the top pushing the caisson into place, and thus eliminating the need for driving. Suction can be used also to resist axial tensile loads. Suction caissons realize economical advantages over traditional driven piles due to the speed of installation, elimination of the pile driving process, and reduction in material costs. There are a number of uncertainties in the design of suction caissons. The state of stress and soil conditions adjacent to a suction caisson may be different from those surrounding driven or bored piles; thus experience-based design methods may not work well with suction caissons. The tensile load capacity of suction caissons depends primarily on the hydraulic conductivity and the shearing strength properties of the foundation material, drainage length, and rate of loading. The relationship between the various parameters affecting the tensile capacity is not clearly understood. Furthermore, during pullout, volume change characteristics of the surrounding soils may change the theoretical suction pressures. A review of the existing knowledge relating to the design and construction of suction caissons is presented in this paper. Experimental results from a number of laboratory studies in sand and clay are also presented along with case histories. Introduction In the last the decade, the use of suction pressure to install the foundations of offshore structures has been transformed from a novelty concept to being a viable alternative to pile driving. This effort has been motivated by the depletion of oil reserves in shallow waters and the need to install offshore platforms at greater depths. The cost of traditional fixed jacket platforms (FJP) increases exponentially with depth due to the increase in material and labor cost. Bullwinkle, the largest FJP, has been installed in 400 m (1350 ft.) of water. Floating structures, such as the tension leg platforms (TLP) are becoming, in some cases, the only economically viable design alternative. For example, the estimated cost of using an FJP in 872 m (2860 ft) of water at the site of the Auger TLP would have exceeded the estimated cost of the oil reserves. Auger, which was ASCE 1995 Civil Engineering Project of the Year was installed at a cost of $1.2 billion (1). Foundations of TLPs are typically installed at great depths and are subjected to tensile and cyclic loading. In this environment, suction caissons realize economical advantages over traditional driven piles due to the speed of installation, elimination of the pile driving process, and reduction in material costs (2). Suction pressures may also be used for developing the axial tensile load capacity of suction caissons. Suction Caissons Suction caissons have been also known as suction piles, suction anchors, bucket foundations, and skirt foundations. Suction caissons can be distinguished from traditional piling in a number of ways. Geometrically, suction caissons have been larger in diameter and shorter in length than traditional piling.

Capitalizing on the UAE's wide experience in the field of land reclamation and artificial island technology, the Abu Dhabi oil and gas industry, represented by ADNOC group of companies, is currently deploying an array of islands across Abu Dhabi's Exclusive Economic Zone in the Arabian Gulf for applications such as new field development and the upgrade or expansion of storage and offloading facilities. With favorable water depths and environmental conditions, land reclamation is often a more economical option for the accommodation of offshore facilities than the construction of fixed steel jacket platforms. ADMA-OPCO is currently engaged in applying the technology to projects such as the Satah Al-Raaz Boot (SARB) field development, a 105,000-bpd development comprising 86 wells on two artificial islands. Simultaneously, ZADCO is making progress on expanding drilling from the Upper Zakum field by constructing four artificial islands (UZAI) to increase field production to 750,000 bpd by 2015. While cost and schedule optimization will be realized with the selection of the artificial island option for these mega projects, designers and contractors are facing the challenge of securing or fabricating building materials in huge amounts for the construction of both the land masses and shore protection structures. Creative solutions to procuring these materials are tabled and investigated. Innovative engineering designs are tried and tested both numerically and using physical model tests. The presence of a soft soil layer within the foundation strata of one of the Upper Zakum islands required special treatment to satisfy island performance criteria. Schedule constraints of construction and fulfilling ADNOC's strategic production objectives continue to be the driving forces behind the resolution of all challenges.