30-09-2013, 02:40 PM
A REPORT ON SUCTION PILES
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ABSTRACT
Suction Piles are used underwater to secure floating platforms. Tubular piles are driven into the seabed and then a pump sucks water out at the top of the tubular, pulling the pile further down.
The proportions of the pile (height and diameter) are dependent upon the soil type. Sand is difficult to penetrate but provides good holding capacity, so the height may be as short as half the diameter. Clay and mud are easy to penetrate but provide poor holding capacity, so the height may be as much as eight times the diameter. The open nature of gravel means that water will flow through the ground during installation causing “piping” flow (where water boils up through weaker paths of the soil). Therefore suction piles cannot be used in gravel seabed.
The operation requirements depend on whether the use of the suction pile, as foundation of mooring and vertically or horizontally. This also determines the anchor line attachment point, the pad-eye. For vertical load this pad-eye is fixed on the top plate, for loads under an angle the pad-eye is attached at 1/3 from the lower end of the pile. Typical (tensile) forces on the top plate vary from 100 to 250 ton.
Recovery is done by lowering the pump onto the caisson it by means of docking cones and attaching onto the caisson with hydraulic latch pins. The installation process described above is then simply reversed, pushing the caisson out. The friction for sand soils is however higher due to the reversed water flow, but one can exert additional tensile force to recover the caisson. The caisson is hoisted back on to the installation vessel, ready for re-employment.
INTRODUCTION
A Deep Foundation is a type of foundation distinguished from shallow foundation by the depth they are embedded into the ground. There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a poor soil at shallow depth, or site constraints. There are different terms used to describe different types of deep foundations including the pile, the pier, drilled shafts, and cssions. Piles are generally driven into the ground in situ; other deep foundations are typically put in place using excavation and drilling. The naming conventions may vary between engineering disciplines and firms. Deep foundations can be made out of timber, steel and pre stressed concrete.
Foundations relying on driven piles often have groups of piles connected by a pile cap (a large concrete block into which the heads of the piles are embedded) to distribute loads which are larger than one pile can bear. Pile caps and isolated piles are typically connected with grade beams to tie the foundation elements together; lighter structural elements bear on the grade beams, while heavier elements bear directly on the pile cap
HISTORY
Suction anchors were introduced to the offshore industry in 1980 in the Gorm field.. The experience gained with installation of these anchors for a loading buoy installed by a heavy lift vessel did not merit repeat usage. The time spent in the field and the total cost was in excess of comparative systems like piles or marine drag and embedment anchors. Excessive jetting and removal of sand inside the anchors were necessary. Because of this somewhat negative experience, it took more than 10 years before a reappearance of these anchors was considered, now largely because of significant experience gained from skirted gravity platforms. Also the large suction anchor installed and retrieved in 1985 at the Gullfaks C site was a key development for accepting this technology. However, purpose of the tests anchor was not for an actual mooring application and again the cost was much in excess of what a mooring application could accept.
Suction Caisson, Why And Where?
Suction anchors have been the preferred solution for a number of mooring applications since 1995. All of these have been permanent moorings for oil industry production units or production related facilities like storage or offloading buoys. In addition to cost the primary reason for selecting suction anchors have been reliability. Reliability is impacted in many ways, but most importantly design capacity predictions and positioning accuracy during installation, I .e. the anchor is installed within very tight tolerances allowing a better prediction of it's capacity.
Suction piles as used today are primarily for mooring application. Other type of applications will be briefly discussed in next chapter.
Soil conditions
Over the years suction piles have been installed in a variety of field conditions spanning world-wide applications in soft and strong soils, shallow and deep water and for many different purposes. When examining the range of soil conditions encountered at these sites it is apparent that suction anchors may be installed in many sites with typical offshore soil conditions. The suction anchor concept is feasible even in mixed conditions, but the design and installation operation requires careful planning and preparation and most importantly it is necessary to understand the mechanisms of how penetration resistance in sand is influenced by suction. This is really the only prerequisite; to understand the geotechnical aspects of suction anchor penetration. The Gorm field represented at the time a real challenge with respect to achieving target penetration depth and layered soils with dense sands and stiff clays are probably the most difficult conditions for penetration, but it has been done: The Yme, Harding and the Curlew fields all had mixed soil profiles and large penetration resistances were experienced.
SOIL BEHAVIOUR
Soil Behaviour set up and consolidation, particularly inside the anchor need some consideration since it may affect the permanent capacity. These effects are more important for low aspect ratios since the behaviour of the internal soil plug plays a more active role here than for the high aspect ratio piles. One mechanism to consider is the re-consolidation of soil close to the wall which may take place at a lower effective stress then the natural overburden since part of the vertical stresses may be carried as soil friction. If the pile is subjected to permanent pull out loads shortly after installation, consolidation of soil close to the wall may be even more affected. Friction is regained at a lower effective stress level causing a reduced friction both inside and outside
In general, all type of vertically loaded anchors, the long term undrained strength of the soil at and below the anchor is suffering a permanent reduction in effective stresses. The upward load direction results in an unloading of the soil at and below the anchor itself and the long term effect on drained and undrained capacity needs to be investigated.