The recent availability of un-tethered AUV (Autonomous Un-derwater Vehicle) survey systems now allows deepwater sur-veys to be carried out in ways and with efficiencies unattain-able with pre-AUV technology. Short, closely spaced survey lines, circular survey lines, and other hybrid and complex sur-vey patterns are now being routinely used to provide detailed characterization of deepwater development sites. This paper describes the use and advantages of an AUV survey for effi-ciently and effectively characterizing details of shallow geol-ogy and soil stratification at proposed deepwater anchor-pile locations. Specifically, results of the first known deepwater AUV micro 3-D seismic survey are presented as a case his-tory. This micro 3-D survey, in more than 5000 ft of water, proved effective in detailing site conditions and, in particular, mapping precise locations of small faults, thus allowing them to be avoided when placing suction-caisson anchors. Such "designer" surveys will find increasing application for detailed characterization of geologically complex deepwater sites be-ing considered for installation of production facilities.
A spread mooring system with anchor piles is among the pioneering approaches for holding deepwater pro-duction structures such as spars on location. Moorings for spars typically consist of three or four groups (or clusters) of from one to four anchors each, arrayed around the spar at a radius about equal to the water depth. A conceptual diagram of a spar with a spread mooring system, and an example of a suction-installed anchor pile are shown in Figure 1. Most in-dividual anchor piles for spars installed to date are either driven piles (typically about 6 to 7 ft in diameter, and 200 to 250-ft long) or suction caissons (typically about 18 to 20 ft in diameter, embedded 80 to 120 ft below mudline). We will limit the discussion in this paper to the use of suction caissons, although the terms "piles" and "caissons" may be used inter-changeably.
Installation of deepwater suction-caisson anchors is initi-ated by self-weight penetration into the seafloor, followed by suction-aided penetration to design depth. Suction is devel-oped by sealing the top of the pile (generally by closing butter-fly valves), except for a port through which the water is pumped out. The resulting difference in hydrostatic head pro-vides the force to penetrate the caisson into the seafloor to design depth.
Resistance to anchor pull-out is developed through fric-tion between the soil and the pile walls (friction piles), or by a combination of friction along the outside wall and reverse end bearing (also referred to as passive suction) developed at the tip of the pile. Details of stratigraphy and geologic features at each anchor-pile site control pile design, installation, and per-formance. For example, thin sands, small faults, or other po-tential fluid-migration pathways near the tip of anchor piles, which depend partly on reverse end bearing (that is, suction) to develop capacity, can reduce their pull-out capacity because of the high permeability of such features.