First-order, second-moment (FOSM) approximations provide an efficient way to assess submarine slope stability across large areas for which digital bathymetric data are available. This is demonstrated using 20m bin 3D seismic seafloor data for a deepwater area with typical geotechnical soil properties. Results are obtained in terms of a factor of safety mean and standard deviation for an infinite slope with pseudo-static seismic loading. From this the probability of sliding is calculated for each bin without the computational burden of Monte Carlo or other iterative methods. Because these types of probabilistic model incorporate parameter uncertainty into their input and output, they can be used to support decisions about the value of additional data collection, or justify more sophisticated analyses that may help to reduce output uncertainties. In addition to providing detailed maps of the probability of sliding, the analysis produces global statistics that allow insight into the broader response of the system to seismic shaking.
Evaluation of deepwater geohazards commonly entails assessment of slope stability either to understand the geologic history of a project area, or to anticipate the risk associated with future events, such as major earthquakes. This can be done qualitatively based on the presence or absence of past landslide deposits; semi-quantitatively using simple measures such as slope angle or gradient; or quantitatively using limit equilibrium slope stability analysis (e.g. Mackenzie et al., 2010). Limit equilibrium methods are widely known and attractive because they integrate the essential physics of sliding and allow evaluation of rare or unprecedented conditions (for example the effects of a large future earthquake). However, they also require specification of geotechnical variables, such as sediment shear strength, thickness and unit weight, in addition to some description of slope geometry (minimally the slope angle).
Driven piles are often used for supporting jackets and providing anchors for mooring patterns. In noncarbonate conditions, driveability assessments are routine and generally reliable. However, piles are now more frequently being installed in areas where carbonate seabed conditions prevail and the database of offshore installation experience in calcareous soil and rock is still relatively small. Offshore pile installation time is costly, and reliable prediction of driveability is essential towards selecting the correct piling hammer and predicting driving conditions including potential refusal. This paper considers a recent project on the North West Shelf of Australia where significant oil and gas field developments are planned. It highlights the challenges of installing driven piles in weakly cemented calcareous sands and calcarenite, and demonstrates how back-analysis of driving data can play an important role in predicting the field outcome, especially that related to risk of premature refusal and the requirement for contingency measures such as relief drilling equipment.
An increasing number of offshore oil and gas fields are being exploited in areas of the world where calcareous seabed sediments (commonly referred to as ‘carbonate soils’) prevail, and the pace of development has been nowhere greater than for the North West Shelf and Bass Strait areas of Australia. Many of the planned developments for these areas will utilise driven piles for anchoring mooring systems and subsea structures, or for supporting platforms, but past experiences have shown that calcareous soils can pose problems for both the pile designers and the installer (e.g. King and Lodge, 1988). At one end of the scale, there are the problems associated with the extremely low shaft resistance mobilised on driven piles when compared to silica sand, with subsequent reduction in capacity such as that documented for the Rankin A Platform (Jewell and Khorshid, 1988).
Traditionally offshore geotechnical drilling has been carried out using drilling systems that are mounted on either floating or fixed platforms on or above the water surface. However, as site investigations have moved into deeper water there is a move toward drilling systems located on the seafloor to provide greater efficiency and improved accuracy. This paper describes a new seafloor drilling system, which is capable of working in water depths of up to 4000m and is rated to drill to a depth of ~150m below the seafloor. The remotely operated system integrates established drilling technology with proven telemetry and controls. The flexible nature of the wire-line drilling technology enables efficient use of standard sampling systems, as well as downhole testing systems (including cone penetration tests (CPT), downhole sensors and borehole geophysical tools). The paper focuses on the new developments and shows results from recent sea trials and projects.
As energy and mineral exploration move into deeper water, there is a growing need for highquality drilling, sampling and in situ testing of near-surface seafloor sediments. Hence, a detailed evaluation of the seafloor sediments has become increasingly important. Traditionally, offshore geotechnical drilling has been carried out using drilling systems mounted on either floating or fixed platforms at or above the water surface (i.e. surface drilling). However, as site investigations have advanced into deeper water, there has been a move toward drilling systems located on the seafloor to provide greater efficiency and improved accuracy (i.e. seafloor drilling). Osborne et al. (2011) provided a summary of existing remotely operated seafloor drilling systems and summarised the advantages and limitations of various systems. Yetginer and Tjelta (2011) evaluated the effectiveness of seafloor drilling technology in terms of productivity, safety and other commercial considerations when compared to conventional techniques.