Differences In Shear Strength Between Jumbo Piston Core and Conventional Rotary Core Samples
- Patrick C. Wong (ExxonMobil Development Co.) | Brian Taylor (Jacques Whitford) | Jean Audibert (Quest Geo-Technics)
- Document ID
- Offshore Technology Conference
- Offshore Technology Conference, 5-8 May, Houston, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2008. Offshore Technology Conference
- 1.6.10 Coring, Fishing, 4.1.5 Processing Equipment, 1.6 Drilling Operations, 4.1.2 Separation and Treating
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In order to reduce costs associated with offshore site investigations, particularly in deep waters, large-diameter piston corers (also called jumbo piston corers) have been used as a cost-efficient tool to supplement conventional drilling and sampling and to obtain continuous seafloor samples as long as 30m. The STACORÒ is believed to be the jumbo piston corer that has the least soil sample disturbance, due to its unique design. There have been indications that samples obtained using the STACORÒ piston coring system yield undrained shear strengths higher than those obtained using conventional rotary borings. For example, comparison of data from five sites offshore West Africa shows that undrained shear strengths measured with the minivane and unconsolidated-undrained (UU) triaxial test tend to be 19% higher in STACORÒ samples than in rotary boring samples. This paper explores the reasons for those differences and attempts to quantify the differences in undrained shear strength of samples obtained with the two methods. The higher strengths associated with the STACORÒ samples appear to arise from two causes. Firstly, comparison of water content profiles appears to show that the STACORÒ "blows by?? (by-passes) the upper 1.5 to 2m of the sediment column, most likely due to penetration of the STACORÒ base into the very soft seafloor soils encountered in deep waters. This depth shift results in an "apparent increase?? of the STACORÒ undrained shear strength profile of 2-3 kPa. Secondly, comparison of minivane and UU triaxial tests on STACORÒ samples and rotary boring samples shows that, on average, STACORÒ samples are less disturbed and, therefore, yield higher shear strengths. Design implications are discussed, suggestions to measure penetration of the STACORÒ base are offered, and the need for alternative characterization of the first 2 to 3m below the mudline is described.
For planned production facilities in deep water where the soils consist of soft silts and clays, the use of large diameter piston coring systems offers advantages over conventional rotary borings. These "jumbo corers?? can be rapidly deployed and recovered, obtain a large volume of continuous sample material, and the sample material tends to be of high quality. They are, however, limited in their depth of penetration. For example, the barrel length on Fugro's STACORÒ system limits recoveries to 24m, as further described below.
The Stationary Piston Corer STACOR®
Development History. Large-diameter piston corers capable of sampling up to 30m of soft sediments have been derived from the well-known Kullenberg corer widely used for oceanography purposes, in theoretically unlimited water depth. However, the degree of soil remolding during the piston sampling process is unacceptable for geotechnical engineering purposes (Borel et al., 2002).
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