Evaluation of disturbance induced on soft offshore sediments by two types of gravity piston coring techniques

Abstract

Sample disturbance is still a key issue in offshore investigations, especially when logistic and financial limitations do not allow the use of drilling equipment. This paper focuses on the comparison between the disturbance induced by a conventional free-fall piston corer (FF) and a modified piston corer (AD) equipped with a velocity control (Angel Descent method). Twin core samples were retrieved in two successions of pelitic sediments with a prevailing non-clayey fraction and a non-negligible sandy fraction. Comparison was based on different acquisition, physical and mechanical parameters ranging from accelerometer data to magnetic susceptibility logs and geotechnical parameters from laboratory investigations, including oedometer compression tests and cyclic simple shear tests. Accelerometer data highlighted the sharp reduction in velocity obtained for AD samples. Magnetic susceptibility logs, characterized by a pattern of peaks induced by several volcaniclastic levels present in the succession, indicated that the AD method significantly reduces core shortening. Among geotechnical investigations, cyclic shear tests provided small-strain shear moduli always higher in AD samples, whilst the response of oedometer compression tests was equivocal. In fact, methods for assessing sample disturbance have demonstrated to bear limited effectiveness when applied to soils with relatively low clay content and significant overconsolidation as it is the case of the studied sediments. 1. Introduction Many geotechnical, geological and geophysical analyses for both research and design purposes, require the knowledge of physical and mechanical properties, e.g. undrained shear strength, strength parameters in effective stresses, stiffness and damping parameters. These parameters are affected by soil disturbance induced by the sampling procedure, which is particularly significant in most offshore projects. Lunne et al. (2006) state that block sampling is by far the least invasive method, and that tube sampling unavoidably induces disturbance that alters the sediment structure (including cementation) with negative fallout on a number of geotechnical test results. A further consequence of disturbance induced by gravity and piston coring is core shortening/elongation, which alters the actual depth of geological horizons used for calculating sedimentation rates or reconstructing sub-bottom stratigraphy. Nevertheless, very often deep water investigations are conducted through vessel-operated gravity/piston corers. Sample depths over 0.7 meter below the sea floor (i.e., that reached by spade box corers) require borehole block sampling and hence drilling devices operated from stable structures (pontoons, barge rigs) or drilling vessels. The former can be used when the water depth is shallow, whilst the latter are too expensive for many research projects. In this paper we present and discuss the results of a coring campaign conducted with a standard piston gravity corer and a gravity piston-corer specifically designed to minimize sediment ...

A state-of-the-art review and prospect of gas hydrate reservoir drilling techniques

“…The recoilless piston corer Advanced Piston Corer (APC) used in ODP, the stationary piston corer STACOR developed by France, the Multiple Autoclave Corer (MAC) and Dynamic Autoclave Piston Corer (DAPC) used in R.V. SONNE cruise in Germany, and the pressure tight piston corer (PTPC) developed by Zhejiang University in China all belong to the above categories (Lunne and Long, 2006;Tommasi et al, 2019). The maximum depth that can be reached by seabed sampling is between a few and tens of meters, but the actual sampling depth is influenced by the geological environment in addition to sampler performance.…”

Numune Örselenmesinin Geleneksel Ödometre Deneyleri ile Belirlenen Konsolidasyon Parametrelerine Olan Etkilerinin İncelenmesi

“…Lim et al (2019) assessed the geotechnical parameters of soils during design, which were obtained by different types of samplers depending on the disturbance in the soil caused by the samplers [15]. Tommasi et al (2019) evaluated the disturbance of open sea sediments using two types of gravity piston coring techniques [16]. Lukas et al (2019) explored the effects of soil disturbance caused by samplers used in low-plasticity soils on undrained shear strength [17].…”