Background and objectives: Motion Decoupled Hydraulic Delivery System sea trials

Background and summary

The ability to measure pressure and permeability in mudstone is critical to understand the impact of fluid pressure/flow on the occurrence, stability, and dissociation of gas hydrate (Hyndman and Davis, 1992; Kvenvolden, 1993; Dickens et al.,1997; Ruppel, 1997; Dillon et al., 2000; Liu and Flemings, 2009), the geometry, structure, fluxes, and earthquake mechanics of accretionary complexes (Davis et al.,1983; Dahlen et al., 1984; Bekins and Dreiss, 1992; Saffer and Bekins, 2002; Screaton et al., 2002), and the cause of submarine landslides (Terzaghi, 1950; Bombolakis, 1981, Haneberg, 1995; Dugan and Flemings, 2000, 2002). Models of these systems await validation because of the sparsity of direct pressure measurements. IODP has started to implement short-term measurements with penetrometers (Flemings et al., 2008) and downhole formation testers (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010) and long-term observatories (Fisher et al., 2005).

Why the MDHDS?

Previously, the T2P and the Sediment Temperature-Pressure Tool (SETP; formerly the Davis-Villinger Temperature Pressure Probe), the two penetrometers used by IODP, were deployed by wireline on the colleted delivery system (CDS). The CDS is analogous to an old-style pointer, in which a series of cylinders slide past each other to increase or decrease the system’s length. In this configuration, the penetrometer is pushed in by the drill string and then the drill string is raised to decouple the drill string from the formation. However, the penetrometer remains connected to the drill string through the CDS, which should expand and contract during ship heave. Analysis of previous deployments showed that when the drill string was raised, the penetrometer was pulled out of the formation >80% of the time. When this occurs, the measured pressure drops rapidly and the measurement is compromised (Fig. F13).

The MDHDS (Fig. F14) was designed to overcome this problem. It is an engineering development intended to serve as the foundation for future penetrometer and other downhole tool formation measurements. The MDHDS allows in situ tools to be hydraulically driven into the formation and then decoupled from the heave of the drill string, which negatively impacts these measurements.

The MDHDS can be deployed either on the coring wireline or on an armored conductor cable such as the logging line. The MDHDS allows the bottom-hole assembly (BHA) to remain 2 m off the bottom of the borehole (Fig. F14). This clearance reduces the possibility of jamming borehole cuttings or other detritus inside the BHA, which could result in coupling between the tool and the BHA. The penetrometer is extracted from the formation by lowering the wireline through the upper latch union, allowing the “running shoe” (designed to retrieve downhole tools by wireline) overshot to latch onto the running shoe fishing neck attached to the upper piston rod. In the case of a hotline deployment, the soft tether will recoil inside the upper piston rod, allowing the running shoe overshot to latch onto the running shoe fishing neck.

The primary objective of the MDHDS sea trials was to test the MDHDS. The goal was to wash to a depth of 100 m coring depth below seafloor, method A, (CSF-A), test the MDHDS in situ for at least 30 min, turn on the pumps to clean the hole, take an advanced piston corer (APC) core, test the tool in situ again, and take three more APC cores. The site chosen for the tests was a reoccupation of ODP Site 1073, New Jersey Margin. This sea trial was the culmination of multiple land-based tests, and a successful deployment means that this tool can be reliably deployed during future expeditions.