Pressure barrels for sampling and preservation of submarine
sediments under in situ pressure with the robotic sea-floor drill rig MeBo
(
Pressure coring is currently the only method that enables precise off-site analysis of gas and gas hydrate volumes contained in marine sediments. Because the manufacture of pressure-coring tools and their operational application are technically challenging, pressure vessels have only been in use for the last few decades. A review of pressure-coring systems used for offshore research from various platforms in the past and today is presented elsewhere (Abid et al., 2015 and references cited therein). These include the Dynamic Autoclave Piston Corer (DAPC) which is frequently in use for pressure coring of shallow (down to 2.65 m below the sea floor, hereafter m b.s.f.) gas-hydrate-bearing sediments at the MARUM – Center for Marine Environmental Sciences since the early 2000s (e.g., Abegg et al., 2008; Heeschen et al., 2007; Pape et al., 2011a).
In 2005, the sea-floor drill rig MARUM-MeBo70 (acronym for
Recently, a pressure-core barrel has been developed within the German
collaborative project “SUGAR” for use with both systems, MeBo70 and
MeBo200. The pressure-core barrel, called MDP (German for
The MDP principally consists of (a) a cutting shoe or core head cutting the sediment core with the required core diameter, (b) a floating piston using hydrostatic pressure to force the penetration of the sediment core into the core barrel, (c) a core catcher that inhibits loss of sediment before the valve is closed, (d) a pressure housing and a valve that closes after the coring process in order to keep the in situ pressure within the core barrel, and (e) a latching device that ensures the correct position of the pressure-core barrel within the MeBo drill string and that activates the closing of the valve when the core barrel is pulled with an “overshot” using the wireline technique (Fig. 1). Titanium was used to manufacture all pressure-holding parts, which facilitates future imaging of pressurized sediment cores by computerized tomography.
Specifications of the conventional core barrels and the MDP operated with MeBo70 or MeBo200.
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As a consequence of required implementations of several activation and lifting mechanisms, the MDP core barrel has a shorter length and a smaller diameter compared to conventional MeBo core barrels (Table 1), resulting in a smaller liner volume. In order to support sediment intrusion into the liner, a piston system has been developed which allows the cutting of a core that has a relatively small diameter compared to the borehole diameter. This system is based on using the physical principle of the increasing hydrostatic pressure when lowering the MeBo from the vessel to the sea floor. A sealed atmosphere within the MDP generates a differential pressure, which at the time of coring drives the piston upwards with the relative downward movement of the drill string. Usage of the differential pressure to support sediment intrusion while coring soft sediments is a unique method (Hohnberg, 2010, patent pending). The piston is locked mechanically before operation and released when a touch sensor strikes the sediment at the drill-hole bottom at the beginning of the coring process. During coring the piston is driven upwards by the hydrostatic pressure at the depth of the borehole. This procedure allows the application of a drilling speed that is higher than that of conventional corers and supports receipt of high-quality cores with comparably high recovery rates (see Sect. 3.2). It adopts the advantages of piston coring but minimizes the adverse impact on the recoverable core-length-to-tool-length ratio. A damping system positioned above the piston regulates the raising velocity of both the piston and the incoming sediment core. After the core has been cut, the overshot is lowered into the drill string in the same way as in the recovery procedure of conventional MeBo core barrels. Once the overshot is latched onto the MDP locking mechanism, the wireline is pulled. Before unlocking the MDP the sealing mechanisms are released by an additional axial lift in the upper part of the tool. Closing of valves at the top and the bottom section of the pressure chamber assures pressure-tight recovery of the core under near in situ pressure. Furthermore, a special pre-configured accumulator is activated in order to compensate for potential changes in pressure due to temperature fluctuations throughout the recovery process and to ensure pressure tightness of the system, thus preserving near in situ pressure during recovery and storage. During pre-configuration the internal pressure of the accumulator is adjusted to as close to the hydrostatic downhole pressure as possible. A significant higher initial pressure (“overcharge”) should be avoided since it lasts on the core and may lead to sample alteration and misinterpretations of core properties.
The MDPs were successfully deployed, with both MeBo200 and MeBo70, during
two campaigns in deep-sea areas in 2016. Cruise SO247 with the German RV
After the MeBo has been recovered and the MDPs were removed from the
magazine, mechanical components above and below the pressure chamber were
dissembled in order to make the MDP's pressure-bearing parts accessible. An
assembly of gas-tight valves and ports (modified after Dickens et al., 2003;
Heeschen et al., 2007; and Shipboard Scientific Party, 1996) and a pressure sensor for
continuous monitoring of the internal pressure were connected to the MDP
pressure chamber. During cruise MSM57/1, pressurized fluid (gas and water)
was released incrementally from the pressure chamber into a gas-tight,
scaled syringe for gas sub-sampling and determinations of fluid volumes.
Quantification of released fluids was carried out on-deck at atmospheric
pressure and ambient temperature (ca. 2 to 6
Repeatedly, after release of a certain gas volume, gas sub-samples were
taken and transferred into glass serum vials for analysis of molecular
compositions (C
The main objectives of the MDP deployments during SO247 were to identify best practice and settings for MDP pressure coring, to technically meet the requirements of both MeBo systems (MeBo70 and MeBo200), and to fit for accomplishing the main goals of cruise MSM57/1. The major objectives of that cruise were to recover cores under pressure and to subsequently carry out a controlled pressure reduction. This degassing procedure would allow for a quantitative determination of the gas in situ amount, which is largely lost when a core is recovered by conventional means.
MDPs have been deployed with MeBo200 during SO247 nine times and with MeBo70 three times during MSM57/1 (Fig. 2, Table A1 in Appendix). During SO247, recovery rates with the MDP of more than 82 % where obtained during five deployments, of which two exceeded 97 %. Virtually no sediments were recovered during two deployments only.
Overview of sediment recovery rates (in percentage of liner
volume) and recovery pressure (in percentage of hydrostatic pressure at the drill
site) obtained in the course of MDP deployments and modifications during
cruises SO247 and MSM57/1.
Two pressurized samples (GeoB20802-6 (2P): sediment core (99 % recovery rate) and GeoB20846-1 (13P): fluid sample from overconsolidated silt section) were received during SO247, both with pressure higher than in situ values (Fig. 2). Although quantitative degassing of the pressure core GeoB20802-6 (2P) could not be executed properly due to technical issues, the two main technical aspects of the MDP (piston coring, preservation of in situ pressure) have been proven to work during SO247. It should be pointed out that the deepest sediments at stations GeoB20803-2, 20824-4, 20831-3, and 20846-1 were collected with the MDP.
During and after cruise SO247, an intense evaluation of the MDP system with regard to core recovery and operation was carried out. Modifications on the sealing concept, the lifting and unlocking mechanisms, and the downstream degassing procedures as well as minor changes of the floating piston led to improvements in handling and operation of the pressure-core barrel and in core analysis. The improvements resulted in an increase of the core barrel's overall reliability and performance, which has been proven throughout MSM57/1 (Fig. 2). Standardized procedures and comprehensive documentation enabled repeated deployment of the MDP on a routine basis. This will facilitate to establish the MDP deployments as a “near conventional” operation in MeBo coring.
During MSM57/1, MDPs have been deployed three times with MeBo70 (Table A1, Appendix). During all operations pressurized samples (two sediment cores and one fluid sample) were recovered. The average sediment recovery of the two pressurized sediment cores was 47.0 %. The deepest sediments at stations GeoB21613-1 and 21616-1 were collected with MDPs (sections 30P and 6P, respectively). The lack of sediment in barrel GeoB21613-1 (29P), which was initially prepared for operation during preceding stations, most likely resulted from a technical malfunction of the piston system. This was probably caused by periodically changing pressure regimes over the course of four lowerings and three liftings of MeBo before final deployment of the MDP.
Except for single MDPs at three sites (GeoB20803-2 (3P), 20802-6 (3P), and 21613-1 (29P)) and both MDPs recovered from site GeoB20846-1, core recoveries with the MDP exceeded average recoveries calculated for all barrels (conventional core barrels plus MDP) retrieved from that site (Fig. 3).
Degassing characteristics of pressure cores (total fluid
volume
Comparative overview of average core recovery rates at individual drill sites (considering conventional core barrels and MDP, in red) and recovery rates of single MDPs at same drill sites (in blue).
These results suggest that the floating piston system of the MDP supports the core intrusion process and, thus, leads to a relatively higher core recovery. However, since different sediment properties and drilling parameters might have affected core recoveries, further investigations are required to evaluate the overall functionality of the MDP piston system.
Both pressure cores obtained during MSM57/1 were degassed quantitatively while the evolution of pressure inside the MDP pressure chamber was recorded (Fig. 4). Degassing characteristics of these cores are given in Table 2.
Degassing characteristics of two pressurized sediment
cores collected with MeBo70 during cruise MSM57/1.
For both pressure cores degassed during MSM57/1, a slight saw-tooth-like
shape of the pressure-time profile was observed at the initial stage of
incremental gas removal (time span ca. 5–40 min). This pattern has been attributed to the presence of gas hydrates in earlier studies (e.g., Dickens et al.,
2000, 2003). However, fluid-to-sediment ratios of ca. 1.3
and 1.0 L L
Analysis of molecular composition demonstrated that gas released from 30P
nearly exclusively consisted of light hydrocarbons, which predominantly
originate from thermocatalysis of organic matter in the deep subsurface
(C
Average molecular composition of gas released from
pressure cores during MSM57/1 (100 mol %
b.d.l.
The results obtained during MSM57/1 substantiate that pressure vessels with full functionality are now available for the sea-floor drill rigs MeBo70 and MeBo200.
Pressure vessels that enable sampling of deep-sea sediments under hydrostatic pressure with the sea-floor drill rigs MeBo70 and MeBo200 were successfully deployed during two cruises in 2016. Core recovery rates usually exceeded those of the conventional corers and core preservation under pressure was achieved for three cores. Successful quantitative degassing and the quality of light hydrocarbons originating from processes in the subsurface clearly demonstrate that the MeBo pressure vessels are now suitable for routine operations.
The MDPs were mainly designed for the recovery and preservation of gas-hydrate-bearing sediments and also for the precise determination of true gas amounts. However, comparably low volumetric gas–sediment ratios obtained from pressurized cores so far show that the MDPs are not only applicable for the retrieval of gas and gas-hydrate-rich sediments but are also appropriate for collecting sediments hosting relatively small gas amounts.
Relevant components of the MDP were manufactured from titanium, which allows for the scanning of undisturbed pressure cores with non-destructive techniques (e.g., with X-rays, gamma rays, sonic waves) during future operations, as has already been done on cores retrieved with other pressure-coring tools (e.g., Riedel et al., 2006; Suzuki et al., 2015). In addition, a MDP subsampling and transfer system will enable segmentation of pressure cores and storage of core segments in smaller pressure cells for the analysis in spatially high resolution and further processing methods in the future.
Overview of sections cored with MDP during SO247 (MeBo200) and MSM57/1 (MeBo70) (in chronological order). Further information on cores collected during the cruises are provided in the cruise reports (Huhn, 2016; Bohrmann et al., 2017).
All data reported are made publicly available through the PANGAEA information system
(Data Publisher for Earth and Environmental Science) sustained by the World Data Center for
Marine Environmental Sciences (WDCMARE). Data from research cruises SO247 and MSM57/1 are publicly
accessible through the PANGAEA information system via
TP conducted degassing experiments with the MDP during cruises SO247 and MSM57/1. HJH designed, built, and modified MDP, tested them in the lab and during cruises with MeBo in 2011, prepared MDP for deployment during SO247, and proposed drill parameters. DW prepared MDP for deployment during SO247 and MSM57/1 and proposed drill parameters. EA participated in lab tests of MDP and designed technical modifications. TF supervised drilling procedures with MDP and MeBo and proposed drill parameters. KH led cruise SO247 and proposed MDP deployment depths. GB proposed development of pressure vessels for the deep-sea drill rig MeBo within the German gas-hydrate-related project SUGAR, led cruise MSM57/1, and proposed MDP deployment depths. TP prepared the paper with contributions from all co-authors.
The authors declare that they have no conflict of interest.
Masters and crews of SO247 and MSM57/1 as well as the MeBo teams are greatly acknowledged for their excellent support during the field campaigns. We are very grateful to the co-chief scientist of SO247, N. Kukowski, Friedrich Schiller University Jena, Germany, for providing opportunities to deploy the MeBo pressure vessels (MDPs). The authors wish to thank Andrew Wright (Geoquip Marine Operations AG), Koji Yamamoto (Technology and Research Center, Japan Oil, Gas and Metals National Corporation; JOGMEC), and the handling editor of the journal, Thomas Wiersberg, for their constructive comments on this manuscript. Design and development of the MDPs were funded by the German Federal Ministry for Economic Affairs and Energy through the collaborative project SUGAR (Submarine Gashydrate Resources; 03SX250B). Edited by: T. Wiersberg Reviewed by: K. Yamamoto and A. Wright