Tomokazu Saruhashi

Rapid growth of mineral deposits at artificial seafloor hydrothermal vents

Seafloor massive sulphide deposits are potential resources for base and precious metals (Cu-Pb-Zn ± Ag ± Au), but difficulties in estimating precise reserves and assessing environmental impacts hinder exploration and commercial mining. Here, we report petrological and geochemical properties of sulphide chimneys less than 2 years old that formed where scientific boreholes vented hydrothermal fluids in the Iheya-North field, Okinawa Trough, in East China Sea. One of these infant chimneys, dominated by Cu-Pb-Zn-rich sulphide minerals, grew a height of 15 m within 25 months. Portions of infant chimneys are dominated by sulphate minerals. Some infant chimneys are sulphide-rich similar to high-grade Cu-Pb-Zn bodies on land, albeit with relatively low As and Sb concentrations. The high growth rate reaching the 15 m height within 25 months is attributed to the large hydrothermal vent more than 50 cm in diameter created by the borehole, which induced slow mixing with the ambient seawater and enhanced efficiency of sulphide deposition. These observations suggest the possibility of cultivating seafloor sulphide deposits and even controlling their growth and grades through manipulations of how to mix and quench hydrothermal fluids with the ambient seawater.

Plan and technological difficulties on NanTroSEIZE long term borehole monitoring system

IODP (Integrated Ocean Drilling Program) scientific drilling proposal 603 (NanTroSEIZE: Nankai Trough Seismogenic Zone Experiment) not only proposes drilling, coring, geological analyzing, and geophysical logging, but also mandates that several long term borehole monitoring systems (LTBMS) should be installed at Nankai trough, where we expect to encounter the mega splay fault and the locked region of mega thrust fault, respectively, in order to understand on the dynamics of seismogenic zone. Borehole is not mere relic after core sampled, but should be sufficiently utilized as the scientific legacy hole to monitor the interior of the Earth. The advantage of LTBMS is to simultaneously manage precise monitoring on various parameters at multiple layers in the same borehole. Its technological difficulties are hiding in establishing the reliable and robust system against deploying long and narrow structure into tiny borehole through the ocean current from the heaving surface vessel. Also, for deeper penetration, higher temperature and pressure cause many problems on the system. This paper describes the plan of NanTroSEIZE LTBMS and its technological difficulties including some results on our development.

Field Experimental Study on Vortex-Induced Vibration Behavior of the Drill Pipe for the Ocean Borehole Observatory Installation

We conducted two field tests aboard the Drilling Vessel Chikyu (D/V Chikyu) in areas of strong ocean currents (i.e., Kuroshio, Japan) during the March 2010 CK10-01 cruise. The relationship between the amplitude of the vortex-induced vibration (VIV) and the ocean current and ship drift speed were investigated to establish a VIV control method for the ocean borehole observatory installation in the Nankai Trough. These tests demonstrate that the ocean current and ship drift speed mainly control the drill pipe VIV. We find that to install sensitive sensors in a strong current area and avoid damage, the key factors are: 1) lowering the sensor assembly in a low current area; and 2) managing the drift speed of the drilling vessel.

Vortex induced vibration suppression of the drill pipe for the long-term borehole monitoring system installation

This study proposes a Vortex Induced Vibration (VIV) suppression method for the Long-Term Borehole Monitoring System (LTBMS) installation in areas of strong ocean currents such as Kuroshio. One of the primary challenges in realizing LTBMS was to install high-precision, sensitive sensors into the borehole without damaging them. Two field tests were performed using accelerometers attached on instrument carrier and/or drill pipes to investigate the characteristics and causes of drill pipe VIV. This test demonstrates that the reduction of the drag on circular drill pipes and tubings and the vortex suppression can be achieved by the suppression ropes and the drill collars. They were suggested to be attached on the drill pipe above the sensor assembly for the actual LTBMS installation. From the VIV monitoring during the installation over a period of several days, the following three points can be drawn for the further VIV suppression: 1) The bottom hole assembly should be lowered in the low current area, with the relative current speed being as low as possible, 2) the drifting speed should be kept well below 1 knot, and 3) the drifting angle between drifting direction and sea current should be kept as small as possible (definitely less than 45°). The results show the VIV amplitude was further reduced to less than 0.5G, which led to the success of the first LTBMS installation.

Real-Time Riser Fatigue Monitoring Routine: Architecture, Data and Results

Recently, the Modal Decomposition and Reconstruction (MDR) algorithm was developed to accurately estimate fatigue damage in marine risers based on measured acceleration and angular rates at several locations. The greatest benefit for drilling risers can be derived by incorporating the method in an online, fully automated system. In this way, fatigue damage estimates are available to the crew on the rig in real-time for risk quantification and mitigation.To this end, the MDR routine was implemented for online assessment of fatigue damage along the entire riser from acceleration and angular rate measurements at typically 5–10 elevations. This paper discusses the architecture, highlights some measured data and provides results for modes, stress and fatigue damage rate for the Chikyu drilling vessel during two scientific drilling campaigns. These campaigns occurred at the Shimokita site (1180-meter water depth) and the Nankai trough site (1939-meter water depth). To the authors’ knowledge, real-time fatigue monitoring of the entire riser has not been accomplished previously.Robust incorporation of the MDR algorithm into an online computational environment is detailed, including incorporation of top tension and mud weight data from the rig, detection and removal of data errors, and streamlined flow of the data through the computational modules. Subsequently, it is shown by example how the measured accelerations and angular rates are used to determine excited modes, participating modes, stress distribution and fatigue damage along the entire Chikyu drilling riser in an online setting.The technology highlighted advances riser integrity management two steps forward by first using measured data at 5–10 locations and the MDR algorithm to reconstruct stress and fatigue damage along the entire riser, and secondly integrating this approach into a fully automated, real-time computational environment. As a result, drilling engineers are empowered with a tool that provides real-time data on the integrity of the drilling riser, enabling informed decisions to be made in adverse current or wave conditions. Measured data also serves as a benchmark for analytical model calibration activities, reducing conservatism in stress and fatigue in future deployments. Furthermore, cumulative fatigue damage can be tracked in each riser joint, enabling more effective joint rotation and inspection programs.Copyright

New ways for research and development using a deep-sea hydrothermal vent system in the Okinawa Trough

JAMSTEC has an international patent for the environmentally-friendly cultivation system of offshore hydrothermal chimneys as the accessory product of scientific drilling program in the Okinawa Trough at deep water of 1,100 mbsl. This cultivation system was inspired by rapid growth of precipitated minerals (sulfide chimney) on the drilled hole. We introduce in this paper, the achievement from the riserlesss drilling campaign in Iheya North field in 2010 as IODP Exp. 331 and in 2014 as SIP Okinawa. After the IODP Exp. 331, the major findings were the massive continuous hydrothermal fluid supply from the drilled hole and the rapid growth of the precipitated minerals (sulfide chimney). The flow rate estimated from the installed producer is around 300 - 500 liter per minute through 3.5 inch pipe in diameter. The Cu-Pb-Zn-rich sulfide chimney having average concentrations of 4.5 wt% Cu, 6.9 wt% Pb, 30.3 wt% Zn and 8.7 wt% Fe at the outlet of the drilled hole was rapidly grown to be from 2 m to 8 m in 6 months. In Exp. 331 two temporary wells are and 5-1/2” tubing pipe were installed and those are producing the hydrothermal fluid. JAMSTEC are planning to install two new monitoring wells in Feb 2016 at Iheya North field and at Noho field. Those wells are sensing and logging flow rate, temperature and pressure. It can capture the precipitated minerals from hydrothermal fluid and those data by retrieving monitoring cell by ROV. The system was built and tested in March 2015. LWD operation was performed in 2014 as a part of SIP program. 7 wells are drilled with LWD for the depth of 200 - 350 mbsf in only 9 days in riserless drilling throughout the risk of 350 deg.C thermal fluid by applying continuous circulation system. The continuous circulation system successfully suppresses the hydrothermal fluid pressure by the circulation back pressure and the maximum downhole temperature recorded at LWD was only 85 deg.C in a short period. At the last our future plan is discussed focusing on our present concerns.

Extreme Directional and Planar Profiles for Kuroshio Current Using Inverse FORM and Proper Orthogonal Decomposition

Kuroshio is a major global current that flows near the east coasts of Taiwan and Japan. Kuroshio is a relatively strong current with typical speeds of 3 to 5 knots at the water surface. It is important to properly understand extreme current profiles of these currents for any drilling activity since the response of deepwater risers is known to be sensitive to the shape of the current profile. This paper presents the derivation of extreme two-dimensional (i.e., directional) and planar profiles for Kuroshio currents at a site in Nankai Trough, Japan; water depth is almost 2000 m. About 6000 currents profiles measured over six months in 2010 by JAMSTEC are used. The inverse first-order reliability method (inverse FORM) and proper orthogonal decomposition (POD) technique are employed. While such methodology is well established, its use for this site posed several challenges. Firstly, the first two modes contribute only about 90% of the energy. Therefore, as many as seven modes were included for accuracy. Since an exact solution requiring joint probability distribution for seven variables becomes quite cumbersome, reasonable simplifications were made for efficient calculations. Second, to preserve the directionality in extreme currents, the inverse FORM problem for the two orthogonal components of the current velocity was simultaneously solved, so that extreme profiles for the two planar directions are obtained. Doing so implies solving a four-dimensional inverse FORM problem, even if the full joint distribution of first two modal weights for each direction is used. This four-dimensional problem was reduced to two related two-dimensional problems, wherein the modal vectors in the orthogonal directions are assumed independent; this assumption was found to be valid for this data set. A set containing a limited number of extreme N-year current profiles is derived using the above methodology. It is found that most of the shapes observed in the measured Kuroshio current data are represented in this set of extreme current profiles. The largest riser response obtained from all these current profiles would be the N-year response. A single extreme N-year profile is often sought in analysis, which is also derived from the set of N-year profiles by selecting the profile which maximizes an assumed response function. In summary, this paper presents extreme currents for a site on which little literature exists, and introduces a methodology to derive extreme directional current profiles from measured data.© 2013 ASME

Operations Summary During Riserless Drilling to >7700 mbsl in the Japan Trench for IODP Expedition 343 & 343T JFAST and Discussion of the Relationship Between Drilling Parameters and Rock Damage.

During IODP Expedition 343: The Japan Trench Fast Drilling Project (JFAST), five boreholes were drilled from the D/V Chikyu in >6800 m water depth. Three of these crossed the main fault target. A logging-while-drilling (LWD) hole that penetrated to 850.5 meters below seafloor (mbsf) (total depth [TD] = 7740 meters below sea level [mbsl]) was documented using a suite of LWD tools. From an adjacent partially cored hole drilled to 844.5 mbsf (TD = 7734 mbsl) 21 cores were acquired that spanned the two main fault targets. During the follow-up expedition 343T a third borehole was drilled to 854.8 mbsf (TD = 7752.3 mbsl) and a simple temperature observatory was deployed in the wellhead. The drilling operation, which lasted 88 days, was very technically challenging. Notably, the drill string had to be withdrawn a number of times due to high seas, and technical issues. In certain intervals, rather than core we recovered loose, subrounded fine gravel clasts of the two major lithologies penetrated to those depths (silt and mudstone). Particle shape and size of these clasts was analysed. Results demonstrate (1) particle shape variations apparent visually are not easily quantified, (2) there are distinct variations in particle size distributions. We discuss whether these relate to variations in drilling parameters.

A Method for Estimating Quasi-Static Riser Deformation and Applied Forces From Sparse Riser Inclination Measurements

A method was recently presented for determining quasi-static and dynamic riser angles using measured data typically found in a riser fatigue monitoring system, specifically acceleration and angular rate data. The riser angles were determined at sensor locations. In this paper quasi-static riser displacement, inclination angle, curvature, and stress are estimated along the entire length of the riser, using only the quasi-static inclinations angles at sparse sensor locations. In addition the distribution of applied forces along the entire riser length is also estimated. A rough representation of the current profile can be calculated using the drag coefficients of riser joints.The riser deformation (displacement, inclination, curvature) and applied forces are estimated by solving the matrix equation f = K*x, where f is the vector of forces and moments, K is the stiffness matrix and x is the vector of displacements and inclination angles. In the equation, force and displacement vectors are unknown and the stiffness matrix is determined using Finite Element (FE) modeling. Constraints are applied, setting the inclination angle at the sensor locations to the values derived from measured data. The remaining highly-underdetermined problem cannot be solved in a classical sense, as it admits infinite solutions. To get a solution that is consistent with the physics of riser deformation, smoothness of the solution is enforced as a constraint. The smoothest solution is solved using quadratic programming methods.Following implementation of the method in Matlab®, the procedure was validated using numerical simulations of a riser in applied current. Both connected (to the wellhead) and disconnected cases were simulated. Estimated riser displacements, slopes, curvatures and applied forces are found to match the simulation results closely.The algorithm was then run using measured data from an emergency disconnect event that occurred on the Chikyu drill ship in November, 2012. The riser displacement, inclination and curvature were determined and found to agree well with results determined using another method.The additional capabilities presented herein further expand the utility of a riser monitoring system. Quasi-static and dynamic riser angles are derived from acceleration and angular rate sensors using previously published methods. Using the method developed herein, the quasi-static inclination angles at the sensor locations can be used to determine the displacement, inclination, curvature (stress) and even applied force along the entire riser. These results are particularly useful in strength assessment, model verification, clashing and emergency event reconstruction analyses.Copyright

Development on long term borehole monitoring system for earthquake and geodetic research

The deep sea scientific riser drilling vessel capable of riser drilling at 2,500 meter water depth, CHIKYU started its scientific operation from 2007, as the first riser drilling platform of the Integrated Ocean Drilling Program, to contribute such important scientific program as the Nankai Trough Seismogenic Zone Experiment and the Japan Trench Fast Drilling. Any of these expeditions involve technological and operational difficulties related to strong current, high wave, strong wind, deep water, and deep penetration. This paper describes the development of long term borehole monitoring system from view points of technical difficulties on deploying it at deep water, deep hole, and strong current.