IODP

doi:10.2204/iodp.sp.341S.2013

SCIMPI installation

What is the SCIMPI?

The SCIMPI is a new subseafloor observatory instrument designed for installation in sediments below the seafloor, providing high depth and time resolution measurements of the physical properties in the sediment (Fig. F1). It will operate for >2 years on internal batteries that can be replenished via remotely operated vehicle (ROV). It can also be connected to cabled observatory systems for real-time data acquisition. With either periodic battery replacement or connection to cabled observatory infrastructure, the SCIMPI provides long-term observations for understanding subseafloor dynamics, such as changes in seafloor and subseafloor gas hydrate systems. The main advantages of the SCIMPI are the ability to tailor the measurements to the targeted seabed characteristics and its relatively small amount of equipment and installation requirements, making it an economical and versatile system for scientific research.

Configuration of the SCIMPI system and specifications

The SCIMPI is designed for dynamic hydrogeological conditions in which the borehole walls collapse around the device once the drill string through which it is emplaced is withdrawn. Borehole relaxation occurs because of two different processes: slower, creep-dominated deformation in fine-grained clays and shales and immediate collapse in unlithified coarse-dominated sediments.

A SCIMPI string consists of multiple measurement modules and a command module connected by varying lengths of cable with a ballast weight at the bottom of the string (Fig. F1). It contains internal batteries but is also able to be powered and controlled via an underwater-mateable connector to either an ROV-replaceable command module or cabled observatory infrastructure. The physical arrangement of modules is serial, but the communications and 9–80 volt direct current (VDC) power busses are multidrop interfaces.

The SCIMPI uses commercial off-the-shelf temperature and pressure sensors that have been successfully used in other marine, industrial, and scientific applications. Individual modules can be tailored to specific measurement missions by configuring the instruments, adding or reducing the instrumentation per module, combining several modules in a single unique casing, or modifying separation between modules by varying cable lengths (Fig. F2). Each sensor module contains a measurement supervisor, a microcontroller for interfacing with as many as four sensors and reporting their data via an RS-485 Mobdus network. A single wire from the logger controls power switching within the sensor modules. The system is optimized to save energy and remain in sleep mode when measurements are not taken. The master controller (MC) is the main processing and data storage component. It consists of a data logger and a supervisor responsible for communications during deployment, communications with sensor modules, and data management for the entire system.

Temperature and pressure in the modules are currently measured using the Seabird SBE-38 and the Paroscientific 410K-101, respectively. These instruments are commonly used in ocean sciences and provide data for characterizing dynamic fluid flow in subseabed environments.

The electrical resistivity smart sensor (ERSS) is custom built as a low-powered four-electrode system that is currently configured for the 0.1–100 Ωm range with alternating polarities at a rate of 100 Hz. This configuration can be adapted for other requirements depending on specific missions. The ERSS does not integrate the correction for temperature in its data output. This should be considered during data analysis in relation to the deployment media. All SCIMPI sensors are manufacturer-calibrated.

SCIMPI modules are powered either by internal battery packs or by connection to a network with an input of 9–80 VDC. The internal battery packs comprise 12 C-sized primary lithium thionyl chloride cells in parallel stacks of four cells in series to provide 26,000 mAh at 14.4 VDC. The batteries are nonrechargeable.

During deployment in a borehole, the SCIMPI will be powered up via the logging wireline cable until release, when the system will automatically shift into recording mode and use the internal batteries. If the SCIMPI is connected to a network, the network acts like the wireline cable, enabling more frequent logging and overriding the internal battery power.

The SCIMPI is configured via a Windows personal computer application (SCIMPI Config) provided by Transcendev (www.transcendev.com/). The configuration is adapted for specific applications by adjusting the rate of measurement for each instrument. Data output of the instrument is recorded in a Universal Time Coordinated time-stamped ASCII file.

This first SCIMPI is designed for a maximum water depth of 1300 m and maximum subseafloor depth of 300 m. However, the system can be adapted to monitor in greater water depths (as much as 6000 m) and subseafloor depths.

Although this first SCIMPI is configured to measure temperature, electrical resistivity, and pressure, the SCIMPI is designed so that it is capable of incorporating other types of sensors.

Where are we installing the SCIMPI?

Why Cascadia?

The Cascadia margin is an ideal location for SCIMPI deployment, as it is in a dynamic hydrate formation environment where a cabled ocean observatory infrastructure (NEPTUNE Canada; www.neptunecanada.com/) is readily available.

The installation of this system in close proximity to preexisting boreholes and CORK observatories (1) enhances our ability to successfully install the SCIMPI, (2) will provide essential information to evaluate the feasibility of the SCIMPI, and (3) is in a location of active gas hydrate research.

Figures F3 and F4 show the location of the proposed SCIMPI installation sites in relation to the features mentioned above.

General science of Cascadia margin gas hydrates and SCIMPI site locations

The northern Cascadia margin is an ideal location for the study of the occurrences and formation of gas hydrate. One of the SCIMPI’s primary applications is to observe changes in subseafloor hydrate formations. The sites proposed are those not only where gas hydrate occurs, but also where venting from the seafloor into the water column occurs.

The proposed site and alternate sites are located in a vent area called Bullseye Vent (Fig. F5). This area has been previously drilled during the Ocean Drilling Program (ODP) (Leg 146, Site 889/890) and IODP (Expedition 311, Sites U1327 and U1328).

Bullseye Vent is the most prominent vent within a seafloor cold vent field ~8 km2 in size. Bullseye Vent has been the subject of many geophysical and geochemical studies (e.g., Riedel et al., 2006a, 2006b).

The primary site proposed for the SCIMPI installation, Site CAS05-CORK, is shown in Figure F6. It is northeast of Bullseye Vent in an area known for vigorous degassing. Installation of the SCIMPI at Site CAS05-CORK will contribute to understanding fluid flux and how it changes during both seismic and aseismic events.

We also include two alternate sites locations: Sites CAS10 and CAS11.

Alternate Site CAS10 (Fig. F7) is located at the northern end of the Bullseye Vent seafloor depression feature where massive gas hydrates occur in addition to a strong bottom-simulating reflector (BSR) (Fig. F8). Site CAS-10 is located less than ~100 m north of the main Site U1328 where logging-while-drilling (LWD) data were obtained.

Alternate Site CAS11 is located northwest of the Bullseye Vent area (Fig. F9) near Site 889 and IODP Expedition 328 Advanced CORK Site U1364. Site CAS11 is at the frontal portion of the Cascadia accretionary prism (Fig. F10) where SCIMPI results could be directly compared with ACORK measurements. The position of these observatories will allow for monitoring pressure gradients associated with subduction-driven consolidation, gas hydrate dynamics, and responses of hydrates to seismic ground motion.

SCIMPI observatory configuration

The nine SCIMPI modules will be configured to maximize capture of dynamic processes at specific depth intervals. The likely configuration is shown in Figure F11 and listed in Table T1. The location of sensor modules was determined applying a k-means cluster analysis to previous IODP core and logging data near Site CAS05-CORK (Lado Insua et al., 2012). This analysis provides an unbiased identification of the major formations characteristics, identifying depths with similar characteristics as the same cluster.

SCIMPI operations plan/drilling strategy

The primary operational objective is to drill a hole to 300 meters below seafloor (mbsf) and install a SCIMPI observatory system. Planned operational steps and time estimates are provided in Table T2.

The preferred location for this installation is at proposed Site CAS05-CORK in the vicinity of Bullseye Vent. This proposed site is ~400 m from Site U1328, which was drilled with LWD and cored to a total depth of 300 mbsf.

The ship will be positioned over the site location coordinates, thrusters and hydrophones will be lowered, and a positioning beacon will be deployed (if considered necessary). A bottom-hole assembly (BHA) will be made up with a 9⅞ inch outer diameter tricone bit and a mechanical bit release (no coring will be conducted). The BHA will be tripped to the seafloor, the top drive will be picked up, and the drill string will be spaced out for spudding the hole. During the pipe trip, all drill collars and pipe stands will be drifted to verify minimum interior diameter of the entire drill string. In addition, the subsea camera system will be deployed to provide a visual verification of the seafloor depth when the drill bit tags the mudline. After recovering the subsea camera, the hole will be spudded and controlled drilling will be used to advance hole to 300 mbsf. The objective will be to maintain a good quality hole for the SCIMPI installation. Washouts due to excessive circulation should be minimized.

After completing the hole to total depth, it will be flushed of all cuttings and a wait period will be conducted to provide feedback on hole stability. The bit will be picked up ~10 m off bottom and ~4 h will spent with the pipe and BHA suspended in the hole without any circulation or rotation—this is intended to mimic the conditions and length of time for SCIMPI assembly and installation. At the end of this period, the drillers will attempt to recover the drill string to ~80 mbsf without rotation or circulation. If this goes well, confidence will be high that the SCIMPI installation can proceed as planned. If the pipe has become stuck and must be worked free, then upon doing so, a full wiper trip using circulation and rotation will be conducted to restore the hole condition to as optimal as possible. Once back on bottom, the hole will be displaced with 10.5 ppg weighted mud, and (time permitting) the wait process will be repeated. It should be noted that the desire is to not fill the hole with heavy mud if it can be avoided, as this could interfere with some of the SCIMPI sensors.

Based upon the results of the aforementioned actions, there are two possibilities for SCIMPI emplacement. The drill bit could be released at the bottom of the hole and end of pipe (EOP) placed at ~80 mbsf. The alternative is to release the drill bit at the bottom of the hole and leave the EOP ~10 m off bottom. Which approach has the highest chance of success will be the subject of shipboard discussions once the hole has been drilled and potential long-term stability has been ascertained.

The SCIMPI assembly, having been preassembled and prepared for deployment, will then be picked up using Yale grips and multiple rig floor tuggers. Once suspended within the drill string, the upper SCIMPI connection will be made up to the Schlumberger cablehead, which in turn is connected to the Multifunction Telemetry Module (MFTM) and then the electronic release system (ERS; also referred to as the electronic RS overshot) on the lower end of the electric logging line. The SCIMPI assembly will then be lowered through the drill string and either into the open hole or to the bottom of the drill string, depending upon the approach chosen above. The MFTM will be used to confirm that all SCIMPI modules are in working order. After everything checks out, the MFTM surface panel will be used to actuate the ERS to release the SCIMPI. The logging line will be recovered and the drill string will then be pulled out of the hole, leaving the SCIMPI assembly in the borehole. Care will be taken when clearing the seafloor to ensure that the ship is positioned over the hole coordinates to minimize BHA swing once clear of the seafloor. The subsea camera system will be used to observe the BHA being raised out of the seafloor and over the top of the SCIMPI installation. About 20 m of the SCIMPI installation will extend above the seafloor for later access by ROV or submersible (underwater-mateable connector, cable, floats, data logger, etc.). A short camera survey of the resulting SCIMPI installation will be conducted before retrieving the drill string.

SCIMPI risks and contingency

Failure of tool during deployment

As part of the SCIMPI project, the primary tool components have all been bench tested, wet tested, and pressure tested. Subsequent to these tests, some modifications have been made to the ERS. Because of this, we plan to repeat the bench test before the expedition. Before deployment on the ship, the SCIMPI will be assembled and tested in the laboratory. The design of the ERS-SCIMPI system allows for real-time communications via the wireline, so we can monitor SCIMPI’s functionality during deployment on the rig floor, while lowering into the borehole, and through final release. We also anticipate sailing the primary engineers responsible for development of the various critical communications components.

Hang up of SCIMPI array prior to arrival at depth

The SCIMPI was limited to a 3 inch maximum diameter with no abrupt diametrical changes to minimize this possibility. Although we do have drill bits that could allow SCIMPI to pass, we plan to drop the drill bit before deploying SCIMPI to provide the maximum opening possible. To minimize the chances that SCIMPI might hang up while being lowered through the open hole, we have the option to deploy the SCIMPI with the BHA near the bottom of the hole. Finally, the real-time monitoring capability of the SCIMPI pressure sensors will provide a proxy for depth, and the SCIMPI can be retrieved before release if it is too shallow because of hole collapse or other problems.

Hole stability

This SCIMPI installation is located near existing drill sites so that we can take advantage of previous drilling and formation information. Drilling operations will include routine techniques for improving borehole stability to prevent the drill pipe from becoming stuck and for enhancing successful SCIMPI installation. In response to the risk that the BHA could become stuck during SCIMPI assembly and deployment, we have inserted a hole stability test in the planned operations. If necessary, wiper trips and heavy mud will be used.

Drill string motion after release

The sea state during final installation, release, and drill pipe withdrawal will be critical. We want to reduce the possibility that the drill string might sever the SCIMPI cable or damage the data module as the ship heaves within the hole and near the seafloor. We do not anticipate using a seafloor structure because we believe this could increase the risk of severing/damaging the SCIMPI. The time to withdraw the drill string out of the seafloor and over the SCIMPI should be minimized, while also ensuring we don’t pull the SCIMPI out of the hole. We will have the subsea camera system deployed while pulling out of the hole. However, only when the very last part of the BHA—the EOP—exits the borehole at the seafloor will we know if the SCIMPI stayed in the hole as planned along with the appropriate extension above the seafloor to allow postexpedition ROV access.

Release mechanism

Based on recommendations from Expedition 342 scientists, a newly redesigned ERS will be used to release the SCIMPI. The ERS has a snap-lock that is latched at the surface to the top of the SCIMPI, and the entire assembly is then lowered into the borehole. An electric motor within the ERS is commanded from the MFTM surface panel to rotate until the RS pulling neck on the SCIMPI is released. This motor can both unlock the ERS from a downhole tool and return the ERS to a locked position so that it can be used to retrieve a downhole tool. The ERS consists of an electronics section, wired through sinker bars, and a motor section that contains the latching mechanism at the bottom of the tool. The MFTM is a downhole sonde that allows real-time communication with the SCIMPI modules through the Schlumberger armored seven conductor cable. It will be used to capture data streams from the different SCIMPI modules before the entire assembly is released and to activate the latching mechanism of the ERS.

Weather/Sea state

The expedition has been scheduled during one of the optimum weather windows for this region. If on-site conditions are too severe to operate (e.g., heave, unsafe rig floor conditions, etc.), the limited contingency time (12 h) will be used to wait for conditions to improve. If offshore conditions are entirely unsuitable for operations, there are currently approved alternate locations to install the SCIMPI.