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doi:10.2204/iodp.sp.325.2009

Operational strategy

Drilling platform

The water depths for the proposed drilling sites on the GBR range from ~30 to 200 m and so require the use of a mission specific platform (MSP) capable of working in shallow waters. The drilling platform, chosen by the contractor and inspected by the European Consortium for Ocean Research Drilling (ECORD) Science Operator (ESO), is the Bluestone Topaz, an International Maritime Organization (IMO) Class 1 dynamically positioned vessel.

The Bluestone Topaz is a geotechnical drilling vessel capable of working in water depths of up to 1800 m. The vessel is equipped with a large moonpool, a Bluestone TT150 derrick, and Foremost Hydraulic top drive.

The compact top drive onboard the Bluestone Topaz is a 150 ton Canadian-built system (Foremost Industries) that is both rugged and powerful. The Bluestone drilling department modified the design of this top drive system to make it ideal for geotechnical or scientific drilling, sampling, and coring activities. Key specifications of the top drive are as follows:

Maximum speed = 200 rpm,

Maximum torque = 32,000 ft lb,

Maximum hook load = 150 tons (300,000 lb), and

Maximum input power = 450 hp (335 kW).

The Bluestone Topaz will have sufficient capacity for food, water, and accommodation for 35 days of continuous operational capability. It is anticipated that there will be one port call for resupply and possible scientific personnel changes.

Coring methodology

General

For efficient drilling/coring it is essential to apply a steady weight onto the drill bit. Vessel heave can reduce or apply excessive weight to the drill bit. The drilling system onboard the Bluestone Topaz is versatile and heave compensated to allow for vessel movement. The drill string is suspended below the top drive and the drill string compensator. The fast line runs over a relative motion compensating heave (3.5 m stroke cylinders) and is connected to the draw works winch. The deadline similarly runs to the anchor point at the drill floor. Compensator loading is ~80 metric tons. Sea water will be used as drilling medium at all times to meet environmental requirements.

The components of the coring system are outlined below:

• Drill string. The drill string consists of 151 joints of drill pipes, 5 drill collars, 2 bottom-hole assemblies (BHAs), and a selection of drill bits. This will make up ~1400 m in total length, thus providing sufficient backup for most operations. The drill pipes are America Petroleum Institute (API) 5½ inch with 5½ inch full hole (FH) tool joints with 4 inch bore inner diameter (ID). The length of each joint is 31 ft. Additional drill pipes, if required, can be placed at the stern of the main deck.

• Bottom-hole assemblies:

• API core barrel. Bluestone has recently collaborated with QD Tech Inc. to produce a versatile BHA for the API drill string that enables sampling, in situ testing, and rock coring to be conducted without having to trip the drill string. Impregnated and stag bits are currently available onboard. Wireline coring can be effected through this BHA.

• Mining-type core barrel. An HQ-size (73 mm outer diameter [OD] and 66 mm ID) wireline core barrel deployed on a mining drill string which fits inside the API drill string. This BHA can also be deployed through the API system, using the API drill string as a conductor or casing.

Coring with API BHA

In normal circumstances, when borehole size and coring speed are less significant, coring of hard formations can be performed with the Bluestone BHA and API drill string as described above. Initially, a camera will be deployed through the API pipe to assess the sea bed and reef structures prior to drilling. The API pipe will then spud in, acting like a stinger and reducing the sea bed footprint, minimizing any potential impacts on the sea bed environment. An internal core barrel will run through the API string and collect standard size IODP cores in standard IODP plastic liners.

The wireline core barrel is deployed and retrieved with an overshot. Both the BHA bit and the core barrel bit will cut through the hard formation. Heave compensation is achieved automatically with the top drive system. The top drive onboard the Bluestone Topaz has excellent control on bit weight to ensure coring stability and quality.

Coring with HQ string

Because of the improved core recovery and time efficiency of using an HQ-size barrel, this method will be employed where core lithology and stability allow. To apply this method, the API drill string is first keyed in or locked into the hard formation. The API drill string is then hung from an elevator at the main deck. The top drive can then be detached from the API drill string, leaving the API string to act as casing. Subsequently, the top drive can commence the HQ string run inside the API drill string. If and where it is allowed, a seabed frame with line tensioner can be deployed to ensure stability of the API drill string. This set up can normally be applied at shallow water depths, probably <70 m.

Coring with HQ string and bumper sub

Alternatively, the method above can be improved with the introduction of a bumper sub in the API drill string make-up. This tool will help compensate heave on the API drill string by providing reliable telescopic movement. The location of the bumper sub has to be carefully planned, taking into account local tidal variation. Currently, a bumper sub with a 1.5 m stroke is available onboard the Bluestone Topaz. Other bumper subs may be considered nearer to the start of the expedition. It is anticipated that this set up will be most suitable for water depths >70 m.

Schematics of the drilling/coring system are presented in Figure F13. If required, the bumper sub can be locked (i.e., when placed at the top of the drill string).

Core run lengths

Typically with mining tools, the maximum core run to obtain optimum recovery is 3 m. Shorter core runs are made if the formation is blocking the bit or it is too friable to withstand a 3 m run. The excellent core recovery obtained during the IODP Tahiti land-based and offshore coring used this strategy and the same core diameter to be used during Expedition 325.

Seawater will be used for drilling in order to meet environmental requirements. Should the core liners start to tear or jam during drilling, a split metal liner will be used.

Core on deck

Once the drilling operation commences and cores begin to come on deck, the coring operations team will be responsible for delivering that core to the curation container for examination and curation. The operation will proceed using a changeover of inner core barrels to ensure continuity of the coring operation in as timely a fashion as possible. Deck operators will deploy an empty core barrel immediately after the full one has been retrieved and then address the core removal and readying of that core barrel for reuse. The cores will be collected in either plastic liners or metal "spoon" liners (before being transferred to plastic liners on the drill floor), with IODP curation procedures being followed.

Gas detection equipment will be carried as part of the total coring operation.

Downhole logging

In all expeditions the downhole logging program will be integrated with the scientific objectives to ensure maximum scientific output. This may include the use of specialist third party tools.

To facilitate downhole measurements and core petrophysics for MSPs, the European Petrophysics Consortium (EPC) has been developing protocols for use both offshore and as part of the Onshore Science Party (OSP).

Unlike the Chikyu and nonriser vessels where the pipe size will be constant and allow a standard set of logging tools to be deployed, MSPs have variable pipe sizes and drill in a variety of water depths, each of which provides constraints on the anatomy of logging operations. Pipe diameter is the controlling factor, and it is envisaged that a wide range of tools, from slim-line memory-mode to standard oilfield suites, may be utilized. Water depth is also an important constraint because some MSP expeditions will operate in very shallow territorial waters where the deployment of nuclear sources may be prohibited or be severely restricted.

Wireline logging

Coring arrangements allow wireline logging to be conducted with the following considerations:

• The OD of the wireline logging tool would have to be smaller than 80 mm to enable clear passage through the HQ string with the core barrel removed. This is required in case it is not possible to pull all of the HQ string, in which case logging can be run through the API bit, which allows a passage of 98 mm diameter.

• The wireline logging winch may have to be placed on the rooster box above the top drive where the rig is heave compensated. Free space may be limited on the rooster box, so a suitable winch size needs to be considered. Alternatively, rigging will have to be done via sheaves and pulleys to the deck.

Upon completion of a borehole, the HQ string will be pulled out and the power swivel reconnected to the API string. The wireline logging tool can run through the power swivel and API string.

This service will be contracted as part of the services for the expedition and will be managed by EPC. The logging equipment and team will be interfaced for a seamless operation, ready to undertake any requirements as the project progresses.

Camera and AUV images

The water depths involved in Expedition 325 range from 30 to 200 m. Of the 38 drill sites, 37 are deeper than 40 m, which corresponds to the limit where the abundance of the living cover (especially the coral cover) decreases sharply and becomes limited. Below this depth, coral coverage is patchy.

At each site, AUV data (if available) will be used to choose the initial site within the 125 m radius buffer zones. The final drill site location will be checked for coral/biota cover using a through-pipe video system. During the expedition, we will core on areas of bare rock or sand. Additionally, the same through-pipe video system will be utilized on completion of the borehole to take photographs to show the effects of coring in the immediate vicinity of the borehole. These before and after photographs will be stored with the drilling data. Experience using this video system during Expedition 310 has shown that it is highly effective at keeping drill sites away from living coral, with camera results revealing minimal or no observed after effects following coring.

Priorities and potential program of work

All aspects of the following work program are subject to change as our knowledge of hole conditions and the timing of operational activities improves.

The GBR transects will be attempted in the following order: HYD-01C (Hydrographer's Passage), HYD-02A (Hydrographer's Passage), NOG-01B (Noggin Pass), RIB-01C (Ribbon Reef 5), and finally RIB-02A (Ribbon Reef 3).

It is important to ensure that the drilling system is operating well, with good core recovery, and that the depth of the Pleistocene reef is established early for each transect. For this reason it is suggested that the sites at each transect are attempted as follows:

1. Target a lower priority site where the predicted target reef lies between 50 and 60 m depth (for example, Site 3 HYD-01C or Site 2 HYD-02A).

2. Progress to the highest priority sites that have the potential to best describe the critical meltwater pulse 1A and the LGM periods (the latter of which were not described during Expedition 310). These are anticipated to lie where the target reef is at 100–130 m depth (for example, Sites 7–9 and 11 HYD-01C and Sites 7–10 and 12 HYD-02A).

3. Progress back along each transect, working shoreward into increasingly shallower waters.

4. Finally, if the above goals have been reached, core a longer hole in both the shallower shelf, at 50–60 m water depth, and in the fore-reef slope sediments (for example, Site 10 HYD-01C and Site 11 HYD-02A).

It is recognized that this survey plan may incur additional time spent on each transect because of transit times between the shallower and deeper sites. This method of working may therefore be altered as our offshore operation evolves.

Open-hole logging will commence after coring, possibly in increments to reduce the risk of hole instability halting the logging runs. The exact pull-back increments will be established on-site once we have a better knowledge of the formations.

It is acknowledged that recording temperature measurements downhole is an IODP minimum measurement that cannot be omitted unless there is an operational reason for doing so. Drilling in Tahiti, and the proposed drilling strategy for the GBR, highlight a number of issues regarding this:

• The proposed hole penetration for 36 of the suggested 40 sites is only 40 meters below seafloor (mbsf). High and potentially laterally variable permeability within the coral structures may well cause a biased geothermal gradient to the seabed, indicating percolating seawater temperatures only.

• If the target depth lies within corals, penetration of the temperature instrument below the bottom of the drill bit may well be impossible without causing damage to the instrument.

• Attempting a temperature reading from the base of the casing while still inside the casing may only provide a biased temperature of the drilling fluids (in this case seawater) being supplied from the vessel.

• No temperature readings were undertaken during Expedition 310 for operational reasons.

It is therefore proposed that temperature measurements not be undertaken during Expedition 325.

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