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The water depths for the proposed drilling sites in Tahiti range from 25 to 310 m and require the use of a mission-specific platform. The Hunter, a dynamically positioned (DP) vessel with a large moonpool, has been contracted as the drilling vessel. The drilling contractor is Seacore of Gweek, Cornwall, who will install a drilling rig on the Hunter over the moonpool and onto part of the aft deck. The Hunter is a class 2 DP vessel, and as such has a minimum dual redundancy in propulsion and navigation systems which meets the Integrated Ocean Drilling Program (IODP) and international environmental requirements for the Tahiti coring operation.
An HQ mining-type wireline coring system with a conductor to the seabed will be used for coring. This is commonly termed piggy back coring because the mining coring rig is installed on top of the conventional API rig, which will deploy the conductor pipe. This allows minimum cuttings due to the small kerf on the bit, a smooth hole profile due to bit type, and the best chance of obtaining high quality and high recovery of core. This type of equipment has been used extensively in other coral reef situations world-wide and has a good track record. For offshore use the drill string is protected by the conductor pipe to seabed. This serves both to avoid excessive string bending and provide a conduit to the vessel for any cuttings that come to the surface. Experience shows that the majority of cuttings are dissipated subsurface while coring.
The coring system for the project is outlined below.
The conductor pipe is a heavyweight drill pipe or casing with an internal diameter (ID) of ~100 mm and a casing shoe to set it ~1 m into the seabed. A seabed template of ~1.5 m diameter and weighing 1012 T will be deployed at the end of the conductor pipe, which will sit on the seabed with the conductor pipe projecting through. This pipe will be deployed with the main drilling power swivel attached to the main heave-compensation system of the drilling rig. The coring system will be installed above this in piggy-back fashion so that all coring is carried out in compensated mode.
Wireline Coring System
A mining-type assembly of triple tube configuration in the HQ range (core = 61 mm) will allow the core bits to pass through the conductor pipe and core ahead. There will be a range of core bits available, including surface set diamond or tailored impregnated bits, which are most likely to be used. As the base of each hole is intended to terminate in basement, a surface set diamond bit would allow the best opportunity to complete the borehole without a change of bit. However, reentry is possible, as there will be a conductor pipe installed between the vessel and seabed.
Coring Run Lengths
Typically with mining tools, the maximum coring run to obtain optimum recovery is 3 m. Shorter coring runs can be made to ensure that this high recovery is maintained in a reef environment, but additional time would be required for the additional wireline trips. The excellent core recovery obtained on the Tahiti land-based coring used 1.5 m runs and the same core diameter to be used in this expedition.
Wireline Overshot and Retrieval System
All inner barrels are wireline retrievable using a well-proven mining core barrel retrieval system. This allows continuous coring operations without having to pull the drill string to recover each core run and the flexibility to drill/spot-core or take measurements in any borehole without recourse to pulling the drill pipe.
Seawater will normally be used for drilling.
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 laboratory 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. The deck operators will deploy an empty core barrel immediately after the full one has been retrieved, then address the core removal and readying of that core barrel for reuse. The cores will be collected in a plastic liner and IODP curation procedures will be followed.
Gas detection equipment will be carried as part of the total coring operation.
This service is contracted as part of the services for the Tahiti expedition and will be managed by the European Petrophysics Consortium (EPC). The logging equipment and team will be interfaced for a seamless operation on board the drill ship, ready to undertake any requirements as the project progresses.
To facilitate downhole measurements and core petrophysics for MSPs, the EPC has been developing protocols for use both offshore and as part of the shore-based party.
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 specialty third-party tools.
Unlike the Chikyu and nonriser vessels where the pipe size is constant and allows 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 for this expedition slim-hole mining-type logging tools will be utilized. Nuclear sources will not be deployed.
Camera and/or ROV
The water depths involved in the general drilling plan related to this proposal range from 40 to 310 m with two exceptions concerning the drilling of keep-up reefs and build-ups on the reefs in the Tiarei and Maraa areas (water depths, respectively = 25 and 30 m). Seventeen of the 19 drill sites 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 (4050 m water depth) to almost zero (below 50 m). For the two drill sites shallower than 40 m above (keep-up reefs), an ROV and/or video camera will be used to accurately determine the location of the drill sites, on sand or bare rocks, and help avoid damage to living coral heads.
Each site drilled within the active reef (and preferably at all drilling sites) will be monitored using a through-pipe video system to ensure that the landing area is free from living coral heads. Additionally, the same camera system will be utilized on completion of the borehole to take photographs to show the effects of the coring in the immediate vicinity of the borehole. These before and after photographs will be stored with the drilling data.
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