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doi:10.2204/iodp.sp.317.2009

Risk and contingency strategy

General contingency plan

Four principal factors could affect the implementation of the drilling plan:

1. Adverse hole conditions at shelf sites (e.g., encountering thick intervals of loose sand that can collapse into the hole),
2. Weather conditions that can limit the ability to drill in shallow water by exceeding the water depth–dependent restrictions on allowable heave,
3. Time delays (e.g., arising from measures taken to respond to hole conditions and changing weather), and
4. Shallow hazards.

Hole conditions

Loose sand at shelf sites will be dealt with in the first instance by using heavy mud to condition the holes. We may also move from primary to alternate shelf sites (e.g., Sites CB-01B, CB-01C, CB-02B, and CB-03A) if it is judged that better conditions may be encountered at such an alternate site, for example, if the lateral extent of problem intervals can be estimated using the seismic profiles.

Ideally, unconsolidated sand would be cased off and a limited amount of casing (up to 600 m) might be available if it is not used during one of the previous expeditions. However, casing, which also requires the installation of a reentry cone, will be employed only as a last resort because it will involve a significant time penalty that would almost certainly lead to the loss of one or more sites from the planned drilling program. The advantages and disadvantages of casing use will have to be evaluated at sea should the need arise.

Weather conditions

Drilling in shallow water on the shelf is more challenging than drilling in deep water. In part, this is because the ship must maintain position to within 3%–8% of water depth (depending on water depth range). Stationkeeping turned out not to be a significant problem when shelf drilling was conducted on the New Jersey margin (Leg 174A). However, drilling in shallow water also involves restrictions on the amount of heave that can be tolerated by the heave compensator. The amount of allowable heave increases with water depth within three depth ranges: 76–300 m, 301–650 m, and >651 m. Therefore, we must be prepared to modify the drilling strategy in response to changing weather conditions.

For example, if weather conditions when the drilling vessel arrives on site are such as to exceed the allowable heave at shelf Site CB-03B (121 m water depth), we will instead begin drilling at slope Site CB-04B (346 m water depth), where greater heave is permissible. If conditions improve sufficiently, we may decide to leave Site CB-04B at a convenient point (e.g., end of XCB drilling or RCB bit replacement) to return to Site CB-03B, dropping a free-fall funnel (FFF) at Site CB-04B if necessary to allow reentry.

Similarly, if the sea state increases beyond acceptable limits while drilling at a shelf site and seems likely to remain high for some time, we may decide to move to Site CB-04B, rather than pull out of the shelf hole and wait for an improvement in the weather. We would deploy a FFF at the shelf hole if reentry is deemed necessary on our return.

In the event that sea conditions are too extreme to operate even at Site CB-04B, which is in the 301-650 m water depth window, we will move to either Site CB-04C (713 m water depth), Site CB-05F (682 m water depth), or Site CB-06B (1158 m water depth). Once again, we would return to Site CB-04B or the shelf when weather conditions allowed, making use of a FFF at Site CB-06B if necessary.

Because all primary and alternate sites are distributed over a small area, transit times between sites are negligible (most sites are within 10–15 km of one another, and the maximum distance between sites is 50 km).

Timing

If significant time is used up responding to shelf hole conditions and poor weather, one option might be to consider drilling Site CB-01B instead of Sites CB-01A and CB-02A. Site CB-01B lies between Sites CB-01A and CB-02A and achieves some of the objectives of each of those sites. Such a decision would only be made following a meeting of the science party.

Shallow hazards

We have addressed these risks by conducting an independent shallow hazard evaluation, applying our standard shipboard operating procedures, and staffing shipboard personnel dedicated to monitoring for signs of potentially hazardous accumulation of hydrocarbons.

The independent shallow hazard evaluation was completed for all proposed sites. The results of the evaluation were reviewed by the IODP EPSP, who forwarded recommendations to the Science Operator. The stratigraphic sections to be penetrated in the proposed boreholes correspond to sequences that do not show hydrocarbons in commercial wells in the study area. Analysis of seismic reflection data indicates that although shallow gas does occur in some regions of the study area, it is not present at any of the planned drill sites. Shallow gas is not seen as a significant issue at the planned sites, but sites were relocated to further mitigate any potential risks.

During the expedition, operations will be guided by our routine safety monitoring procedures (described in Pimmel and Claypool, 2001) to ensure that the sediments being drilled do not contain greater than expected amounts of hydrocarbons. The objective of hydrocarbon monitoring from a safety standpoint is to distinguish potentially hazardous accumulations of hydrocarbons from the background of the normal increase in biogenic hydrocarbon content with depth. Identification of chemical composition and physical properties of any gas or solid petroleum substance is critically important for recognition of the presence or possibility of dangerous accumulations. The composition of gases may enable distinction between biogenic gas and thermogenic gas that has leaked upward from an underlying oil and gas accumulation. The prevailing guideline is that drilling should be stopped if hydrocarbons (or hydrocarbon indications) suggesting the presence of substantial accumulations of gas and oil are encountered.

All recovered cores will be routinely monitored for signs of a potentially hazardous accumulation of hydrocarbons described in Pimmel and Claypool (2001). Immediately after the core is recovered on deck, sediment headspace samples will be taken from the cores to conduct gas analyses by gas chromatography (GC). If the core shows signs of free gas accumulation visible through the clear plastic core liners (e.g., bubbling, gas pockets, expansion, etc.), a liner penetration tool will be used to collect gases that will also be analyzed by GC. Facilities are also available on board the ship to determine the type, amount, and thermal maturity of organic matter using a pyrolysis Source Rock Analyzer should this assist in the interpretation of potential risk.

We sail shipboard personnel dedicated to the safety monitoring program. Two full-time chemistry technicians' primary responsibility is to provide 24/7 collection and analyses of the safety monitoring samples. In addition, we will sail an organic geochemist to provide the Operations Superintendent (responsible shipboard science operator representative) and Co-Chief Scientists with advice concerning interpretation of these analyses and potential risks.

Alternate sites

Seismic profiles of all proposed sites discussed below (and of all primary sites discussed above) are included in the site sheets of this prospectus.

Shelf sites

Sites CB-01B, CB-01C, and CB-02B

These sites (101, 97, and 116 m water depths, respectively) serve as alternate sites for primary Sites CB-01A and CB-02A. During the review process of Proposal 600, the value of penetrating sequence boundaries on their clinoforms was stressed. We have therefore included Sites CB-01B and CB-02B, which also have the benefit of being in slightly deeper water than Sites CB-01A and CB-02A, respectively, though at the expense of greater penetration depths. On balance, we prefer Sites CB-01A and CB-02A over the respective "B" sites both for scientific reasons and to minimize penetrations in shallow water.

Because of an underlying mounded drift deposit at Site CB-01B, it was suggested (EPSP December 2005 meeting) that an additional alternate site be selected. Therefore, Site CB-01C is included as an alternate to Site CB-01B. It was necessary to locate Site CB-01C on a different dip profile (EW00-01-70) in order to avoid the drift (Fig. AF1).

Site CB-03A

Site CB-03A (125 m water depth) was not chosen as a primary site because of high amplitudes observed in the seismic data at ~1.05 s.

Slope sites

Site CB-04A

This site (340 m water depth) is located in the immediate vicinity of Site CB-04B. Because of high amplitudes at 1.6–1.7 s in the seismic profiles at Site CB-04A, EPSP approved penetration only to 1270 mbsf. However, this is insufficient to reach the Marshall Paraconformity at this site, and therefore Site CB-04B was chosen as a primary slope site. Both Sites CB-04A and CB-04B are close to a seafloor depression on EW00-01 Profile 23 (Fig. AF9), raising concern that the depression might represent a large pockmark. No precise bathymetric data exist for the area, but the feature also appears on the next basinward strike profile, indicating that it is an incised slope canyon.

Sites CB-05B, CB-05C, CB-05D, and CB-05E

These sites (389–402 m water depths) target sediment Drift D11. Site CB-05C, which drills into the crest of the drift, is considered the primary site for drilling Drift D11. The crest of Drift D11 is less deeply buried at Site CB-05C than the crest of Drift D10 at Site 1119 (Leg 181). Although drilling sediment drifts is no longer a primary objective of Expedition 317, a CB-05 site could yet be drilled as a contingency site if shallow shelf drilling turns out to be impossible, either because of hole conditions or poor weather, and sufficient time remains available. It is preferable to drill the thickest part of Drift D11, just basinward of profile EW00-01-23 (Fig. AF12), at Site CB-05C. Site CB-05D was added, slightly downdip from Site CB-05B, to avoid a high-amplitude reflection at 1.8–1.9 s. Site CB-05D is on commercial crossing strike profile CB-82-47 (see site sheets). Finally, Site CB-05E on EW00-01 profile 23 was added as an additional alternative site to avoid some high amplitudes near the seafloor at Site CB-05B. Site CB-05E is not at a line crossing but is ~200 m away.

All CB-05 sites are assessed as having a "low," as opposed to "negligible" or "zero," risk of shallow gas. This is because of a widespread interval of high amplitudes that occurs at all of Sites CB-05B to CB-05E at ~850–950 ms (Figs. AF13, AF14, AF15, AF16). This high-amplitude interval is ubiquitous in this area and cannot be avoided. However, a similar facies occurs at a similar depth at Site 1119 and was penetrated safely there. The high amplitudes may result from higher carbonate content and higher frequency of occurrence of silty sand beds than in overlying sediments. Note that Site 1119 is over a different drift (Drift D10) from that to be penetrated at the CB-05 sites (Drift D11). However, the high amplitudes at Site 1119 occupy the same stratigraphic interval as those at Sites CB-05B to CB-05E (part of the interval between Unconformities U11 and U14), suggesting a similar origin.

Site CB-04C, Site CB-05F, and Site CB-06B

These sites (713, 682, and 1158 m water depths, respectively) are the only sites within the water depth range with the least restricted heave limits (>651 m). Therefore, they serve as global alternate sites in case the sea state does not allow operations at any primary sites in <650 m water depth. Furthermore, basinward of the toe of the modern slope, as at Site CB-06B, the Marshall Paraconformity can be reached with greatly reduced penetration (<1100 mbsf). Sites CB-04C and CB-05F have the same scientific objectives as Sites CB-04B and CB-05C, respectively, with the deepest target being the Marshall Paraconformity. However, Sites CB-04B and CB-05C remain the preferred drilling locations for these objectives because of the reduced number of sequences penetrated at the deeper sites.

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