SITE 1091
(Proposed Site TSO-5C)

Moderate winds from the west-northwest prevailed for the transit to Site 1091 on the western flank of Meteor Rise. The 281-nmi transit was made in 30 hr, with an average speed of 9.3 kt. The positioning beacon was launched at 1845 hr on 1 January 1998.

Hole 1091A
Predicted severe weather (other than cold rain showers) failed to develop. Hole 1091A was spudded at 0445 hr on 2 January with the bit positioned at 4369 mbrf. The first core recovered 6.9 m and placed the seafloor depth at 4371.6 mbrf. Continuous APC cores were recovered, with cores 3H and 16H requiring a second wireline trip when the APC failed to actuate. Orientation began with Core 3H. A growing swell caused increasing pitch and vessel heave while coring was in progress, and a second swell added rolls of up to 7°. For the safety of the drill string, knobby drilling joints were used from Core 17H, which was the first use of knobbies for APC coring in ODP history.

Weather and motion conditions moderated, and the six knobby drilling joints were removed after Core 29H. Four additional cores were taken to 310.9 mbsf, when coring was terminated. The APC refusal point had not been reached, as cores were achieving full stroke and the maximum withdrawal overpull had been 60 kips. Most of the scientific objectives had been reached, however, and the forecast of imminent and exceptionally severe weather prompted the decision to pull out of the hole. The seafloor was cleared at 0315 hr on 4 January to end Hole 1091A.

Hole 1091B
The severe weather did not occur. The vessel was offset 10 m and the bit was positioned 3 m higher than it had been for Hole 1091A. Hole 1091B was spudded with the first APC core at 0455 hr on 4 January. Orientation began with Core 4H but was discontinued after Core 13H because the paleomagnetic data from Hole 1091A did not warrant the additional operating time.

By the morning of 5 January, wind gusts were approaching 40 kt and swells exceeded 20 ft in height. The severe heave conditions affected core recovery adversely and seemed to increase the frequency and severity of core liner failures. Coring continued to the target depth of 274 mbsf, but recovery fell to about 60% for the final ten cores.

Hole 1091C
The rig was offset by 10 m for a repeat section. A seafloor core was shot from 4372 mbrf, 3 m deeper than the original Hole 1091A mudline core. The driller's pressure gauge showed a mechanical actuation of the APC which significantly reduced prospects for a high-quality core and it was requested that the interval be recorded.

Hole 1091D
Normal actuation was indicated as a new hole was spudded from the same pipe depth (4372 mbrf). The severe weather and motion conditions persisted for the first few hours of coring, but began improving rapidly in the evening hours of 5 January. Core recovery improved with depth and with improving weather conditions. When the target depth of 203 mbsf had been reached, the drill string was again pulled above the seafloor for a final attempt to fill gaps in the upper stratigraphic section.

Hole 1091E
The vessel was positioned 10 m north of Hole 1091B and Hole 1093E was spudded at 1440 hr on 6 January. Again the bit depth was 4372 mbrf because an important sediment gap still existed in the uppermost core interval. Environmental operating conditions were favorable as six APC cores were taken to a depth of 51.7 mbsf. However, recovery was again limited to about 61%. Nearly every core experienced some degree of core liner failure, and some of the failures contributed directly to reduced recovery.

Coring operations at Site 1091 were terminated as the allotted operating time expired. The drill string was recovered and the drill ship was underway at 0545 hr on 7 January.

To 177 Operations - Site 1092

To 177 Table of Contents

ODP Publications

ODP Home