Engineering Implications Of Deepwater Geologic And Soil Conditions, Texas-Louisiana Slope

Campbell, Kerry J.; Hooper, James R.; Prior, David

doi:10.4043/5105-ms

Cited by 7 publications

(3 citation statements)

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“…Mounds containing hydrate have been reported from the Gulf of Mexico (Campbell et al 1986;MacDonald et al 1994). The growth of gas hydrates in near-surface sediments has been attributed as a possible mechanism for the formation of these mounds in the Gulf of Mexico (Prior et al 1989).…”

Section: Discussionmentioning

confidence: 99%

Possible hydrate mounds within large sea-floor craters in the Barents Sea

Long

Lammers

Линке

1998

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Interpretation of several surveys across a 'crater field' in the Barents Sea provide further evidence that the craters (large depressions, 300-500m diameter, 10-30m deep) are related to gas escape after deglaciation some 15 000 years BP. The disposition of the craters suggests that the flow of gas was controlled by fractures within the Triassic siltstone bedrock. Topographic highs within several craters, comprising angular blocks of rock locally rising above the level of surrounding crater walls, are interpreted as hydrate mounds indicating that gas flow continued after the formation of the craters. This may be the first reported occurrence of hydrate mounds in lithified sediments. Assuming the gas was methane and seabed temperature was similar to that at present then the hydrate mounds were formed at a time when the sea-bed was between 280 and 340 metres below sea level (mbsl) (i.e. 10-80 m lower than at present). Geochemical studies provide evidence that gas hydrates in the sub-bottom adjacent to the crater field are presently decomposing in accordance with seasonal temperature variations.

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Section: Discussionmentioning

confidence: 99%

Possible hydrate mounds within large sea-floor craters in the Barents Sea

Long

Lammers

Линке

1998

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“…Geophysical survey work was carried out in 1986, 1987, and ROV work was done in 1989 There are several reasons for the multiple surveys, but the most important is the complex and difficult conditions found in some areas. This section describes survey planninq and the reasons for each survey.…”

Section: Survey Planning and Historymentioning

confidence: 99%

Geohazards Surveying and Complex Seafloor Conditions Along Deepwater Jolliet Pipeline Routes

et al. 1990

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This paper discusses pre-lay survey work and the difficult conditions found along the deepwater portions of the Jolliet pipelines. Water depths range from about 1760 ft at the TLWP in Green Canyon Block 184 to about 616 ft at the CPP in Green Canyon Block 52, a straight-line distance of about 9 miles. Several surveys were required to adequately define conditions and find acceptable routes. A total of 380 line-miles of geophysical survey work was done during two surveys in 1986 and 1987, and 30 piston cores were taken. A small amount of geophysical survey work was done in 1988, and an ROV survey for visual confirmation of conditions in a rocky area was done in 1989. Difficult conditions include: irregular, rocky seafloor with sharp relief of tens of feet; fault scarps as much as 240 ft high; and areas of possible landslides. Because of these difficult conditions, the original proposed pipeline route was changed and the length of flexible pipe was increased. Geophysical survey tools included: a narrow-beam water-depth recording system with velocipede calibration; a scale-corrected side-scan sonar system; a 3.5-kHz shallow-penetration profiler; a one-kilojoules intermediate-penetration profiler; and a seismic Control Module to reduce vertical exaggeration in rugged topography and increase horizontal resolution. A 10- to 20-ft-Iong piston corer was used for soil sampling. The difficult conditions encountered on this project are expected to be common in some other deepwater areas. Simple, straight-line pipeline routes connecting two points will not always be feasible. Consequently, schedules and approaches that have become customary for pipeline route surveys in less than 600-ft of water in the Gulf of Mexico may not be adequate for many deepwater pipelines. Careful planning, special survey tools, more detailed surveying, and more time for data collection and analysis may be needed. INTRODUCTION This paper discusses the planning and execution of the pre-lay geohazards pipeline route surveys for the deepwater portions of the Jolliet pipelines and the analysis of difficult conditions found along the routes. This paper also discusses why the conventional approach to pipeline route surveys will not always be adequate in deepwater, and points out considerations for future deepwater pipeline route investigations. In this paper the terms "deepwater" and "continental slope" are used interchangeably and indicate water depths in excess of 600 ft. The term "continental shelf" refers to areas where water depths are less than 600 ft and the seafloor is generally flat and featureless. This paper is mostly restricted to discussion of the geophysical survey work and piston coring for the process gas and process oil/ water pipelines extending about 12.2 miles between the Tension Leg Well Platform (TLWP) production platform in Green Canyon Block 184 and the Central Processing Platform (CPP) in Green Canyon Block 52 (Figs. 1 and 2). Geophysical survey work was carried out in 1986, 1987, and 1988. Visual inspection of a rocky section of the route was done in 1989 using an ROV, but detailed discussion of this ROV work is beyond the scope of this paper. Difficult conditions along the deepwater portion of the 12-in.

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“…1,2 In addition, the engineering properties of soil deposits may vary horizontally across different geologic provinces as anticipated, but can change unexpectedly with depth as well. The geologic conditions can include: (1) steep slopes of 15 degrees or more; (2) irregular and sometimes rocky topography with relief ranging from several to tens of feet; (3) active faults with seafloor relief of up to more than 200 ft; (4) modern and relic landslides covering large areas; (5) gas hydrates; and (6) seafloor erosion of tens to hundreds of feet of sediment.…”

Section: Introductionmentioning

confidence: 98%

Deepwater Soil Properties and Their Impact on the Geotechnical Program

Quirós

Little

2003

All Days

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The sometimes unpredictable and highly variable nature of soil conditions in deepwater regions must be recognized in planning geotechnical site investigations. Examples of four different soil stratigraphies in deepwater are presented to demonstrate the variability and peculiar nature of some sediments and how important it is to plan for comprehensive programs both in the field and in the laboratory phases of a geotechnical site investigation. The authors will show how reliance on correlations based on index properties and failing to understand their limitations can lead to misinterpretation of soil properties.

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Engineering Implications Of Deepwater Geologic And Soil Conditions, Texas-Louisiana Slope

Cited by 7 publications

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Possible hydrate mounds within large sea-floor craters in the Barents Sea

Possible hydrate mounds within large sea-floor craters in the Barents Sea

Geohazards Surveying and Complex Seafloor Conditions Along Deepwater Jolliet Pipeline Routes

Deepwater Soil Properties and Their Impact on the Geotechnical Program

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