Geohazards Surveying and Complex Seafloor Conditions Along Deepwater Jolliet Pipeline Routes
Authors: Campbell K.J. , Tillinghast W.S. , Hoffman J.S. , Roulstone J.A. 
Published: October 2008

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.