Assessment Of Detailed Seafloor Mapping Techniques For Pipeline Span Analysis And Platform Siting
Authors: Mairs H.L. , Clukey E.C. , Crossley C.W. 
Published: October 2008

ADSTRACT

High resolution swath bathymehy was collected with a deeptowed vehicle, along two pipeline route corridors in the Mississippi Canyon region of the Gulf of Mexico (GOA$. Swath bathymetry was also collected with shallow-towed and hull-mounted swath systems over portions of the same region. The recent installations of these two pipelines and a platform jacket have provided an opportunity for assessing the electiveness of swath bathymetric mapping techniques for evaluating pipeline spans and platform levelness. The effectiveness of the swath technology was evaluated based primarily on the comparison of results obtained from the regional deep-tow survey and the post installation observations. Data from all three swath systems were also compared. Overall, the survey results provided accurate data concerning the water depth and jacket inclination. Pipeline spanning predictions for the pipeline were likewise very encouraging.

INTRODUCTION

High resolution geophysical surveying is an important component of offshore facilities and pipeline sitting. The irregular seafloor conditions in the deepwater GOM can be challenging for pipeline routing and platform installation. Therefore, it is important to identify& regions where significant lengths of pipeline may go unsupported and to identify local slopes that could effect platform levelness. Conventional methods of single beam bathymetric data collection may not provide the data resolution and seafloor coverage necessary for performing detailed pipeline span and jacket leveling analysis.

Recent advances in surveying techniques have significantly improved the capabilities for accurately defining seafloor conditions. One of the most important new advancements has been the development of swath bathymetry. This technology provides the capabilities for more accurately efining seafloor conditions by collecting data across the entire survey swath rather than just underneath the survey vessel or tow fish. This added capability provides the resolution required to determine pipeline spans. Several methods are available for collecting swath data. Sources may be towed near the seafloor (deep tow) or higher up in the water column (shallow tow), or may be mounted on the hull of vessel (hull mounted). The importance of accurate high resolution geophysical surveying increases in areas with irregular seafloor. The Mississippi Canyon, where two pipelines and a steel jacket have been recently installed, qualities as such an area (Figure 1). The platform is located on the Alabaster Knoll, on the western flank of the canyon. The knoll formed as part of the erosional processes that formed the Mississippi Canyon. It is very rugged and ranges in water depth from about 460' at the crest to about 1200' at the base.

One of the pipelines, a gas flow line, runs from Alabaster to the Zinc subsea template, Located near the center of the canyon in approximately 1460' of water. The seafloor features on the canyon floor itself are relatively mild and the seafloor slopes to the southeast at about 1.30A. The flow line from the seafloor template is routed to the west and up the Alabaster knoll in a southwest direction to the Alabaster platform. The total length of the gas flow line is about 31,000'.