Developing Engineering Design Criteria for Mass Gravity Flows in Deepsea Slope Environments

Abstract

Abstract The ocean floor, from the outer edge of a continental shelf to the abyssal depths, is subject to a range of fast sedimentcharged bottom currents, such as turbidity currents, mud flows and debris flows. These currents are generally called mass gravity flows. The recent expansion of the offshore industry to the deep ocean hydrocarbon sources and the laying of ultradeep pipelines have brought about the need for engineering design criteria for these flows. We have developed a system for establishing engineering design criteria for mass gravity flows that includes methods for acquiring appropriate field data, the application of nuclear age dating and numerical modeling to produce relationships between the magnitude and reoccurrence intervals of these episodic events. The methods are derived from the concept that, although it is impractical to solve this problem with direct measurements of mass gravity flow events in an intended project area, it is possible to collect sufficient data from seafloor features and sediments to adequately constrain numerical models of the flows. The observed erosional and depositional features are synthetically reproduced. Thus the current speeds, heights, width and densities determined in the modeling are available for design criteria analysis. Introduction Designers of ultra-deep pipelines in the Indian Ocean, South China Sea, and Black Sea have all encountered the need to evaluate risks posed by mass gravity flows. These include turbidity currents along the Oman India Pipeline (Mullee, 1995) and the Malampaya Pipeline (Schneider and Groenveld, 1998) as well as both turbidity currents and debris flows along the Blue Stream Pipeline (Niedoroda et al, 2000). Mass gravity flows are rare and generally unpredictable events, making risk evaluation from direct measurements impractical. Scale laboratory experiments are difficult to relate to prototype conditions, because scaling relationships are not well known. Because of these difficulties in developing engineering design criteria, an alternate approach was developed. Several common features of mass gravity flows have been exploited. Their physics are relatively simple, involving balances between the entrained mass of sediment, friction and gravitational acceleration. Mass gravity flows reoccur in seafloor canyons and channels that can be accurately mapped, so the slopes and channel widths are known. These flows consist of moving masses of sediment and water that have distinct patterns of seafloor erosion and deposition. In most instances the seafloor deposits have formed from the action of a large number of individual events. Examination of sediment cores allow the properties of these deposits to be described. Background The nomenclature for mass gravity flows in the scientific literature is not standardized, and different names have been given to the same phenomena by different authors. Types of mass gravity flows can be categorized by their conditions of initiation, the details of the fluid behavior during their travel, the means by which they dissipate energy, or their sediment characteristics.