Bottom Current Extremes in the Deep Central Gulf
Authors: Cooper Cortis K. 
Published: May 2011

Over the next few years, a multi-billion dollar pipeline infrastructure will be built to transport oil and gas from new production fields in ultra-deepwater frontier regions in the central Gulf1 north to the existing pipeline infrastructure. Some of these lines may reach more than 100 km in length and will pass through a region with complex, steep bathymetry. Ocean currents will exert the extreme environmental loads since the water is deep and surface waves are of no consequence. Little has been published about extreme bottom currents in the deeper regions (&gt; 500 m) of the central Gulf. This paper analyzes several long-term measurement sites along with output from a hurricane model and a Loop Current eddy model. The results suggest that the stronger bottom currents can come from three sources: inertial storm currents, Topographic Rossby Waves, and Loop Current eddies (LCE)2. Nominal n-yr conditions are calculated at water depths of 500, 750, and 1500 m. In 500 m of water, the 100-yr current is governed by LCEs but just barely. In deeper water the 100-yr value is probably controlled by the inertial current generated by the passage of a hurricane. At all depths, the 100-yr near-bottom current speed is estimated to be about 40 cm/s but there is a lot of uncertainty. Both the hurricane and LCE models need further improvement in water depths greater than 500 m. However, the biggest contributor is the uncertainty in the vertical gradient of the velocity near the bottom. Existing measurements, albeit limited, suggest bottom intensification can be substantial, e.g. the speed at 17 m above the bottom can be double that at 100 m. This presents a major challenge since pipeline designers need the estimates within roughly 1 m of the bottom yet most historical measurements are taken 10's of m above the bottom. Nor do the present numerical hindcast models offer sufficient vertical resolution to resolve this challenge.

Introduction

Over the past decade there have been many new discoveries in the Wilcox and Miocene trends in the ultra-deepwater of the central Gulf. Examples include BP's Atlantis, Chevron's Big Foot and Jack-St. Malo, Petrobras's Cascade/Chinook, and Shell's Stones. Most are connected by SCR's which are sensitive to current-driven fatigue at the point of seafloor contact. More significantly, the facilities require a new infrastructure of pipelines some of which may stretch 125 km.

The deep central Gulf depicted in Fig. 1 is characterized by a broad plateau that gently slopes from the continental shelf break to about 2000 m over a horizontal distance of 50-125 km. While the region is gently sloped on a large scale, it is dotted with steep salt domes and minor escarpments with scales of order 10 km horizontally and 500 m vertically.

Environmental loads on the new pipeline infrastructure will be driven by the so-called ‘steady’ bottom currents3. Accurately quantifying those bottom currents is especially important because stabilization (e.g. added weight coating, minimizing spans, etc.) can add major costs given the length of the pipelines and the water depth.

Little has been published about extreme bottom currents in the central deep Gulf. The notable exception is DiMarco et al. (2001, henceforth " DHJ??) who, after carefully compiling all available current measurements in the Gulf as of 1999, spent considerable attention on extreme currents. However, the highest extreme they found at the time in the central Gulf was only 37 cm/s measured during a cyclonic eddy event in 674 m of water from a mooring that was deployed for only a few months. The second strongest current was less than half that (18 cm/s) measured in 845 m during a tropical cyclone.

The next section identifies the various data sources while the third section examines and discusses the extreme events and provides nominal n-yr values. This section is broken into four subsections according to the depth: 500, 750, 1000, and 1500.