Abstract

In 2008 BP Alaska required information on ice parameters offshore Northstar Island in connection with an engineering review of the island?s armoring system. To acquire such data Woods Hole Group Inc. designed, built, and deployed three bottom mounted oceanographic instrument platforms. Two of the bottom platforms were equipped with ASL Environmental Science Model IP-5 Ice Profilers and Nortek AWAC Acoustic Surface Tracking (AST) Wave and Current Profilers. The platforms were deployed in August 2008 in water depth of about 35-40ft and were recovered in August 2009. The BP Alaska 2008–09 measurement program was the first use of Nortek?s AWAC AST with the Nortek Internal Processor (NIP) and extended memory for a long-term (1-year) measurement program under Arctic ice. The instrument was configured to collect current profiles and wave data in the usual way, and in addition it collected special " diagnostic?? high-resolution echo amplitude profiles sampled at a 1 Hz rate for 512 second bursts, alternating with the wave bursts, for measurement of ice draft. The advantages of this instrument lie in its combination of current, wave, and detailed ice draft measurements and its ability to record large quantities of data over a long period of time. Quasi-simultaneous measurements of ice parameters and ocean currents allowed estimation of the size and mass of the floating ice floes. The measurement program was successful and it highlighted strengths and weaknesses of the measurement systems used for current, wave, and ice measurements.

Introduction

Information about sea ice presence, ice keel draft and floe size is important for various maritime applications. For example, in connection with an engineering review of the armoring system, BP Alaska required information on ocean currents, waves, and ice thickness (ice draft), and mass of ice floes offshore Northstar Island in the Beaufort Sea. To acquire such data Woods Hole Group Inc. designed, built, and deployed bottom mounted oceanographic instrument platforms. Two of the bottom platforms were equipped with ASL Environmental Science Model IP-5 Ice Profilers and Nortek AWAC Acoustic Surface Tracking (AST) Wave and Current Profilers. The 2008-2009 BP Alaska measurement program was one of the first applications of Nortek's 'diagnostic burst' technology for long-term ice draft measurements. The detailed time series of echo amplitude profiles collected by the AWAC AST during each diagnostic burst permits (under certain assumptions) estimation of the size of floating ice blocks during times when the sea surface is partially open. The ability of the AWAC AST to collect this type of detailed ice draft information, together with directional wave measurements and current profile measurements, was a key factor in the selection of this relatively new technology for the BP Alaska program.

BP's Northstar Island is located several miles northwest of Prudhoe Bay. The continental shelf of the Beaufort Sea in this region is very gently sloping and the water depth is about 10-12m. In winter, the landfast ice of the Beaufort Sea completely surrounds the island, and in the fall and spring floating ice, especially ice blocks that can be tossed by the waves, can impact the island?s protective armor.

The platforms were deployed in August 2008 on an east-west line about one mile offshore of Northstar Island and were recovered in August 2009. The platforms were positioned not far from each other. Therefore, spatial differences in the wave and ice climates are not discussed in this paper.

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