Design and Construction Procedures for Proposed Arctic Offshore Structures
Authors: Gerwick Ben C. , Lloyd Ronald R. 
Published: October 2008

The long, shallow, continental shelf area of the Arctic Ocean immediately offshore the North Slope presents a number of major challenges to offshore construction. Ice, of course, is the predominant consideration. Several zones are identifiable proceeding seaward the lagoon, the islands, the shore fast areas, transition area, and pack area. The transition and pack area structures must be designed for a number of ice conditions: compressive failure of the sheet ice, rafting, pile up, and pressure ridges.

The combination of hitherto unprecedented lateral forces with the logistical problems of the North Slope and Arctic Ocean have required the development of new design concepts and construction procedures for offshore Arctic structures.

Several other factors are of prime importance the low temperature effect on construction material behavior under impact, the short open water season, and the extremely high labor and equipment costs.

After consideration of numerous alternatives, floating caissons and barges have been chosen with maximum pre-installation of equipment. Reinforced and prestressed concrete has been selected as the basic construction material because of its rigidity, structural behavior at low temperatures, and economy. Structures have been designed to be primarily self-contained for installation.

This prestressed concrete caisson barge approach has provided satisfactory solutions to the design and construction requirements and, in addition, provided the benefits of adaptability to variations in soils, provision of storage capabilities, and enclosed spaces for quarters and machinery.

Specific designs have been developed to meet the parameters of the several offshore zones, and construction procedures evolved to provide maximum flexibility for contingencies such as variation in sons, or early ice. These are presented in some detail, and an evaluation made of structural suitability, stability under tow, construction procedures, and economy.

Environmental Considerations

The Arctic Ocean adjacent to the North Slope is characterized by its shallow depth and gradual slope to deep water. At only a few locations can 100 foot depth of water be found closer than 25 miles offshore. The sea-bed consists generally of sedimentary deposits of silts, sands, and gravels, apparently alluvial fans from glacial streams during the ice age. Occasional subsurface ridges have contours resembling moraines; although perhaps they have merely been scoured up by deep ice ridges. Permafrost is believed to exist only in odd spots outside the chain of islands.

Air temperatures range from -40° F to +50°F with extremes of -65°F and +75 ° F. The water is very uniform in temperature, from +28°F to +30°F and very saline, except in the lagoons opposite the rivers.

Winds are predominantly from the east, 10 to 15 miles per hour, with a maximum of 50 to 60 knots. Fog is common offshore during the summer, especially along the edge of the pack ice.

Wind-generated waves are less than 5 feet high for 90% of the time that the water is free from ice. Tidal range is only one foot, and current is generally less than one knot, although some higher wind-generated currents have been reported.