Pipelining in 6000 Feet of Water- A Case Study of Washington Natural Gas Company's Puget Sound Crossing
Authors: Bomba John G. , Seeds Kenneth J. 
Published: October 2008

An important consideration in a deep water pipeline project is close coordination between the client, the engineer and the contractor. The Puget Sound gas pipeline project is an excellent example of this cooperation. Several construction methods were studied. A tight construction schedule, equipment availability, contractor's experience, and lower mobilization costs were factors in selecting the pull method for these crossings.

Design calculations by Williams Brothers established negative buoyancy for stability during and after installation, minimum allowable radius of curvature, and maximum allowable tension for installation. From this information, Mid-Mountain Contractors selected equipment and laid out work for maximum efficiency and security.

Amalog inspection of, all pipe at the coating yard was employed. Five non-repairable defective joints were discarded.

The careful attention to details in preconstruction planning by Washington Natural, Mid-Mountain and Williams Brothers was rewarded by the smoothness of the actual installation operations for the world's deepest operating natural gas pipeline.

INTRODUCTION

The deepest operating hydrocarbon pipeline in the- world was installed by Washington Natural Gas Company across Puget Sound near Seattle, Washington, in July and August 1969. Dual 8" crossings were made across the East Passage, between South 240th Street near Des Moines and Point Robinson on Vashon Island, and across Colvos Passage between Robinson Point and the Kitsap Peninsula (Figure 1).

The East Passage was the longer and deeper of the two (Figure 2). Horizontal distance was approximately 11,700 feet. Maximum water depth was 670 feet below mean sea level. Colvos Passage was a shorter, shallower crossing, with a horizontal distance of approximately 6, 600 feet and a maximum depth of 397 feet.,

The unusual design and construction aspects of these crossings were many and varied, as suggested by the following facts. Tidal variation during construction ranged form.-2 feet to +13 feet above mean sea level. Colvos Passage is known to have fairly high currents flowing through it. The 670 foot, depths at mean sea level of East Passage is equivalent to an external pressure of 298 psi. the shore approaches on each crossing were determined by available right-of-way rather than construction ease.

Project History

Early in 1963, Washington Natural Gas Company began a series of studies for supplying gas by pipeline to the Seattle Tacoma area. These studies indicated that the most obvious means was the installation of a supply main across the Tacoma Narrows. On this basis, Washington Natural applied to the State Highway Commission for permission to install a single 6?? supply main on the Tacorna Narrows Bridge.

After two year s of lengthy public hearings, the Highway Commission rejected the application, and the Gas Company was faced with the alternative of finding a suitable underwater crossing site. Available onshore right-of-way was the determining factor in selection of a crossing site, and accordingly, the actual route was restricted to relatively minor deviation from specified point to point line, regardless of environmental circumstances.