Technical Capabilities in Offshore Pipellne Operations to Maximize Safety
Authors: Milz Elmer A. , Broussard D.E. 
Published: October 2008

Design, construction, and operation of offshore liquid and gas trunklines have many similarities to lines onshore. However, there is one major difference; cost. Offshore pipelines are significantly more expensive than those onshore to construct and to operate, and failures are also more expensive. Other factors which make offshore pipelines different from those onshore are environmental conditions (wind, waves, and currents; bottom soils which are usually soft and easily deformed; and seawater as a corrosive medium), inability to physically see the pipe once it is in place, and relatively great distances between locations at which contact is made with the pipe. For these reasons, offshore pipeline construction is usually conservative and offshore pipelines are operated and maintainer so as to minimize risk of failure,

This paper reviews the design, construction and operation of offshore pipelines, with emphasis on steps which are normally taken to maximize safety and thereby to minimize the cost of failures. Offshore pipeline failure experience is reviewed and governmental regulations which govern offshore pipelines are discussed.

INTRODUCTION

Pipeline transportation is the world's safest method for moving bulk materials. U. S. Department of Transportation statistics for the period 1963 through 1968 (Figure 1) show that the rate of fatalities per billion ton miles of commodities transported was only 0.011 for pipelines as compared with 10.9 for highway truck transport, 2.5 for rail and 0.31 for marine (a factor of 30 safer than the next safest means and a factor of 1000 safer than highway transport.) Marine pipelines (total trunk lines in marine environments now equal about 13,000 miles) are presently safe and capability exists to continue to construct and operate in a safe mode even in much deeper water. (Present maximum depth of marine pipelines is about 400 feet.)

U. S. Geological Survey (U.S.G.S.) statistics (Table 1) indicate that only four major pipeline breaks world-wide occurred in marine environments during the past 14 years. (Statistics cover spills of 1,000 barrels or more.) This compares with a total number of major oil spill instances into marine environments (1,000 barrels or more) of 49 during the same period.

The purpose of this paper is to examine offshore liquid and gas trunk line design, construction and operation practices and capabilities as they relate to potential hazards.

DESIGN

There are several aspects of the marine environment that require special design considerations. The high cost of construction and repair give economic incentive to utilizing the highest quality of materials, both for the pipe and for corrosion mitigation. Rigid specifications augmented with careful inspection and quality control procedures are required for marine pipelining.

A great deal of knowledge exists with regard to the behavior of steel and steel fabricated tubes. The oil and gas industry, in cooperation with the steel manufacturers, have developed high quality, high strength steel which is manufactured into pipe to rigid specification Substantial research has been done and is presently in progress to gain a complete understanding of the behavior of steel pipe in both the elastic and plastic stress regions.