Wave-Induced Scour Around Offshore Pipelines

ABSTRACT The size, numbers and applications of offshore pipelines are steadily increasing; pipelines carry crude oil from oil fields to shore; refined oil from offshore ports; municipal and industrial waste from shore into deeper water; in the future, bulk commodities may be carried from offshore ports to shore. In all cases, a pipeline must cross the surf zone before getting into deeper water and in most cases, the pipelines are in the zone of intense dynamic action caused by waves during storms, appreciable movement of sediment, onshore currents and littoral currents. There have been many failures of offshore pipelines during the last ten years and there appear to be several reasons for the failures, notably inadequate cover and low "specific gravity" of the pipeline. Under the first category the depth of burial may be insufficient, the quality of burial soil may be inferior to the soil outside of the trench, or the compaction of cover soil may be inadequate. Under the second category the pipe may actually float to the surface of the ocean bed as the soil around the buried pipe liquefies. An extensive laboratory study was conducted to evaluate the development of underwater bars and scour patterns around the pipe with the pipeline buried at various depths below the ocean bed. The analysis of experiments indicates that depth of scour around offshore pipelines under certain wave conditions is greater than anticipated. The pipe itself causes additional scour around it. Recommendations as to the minimum depth of burial are given. INTRODUCTION In more recent times, the energy crlS1S has caused acceleration of exploitation and exploration of the oil and gas reserves under the continental shelf. The most economical way of transporting oil and gas is through pipelines from offshore platforms or underwater completions to shore. In another widespread application of pipelines, environmental considerations and regulations call for the disposal of municipal and industrial waste in deeper water, both in lakes and oceans, than previously. Here, as in all engineering design work, a better understanding of soil-pipe interaction and magnitudes of various forces acting on an unburied pipe is required before good design criteria and engineering practices can be well defined. Experimental studies, mainly in the laboratory, but some in the field, have been conducted in many countries, notably in the U.S.A., the Netherlands, and Japan. While the petrochemical industry is evaluating pipeline failures and design practices, such technical information is generally restricted and not available. Since design engineers have been made more conscious of failures in more recent years, pipelines are sometimes over-designed and excessively expensive to construct. Over the past 10 years or so, the incidence of reported submarine pipeline failures, particularly of large diameter pipelines, has increased markedly. The American Society of Civil Engineers appointed a Task Committee on Pipelines in the Ocean in 1969 to define the problems of pipelines in the oceans and to determine the state-of-the-art in the design and construction of pipelines in the oceans.