Abstract

Routine surveys of a subsea pipeline off the NE coast of Scotland in 2000 and 2002 revealed the presence of large numbers of encrusting mounds. These were perceived as a potential risk to the integrity of the pipeline and were subject to investigation.

The mounds are constructed by sabellarid worms. Individual worms trap sand from turbulent waters, cementing grains to form robust tubes and collectively forming structures up to 0.75 m in diameter, extending over kilometres of pipeline. Growth rates appear to be relatively rapid and the areas occupied increased significantly within the initial period of observation.

The mass and surface area of these structures could affect pipeline integrity in two ways: where the pipe is in span a dead load is applied as a direct consequence of the presence of the mounds. In addition, there is an increased lateral live-load as a result of the increase in surface area presented to currents. By contrast, overgrowth of the pipeline may ultimately offer additional protection. This account describes the nature of this novel addition to shallow water pipelines and assesses its likely impact. Observations in 2004 and 2005 indicated substantial reductions in the areas covered by sabellarid colonies and here at least whatever risk they may present has so far proved transitory.

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