Trawl Gear Protection Within Platform Safety Zones

This paper deals with the risk to human lives, installations and the environment that trawling activity within the platform safety zones may cause. Due to the high consequences associated with failures of pipelines and subsea installations in the vicinity of offshore platforms, the zone around (manned) platforms are in some design codes, such as the DNV-OS-F101 for submarine pipelines [1], defined as a safety zone. Such a zone implies that pipelines and risers are designed to a higher target probability of failure, i.e. an extra safety margin is incorporated. To further reduce the probability of failure and, hence, increase the safety, extreme load like trawl interference may be avoided by declaring the safety zone as a trawl-free zone. This means that no fishing gear, such as bottom trawl gear, shall intrude this zone. Because this zone is defined as a trawl free zone, it is common practice not to consider trawl loads within this area, i.e. pipelines and subsea installations are not necessarily designed as over-trawlable. The paradox is that in some areas with extensive trawling, the manoeuvring of trawlers may easily cause the bottom trawl gear to intrude the safety zone and thereby expose the trawler, its crew and the platform with its crew to significant risks. This paper discusses one case where a trawl caused some damage to subsea installations well within the safety zone. It is demonstrated how the bottom gear may intrude the safety zone as the trawler round the platform zone. Different measures to avoid this behaviour or remove/reduce the associated risk are also discussed with background in the real case.