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Chapter 11

Trawl Impact, Pullover and Hooking Loads

11.1 Introduction
The interaction between fishing gear and a pipeline is one of the most severe design cases for
an offshore pipeline system. The reason for the seventy of the impact, pullover and hooking is
not well described by the industry today. The damage to the pipeline (and to the fishing gear
& ship) is very dependent on the type of fishing gear and the pipeline conditions, e.g. the
weight and velocity of the fishing gear and the wall thickness, coating and flexibility of the
pipeline. The most important issue with respect to design of fishing gear resistant pipelines is
the ability to make a realistic description of the applied loads and their time history, and
pipeline resistance, i.e. the pipeline configuration on the seabed including freespan and the
pipe stiffness. The summary of loads, response analysis and acceptance criteria are listed in
Table 11.1.
Table 11.1 Summary trawl impact, pullover and hooking.
I I Time I Load I Solution Design acceptance Parameters
Design
criteria
Impact mseconds mass velocity mass-spring dent damage in energy absorption

seconds

minutes

11.2 Trawl Gears

11.2.1 Basic Types of Trawl Gear
Bottom trawling is typically conducted with two typ s of trawl gear in North Sea: Otter and
Beam. Otter trawling occurs down to depths of more than 400 m. Generally beam trawling
occurs in water depths down to 100m. The Otter trawl board is a more or less rectangular steel
board which holds the trawl bag open, while the beam trawl consist of a long beam which
holds the trawl open. The beam has beam shoes on each end and an impact is assumed to be
from these beam shoes.