Scope and application

The objective of this RP is to provide rational criteria and guidance on design methods for pipelines subjected to interference from trawling gear; including the impact, pull-over and possible hooking phases. Design criteria are given as well as guidance on applicable calculation methods.

For pipelines subjected to global buckling, such as:

1) pipelines with release of effective, compressive axial force (buckling) prior to trawling, or

2) pipelines with release of effective, compressive axial force simultaneously with trawling, i.e. the trawl load triggers global buckling.

Global buckling pipeline response analyses is required in combination with the trawl load assessment. The global buckling analyses needs to include sensitivity studies of governing parameters such as pipe to soil interaction in accordance with DNV-RP-F110, to establish the proper functional load condition factor.

This document is applicable to rigid pipelines with outer steel diameters larger than 10" (i.e. the smallest pipe diameter tested in model tests have been used as basis for this RP). However, if measures are taken in the analyses to account for scaling effects outside of the model test validity range, the methodology reflected in this RP may still be applicable. Typically, this would involve special assessments of soil to pipe interaction factors (ref. Figure 3-3), global bending stiffness of pipeline and factors related to relative sizes between pipe and trawl equipment.

For pipelines with outer steel diameter less than 10", the soil reduction factors based on Figure 3-3 should conservatively be based on 300 mm outer diameter for impact load calculations.

Loads and load effects on flexible pipelines can be determined, provided relevant flexibility characteristics and acceptance criteria are specified.

Trawl pull-over and hooking may govern the acceptable free span lengths and gaps. These aspects are covered within this RP, while the effect of environmental loads on free spans is covered by DNV-RP-F105.

This document does not cover pipeline attachments such as anodes. However, it is envisaged that such equipment is designed for trawl loads both with respect to impact and pullover including abrasion from trawl wire. Additionally, edges and protrusions and or bolts that may snag trawl nets need to be avoided.

Further, a methodology for qualifying possible DEH cable attached to the pipeline inside a protective structure is given in Appendix A.

This RP is intended for use on a world-wide basis. However, the collection of trawl gear data has been carried out for the North Sea and the Norwegian Sea. Data is given appropriate for otter, beam and twin trawling equipment in use in 2005 and expected for use in the near future in these areas.

The following design aspects are considered:

— coating damage due to impact

— pipe denting due to impact

— overstress due to pull-over or hooking.

This comprises the following topics:

— most critical trawl equipment

— frequency of trawl impacts

— effective impact energies to be absorbed by the coating and the pipe

— requirements to structural modelling

— recommendations for pull-over loads

— recommendations for lifting heights due to hooking

— acceptance criteria.