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 response analyses are 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 (see 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 pull-over 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 App.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 2014 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.

The following tests are included for qualification of coating and strapping system:

— impact tests

— scraping tests.

Scraping tests are new to the present revision of this RP and has been included due to observed coating damage as the trawl boards scrapes over the pipeline when it is being pulled over.

The design aspects are covered for bottom trawl gear, hence pelagic trawl gears are not considered herein.

Figure 1-5 shows part of a flow chart for a typical pipeline design. After deciding on diameter, material, wall thickness, trenching and coating for weight and insulation, the trawling design is performed. It must be emphasised that the trawl pull-over assessment must be based on a realistic estimate of the effective axial force, and any changes due to sagging in spans, lateral buckling, end expansion, changes in operational conditions etc., must be properly accounted for. Unacceptable results from trawl impact, pull-over and/or hooking may change the sequence in this design flow chart (Figure 1-5) as it may have impact on coating and burial of the pipeline.

The response to trawling over anchor lines, mooring lines, catenary risers and etc. is not covered by this RP.

For guidance on pipe-soil interaction related to interference with pipelines, reference is made to DNV-RP-F114.

DNV-ST-F101 Sec.1 states that in case alternative methods and procedures are used, it shall be demonstrated that the obtained safety level is equivalent to the requirements in DNV-ST-F101, i.e. that the nominal failure probability is equal or less than the target values (see DNV-ST-F101 Sec.2). For components, equipment and systems, which are not already covered by a validated set of requirements such as given in DNV-ST-F101, technology qualification shall be performed to document acceptable performance and margin to failure (recommended practice on technology qualification is given in DNV-RP-A203).

Further, DNV-ST-F101 Sec.2, requires that the bursting limit state must be satisfied. This opens the possibility for using different methods or procedures than those presented in DNV-ST-F101 or in any DNV recommended practice.