What’s is bottom roughness?It’s an analysis to predict the location of span along subsea pipeline after installation or during operation. So, what’s the different between free span analysis?free span analysis is “only” determine the maximum allowable length of pipeline that unsupported when laid on seabed. The criteria of allowable span is divided into VIV(Vortex Induced Vibration) and ULS (Ultimate Limit State or Static Criteria). Why bottom roughness analysis is so important?Without bottom roughness analysis you don’t know how many support (sand bag, grout bag, etc) that you need to rectify of over span. However, it’s only prediction, so you need further inspection to ensure the location of over span.

2D OBR analysis is very simple, you can use any FEA softwere such as ABAQUS, ANSYS, etc..For this note, I used ABAQUS FEA as mny OBR simulator. The pipeline can be modelled using 1D element (PIPE31H) and the seabed can be modeled using anlaytical rigid of surface. The common step that used for Bottom Roughness is adopted from Subsea Pipelines & Riser book (2005,Yong Bai & Qiang Bai)

After ODB file is produced from your ABAQUS FEA machine, for the next step you need to extract COORD1 (coordinate of pipeline KP), COORD3 (water depth of your Bottom of Pipe [BOP]), and CPRESS (Contact pressure as indicator only to make sure each node of pipeline BOP is touching seabed or not). You can use python file or extract manually using Abaqus Viewer.

Before you try process further with your excel spreadsheet, you must interpolate your water depth of seabed (that has been inputed in abaqus) against your COORD1 data (coordinate of pipeline KP), then you will get new water depth of seabed with same KP with your COORD1 data. Divide your Excel spreadsheet into 2 Sheets.

In the sheet no.1 you need to set your sheet as a sample below. You will see any number span start & span end along the pipeline. Please make sure during pipeline embedded (negative value of span gap) there are any value of CPRESS and vice verse. The maximum gap value along pipeline must be inputted into allowable free span analysis (onset/screening/fatigue criteria as per DNV RP F105)

After that, you have to “sort & filter” each span start & span end, then copy this KP into sheet no.2 as a picture below. From each this formula you will get the overspan results. Please notice that this screening will used maximum gap value along KP start and KP end.  The minimum span gap value that influence VIV, is 30% OD (Ref: OTC No. 4455, 1983, The Influence of Boundary Layer Velocity Gradients and Bed Proximity On Vortex Shedding from Free Spanning Pipeline). Some people used 20% OD (Ref: Sumer and Fredsoe, 1989, Hydrodynamiccs around Cylindrical Structures) as conservatism result

The analysis has not over yet. First, you need to check with centre third of pipeline criteria. If the average center third of pipeline below allowable gap value, you can ignored this over span if it is caused by VIV only (not ULS/Static criteria). But if, you want get the conservatism result, you can consider this span is over span (But it will affect your span rectification cost). Second, you need to check the uplift possibility (or look like upheaval) in your ABAQUS FEA model. It should be not happened because the weight of pipeline and wave/current will forced this uplift section to drop on seabed as lateral buckling. So, if your over span is occur in this uplift section, please ignore it as a span, but you must check the integrity of this section (Using Load controlled Check & Displacement Control Check as per DNV F101). Third, you need to reassessed again each over span section with each gap value and recalculate again in allowable free span assessment, so you will get new allowable span that longer than before.

Your 2D Bottom Roughness Analysis is end from here. But if you want, you can do re-analysis (start form sheet-1) with interactivity phenomenon [during VIV, your span is will induced vibration with its neighbor span, so make this effective length is longer than isolated span]. As per DNV RP F105, you can determined each  span is a kind of isolated or interacting span as a diagram below.

Source : Danar Yurindatama, " 2D Pipeline Bottom Roughness", January, 2013