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How to plan my sparse LBL array (Guidance Note)

21 June 2021

This article aims to help planners avoid the standard pitfalls of planning a Sparse LBL array.

Planning an array ahead of time

It is best practice to plan a transponder array deployment before it is placed on the seabed. This can be for a number of reasons and can highlight potential issues before the job has started:

  • Avoid areas such as overage curves anchor scour locations areas deployment corridors etc
  • An array plan will include an SV analysis that will define the expected range of your Compatt transponders at different depths. This will avoid  a situation where a Compatt that you need, maybe  is out of range
  • The seabed is not always flat, strategically placing Compatts at the top of mounds for example, can lead to less Compatts being needed so less vessel time is needed and money is saved.
  • Do you need specialist software to do an array plan, arguably yes. Though Sonardyne does offer an inhouse array planning service and will soon offer a cloud-based planning portal for all those who want to create their own arrays.

In order to perform an array calibration, you will need:

  • A Sound velocity profile (SVP) taken at the location in question (preferably at the same time of the year) and covering all depths at that location (SV smoothing / QC may be necessary depending on the quality of your SVP)
  • A .xyz Digital Terrain Model (DTM) of the area in question preferably under a Gb in size .
  • A .dxf drawing that covers all relevant subsea infrastructure. Preferably one that does not have every nut and bolt included as the smaller the drawing the more efficiently the system will run.
  • A list of x y coordinates for the Compatt along with transducer heights (usually depending if you are using stands / floats or a combination of) (Z is usually defined by the system as it plots the x y and adds the transducer heights).

Once you have everything it is worth working out the distance that you want to cover and defining the type of array that you are going to be using:

  • Standard LBL,
  • This will require two box-ins or two known coordinates, at least one SV Compatt and a standard ROV setup
    • No INS needed.
  • 2D SLAM calibrated LBL,
  • This will require two box-ins or two known coordinates at the beginning and end of a predefined route and no more than 6 Compatts between box-ins
  • Or if working in an array you could conceivably calibrate the array relative to one Compatt/boxin
    • INS needed.
  • 3D SLAM calibrated LBL
  • This will at least one box-in or a known coordinate.
    • INS needed.

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