Placing the MRU, one of the most delicate (and forgotten) parts of Mobilization planning

by Umberto on 18 October, 2020 in Online

I have seen it way too often.

Mobilization plans detailed up to the centimeter of welding needed and the minutes of crane utilization.

But do you know what I have seen often too?

The same mobilization plans lacking in info about the vessel for the surveyors that were going to perform the job.

Info like any free space on the monkey island for the DGPS placement or where it was possible to install the MRU.

Well, for the DGPS, if you get the offsets right, every place is the same (as long as the antenna isn’t covered or next to large metal surfaces that could lead to multipath or next to antennas that could disturb).

Unfortunately, the same way of thinking cannot be applied to the MRU. Its placement influences heavily the results of the survey, especially if it is an MBES survey.

Iportant Premise - MUST READ

Here a little note is mandatory.

Most of my experiences are about mobilizing a vessel for one job only.

That means using supply vessels for survey purposes and installing on them temporary instruments, like LARSs, multi-sensor poles, and so on and removing them once the job was done (usually one or two months later).

Unfortunately, I have never participated in the mobilization of a survey vessel meant to stay that way for all its life.

Therefore, here you will find my point of view only on this side of the job, probably also biased by the fact that sometimes the vessel becomes available only a few weeks before the job, giving a really tiny time frame for the planning.

If you are ok with this we can jump right in.

Let’s see what I’m talking about.

MRU: what is it? And why is it so crucial to position it correctly?

But, before we dive in, I must be sure that we are on the same page. So here it is a description of what an MRU (Motion Reference Unit) is according to Wikipedia:

Motion reference units are a kind of inertial measurement unit with single- or multi-axis motion sensors. They utilize MEMS gyroscopes. Some multi-axis MRUs are capable of measuring roll, pitch, yaw, and heave. They have applications outside the aeronautical field, such as:

Antenna motion compensation and stabilization
Dynamic positioning
Heave compensation of offshore cranes
High-speed craft motion control and damping systems
Hydroacoustic positioning
Motion compensation of single and multibeam echosounders
Ocean wave measurements
Offshore structure motion monitoring
Orientation and attitude measurements on Autonomous underwater vehicles and Remotely operated underwater vehicles
Ship motion monitoring

Source: Wikipedia

As I would say it:

"The MRU is an instrument that lets you understand the angular movement of the object it is installed on."

Since those angular movements are used to correct position results for the Pitch, the Roll, and the Heave movements of the ship it is crucial to position it good to let us have the best correction we can.

You may think since it measures the angular movement and since the angular movement is the same along an axis that the results will be independent of its installation position.

Unfortunately, it isn’t like that at all. Depending on its position, the MRU could record erroneous data and, therefore, corrupt the rest of your results.

Let’s see of which error I’m talking about.

Which error may be caused by the wrong MRU placement?

To clearly see the errors we have to understand that the MRU doesn’t simply measure angles (like we like to think in our world made of perfect AutoCAD drawings), but it measures movements and forces and translates those in angles.

That’s the main reason we can have errors in MRU readings that lead to not-so-good results.

I’ll try to give you an example.

Multibeam Survey. Over-the-side pole.

Have you ever seen a survey performed with the MRU on the pole? Some surveyors think that it is the best place to install an MRU for an MBES Survey because it will record the actual MBES movement, giving the best correction possible.

I was one of those.

Until I understood how wrong I was.

Let me clarify this with a picture.

Error of placing MRU on pole

Representation of the error due to MRU placement.

As you can see in the image above, when the vessel rolls, the MBES, together with the sideways motion, is also lifted.

Since this, if the MRU is placed next to the MBES it will record some of that roll motion as heave motion, giving erroneous correction values to the navigation system.

If you were a surveyor on that boat you would end up continuosly worrying without understanding why your MBES data isn't properly corrected and is more "wobbly" than it should.

Possible Application of MRU on MBES pole

I’m not saying that this configuration can never be used though.

It is particularly useful in shallow water jobs using small vessels (2-4 meters long), also because with a small boat the MRU error is drastically reduced.

But why some companies still use this approach also on a bigger vessel?

Because putting everything together simplifies the mobilization and reduces the mob/demob times, thus saving money.

The three main ways of installing an MRU. Which is the only one that will lead you to a good survey result?

So which is the only right way to install an MRU?

Let’s start seeing what are our options:

  • On the survey pole
  • Next to/Into the survey room
  • Next to the vessel’s COG

Well, we have already discovered that the pole, even though it could seem the smartest point to install it, is not the best place if we want to have accurate MRU readings and corrections.

And you know what?

The same applies to the survey room installation.

It could be easy, both to install it there and also to check on it from time to time. But the only downside of it is that it probably is far from the real rotation point of the vessel, causing wrong readings due to the bigger movements.

It should be clear by now that the best option should be to install the MRU on the COG of the vessel, or at least as near to it as it is possible.

We know that this is rarely feasible, sometimes we have short cabling, sometimes no power in the right place, some other times no way to pass the cables.

So how can we overcome this? Here are a few tips I learned during the last years.

  1. If you cannot put it right on the COG, try, at least, to put it on the central axis of the vessel.
  2. If the vessel backdeck is empty (supply vessel used as a survey vessel) its COG will be more forward so it could probably be installed inside the living quarter and still be next to the COG.
  3. Try to get the input of the vessel MRU (they are usually on the COG and sometimes are survey-grade) and run periodic comparisons to the one(s) you installed.
  4. Ask the crew where the vessel MRU is placed and use the vessel arrangement plan to estimate how far it is from yours.

FREE Resource: one of the best manuals I have ever seen.

I wanted to leave you a free manual that I think is one of the most comprehensive materials about our job.

It is disguised as an R2Sonic 2024 manual but it is more than that.

It explains the theory behind a whole MBES survey, from the DGPS to the CTD, to the MRU to the MBES.

Look for the Appendices, starting from page 76. It is worth to give it a read in your free time.

Believe me, you won’t be disappointed.

Here is the link to it: LINK

Catch you around us fellow Smartest Surveyor. If you have any question feel free to comment below.

Chat soon,

Umberto


Tags

Instruments, Tips & Tricks


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