Some of the comments in the oildrum.com are really top quality information, but they vanish into the ethersphere far too quickly as the next day’s enormous comment thread commences.

Today shelburn did a great job explaining how to understand/interpret ROV video footage.

shelburn on May 24, 2010 – 11:20am Permalink | Subthread | Parent | [Parent subthread ] Comments top

VIEWING ROV VIDEO

Apologies to those who already read this on a previous thread.

There should be a short course in viewing ROV videos to go along with this live streaming. There is always a learning curve when a new oil company man comes onboard until he understands the peculiarities of ROV video. A lot of this is not intuitive and no one should feel stupid because things in the pictures confuse them. I’ll try to hit just a few of the highlights.

Debris in the picture – This area of the Gulf of Mexico is covered with incredibly fine soft mud. If an ROV is working near the bottom and he has to move upwards his thruster wash will hit the bottom and stir up this mud and any other light debris (hydrates, tar balls, strings of heavier petroleum products, etc). The currents at this depth are usually slow and it can take a long time, as much as 15 minutes or more, before the visibility clears. And if the ROV is working anywhere near the bottom it is very difficult not to occasionally kick up some mud.

There are a number of other ROVs working in the area (any light you might see is another ROV vehicle or cage). So if any ROV upcurrent from the one feeding the live video stream kicks up some mud and debris that will drift over the area and cloud or obscure the picture.

It is also possible that another ROV is doing some excavating which will stir up LOTS of mud.

If there were any actual “eruptions” of gas coming from around the casing or elsewhere I doubt conditions would return to normal.

I would guess the “snowstorms” of white material are methane hydrates. They are near neutrally buoyant so chunks would float around just like the mud does. Methane hydrate at the surface melts at about the same temperature as water, 0 deg C (32 deg F), but at that depth the ambient pressure is over 2,000 psi and the melting point of methane hydrates is 18 deg C (64 deg F) and the surrounding water temperature is about 4 deg C (40 deg F) so they stay “frozen”.

Color – Be careful about making any judgments based on color. There is no ambient light at that depth so everything you see is being illuminated by the lights from the ROV. ROVs typically carry lights that may have different color temperatures and the angle of the light, the intensity and the distance between light and subject, the amount of turbidity (mud) will all have an effect on the apparent color your eyes see. I noticed that the leaks coming out of the kinked riser often seem to be different colors. I think that they are actually all the same and the visual differences are due to the factors I just listed.

Distortion – ROV cameras are usually ultra wide angle and have considerable distortion. A straight length of pipe can look like it has a very pronounced curved bend in it when viewed from one angle and then look straight or even bent the other way when viewed from different angles. This is one of the factors the task force measuring the flow will have to compensate for if they try to measure flow based on videos.

For some reason the human eye is comfortable with very high levels of distortion. For example how many people do you know who stretch a standard 4:3 TV picture to fit their 16:9 wide screen. They would rather view a picture with 33% distortion that to have the smaller but undistorted picture.

Also the apparent difference in the size of a leak between a video you saw earlier and one you are looking at now could be because the pilot has zoomed his camera in or out or has switched to a different camera with a different focal lens. ROVs typically carry several cameras. Or it could be because the volume of the leak has changed.

Holes in the Seabed – Originally the broken end of the riser was partially buried in the mud. Before the RITT was deployed the ROVs spent a lot of time excavating around the riser to get access to insert the RITT and probably to try to get the mud down some feet below so they wouldn’t stir it up and obscure their visibility. Typically these excavations are a series of large holes as the ROV often has problems seeing where he is working. When the ROV pans around and you see holes in the seabed that is most likely what you are seeing.

This entry was posted on Monday, May 24th, 2010 at 20:11 and is filed under Our World. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.