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ROVworld Subsea Information FAQ (Frequently Asked Questions)



Category: Main -> Technical FAQ's

Question
•  We are trying to protect Aluminium with Magnesium anodes instead of Zinc as my client thought the Zinc anodes weren't active enough?
•  How do I splice a cable?
•  What is Broadband?
•  How Does Broadband Work?
•  How do broadband and dial-up compare?

Answer
•  We are trying to protect Aluminium with Magnesium anodes instead of Zinc as my client thought the Zinc anodes weren't active enough?

Cathodic protection is an electrochemical process which halts the natural reaction (corrosion) of metals in a particular environment by superimposing an electrochemical cell more powerful than the corrosion cell. Sacrificial Anodes are fitted or bonded to the metal to be protected which in turn as it has a greater electrical potential than the anode material becomes cathodic and causes the anode to waste instead of itself. The number and size of anodes is determined by the type of material and the surface area being protected.

The majority of ROV frames are made of aluminium (some TMS's are mild steel, other frames can be Stainless so corrosion is not a problem). If you look at a table which lists the potential differences (voltage) between various metals you always want to choose an anode which is more negative (i.e. more reactive) than the metal you wish to protect.

Doing this, you would expect that zinc is not suitable to protect your aluminium ROV, that you should use Magnesium.

Magnesium is good for protection, however, with pure Magnesium anodes they will have to be replaced on a regular basis, giving a high maintenance cost. It might be worth while looking into an alloy anode (with a higher percentage of Mg) if that's what is required.

If ROVs operated in freshwater then that would be that, however they mainly work in seawater. MGDuff, one of the major anode suppliers in the Uk for ROVs, say that Zinc should be used for seawater. Be careful if you use too many anodes (or Magnesium anodes), as if there is a great potential difference between the anode and the metal being protected corrosion will actually accelerate and the electrical flow does cause noise on CP and Pipetracker systems.

Anodes form a white coating of oxide on the outside, this should be cleaned off occasionally to ensure the anode material stays in good contact with seawater.

Here are a list of some websites that could offer some more help;

MGDuff, manufacturers of Anodes http://www.mgduff.co.uk/leisure-craft/cathodic-protection.html
NACE International—The Corrosion Society - http://www.nace.org
Unilox Industrial Corporation - http://www.unilox.com
Galvotec Alloys, Inc - http://www.galvotec.com/mainset.html
A H Latham Marine - http://www.zincsmart.com

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•  How do I splice a cable?
The 'Black' art of splicing is probably one of the most contentious jobs that an ROV crew can perform. If you do it well and everything works in the water, then you've done your job. If it doesn't, then you will be ridiculed for life.

1. Bare both cables to be spliced around two inches from the end, dependent on the number of conductors.
2. Bare the ends of the conductors approx 20mm, and crimp together, using the adhesive heat shrink insulated crimps and the correct crimping tool. Shrink the insulation using aheat gun and the joint is complete.
3. Also attempt to step out each joint if possible, so that the finished splice does not resemble a 'monkeys fist'.
4. Alternatively you can solder the ends together, but you'll first have to slide some heatshrink sleeving up the conductor sheath, then position over the solder joint before shrinking.
5. Next, wrap some self-amalgamating tape around each jointed conductor ensuring that you create a good seal.
6. Remember to stretch the tape sufficiently to allow it to amalgamate correctly to the layer beneath.
7. Follow up by, lightly abrading the main cable sheathing an inch or two above the ends and wrap the whole lot with good old self-amalgamating tape, again ensuring that you cover at least two inches of the cable outer sheathing at both ends.
8. Finally, cover the self-amalgamating tape with a layer of electricians PVC tape, to improve the abrasion resistance, and give some mechanical strength.
...There you have it!

Some variations on the theme, include the use of sealing adhesives, adhesive lined heatshrink for the final covering, etc.
For obvious reasons, the soldering technique does not work well with aluminium conductors!

It is worth noting that these joints are not mechanically as strong as the original cable and should not be subjected to tension forces if possible.

An Extract from the ROVworld forum posted by Equlizer

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•  What is Broadband?
Broadband is a high-speed way of connecting to the internet. It is typically 10 times faster than so-called dial-up net services that use a modem and standard telephone line. Broadband can be accessed via the phone line (known as Asymmetric Digital Subscriber Line - ADSL) or by cable, wireless or satellite. In the UK cable can be accessed only in areas where either Telewest or NTL supplies it. Wireless or satellite tend to be available in more remote spots.
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•  How Does Broadband Work?
ADSL uses technology which allows data to be carried alongside normal calls on standard telephone lines. Cable broadband is carried via optic fibres, which also provide digital TV and telephone services. In each case, the data is carried along lines or cables that have more capacity than conventional ones. Access speed is measured in bits per second: 512 kilobits (512k) is 10 times the speed of dial-up; 1 megabit (1mb) is 20 times faster.
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•  How do broadband and dial-up compare?
The two big advantages of broadband over dial-up are that it frees up the telephone line for voice calls and can be left on permanently.

It still requires a modem, which is paid for when you sign up.

Broadband finally became regarded as a "mass market" product in the UK in 2004 when prices fell below £20 a month for the 512k version.

And the gap between charges for broadband and dial-up services is continuing to narrow.

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