Offshore Cable

Applications  This is an excellent solution for online applications where the risk of losing a surface buoy is too great.  Almost all installations have the cable lying on the seabed, however there have been some installations with cables to buoys.  This configuration is generally discouraged unless there is certainty that surface conditions never become severe (e.g. small enclosed bodies of water).

 A cable is suitable for providing  power and communications to Nortek instruments (typically the AWAC) for distances of up to 5 kilometers.  The RS422 protocol is used for applications where the cable length is greater than approximately 70 meters.  . 

For distances greater than 1000 meters it is often encouraged to use a cable composed of segments (e.g. 500 meters), This help minimize the expense of replacement in the event of cable damage.

Cable Description  Nortek's offshore cable has seven leads which is covered with two layers of vinyl which sandwiches a stainless steel armored layer.

Cautionary Notes  Cables arguably provide the most dependable means of communication.  The not so obvious drawback is that do to their exposure there is an element of survivability which is very dependent on the location where they are laid. It is important to be aware of these risks and know what type of precautions can be taken to minimize the risk.  The following is a partial list of these items:

• Segmented cables in the event of a cable break the cable will have to be replaced.  This can quickly become an expensive endeavor if it happens more than once.  One solution is to construct the cable as a series of cables.  If there is a cable break then just a segment needs to be replaced.  Since connectors represent a large part of the cost of there is a trade off as to the length of the segments.  Nortek’s experience is that segments that are 500 meters are reasonable for the bottom and 200 meters long for the portion that extents through the energetic portion of the water column (e.g. sea-air interface, surf zone, etc.).
• Weighting Cables is important for reducing exposure.  Exposed cables are more susceptible to being snagged by fishing gear and anchoring.  Cables that are not firmly on the bottom are also more susceptible to wear and tear.
• Strain Relief The concept here is that when the cable experiences heavy loading that the connectors stay connected but separate before the cable breaks.
• Voltage Drop  The amount of voltage drop (Vdrop) along a cable is a function of the current draw (I), linear resistance of the cable (Rl), and the cable length (L); Vdrop = I*Rl*L.  The typical current draw during the measurement cycle for an AWAC is 1 amp.  The Nortek offshore cable has a linear resistance of 10.6 Ohms/km.  Therefore a cable that is 1 km long will have an expected voltage drop of 10.6 volts.  Since Nortek equipment typically operates between 8.5-16 volts, it is often easier to use a DC-DC convert at the instrument.  Nortek offers the AWAC with a 48 volt DC-DC converter.

Related Item: Interface Box For instruments connected on long cables, Nortek offers a interface unit which manages the necessary power conversions (48 volts/ 16 volts) and communications conversions (RS422 / RS232).  The interface box also has power surge protection.