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 communication to Nortek instruments (typically the AWAC) for distances of up to 5 kilometers. The RS422 protocol is used for applications where the cable is longer than approximately 70 meters.

For distances greater than 1000 meters, using a cable composed of segments (e.g. 500 meters) is often recommended. This helps minimize the expense of replacement in the event of cable damage.

Cable Description  Nortek's offshore cable has seven leads that are covered by two layers of vinyl sandwiching a stainless steel armored layer.

Cautionary Notes  Cables arguably provide the most dependable means of communication. A less obvious drawback is that due to their exposure there is an element of survivability which is very dependent on the environment around the cables. It is important to be aware of these risks as well as the precautions that can be taken to minimize them. 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 become expensive if it happens more than once. One solution is to construct the cable as a series of cables. If a cable breaks, then just a segment needs to be replaced. Since connectors represent a large part of the cost, there is a trade off as to the length of the segments. Nortek’s experience is that segments of 500 meters are reasonable for the bottom and segments of 200 meters for the portion that extends through the energetic portion of the water column (sea-air interface, surf zone, etc.).
• Weighting cables This is important in order to reduce the exposure of a cable. Exposed cables are more susceptible to being snagged by fishing gear and anchoring. Cables that are not firmly weighted to the bottom are also more susceptible to wear and tear.
• Strain relief The concept here is that when the cable is exposed to heavy loading, the connectors will 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. A cable of 1 km will accordingly 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 converter 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 an interface unit which manages the necessary power conversions (48 volts / 16 volts) and communication conversions (RS422 / RS232). The interface box also has power surge protection.