Real-Time Oceanography with Inductive Moorings

Figure 1: Inductive Mooring

Rapid progress in the development of satellite, RF, and cell-phone telemetry has made real-time, unattended, remote oceanography increasingly practical. However, before these new telemetry techniques can be exploited, the data must first be brought to the surface.

In the past, underwater-to-surface data transmission was accomplished using direct cable connections. Such cables were bulky, expensive, unreliable, and the positions (and number) of individual sensors were fixed once the cable was designed and manufactured. More recently, acoustic telemetry has been investigated as a substitute for direct cables. However, acoustic methods are even more expensive, require additional battery packs, have limited range restricting instrument depth, and are subject to a multitude of error sources and failure modes. Today, the improvement of inductive modem technology provides a solution that is convenient, economical, reliable, and flexible; allowing up to 100 instruments to be positioned or repositioned at any depth (Figure 1).

Figure 2: Basic Transformer Operation

Figure 3: Sea-Bird Inductive Coupling

A transformer has two or more coils that share a magnetic field. Materials such as ferrite can be used to form a transformer core that ensures the necessary sharing of magnetic fields (Figure 2). An AC voltage applied to an 8-turn primary winding will induce a voltage half as large on a 4-turn secondary winding.

A transformer is similarly employed in IM systems. Figure 3 illustrates the operation of the transformer defined in the Alternative IM System (section IVb), which is conceptually the simplest type. Note that the secondary winding (in this case, the mooring cable) does not have to be wound tightly on the transformer core. The winding must only pass through the hole in the core.

In the Preferred IM Configuration (Section IVa), the ends of the mooring cable are grounded to the seawater. This causes a current to flow through the mooring wire and seawater. The Inductive Cable Coupler (ICC) senses this current providing a voltage for presentation to the Surface Inductive Modem (shown pictorially in Figure 4 and schematically in Figure 5.)

Figure 4: SBE Inductive Coupling

Figure 5: Inductive Coupling Schematic

Figure 6: Preferred Inductive Mooring Configuration

a. Preferred Inductive Mooring 

A typical ocean mooring is shown schematically in Figure 6. The mooring cable is a plastic-jacketed galvanized steel wire rope, a type frequently used for non-inductive oceanographic moorings because of the corrosion resistance provided by the plastic jacket. The ends of the wire rope are terminated with steel thimbles or swaged eye terminals. Deep moorings typically employ synthetic line below the lowest instrument.

Individual inductive modem instruments may be clamped along the mooring cable at any position. It is not necessary to break the cable at the instrument positions or to provide any electrical connection between instrument and cable. Each instrument can be freely moved up or down the cable.

The surface buoy contains a Sea-Bird Surface Inductive Modem (SIM, see Section Vc) that communicates with the underwater components via a through-hull bulkhead connector, polyurethane cable, and Sea-Bird Inductive Cable Coupler (ICC, see Section Vd). Like the IM instruments, the ICC simply clamps anywhere along the plastic jacketed portion of mooring cable — no actual electrical connection is required. The buoy controller / CPU (not supplied by Sea-Bird) links to the SIM using RS-232 to permit two-way half-duplex communication with the entire suite of IM instruments.

 
Figure 7: Alternative Inductive Mooring Configuration

b. Alternative Inductive Mooring

An alternative system can make use of a direct electrical connection between the surface buoy and the top of the mooring cable as shown in Figure 7. In this configuration, the plastic-jacketed cable is brought inside the buoy. The buoy must be mechanically secured to the top of the cable without compromising the electrical insulation of the plastic jacket. The SIM connects to the galvanized steel wire and to the buoy hull (or other seawater return). This configuration eliminates the need for the ICC, but may complicate connecting the buoy to the mooring wire.

Figure 8: Cable-to-Shore Mooring

c. Cable-to-Shore Mooring

As shown in Figure 8, instruments may be placed at any position along a jacketed wire rope leading from shore. Here the problem of insulating the inner conductor at the shore end is easily solved, making the direct-connection approach the preferred method (Section IVb).

Figure 9: Through-Ice Mooring

d. Through-Ice Mooring

Figure 9 shows the arrangement permitting IM moorings to be used for through-ice applications.

It is essential that both ends of the mooring cable are immersed in the seawater to complete the circuit.

Figure 10: Fresh Water Mooring

Figure 10 shows an arrangement permitting IM moorings to be used for fresh water applications, for a mooring with a SIM-Coupled Surface Inductive Modem and an Inductive Cable Coupler.

It is essential that you make a good contact to the bulk water to complete the circuit. For fresh water applications, this requires a large electrode in the water at the top and bottom of the mooring. Use at least a 0.7 m length of 2 inch (outer diameter) or larger conductive pipe (galvanized or stainless steel). Strip the end of the mooring line and clamp it to the pipe. Do not let the electrode pipe lie on the bottom; clamp the pipe to the mooring line at least 2 or 3 meters above the bottom. A mooring with this design should provide good communications over 1000 meters of mooring line in water with conductivity as low as 100 µS/cm.

For a mooring with a SIM-Direct Surface Inductive Modem, the bottom connection would be as shown in Figure 10. The connection to the buoy is as shown in Figure 7; several square feet of un-painted surface on the buoy is required to provide the large electrode at the top.

Figure 11: Swaged Eye Terminal

a. Mooring cable  

Mooring cables are usually polyethylene-jacketed galvanized steel wire rope. Provision for electrical contact with the water must always be made at the lower end of the cable. Typically a few centimeters of insulation are removed and a swaged eye terminal (Figure 11) is installed, providing both a means of mechanical attachment and a suitable seawater ground. Sources of mooring cable and cable assemblies are given in Section VI.

Figure 12: SBE 37-IM

b. Underwater Sensors

While the IM system can support sensors operating in command mode only (i.e., the sensor is asked to take a sample and it returns the data), it is desirable that IM sensors can also operate autonomously. This requires each sensor to sample and self-record data using its internal clock, while allowing the IM system to command for the most recent data. This approach has the advantage that data can ultimately be extracted from individual sensors despite a failure in the IM, buoy, or telemetry components. The sensor’s ability to avoid conflict between its own clock-driven sampling and the separately clocked commands issued by the buoy controller is also useful.

1. Sea-Bird MicroCAT CT/CTD Recorder, SBE 37-IM and 37-IMP. 

The SBE 37-IM (Figure 12) is a self-contained CT (optional pressure) recorder for depths to 7000 meters. The SBE 37-IM can be commanded to sample via the IM link. However, it is usually desirable to make use of the MicroCAT's Autonomous Mode based on its internal clock and memory. In this mode, samples are taken and recorded in non-volatile memory at fixed time intervals, while the buoy uses the IM link to request the most recent results.

The SBE 37-IMP combines the features of the 37-IM with an integral, internal pump.

Figure 13: SBE 39-IM

The SBE 39-IM (Figure 13) is a self-contained Temperature (optional pressure) recorder for depths to 10,500 meters. This operates in command or autonomous mode, just as the SBE 37-IM.

Figure 16: Underwater Inductive Modem

Figure 17: SIM Board

c. Surface Inductive Modem

A small printed circuit card mounted in the surface buoy, or on-shore in the case of cable-to-shore systems (Figure 17). The SIM is offered in two configurations:

1. Sea-Bird SIM-Coupled for use with a Sea-Bird Inductive Cable Coupler (Preferred Inductive Mooring System, Section IVa)

2. Sea-Bird SIM-Direct for direct connection to the mooring cable (Alternative Mooring or Cable-to-Shore Systems, Section IVb, c)

SIM Board Specifications:
Board dimensions -- 4.25 x 5.83 inches (108 x 148 mm)
Power required -- 7 to 25 volts at 10 microamps quiescent; 60 milliamps communicating

Figure 18: Inductive Cable Coupler

d. Inductive Cable Coupler (ICC)

Used with SIM-Coupled for moorings similar to Figure 6. The ICC (Figure 18) is available in a number of sizes for clamping to mooring cables with diameters from 1/4 inch to 16 mm. The ICC cable can be specified for the desired length, with a pigtail or a molded connector.

Figure 19: Typical Surface Buoy

f. Surface Buoy

The surface buoy houses the Surface Inductive Modem, Buoy Controller/Data Logger, telemetry transmitter/receiver, and necessary batteries (Figure 19).

This item is not presently supplied by Sea-Bird.

Figure 20: Typical IM Coupler Installed in Instrument End Cap

Qualified manufacturers of serial-output sensors can obtain OEM Underwater Inductive Modem (UIM) circuit cards and other components needed to convert new or existing instruments to IM operation. The present circuit card format is a 4.33 inch (110 mm) long three-board rectangular card set for longitudinal (lengthwise) mounting in 2 inch (51 mm) housings. Sea-Bird will make UIM board sets in a format to suit OEM requirements for minimal expense.

The card sets operate from supply voltages in the range of 7 to 16 volts and are compatible with the battery packs typically used in ocean instruments. Data interface is full-duplex RS-232 (9600 baud standard; other baud rates available).

In addition to the UIM boardset, a split toroid core and transformer winding is required. In designing the mechanical mount for the IM coupler, note that one half of the toroid is to be captured within the instrument and surrounded by the free-standing winding. Single-pin seal pins (not supplied) are typically used to support the winding and bring the electrical connections into the instrument. The winding and core half are then urethane potted. The potting material must adhere to the seal pins; no other adhesive bonds are critical. An important feature of this configuration is that the winding is entirely surrounded by the potting material so that water intrusion along the toroid surface is of no consequence. The toroid itself is unaffected by water exposure.

The OEM must make provisions to lightly clamp the other toroid half into position. Figure 20 shows various aspects of the basic design. Additional details and design assistance are available from Sea-Bird.

Figure 21: IM Coupler on Flexible Lead