Reference electrodes ships

Since it is important to keep the internal element wet and surrounded by a filling solution of known concentration, manufacturers of reference electrodes ship the electrode filled with filling solution. In order to prevent leakage of the filling solution through... 

To measure the potential, reference electrodes are lowered on unbreakable ropes tensioned with 20 kg of lead as near as possible to the ship s side. IR errors can be neglected because of the good conductivity of seawater [see Eq. (2-34)]. In contrast to fresh water, the switching method in seawater is not necessary (see Section 3.3.1). 

At the same time, the practical ship protective potential of 37th, 72nd, 128th, 168th and 228th frame was measured when the ship berthed at port by Cu/CuS04 reference electrode. Compared with the measured results, the calculation results showed the same trend as the practical measured data and were near the same values as the measured results, as shown in figure 4. That is to say the calculation results are accurate. 

The ship is connected by cable to the platform and the reference electrode is steered by a remote-operated vehicle (ROV) or—in shallow waters— directly from the ship over the pipeline. In the case of long pipelines of more than 20 km, there can be considerable difficulty in running the cables. 

On a supertanker, the initial current of 10 A may rise to over 1000 A during the course of its operational life. Modern ICCP ship designs usually place anodes in symmetrical dispositions, but in bulk carriers, there is a need for internal access and for cable-runs to be away from anodes and reference electrodes. This usually precludes electrodes from being sited external to storage tanks. Instead electronics are placed either well forward or well aft, where the adjacent machinery spaces provide convenient access to the various pieces of equipment (Fig. 13.6). There are several reasons why potentiostatic systems frequently fail to provide adequate protection [8] ... 

It was found, for example, that a protection current density of 30 mA/m in quiet seawater rises to over 110 mA/ m at 15 knots and above. Ship ICCP anodes are few in number and very small in relation to the cathodic surface area, thus the distribution of potential from them is inevitably uneven. In the design process, it is the difficulty of knowing the expected potential distribution over the structure that leads to reliance on current density measurements as a means of assessment. Yet many ship ICCP designs utilize no more than two or three reference electrodes and these electrodes can only measure the potential in their adjacent vicinity. 

Figure 13.8 Effect of reference electrode location on potential distribution on a ship hull in both static and flow conditions (a) with one reference electrode amidships and (b) with an extra reference electrode aft and the original electrode repositioned ahead of the forward anode.

Figure 13.44 Optimized reference electrode locations on BE meshed ship hull. (Courtesy of Robert A. Adey, Computational Mechanics BEASY)...

Fig. 10.29 Cathodic protection using impressed current, (a) A circuit showing the principles the signal from the reference electrode is passed to a power-unit control where it is compared with a preset level. The resultant error signal is amplified and used to control semiconductor power devices which allow a controlled current to pass through the anodes, (b) A typical layout of components in various types of vessel, (c) Transformer/rectifier power units for marine use. (d) Platinized titanium or lead-silver alloy anodes being installed on a ship s hull. The anodes are insulated from the hull and have special insulating, backing shields which help to improve potential distribution and prevent over-protection, i.e. too negative a potential (Photographs courtesy Corrintec (UK) Ltd.)...

Fig. 10.31 Portable reference electrode probe, (a) Surveys of electrode potential distribution on ships hulls, submerged offshore pipelines, oilrigs, platforms, jetties and docks are often carried out by manual diver-held probes, (b) These battery-powered devices usually incorporate a tip spike, a silver/silver chloride reference electrode and a digital voltmeter (dvm) within a tough, insulated and sealed pistol. The shroud surrounding the electrode element defines the sensing area. These probes are used to depths of approximately 300 m and are capable of measuring to a precision of 1 mV. Rechargeable Ni-Cd batteries are used. Sharpened tips may be used to penetrate coatings. (Courtesy Corrintec (UK) Ltd.)...

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