Cathodic protection for offshore drilling and production platforms

For the corrosion protection of offshore drilling platforms, the combined corrosion protection method of coating and cathodic protection is adopted. Cathodic protection is adopted for steel pipe piles or steel sheet piles under water and in the splash zone, while the method of covering above the average low tide level is adopted. Cathodic protection can be sacrificial anode cathodic protection, forced current cathodic protection, or a combination of the two, depending on the structure, corrosive environment, power supply, equipment reliability, operation management and other factors of economic and protection effect.

Generally speaking, after the adoption of cathodic protection, the protection current density decreases with the extension of time, mainly because of the formation of cathodic protection deposition film on the surface of the structure, which can hinder the diffusion of oxygen. After the structure with coated layer adopts cathodic protection, the protection current density increases with the extension of time, mainly because the coating layer gradually aging, the damage rate increases, and the cathode deposition film formed is not enough to reach the current density as small as that under the protection of intact coating layer. The coating structure can improve the distribution of cathodic protection current density and protection potential, especially for the forced current cathodic protection system.

Forced-current cathodic protection should be given priority to estuary port facilities where water quality varies greatly. To prevent overprotection, the sacrificial anodes can be kept far away from the protected surface, or a variable resistor can be connected between the sacrificial anodes and the protected body to control the output current.

In the ocean drilling platform facilities corrosion, tidal range zone in the above parts should ShiJiaHou type slurry coating anticorrosive, for the underwater part, most of the past with impressed current cathodic protection, with the improvement of the performance of the aluminum alloy anode and cathodic protection of accumulating experience parameters, using sacrificial anode cathodic protection of ocean drilling platform choose what way the implementation of protection, It is mainly considered from the following aspects: 1, the reliability of the protection system 2, the influence of adjacent structures 3, the protection current requirement 4, the complexity of the structure 5, the structure life 6, environmental conditions and so on.

Sacrificial anode layout and installation:

The sacrificial anode should be evenly arranged on the protected structure facilities and should be 300mm lower than the lowest tide level. The sacrificial anode of the water drum cable should be arranged symmetrically at the bottom of the water drum.

There are two requirements for the installation of the sacrificial anode: 1. Ensure sufficient strength; 2. Ensure that the contact resistance between the sacrificial anode and the protected body is less than or equal to 0.005 ω.

The sacrificial anode can be installed by welding method or bolt fixing method. The commonly used installation methods are: 1, hanging plate fixing method 2, hook fixing method 3, Angle steel fixing method 4, bolt fixing method

Impressed current cathodic protection:

1. Calculate the protected area of the protected body, in addition to the average low tide immersed sea water and sea mud area, the area of the tidal range area and the area of the accessory structure should be calculated together.

2. Calculation of protection current: according to the protection area and initial protection current density or maximum protection current density, the protection current required by the cathodic protection system is calculated, and 1.25-1.5 times of it is taken as the total current requirement.

3, the selection and quantity of auxiliary anode: used for port terminal forced current cathodic protection of auxiliary anode: high silicon cast iron anode, noble metal oxide anode.

Specific anode layout is divided into three:

1) Caisson type. The anode is fixed in a preset box and cage, about 100-200 meters away from the protected steel structure. This way is relatively simple, and the protection potential is relatively uniform, the construction is relatively simple, the disadvantage is that the safety of the cable is relatively poor, easy to be affected by passing ships or waves and other factors and broken.

2) Side wall. The anode is fixed on the main body of the steel structure to be protected, and the anode is installed on the support. Attention should be paid to the inner side of the installation anode to install a shield plate, so as to avoid the over-negative potential near the point resulting in over-protection and accelerated corrosion. The installation of this protection method is more complicated, the construction is more difficult, and the protection potential is relatively not very uniform.

3) Suspension type. According to the platform structure, the geometric center point is suspended, so that the anode is equidistant from the protected structure. The suspension device is a difficulty, which requires the nylon sinew and the anode frame that are resistant to seawater corrosion, and a heavy tuo is suspended below. This way is the simplest construction, the most uniform protection potential. The potentiostat is usually in the cathodic protection room. If exposed outside, the explosion-proof potentiostat is required.