Offshore Pipeline

An undersea pipeline is an installation of pipelines under the sea, which can be divided into the following categories: subsea pipelines, coastal pipelines, deep sea pipelines, including pipelines for the transportation of oil, natural gas, seawater, freshwater, and wastewater. On average, the low electrical resistance of seawater simplifies the operation of cathodic protection for undersea pipelines. The most suitable anode for protecting undersea pipelines is the bracelet-type anode. The factors that affect the current demand in seawater are temperature, salinity, and currents. Temperature is usually the most important factor. Bracelet-type anodes for undersea pipelines are usually arranged along the pipeline with a certain interval. The standard material for bracelet-type anodes is aluminum-zinc-indium, but zinc anodes are sometimes used. If the surface temperature of the pipeline exceeds 50°C, bracelet-type zinc anodes are not suitable. When the pipeline is in a high-temperature environment, we recommend using other types of anodes that are arranged along the pipeline and connected to the pipeline with a cable.

The electrical connection between the pipeline and the anode is made using copper cables, which are thermally welded to the iron core and the pipeline. The cathodic protection of the pipeline requires sufficient current to be provided on the surface of the pipeline, which can lower the potential difference between the pipeline and the electrode, making external corrosion very weak.

In order to achieve better corrosion protection, the surface of the pipeline can use the method of cathodic protection combined with suitable coatings. Short pipelines can have anode installed at both ends. This method is particularly suitable for pipelines within offshore platforms. If there is an electrical connection between the pipeline and the platform, the anode can also be installed on the platform, which can achieve the goal of protecting the pipeline.                            

The selection of cathodic protection parameters mainly includes the protection potential and the protection current density.

1. Protection Potential

The cathodic protection potential of the steel structure of subsea pipeline is shown in Table 1.

Table 1 Cathodic Protection Potential of Steel Structure of Subsea Pipeline

2. Protection Current Density

The size of the protection current density is mainly related to the type of subsea sediment, the factors affecting subsea sediment, oxygen content, salinity, and anions, etc. These factors can cause the protection current density to fluctuate within a wider range in different subsea environments. The protection current density for steel structures in different subsea environments is shown in Table 2.

Table 2 Minimum Protection Current Density

Note: When subsea pipelines are carrying hot fluids, the current density required for thick film coating protection increases by 0.2 mA/m2 for every 1°C increase in temperature (starting from 25°C), while the current density required for thin film coating should increase by 1 mA/m2 for every 1°C increase in temperature.

Subsea pipeline sacrificial anodes are generally available in two shapes: bracelet-type anodes and bar-shaped anodes. If the ambient temperature is low, bracelet-type anodes can be fixed on the pipeline; if the ambient temperature is high, to avoid the sacrificial anode consuming too quickly, a sacrificial anode bed can be used, which is similar to sacrificial anode protection on land.