Insuring LNG facility integrity

Engineers and operators face many challenges when erecting above-ground liquid natural gas (LNG) storage tanks on wet, marshy lands. Providing a stable base to support the immense weight of the structure represents one of the greatest challenges. Stability can be accomplished by constructing a deep foundation, consisting of non-marine steel piling driven deep into the ground. These steel piles are often subject to low-resistivity, highly corrosive soils. To extend the life of the steel pilings supporting the LNG tank, engineers and operators turn to cathodic protection (CP).

Cathodic protection systems for LNG tanks serve a critical purpose, and there is only one chance to get them right. Once cathodic protection systems are installed, foundations are poured, and tanks are erected, there is no opportunity to go back and fix the system. Not only must they be designed properly, but they must be designed to install easily and with minimal risk of damage during installation.

Several factors need to be evaluated before designing a CP System:

• LNG tank design (pile layout)
• Soil conditions
• Current requirement
• Current distribution
• Installation.

Large-diameter LNG tanks can utilize several hundred steel piles to support their immense weight. Two types of piling can be used in wet, marshy lands: end-bearing piles or friction piles. With end-bearing piles, its base rests on a relatively firm soil such as rock, dense sand, or gravel. The load of the structure, which is sitting on the tops of the piles, is transmitted through the pile and into the firm soil below.

Friction piles are used in situations where the firm soil is at a considerable depth and out of reach. In such situations, the piles are driven through the soft soil for some distance. The piles work by transmitting the load of the structure by means of skin friction or cohesion between the soil and the embedded surface of the pile.

Designing for LNG facilities

When designing a CP system for steel piles, corrosion rates are highest near the surface and decrease as you move further down the pile. A point comes at which the oxygen level is sufficiently low enough that no further corrosion risk exists. In that case, all that needs to be protected is the top portion of the piles, which is usually the top 50 ft.

Cathodic protection of closely spaced steel piles requires the anodes be located in a distributed pattern in and amongst the steel piles. In theory, deep-anode ground beds can be employed to cathodically protect steel piles used for foundations. However, the quantity of piles and the current demands prohibit it. Deep-well anode systems are often used to provide large amounts of current from a single point located remotely from the structure or structures that require protection. A single deep well can provide current for many miles to a well-coated pipeline. A single deep well can typically also be used to protect coated plant piping systems in a congested plant environment. They can be employed on some piling applications where the number or concentration of piles is relatively small. Deep wells cannot be used in an LNG tank pile application.

The current density required for an impressed current cathodic protection system ranges from 10 to 30 mA/m2 (or approximately 1 to 3 mA/ft2) surface area of bare pipe, which is a general requirement. Given the typically low soil resistivities encountered for wet, marshy soils, the 2-3 mA/ft2 value would generally be recommended. Current density has minimal impact on the anode quantities as current distribution will determine the number of anodes required; however, it will affect the rectifier sizing and individual anode rating. It is important to note that current will flow from the anode to depths farther down the pile. This current will extend protection well below fifty feet even though oxygen levels are not sufficient for significant corrosion. When designing a CP system, it is important to add more current to the system as a safety margin.

Another critical design factor is ease of installation. Conventional distributed anode design would require that a small hole, typically six inches in diameter, be drilled or augured into the soil and then the anode would be lowered into the hole followed by coke backfill. It is unlikely that conventional installation would work in a wet, marshy environment. The soils would not support keeping a hole open long enough to drop the anode in place. This might be overcome by casing a drilled hole or driving a pipe into the ground to allow the anode to be inserted, but both are costly and time consuming.

It is critical to the operation of the cathodic protection system that the piles be electrically bonded together. This can be achieved by welding rebar to each of the piles — the preferred method — or by welding (via Cadweld or another equal method) electrical bond wires from pile to pile.

With large LNG tanks, the control system for the cathodic protection system would consist of a series of zones developed for the LNG tank pilings. Separate rectifiers would allow each zone’s output to be set independently. This allows for greater control and flexibility of the system. The reference electrodes and anode wires would run into separate junction boxes for each zone. The specifics would be determined during the design phase.

Available systems

MATCOR has developed an innovative, prepackaged anode design that reduces installation time and materials when installing cathodic protection systems in a wet, marshy environment. The anodes are meant to be driven or pushed into the ground. This method differs from traditional methods, which require drilling a hole into the ground first and then placing the anodes into the hole. This driving style anode has been successfully utilized for several projects, reducing installation time and minimizing material costs.

The proprietary driving-cone design is a proven solution specifically meant for driving anodes into wet, marshy soils. These anodes were first used in 1983 and have been successfully used in a variety of projects around the world. The real secret to the direct-drive design is in protecting the cable connection to the anode during installation. The driving anode assembly installation uses specially engineered driving tools to protect the cabling during installation.

Anode placement in and amongst the piles is critical to assuring thorough current distribution. For a typical design, the driving anodes are 10 ft long and are driven down 20 ft in order for the anode to be located amidst the piles with the active area between 20 ft and 30 ft. At this depth, current will distribute to the top of the pile and down the length of the pile, providing the necessary current distribution to protect the critical top 50 ft of the pile. The standard design life of the anode is typically 25 to 30 years at the rated operating current in soil; however, longer design life anodes can be utilized with little additional cost.

Cathodic protection systems prevent corrosion of the protected structure only when functioning properly. As a result, their design should incorporate sufficient reference electrodes to allow confirmation that the system is indeed operating as intended. To assure that the system output is properly calibrated to meet cathodic protection criteria, provisions for testing must be incorporated into the design. Typically, monitoring systems are installed at a minimum of 5% of the piles. The monitoring system consists of a special driving reference electrode assembly installed in close proximity to the steel pile. To properly monitor the system operation, the reference electrodes are typically installed at 10 ft and 40 ft depths and distributed throughout the piling system.

Installing the reference electrodes provides a similar installation challenge to the installation of the anodes, and MATCOR has developed a proprietary driving reference electrode assembly that has a similar installation to that of the anodes. The electrodes will be assembled into housings with integral driving cones for direct installation into the soil.

LNG tanks are critical service components to any LNG receiving terminal and these heavily engineered tanks require sound foundations. When these tanks are located in marshy coastal areas and require extensive structural piling support, cathodic protection of the steel piles is a significant issue.