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Offshore Platform & FPSO
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Offshore Platform & FPSO

Offshore Platforms

With the term offshore prlatforms we refer to objects such as production platforms. This structure located remote in open sea and cathodic protection required to keep this equipment on safe operation for their design life

Corrosion is at maximum at a small distance below the water line and gradually decreases with depth. In mud, corrosion is usually much less severe. The submerged steel surface and the steel in the mud can be cathodically protected.

Corrosion in the tidal zone or the “splash” zone above low water level may be severe and cannot be fully protected by cathodic protection. Protection shall be given by the use of heavy-duty coatings and adequate corrosion allowance.

The submerged sections of production platforms are normally uncoated and rely completely on cathodic protection. Cathodic protection systems for offshore structures shall be designed in accordance with DNV RP-B401, a well established recommended practice for offshore Cathodic protection.

Either sacrificial anodes or impressed current systems or a combination of both may be used to protect offshore structures. Impressed current systems should be used with utmost care as the risks of inadequate protection are considerable.

 

Current requirements

The current required for cathodic protection varies with the salinity, temperature (oxygen content) and flow conditions of the water. For the design of cathodic protection for offshore

Structures.

 

Sacrificial anodes

For offshore structures, sacrificial anodes are the standard type of cathodic protection system. It is more attractive to use aluminium-indium alloy or zinc anodes for Cathodic protection, for reasons of high dependability, immediate availability and no subsequent maintenance during the design life. The anodes on platforms are welded onto the structure prior to installation of the platform. The system designed to last for any length of time.

For offshore structures, aluminium-indium alloy is mainly used as this material provides the highest long-term output per unit weight. The design of sacrificial anodes on uncoated structures shall take into account the high current required to polarise the steel and occasional high currents required to repolarise after heavy storms.

 

Impressed current

Impressed current system’s are not normally used on steel offshore platforms because of the mechanical weakness of cables and anode systems and the frequent monitoring and maintenance requirements.

 

 

FPSO Protection

Oil and gas production operations are increasingly using floating production units, often made from converted ships or purpose built vessels. These vessels are normally anchored by multiple anchor chains (uncoated) and production vessels are connected to wells and other sub-sea equipment.

FPSOs are a special case for protection, as the CP systems are required for the field life (for example 30 years); FPSOs are not dry docked every 5 years as is common with ships.

The common approach is to use aluminium-indium alloy anodes. Moreover, the service of the tanks of a FPSO is different from that of tanks on normal trading vessels. Tanks are used for the processing of produced crude and may contain large amounts of production water, stationary ballast or mixtures of (sour) hydrocarbons and water. This may require a different cathodic protection approach.

 

For FPSOs the intervals between dry-docking are much longer than for ships. Therefore the design life of sacrificial anode systems shall be adequate to cover the full operational period of the vessel.

EXTERNAL CATHODIC PROTECTION

Paint coating provides the external corrosion protection of the primary structure of an FPSO. As the vessel is stationary the external damage of this coating by anchors and mooring is less likely. Some damage can be expected by support vessels and degradation of the coating system by ageing.

Current densities required for protection are similar to those required for fixed offshore structures. The seawater temperature at the permanent mooring location is required to determine the required current density.

Impressed current

If continuous power is available, impressed current systems can be employed. The anode construction can in principle be the same as for ships, although there is more freedom for construction and distribution of anodes on the hull.

 

Sacrificial anodes

Sacrificial anode systems can also be used because of the low current requirements compared to ocean going vessels and the likely absence of major anchor damage. As for fixed offshore structures, aluminium-zinc-indium anode material should be used.

 

PROTECTION OF TANKS

Cathodic protection of FPSO tanks is normally acomplished by means of sacrificial anodes. The types of tanks requiring cathodic protection are ballast tanks, crude storage tanks and production tanks (receiving tanks and slops tanks).

Design life of tank cathodic protection systems is the same as for the whole structure as retrofitting is not possible or very costly because it may interrupt production.

With sacrifical anodes inside tanks there is a risk of hydrogen build up. Tanks should be properly ventilated to minimise the risk of explosive atmospheres building up in the top of the tanks.

 

Suzhou SYNCOR Cathodic Protection Co., Ltd. working more than 30 years in Cathodic protection doing design, supply materials, installation and commissioning’ trying to deliver some of most complex project in the world, on budget and on time for our clients.

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