and inspections of offshore hulls can cost upward of US$500,000 a year. Yet it is known that ultra large crude carriers (ULCCs) can have the submerged sections of their hulls cleaned in a matter of days if they use foul release coatings. Tis can have a dramatic effect in reducing operating expenditure. “Te thickness of fouling varies around


the world. Engineers designing jackets for offshore platforms have to consider the additional weight loadings on the steel due to potential macro-fouling build-up. Teir response is oſten a requirement for thicker steel on the jacket legs. In turn this can cause handling issues due to the increase in the jacket’s weight.” However, the International Paint


spokesperson adds that coatings that offer equivalent protection against corrosion in Arctic


conditions are still under


development.“Conventional abrasion resistant coatings, such as glass flake epoxies, do not have these characteristics and, as a result, are unsuitable. With the hard film resisting ice abrasion and reducing corrosion, hull steel thicknesses can be reduced. “Te coatings industry will also have to


be innovative to confront new risks. For example, engineers are looking at coatings to avoid the risk of ice building up on superstructure topsides. Current regulations regarding the accumulation of ice on marine vessels will be equally applicable to oil and gas operators.”


Dress to impress While investment in coatings is a priority in the fight against corrosion, this strategy can also be accompanied by investment in impressed current cathodic protection (ICCP), as a way to mitigate against further corrosion when coating protections begin to fail. Ultimately, either due to fouling organisms


either eating away at the coating, or because abrasive elements such as ice have rubbed off the coating, hull coatings only go part of the way to providing protection against corrosion and are seldom the complete answer, while industry experts further warn that porosity or small imperfections over the surface of coatings can lead to areas of corrosion over a period of time. Te type of corrosion that most commonly manifests itself on offshore structure is galvanic corrosion, when dissimilar materials – such


as steel hull plates, bronze alloy propellers and stainless steel shaſts – are connected electrically and exposed to an electrolyte like seawater. In a bi-metallic cell, the more noble metal


in the galvanic series becomes the cathode, while the less noble metal becomes the anode and is subject to corrosion, and the greater the difference between the metals, the greater the voltage driving the current in the bi-metallic cell, leading to increased levels of corrosion. Tis can be protected against by cathodic


protection, which by the application of a more powerful external current in the opposite direction to the corrosion current, neutralises corrosion cells through suppressing all electro-chemical activity. The most common defence against


galvanic corrosion are sacrificial anodes fixed to a vessel’s hull. Normally bars of zinc or aluminium that produce a fixed output on installation. However, as the anode is ‘consumed’ during the electrochemical process,


its output


gradually diminishes and the performance in neutralising corrosion is therefore unknown and unmeasurable. The alternative, more technologically


advanced, method for combating galvanic corrosion is employing an ICCP system, which uses anodes made from very stable materials such as titanium, tantalum, or niobium with perfectly engineered coatings to increase that stability and durability during operation. The latest manufacturing techniques additionally mean that these anodes do not


Offshore Marine Technology 1st Quarter 2012


Operators should be aware that, when slack, FPSO and FSO mooring chains can lose the connection for corrosion currents, necessitating a different ICCP arrangement.


require replacement for 15 years or more, making them ideally suited to the long term requirements of semi-submersibles and FPSOs, while sacrificial anodes are normally replaced every five years when cargo ships are traditionally dry-docked. However, the most important difference between the two types of equipment is that ICCP systems have reference electrodes which constantly measure the electrical potential at the seawater-hull interface. They send a signal back to the control panel which automatically raises or lowers the output to the anodes, ensuring that the hull always receives the optimum level of current to eliminate corrosion.


Cutting costs Stephen Ellis, business development director at UK-based anode producer Cathelco, which specialises in the production of ICCP systems, says that the greatest motivation for offshore vessel owners and operators to switch from a sacrificial anode system to an ICCP system is the increasing cost of raw materials. “Te fact is that, with the increase in the costs of raw materials such as aluminium, the cost of going for sacrificial anodes has risen markedly, while an ICCP system is increasingly becoming more cost-effective,” he says. Cathelco recently completed the retrofit


of an ICCP system on the Ocean Rig drillship Leiv Eiriksson, which had finished a drilling project in the Black Sea, off the northern Turkish Coast, and was due to be relocated to the Atlantic to work off the coast of South America, in waters that will


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