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Science of Stuck Part 1
Numerous subsea components are designed to be retrievable. This is due to the need to periodically or reactively replace parts which are not expected to last the full design life of the field. Retrieval is typically conducted by work class ROVs, operated from inspection, maintenance and repair (IMR) vessels. A subsea component should be recoverable in a matter of hours, not days and certainly not multiple campaigns. Considering IMR vessel charter, and potential production implications, the cost of retrieving stuck components can quickly run into six or seven figures.
Retrievable components are typically manufactured from a variety of metallic materials. As these are to operate subsea, a strategy to combat the corrosive nature of seawater is required.
Corrosion takes place when metals oxidise (e.g. react with oxygen) in an electrolyte (conductive medium). Seawater is a highly conductive electrolyte due to the concentration of ions in the water. These ions, a distribution of which can be seen to the right, increases the conductivity, and in turn the corrosivity, of seawater dramatically when compared to potable or distilled water.
One of the strategies to prevent corrosion, along with the use of expensive, corrosion resistant alloys or protective coatings, is cathodic protection. For an overview of how cathodic protection works, see the graphic below.
Two different scenarios are presented in the above graphic:
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Left – an iron-based metal (steel) is freely corroding in seawater. This is a thermodynamic inevitability when exposing common carbon steel to a corrosive environment such as seawater in lieu of any measure taken to prevent corrosion. The corrosion is an electrochemical process, with an anodic (oxidation and metal loss) and a cathodic (reduction) site where reduction of oxygen takes place. Note that this picture would not apply to a corrosion resistant alloy.
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Right – the same steel is now electrically connected to a “less noble” metal, in this case zinc or aluminium, or a mixture thereof. We call this a sacrificial anode. The term sacrificial stems from the fact that the anode will preferentially corrode and in the process donate electrons to the material being protected, thereby allowing the steel to take part in the oxygen reduction process without actually undergoing metal loss. Note also that the connection of the sacrificial anode increases the driving voltage for the various electrochemical reactions to take place from typically -650 mV to -1050 mV.
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The hydroxide ions (OH-) produced through the oxygen reduction reaction are a crucial early building block in the promotion of calcareous deposits, as we shall see.
It is clear we need to understand the science of deposits and their formation in order to address sticky problems, a closer look is presented below.
Calcareous deposits is a term applied to a number of different crystalline salts which may form on surfaces submerged in seawater. The two main constituents are calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2). Calcareous deposits are typically observed on steel surfaces connected to cathodic protection systems. Depending on the function of the metallic structure or component, the formation of calcareous deposits can have positive or negative effects:
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Static carbon steel structures benefit from deposits as they provide additional corrosion protection where coatings may have broken down
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Retrievable, typically CRA, components frequently seize in place due to calcareous deposits, necessitating costly, time-consuming deposit removal efforts. In the case of CRAs, the CP system is a requirement for the sufficient production of hydroxide ions on the metal surface.
Case Study – Stuck Electrical Flying Lead (EFL)
Now that we’ve been introduced to some of the factors which influence corrosion and cathodic protection, let’s have a look at a case study, involving a retrievable electrical connector which frequently gets ‘stuck’ subsea.
If we were to perform a root cause analysis, in this case a 5 Whys, you would typically return the answers seen to the right.
What should be noted is that the answer to the last "Why?" addresses a separate issue, namely preventing external corrosion. We have, in other words, addressed one problem while creating another.
Now that we understand how deposits form, and the fact that cathodic protection can be helpful or problematic depending on the situation, we are ready to think about solutions to the problem.
In the next two posts in this series we will explore how excessive unwanted deposits can be dealt with – during operations and, preferably, during design.
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