Science of Stuck Part 2

In the first article in the Science of Stuck series we saw how calcareous deposits form, and how this can be either positive or negative depending on the component the deposits form on. The following key take-aways were presented:

 

  • Seawater is inherently corrosive, and a strategy to combat corrosion is necessary when placing equipment on the seabed 

  • Preventing corrosion via cathodic protection frequently leads to a situation where calcareous deposits form on retrievable equipment

  • The additional cost of retrieving components which have become stuck due to deposit formation is considerable

 

In this second article, we will discuss the available methods to remove unwanted deposits, and some of the things to keep in mind when doing so.

The picture to the right shows a subsea electrical connector, which has been in service for a number of years before being retrieved. Deposits can be seen on all metallic surfaces. The following is noted:

 

  • Surfaces which have been “out in the open” (area circled blue) have more deposits than surfaces where the free flow of seawater has been restricted (area circled white)

  • This type of connector is typically constructed of super duplex stainless steel, and does not require cathodic protection

  • Isolating these types of connectors from the CP system is recommended by the supplier

electrical,connector,efl,a60n,calcareous,deposit,perth,subsea
lemon,a60n,citric,acid,subsea,cleaning,calcareous,deposit,perth

Commonly used methods to remove unwanted calcareous deposits from subsea equipment can be classed as follows:

 

  • Chemical methods – primarily the use of acids to dissolve deposits

  • Mechanical methods – the use of various tools to vibrate components to break the deposits apart

 

In the following we will take a closer look at a case study involving the use of an acid to remove deposits and discuss the various consequences of doing so.

The use of a CP system promotes localised very basic conditions (hydroxide ion rich) at metal / seawater interfaces, which in turn makes the formation of calcareous deposits thermodynamically favourable. With the introduction of acids this situation can be reversed (going from basic to acidic conditions) to aid in the removal of the deposits.

 

All metal carbonates (e.g. calcite) react with dilute acids and produce salts, CO2 and water upon reacting, as depicted below

Commonly used acids (sometimes used as mixtures)

  • Citric acid

  • Sulphamic acid

  • Acetic acid

  • Formic acid

  • Phosphoric acid

calcite,citric, acid, cleaing, a60n, subea, perth, calcareous, diagram,graphic,tom,hitchmough

Calcite, as used in the above example, is a crystalline solid containing ordered, repetitive units of positive calcium ions and negative carbonate ions. The defining characteristic of an acid is its ability to donate protons when in an aqueous solution, and the degree to which this happens sets the limits for the pH achievable with a particular acid. The lower the pH, the more protons are available to react with the deposits, as seen above in the intermediate step.

 

Most acids could, in principle, be used to remove unwanted deposits, but there are a number of potential issues which need to be considered. These are listed below.

Integrity / HSE considerations

  • Acids are inherently corrosive, and the following material classes could all be negatively affected:

    • Metals

    • Polymers

    • Coatings

Materials testing programmes should be initiated whenever there is uncertainty.

  • The use of some acids will have a negative effect on the surrounding subsea environment, and environmental studies may be required

  • Some acids are not safe to handle without special PPE, and offshore personnel exposure needs to be considered

Practical Considerations

  • The ongoing use of large amounts of acid to retrieve stuck components is expensive. Various acid products come at various prices and levels of effectiveness

  • Access to the critical areas of a subsea component is often very hard – equipment / component design frequently does not consider future need for acid cleaning

  • Sealing a component in an effort to increase acid concentration and residence time is often very difficult. The use of custom shrouds and/or solid acids can often help in this regard

  • Various acids will produce various salts when reacting with deposits, and these salts will have various solubility in seawater. This can be important when e.g. treating components in very confined / low water volume spaces.

We now understand how and why deposits form, and how they are typically dealt with during subsea operations. In the next instalment in the Science of Stuck series we will discuss how deposits can be prevented from forming altogether, preferably during design.

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