Science of Stuck Part 3

In the first two articles in the Science of Stuck series we discussed how calcareous deposits form and how we can remove them from critical subsea equipment. 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

  • Deposit removal using acids, which is the typical method, is expensive and necessitates numerous practical and integrity / HSE considerations.

 

As always, prevention is better than cure, so in this third article, we will discuss methods to prevent unwanted deposits, and some of the things to keep in mind when doing so.

The Venn diagram shows the three factors that are needed to produce excessive calcareous deposits. Removing one or more of these contributing factors is the preferred strategy:

  • Coat, or otherwise change metallic surfaces to decrease their electronic conductivity

  • Electrically isolate wetted metallic surfaces from the cathodic protection system 

  • Exclude metallic surfaces connected to general structures, and thereby to the cathodic protection system, from seawater


Each strategy comes with unique challenges, which we will explore in the following sections.

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Coating surfaces to prevent calcareous deposits.

In some scenarios, this is a good solution, but typically there are numerous drawbacks to the strategy:

  • Uncoated CRA components are typically designed to move, either as part of a retrievable module or component, or during normal operations

  • Coatings typically suffer damage after repeated movement and associated frictional forces, potentially compromising functionality

Isolating modules or components from the CP system to prevent calcareous deposits:

Some CRA components (those with sufficient corrosion / pitting resistance) could in principle be isolated from the CP system to prevent excessive deposits. Drawbacks of this strategy include:

  • CRA components are typically part of a complex network of tubing, structural steel, other components, etc. and it is not always possible to isolate from the CP system

  • Not all CRAs corrosion resistance, in certain scenarios, can be guaranteed without CP

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Mechanically isolating selected components, or parts thereof from seawater contact / open exposure.

This strategy is very similar to coating components, but does not suffer from some of the same drawbacks:

  • Allows the metallic surfaces to function without the addition of coatings and without difficult isolation from CP system

  • Allows lower grade CRAs to be used and connected to the CP system

  • Allows for retrofitting onto already installed equipment

Examples of components which have been excluded from seawater contact in critical areas:

  • Electrical connectors with a deposit management sleeve protecting the internals of the connectors from deposits

  • Linear valve override stems (copper coated) with / without deposit management sleeve

Hydraulic couplers after 9 months subsea Browse basin, LHS couplers contained within deposit management system. DMS designed to fit between the underside of a subsea control module and its mounting base.


This system offers the following:

  • Protection of hydraulic couplers and electrical connectors from excessive deposits

  • Options for filling with inert hydraulic oil, or for acid flushing components pre SCM recovery

SEM micrograph of the deposits grown between male/female parts of the hydraulic coupler, responsible for the 10x force multiplier to separate the coupler after subsea / CP exposure. Gap between coupler parts is approx 200 microns.

Load test results from the seawater exposed coupler, showing a 10x on the force required to separate coupler. An SCM is typically equipped with 10-20 hydraulic couplers

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