Cathodic protection design for rigs and mobile offshore units

Cathodic protection for new-build, conversion and life extension project is designed based on established codes as well as a fair share of experience.

Our designers apply established codes and best practice for cathodic protection design, installation and maintenance.

Life extention and conversion of floaters

FPSOs and similar units are often based on modified vessels, usually crude oil shuttle tankers – already fitted with a hull mounted ICCP system. For this reason, many operators find it easy to use the same ICCP system to protect the vessel in its new role. Nevertheless, an FPSO cannot be considered a regular tanker when it comes to corrosion protection.

A corrosion protection strategy should be established early to avoid problems later on in the vessel’s lifetime.

Whether it is a new-build, conversion or life time extension project, this will involve some interesting corrosion protection challenges:

• Should ICCP or sacrificial anodes be applied for hull protection?
• How to protect the ballast tanks?
• What about corrosion protection of mooring and riser systems?

A front-end engineering and design (FEED) for life extension, conversion or upgrade will benefit from early involvement and timely provision of relevant and necessary information.

Relevant design guidelines to follow can be summed up as:

1. Contingency must be included in the system design.
2. Number of ICCP anodes must be increased, for better current distribution and reduce the consequense of possible malfunction of a few anodes, at a later stage.
3. Active MMO layer of the ICCP anodes must be designed for the expected lifetime of the project.
4. Reference cells must be of a long-life design type.
5. Dielectric anode shielding to be dimensioned for the anode capacity and system lifespan.
6. External hull components should be diver replaceable.

SPS and class renewal of rigs

Classification societies require different levels of SPS surveys to be conducted every 3, 5, 10 and 15 years. This involves investigation and surveys to ensure the integrity of the cathodic protection system.

Cathwell has a track record of delivery on SPS and classification projects, both offshore and at yard stay. The scope of work would typically include:

• Testing and tuning of ICCP control cabinets.
• Operational testing and inspection of ICCP anodes and reference cells.
• Consistency check of protection levels.
• Visual check of underwater ICCP items.
• Recommendation of possible measures (repair and replacements).
• Adjustment and tuning of protection level.

If not otherwise specified, the cathodic protection related class renewal work would be conducted in adherence with the following codes:

DNV-RU-0101: “Rules for classification – Offshore drilling and support units.”
DNV-RP-B401: “Recommended practice for design, manufacture and installation of sacrificial anodes.”
DNV-RP-C302: “Risk based corrosion management.”

Design input request

What do we need to assist you?

This section applies mainly for newbuild and retrofit, althoug similar details may be relevant in verifying existing designs.

By providing the below listed information (for the system in question), we would be able to carry out a proper design. If not all details are available right away, we can usually start with what you have, and adjust as we go along.

You may direct your input, questions or requests here.

Protection of underwater hull (by ICCP or sacrificial anodes)

To calculated cathodic protection of the external underwater hull, the following input would be requested:

1. GA drawing.
2. Area of operation (assumed).
3. Specification to be applied (e.g. DNV-OSS-101, NORSOK M-503 or similar).
4. Coating condition (coating category or assumed breakdown).
5. Protection preferences, if any (ICCP or sacrificial anodes).

Adhering to established requirements is obvious. Deciding what is “reasonably practicable” requires a good share of experience.

Anodes for ballast tanks

To calculated cathodic protection of external underwater hull, the following input would be requested:

1. GA drawing.
2. Tank type (side tank, double bottom, fore peak, aft peak or similar).
3. Tank area/volume.
4. Specification to be applied (e.g. DNV-OSS-101, NORSOK M-503 or similar).
5. Coating condition (coating category or assumed breakdown).
6. Tank content (regular sea water, fresh water, other).
7. Ballast time and level (assumed).
8. Design lifetime of anodes (2-3 dry-docking periods are typical).

Antifouling for seawater intake system (ICAF)

Details requested to calculate a proposal for antifouling treatment of sea chests, mud box and seawater pipings:

1. GA drawing and sea chest arrangement drawings (if available).
2. Docking interval (assumed time of operation between dry-dockings).
3. Pump capacity (flow rate in m3/h) from the sea chests subject to treatment.
4. Material quality in seawater pipes.
5. Area of operation (assumed).

Propeller shaft grounding (slip ring system)

To design and propose a shaft grounding system, the following information would basically be requested:

1. GA drawing.
2. Shaft diameter.

You may direct your input and requests here.