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A caisson replacement job well performed

As more and more platforms on the Norwegian continental shelf are reaching their design life, installation components are starting to show wear and tear caused by the ravages of time existing in a harsh offshore environment. An example of this is damages caused by corrosion. Salt, water, air, and rough mechanical impact on metal are all ingredients in a corrosive meal.

Ocean waves are constantly crashing into steel platform legs and pipelines that are supporting the platforms and have been doing this for 30-40 years at many offshore installations. Hence, many of the Norwegian offshore installations from the 80’s and 90’s are in need of inspection, repair and modification today.

OceanTech specializes in performing work in the splash zone where the waves create a sometimes very hostile environment – and a very corrosive one. Due to the very nature of the splash zone, being both just below and just above the sea level, not to mention the waves applying strong forces to all their surroundings, utilizing divers and ROVs in this area is rarely an option. OceanTech uses robotic solutions and other access tools to perform cleaning, inspection, repair, and modification work in the splash zone.

Equipped with solutions

During the summer season of 2022, OceanTech was involved in replacing a section of a caisson on a platform in the North Sea. Due to corrosion, it was necessary to replace a section ranging from 10 meters above sea level to 10 meters below sea level.

OceanTech adjusted the setup for a robotic solution called the Clamp Access Tool (CAT) to perform various subsea tasks related to the caisson replacement. A robotic arm was mounted to the CAT in order to be equipped with tools necessary to execute the subsea work. The tools were put in a basket mounted to the CAT for the robotic arm to be able to pick them up as required.

First, the CAT was used for cleaning the entire length of the relevant section of the caisson. Subsea cleaning is often considered necessary before commencing other subsea operations, like an inspection, because of marine growth covering the surface of components.

A subsea saw was used to cut out the damaged caisson section. To be able to perform correct sawing, the robotic arm on the CAT was equipped with marking tools – picked up from the CAT basket – to mark the cutting elevation. During the sawing, OceanTech used inspection cameras and lights mounted to the CAT for monitoring.

After removing the caisson section, the robotic arm was equipped with a subsea grinder to smoothen the cut area. Also, the area was cleaned using innovative cleaning techniques, a rotating rubber tool that effectively cleans subsea components.

After having positioned the new caisson section into place, the CAT robotic arm was equipped with a subsea torque tool to tighten the bolts on a subsea clamp securing the new caisson into place below sea level.

Preparation is key

OceanTech has a wide range of splash zone access tools and techniques available. In addition, OceanTech is well familiar with setting up the access tools to suit various client requirements.

Before performing the offshore work replacing the caisson section, the CAT was set up at OceanTech’s workshop at Dora II in Trondheim to meet the client requirements. The engineering was performed in-house by skilled engineers who also performed the method planning for the offshore scope of work. Also, project specific subsea equipment was made by the project team.

The CAT and required tooling were assembled and tested at OceanTech Subsea Test Center. This was to get the right quality assurance to the setup upon delivery.