Equinor’s Heidrun field has been producing oil and gas since 1995 using the world’s first tension leg platform (TLP) with a concrete hull. The Heidrun platform is installed at 350 meters of water depth and is the world’s largest floating concrete platform. It is also supported by four circular columns, giving the construction a total height of 249 meters. The platform deck is 32 meters above Mean Sea Level (MSL).
The height of the platform deck, combined with the large distance between the columns, means that riser protection nets are required to protect the risers from vessel impact in the splash zone. These nets are stretched between the platform’s columns to arrest encroaching supply vessels and prevent them from striking one or more of the 60 vulnerable risers.
The design lifetime of the Heidrun net was 25 years. Following initial damage to the ropes in 2015 and a subsequent study confirming fatigue, Equinor decided to hire OceanTech to remove two of three nets and install another one.
– The project was initiated in September 2020, Ståle Karlsen, Senior Subsea Engineer and Project Manager at OceanTech, said.
Rigorous onshore planning and testing
The original Heidrun riser protection nets were attached in a dry dock in 1995. To complete the replacement offshore, the OceanTech team developed new methods and techniques for work in the splash zone.
– Replacement in harsh marine environments immediately below the ocean surface cannot be completed by using traditional methods such as ROVs or divers, Karlsen added.
The project encompassed an 18-month planning process. The approach included prefabrication of structures and tools, staffing, and rigorous testing at OceanTech’s Subsea Test Centre in Trondheim. Karlsen believes the center serves as an important contributor to the development of innovative tools and techniques.
– By testing onshore, we are able to explore whether our techniques and tools are able to perform specific tasks, and make necessary adjustments to minimise costly repairs and waiting time offshore, he said.
Karlsen also highlighted the additional benefits onshore testing brings to personnel training:
– Our personnel have already completed similar tasks when they arrive at the platform offshore.
Solution-oriented collaboration is key to project success
The project’s offshore rigging included handling large units like nets and wing ropes, using up to 50 industrial air hoists, and working on three columns simultaneously. Coordinating five teams of rope access technicians added to the complexities of the project.
– The size of the project, and the distance between the columns, required significant planning efforts and radio communications in order to coordinate the climbers, he said.
In total, more than 60 rotational workers, including rope access technicians and subsea engineers, were involved offshore. Karlsen commended Vertikal Service’s highly skilled workers who worked on the project:
– Finding the right people with the right skill set is always a challenge, in particular for projects of this stature. We have been working with Vertikal Service on several projects, and they always provide trustworthy and skilled offshore workers, he commended.
Karlsen also praised the collaboration with Equinor, highlighting a mindset that enabled his team to explore solutions to challenges rather than avoiding issues that could hinder progress.
– Pragmatism and solution orientation are vital to succeeding with such a complex project. The collaboration with the Equinor staff was excellent, and we really appreciated their efforts in assisting us in overcoming challenges.
Avoided costly operational disruptions
OceanTech deploys various tools and techniques in splash zone operations, from lightweight robotic solutions to heavy-duty solutions with work-class robotic arms. All the tools and techniques avoid any use of divers, ROVs, or support vessels. The team of engineers and technicians constantly seeks new and innovative working methods. This maintains OceanTech’s position as the preferred supplier of splash zone solutions on a global scale.
At the Heidrun platform, the project team performed underwater cleaning and structure cutting using diamond wire saws and installed new specially designed structures and ropes.
– An immediate consequence of not replacing the net would have been operational disruptions with possible shutdown in periods. The old net was in a shape that would have enabled it to last only for a few more years. Without the new net it would simply have been almost impossible to use supply vessels on the west side of the platform, Karlsen concluded.