Offshore

Threatened by iron leakage from steel bolts that are over half a century old, the oak hull of the Swedish royal warship Vasa has had more than 4,000 bolts replaced with bolts manufactured in super-duplex and hyper stainless steel from Swedish stainless steels and special alloys manufacturer, Alleima.

Corroding bolts became a major threat to the main attraction of the Vasa Museum in Stockholm – as iron from these fasteners proved to be a catalyst for the deterioration of the ship’s oak beams and planks. Vasa, once a powerful warship, was completed in Stockholm in 1628 but tragically sunk on her maiden voyage after less than one nautical mile. The shipwreck was relocated in 1961 and formerly restored to its full glory.  

Built from more than a thousand oak trees, Vasa’s hull was originally held together with iron emitting bolts. After 333 years on the bottom of the Baltic Sea, the bolts had corroded away and were replaced by new ones made from steel during the ship’s restoration. However, after fifty years in the museum’s humid environment, those replacement bolts have also become a serious problem. To preserve this international treasure for future generations, a decision was made to replace the ship’s corroded fasteners.   

In order to stabilise Vasa’s hull, a new bolt was designed by engineers at the Vasa Museum. At the same time, methods and machinery for this extensive replacement project were also proposed. New super-duplex stainless steel bolts were one possibility the museum considered for the new bolt material. Following a positive evaluation of SAF 2507^® super-duplex stainless steel performed by Chalmers University of Technology in Gothenburg, the museum turned to Alleima, headquartered in Sandviken, to provide technical advice on the bolt design, as well as material for testing purposes.  

“We decided to go with the high alloyed stainless steels because they proved to be the most suitable, versatile and from our point of view, the easiest to work with,” explains Jacob Jacobson, head of the Vasa unit at the museum. Alleima formed a technical team to determine which materials would be best suited to support the 900 tonne hull as it constantly shifts in its cradle while leaving it unaffected by chemical compounds. A project to further develop the machining of advanced duplex stainless steel grades was initiated and the technical team found the humid atmosphere of the Vasa Museum to be the perfect place for long-term testing of advanced stainless steel grades.  

“Having the opportunity to test our material in a meticulously controlled environment, such as the one Vasa offers, is very important for us. The time frame for the project gives us a unique opportunity to monitor the material in a way that would otherwise be nearly impossible to mimic,” states Jesper Ederth, head of R&D at Alleima strip division.

The cooperation resulted in a modified design of the original bolt – a combination of SAF 2507^® and SAF 2707 HD™ super-duplex and hyper-duplex stainless steel grades with dual properties vital to this project – corrosion resistance and exceptional mechanical strength that could support the weight of Vasa’s hull. Additionally, Alleima provided the museum with specialised equipment to measure the status of bolt movements over time. “We have 10 people engaged full time to ensure Vasa is continuously being preserved,” highlights Lisa Månsson, director of the Vasa Museum.  

She adds: “Vasa is a tremendous source of Swedish pride and is an international maritime treasure. In its time, the flagship of King Gustaf II Adolf’s fleet represented state of the art shipbuilding expertise. Today, the very latest metallurgical technology is applied to preserve this unique 17^th century ship for future generations of historians and visitors to the Vasa Museum.”