The Museum of Cultural History in Oslo considers to move the more than 1,000-year-old Viking ships through Oslo to a new museum on the other side of town. DNV assists the museum in preparing a basis for decision making.
The Viking ships from the Tune, Gokstad and Oseberg discoveries, all of which contained rich burial goods, were built in the 9th and 10th centuries and are among Norway’s foremost cultural monuments. They are currently in the Viking Ship House at Bygdøy (Oslo), which is Norway’s most visited museum and a part of the Museum of Cultural History.
Eighty years after the first part of the exhibition was opened, the number of visitors to this museum has increased 10–15 fold, with more than 200,000 visitors in the summer months of July and August every year.
Expected to be worst case
DNV has conducted a strength analysis of the Oseberg ship, which is the most fragile of the three Viking ships in the museum. The Oseberg ship on exhibition was put together from more than 2,000 pieces after being excavated in 1904. The analysis of this ship was expected to represent a worst case for the conditions of the three Viking ships.
Finite element model established
A finite element shell model of the Oseberg Viking Ship was established for strength analyses. The geometry was taken from available scan results, providing 3-D coordinates along each board of planking in the ship. The material properties of the archeo-logical oak were assessed based on laboratory tests of available test specimens, data from the literature and engineering judgement.
The material was divided into three different categories according to quality. Based on visual inspection carried out in cooperation with the staff at the museum, the properties of the different categories were assigned to different parts of the finite element model of the ship. In addition, a relatively large number of existing through-thickness cracks were modelled by splitting up the connections between adjacent finite elements in relevant positions. The contribution from the internal framing to the global strength and stiffness of the ship was ignored due to a large number of cracks and lack of effective connection between the frames and the planking.
Improving the support
The ship is continuously supported along the keel, and in addition around 10 individual support pillars on each side contribute to carry the weight. Although these support pillars are essential to prevent the ship from collapse, they provide concentrated loads on the planking with associated stress concentrations.
During the 80 years on display, a gradual weight transfer from the keel to these pillars has taken place as a consequence of long term deformations. In-field measurements of the loads in some of the pillars and also of load versus deflection were obtained by jacking the pillars up a few mm from the floor. The results were compared with similar results from the finite element model, for verification purposes. It has been recommended to improve the permanent support of the ship.
Handle with care
The model has also been used to analyse some examples of accidental events and various examples of undesirable events that one may think of as a consequence of preparing the ship for transportation and of transporting the ship to a different location in Oslo.
The model can be used to document how unloading of highly stressed areas can be achieved by introducing additional supports underneath the ship. This is relevant both in the case that the ship is on permanent display but also in the case of transportation to a new location.
Furthermore, the model can be used to establish tolerances related to the ship handling and transportation, where the main philosophy is that the ship must be handled in such a way that the stress level is always favourable compared with the stress level in the present situation.
Better than expected
The preliminary conclusion of DNV’s analysis says: “Moving the Oseberg ship will involve a limited risk of failure provided the ship is given extensive support during transport and is handled within defined limits, and provided no accidents occur.”
”The ship is in better condition than expected, although the wood in it is much weaker than new wood,” says DNV’s project manager Torfinn Hørte. “We have focused on the loadcarrying parts of the material in the ship, whereas the report does not conclude anything about locally weaker parts that are glued, made up of splinters, and suchlike.”
Overall risk assessment
In addition to the strength analysis, an overall assessment is currently being conducted of the risk related to moving the artefacts from the Viking ship discoveries. The security and storage factors, visitor arrangements and opportunities that may emerge from putting the Viking ship treasures into a new context – professionally and experience-related – are also important.
“All kinds of imaginable factors must be considered and weighed against one another – both advantages and disadvantages,” states the museum in a press release.