Maritime Tales: A Company History Series | Episode 7

Words by Dave McMillan, former CEO of Vard Marine Inc. (1989-2020)

In the spring of 2002, the company was awarded a contract by Exxon Mobil to design free-standing storage tanks to be used in their Adriatic LNG storage and re-gas facility approximately 15 km offshore from the Italian port city of Porto Levante.  This project grew into the largest contract for the company to that time and was very challenging from many aspects.  First off, the tanks were made from 9% Ni steel as the material for cryogenic storage, and secondly, rather than being subject to marine loads, the unit was bottom founded on the seabed where seismic loads were the dominant loads imposed on the structure.

While the storage tanks looked like simple boxes, their appearance belied the design effort that went into them.  The shell of each tank was of stiffened panel construction with web frames and stringers, similar to a ship, and designed for fabrication on a shipyard production panel line to reduce man-hours.  The internal structure, however, was very different from a ship, using internal bracing with transverse, longitudinal, and diagonal structural members.  The shell and the internal structure were designed to resist both static pressures from the LNG and vapour and the dynamic sloshing pressures that could be generated by a large earthquake.  Low-cycle fatigue due to tank fill-empty cycles were also a major design consideration.  Although having very high strength and ductile at low temperatures, the 9% Ni steel was only available in plates, was prone to cracking when formed, and had undermatched weld material.

We worked with a seismic specialist from California who provided us with earthquake loads that would be transmitted through the concrete gravity base structure (GBS) main structure and into the LNG storage tanks.  This whole design process was extremely complex with the end goal to ensure that transmission of the earthquake through the seabed under the concrete structure was predicted accurately.  A Vancouver subcontractor, Ausenco Sandwell, applied their seismic design knowledge to the bracing design, helping to ensure that the tank structure would absorb the loads safely.

The tanks were 155 m long by 33 m wide and 27 m high, giving a total capacity of 125,000 m3 each.  The two tanks were constructed in South Korea and dry-transported to the final assembly site in the south of Spain where they were installed in the concrete gravity-based unit (Photos 1, 2 and 3).

This was a very complex and significant FEA undertaking with Mark Munzel taking on the role as the Project Manager and structure lead.  Current President, Wade Carson, was responsible for most of the fatigue analysis.  To this day, I still don’t believe we appreciate the immensity of this undertaking and the fact it all worked well from the design, through construction of the tanks at Hyundai Heavy Industries in Korea, to installation in the GBS near Gibraltar and final commissioning off the coast of Italy (Photo 4).  A great credit to all involved in a very unique and challenging project.