The aim of this work is to get a better understanding of how optimal weight savings of the cylindrical shell plates in storage tanks can be reached using higher strength duplex material. The design criteria will be based on the requirements given by the American Petroleum Institute standards API 650 for welded storage tanks and API 12B for bolted storage tanks. The expected result is that use of duplex stainless steel instead of austenitic stainless steel can reduce the weight of the material needed to build a storage tank. A comparison between welded and bolted storage tanks will also be included in the work to find which joining method that is preferable. Additionally, an evaluation of the software TANK^TM by Intergraph will be performed. Matlab® will be used in order to perform the evaluation to compare the results.

The main difference between the standards API 650 and API 12B is that there are already given standard sizes of bolted storage tanks API 12B. Therefore, only storage tanks up to 1 625 m³ could be compared. When comparing storage tank sizes up to 1 625 m³ the result is that bolted storage tanks require a smaller mean thickness of the cylindrical shell than welded storage tanks and therefore less material and total weight of the shell. If the trend continues also for larger tank sizes, bolted storage tanks will be preferable to reduce the total weight of the storage tank.

By use of duplex stainless steel instead of austenitic stainless steel in storage tanks, the minimum required thickness can be reduced because of the higher strength of duplex stainless steel. This leads to a smaller mean shell thickness and therefore a reduced weight compared to storage tanks made of austenitic stainless steel. For most dimensions of storage tanks, duplex stainless steel is cheaper than austenitic stainless steel, but for some smaller dimensions, grade 1.4307 (austenitic) is cheaper than LDX 2101® (duplex).

Instability calculations have been performed for welded storage tanks, while no instability calculations have been performed for bolted storage tanks. With the wind speed 190 km/h (proposed by API 650), the welded storage tanks need to have wind girders to stiffen the cylindrical shell and in some cases also be anchored to the ground. Anchor is not required for large dimensions of storage tanks.

The evaluation of TANK^TM showed that most parameters agreed to the computed value of the same parameters in Matlab®. Why a few parameters not agrees is not fully known, therefore better explanations of the parameters used in the equations in API 650 would be preferable. To see the difference between Matlab® and TANK^TM it would also be of interest to see how the calculations are performed in TANK^TM, as only the results and inputs are printed at the moment.