An integrated petrochemical plant for hydrocarbons processing in Al-Shuaiba, Kuwait, is being expanded by the addition of numerous tanks with varying purposes. The Italian construction company Paresa SpA was responsible for the on-site erection of two spherical tanks. One, of 7200 m3 capacity, in carbon steel, is for the storage of polypropylene. The other, of 5500 m3 capacity, in stainless steel, is for the storage of polyethylene. Polypropylene and polyethylene are raw materials for the production of plastics.

Paresa SpA, Cesana, Italy, is an international building company specialising in the on-site construction of underground storage tanks, tanks for cryogenic liquids and cylindrical and spherical pressurised tanks for the petrochemical industry. Paresa is renowned for its highly experienced personnel, research and training and quality and safety programmes. Accreditations include ISO 9001 quality management certification, ISO 14001 environmental management certification, and prestigious ASME authorisation for the use of U and U2 symbol stamps in accordance with the ASME Boiler and Pressure Vessel Code - particularly important for the project in Kuwait.

The spheres
The spherical shape represents the optimal ratio between volume and surface. A natural phenomenon, this characteristic is very popular in engineering. However, copying the quality and perfection of nature in a man-made construction is not easy, as explained by Paresa's Technical Supervisor, Raffaele Cedioli and Quality Control Manager, Nicolo Amodio.

"The carbon steel tank is 24m in diameter with a wall thickness of 34-38mm. The stainless steel tank is 22m in diameter with a wall thickness of 30mm. The latter is particularly interesting as, to our knowledge, it is one of the largest ever constructed in this material. It needs a series of complicated and closely coordinated operations to be able to meet the final building and quality requirements."

"The choice of stainless steel", says Mr. Cedioli, "is not primarily governed by the aggressiveness of the chemical substance it is to contain, but merely by the operating conditions. The contained product reaches a temperature of -89°C and therefore the selected material is stainless steel SA 240M type 304 (EN 10204 3.2/BV)."

"A sphere is essentially constructed from a great number of segments forming the two hemispheres, completed with two caps; the north and south poles. In the design stage, the ratio between number of segments and dimensions has to be decided. More pieces of smaller dimensions are easier to transport and install - but they involve longer welding times. Fewer pieces of larger dimensions simplify the welding operations - but complicate transport and movement. Once decided, not all suppliers may be capable of supplying sheets in the required material grade and dimensions. Considerable experience is needed to solve this puzzle."

Prefabrication
In Paresa's Cesana production facility, the steel sheets were cold-pressed to the required curvature after which the edges were bevelled. In the case of stainless steel, to achieve the required cut quality, this operation was performed by plasma cutting using an ESAB Suprarex SXE-P cutting machine equipped with a plasma VBAExpert head. It is the first cutting station of this type in Italy and only the second in Europe. Finally, the components were transported to Kuwait for assembly, each individually marked with its complete chemical and mechanical history, acceptance tests performance and with a code for its eventual position in the construction.

Welding and assembly
For practical and environmental reasons, the decision was taken to use the MMA (SMAW) process with 2.5, 3.2, 4.0 and 5.0 mm diameter ESAB OK 61.35 (EN1600: E 19 9 L B 22/SFA/AWS A5.4: E308L-15) electrodes. Due to safety regulations surrounding the project, the weld material was supplied with 3.2 chemical and mechanical testing certification carried out in the presence of the Bureau Veritas inspection authority.

The welding operation involves joining the segments (62 for the stainless steel sphere) into the two hemispheres to form a globe and joining the caps to the hemispheres - easier said than done!

Deformation control was a major obstacle in obtaining perfect globe dimensions. It affected a large number of components and welds, with different joint designs (1/3 and 2/3 X-joints) at various positions in the construction. Even though starting from an ideal situation, in which all the components of the structure have been prepared with extreme precision, the human element becomes the decisive variable.

Where possible, certain segments were preassembled flat on the ground, coupled in pairs for the northern and southern parts of the hemispheres and in trios for the equatorial zones. This considerably simplified the welding and dimensional control, but complicated the assembly, because heavy weights, often in excess of 20 tonnes, needed to be lifted and aligned with the construction. It involved full penetration welds with opposite runs, but the bead sequence needed to be judged case by case, in order to control deformation and obtain a perfect shape.

Quality control
Quality control requirements were extremely high, due to the safety criteria surrounding this project. "Fortunately we have long experience in control, safety and quality", says Dr. Amodio. "The requirements of the inspection authorities on site were tough. Ultrasound testing, magnetic partical testing, dimensional control, and control of physical and chemical properties, were carried out in accordance with the ASME stamp under the supervision of the authorised inspection agency. It has been an excellent opportunity, though, to show our capability and create an excellent reference for future projects".

"ESAB's OK 61.35 stick electrode is a quality product with a great reputation in our industry –a really dependable factor. Furthermore, VacPac vacuum packaging enables us to skip re-baking procedures which, otherwise, would have been unavoidable under the warm and humid climatic conditions in Kuwait. Moreover, ESAB has given us valuable support, throughout the project".

By Bruno Malagoli, Product Manager Consumables a ESAB SPA., Italy.