This is the third blog in a 5-part series about tanks. If you want to read the first entry about why tanks are important click here, or for the second part about tank design considerations click here.
After designing a tank, the next step in the lifecycle is to build the tank. As with design discussed in the last blog, there are many factors that must be considered in the fabrication process acting on a storage tank. As per our previous article on design, there are some useful standards for design and construction of storage tanks, notably API 650 and API 620, for anybody in need of guidance on how to successfully design and construct a tank, those standards give a quite comprehensive overview on many of the problems one might encounter and offer a wealth of information on solutions to overcome such problems. This article is in no way a complete overview of the construction process however it should offer some guidance on some of the considerations that must be made.
Storage Tanks – Foundations
The first part to consider when building our tank is the foundations on which it will sit. As the foundations will support, he full load of the tanks, (which could be well over 100T in some cases) it’s critical to make sure they are correctly constructed to reduce any chance of failure going forward.
The purpose of the foundations is to ensure the load carried by the foundations is properly transferred to the ground below to prevent any part of the ground sinking or moving in a way that would damage the integrity of the tank, thus we must consider the ground itself during the installation of our tank foundations.
Normally a tank design will specify what sort of ground conditions must be met prior to the installation of the foundations, as a minimum the ground will need to be compacted, however in more complicated scenarios with high loads, or poor ground conditions it may be required that a bedding is be laid above the soil.
In most circumstances once the ground underneath the tank has been appropriately prepared a concrete foundation will be poured, the type and density of the concrete will be specified by the designer of the tank, and the concrete supplier should be able to provide detailed instructions on the appropriate way to ensure installation goes as smoothly as possible. As with all concrete installations, there are steps to follow and considerations to be made (which are beyond the scope of this article) to prevent future issues.
Storage Tanks – Fabrication
As discussed in previous articles, the most common material for storage tanks is steel and thus we will focus on steel tanks in this section of the article. Once the foundations have been completed the next step if the fabrication of the storage tank, this begins with the installation of the tank floor, followed by the walls and finally the roof (for covered tanks). After these steps pipe connections, ladders and other required welding tasks will be performed to finish off the main structure.
Steel tanks are built by welding together several steel plates to form the overall structure of the tank. Welding can be a tricky process with numerous different things to manage, some considerations that should be made in the fabrication process are as follows:
- Ensure an appropriate weld procedure has been created or selected for these materials and conditions.
- Ensure any boilermakers or fabricators are qualified and coded to the weld procedure that is used.
- Ensure welding consumables have their material certificates available and match those specified in the weld procedure.
- Ensure appropriate NDT methods have been selected and organized to be completed on the welded structure.
All these concerns (and any others) should be addressed in a suitable inspection and test plan (ITP) that will be followed and signed off on at various stages throughout the fabrication and installation of the tank. ITP’s are critical documentation and should be used in every tank construction job, through the use of ITP’s, errors can be avoided and when non-conformances do occur they can identified quickly.
Storage Tanks – Protective coatings
Following the creation of the tank structure and any follow up inspections to ensure its integrity, tanks will usually have some form of protective coating applied. In corrosive environments, or when working with hazardous chemicals, some form of lining may be applied to the inside of the tank to protect it from damage.
Even in scenarios where a tank is filled with nothing but water, or something equally non-reactive tanks will still have their insides and outsides painted. Painting is a critical step and is not done just to make things look pretty, as painting steel structures will significantly slow and in many cases completely halt the rate of corrosion. Fun fact, it’s for this reason the Eiffel Tower is repainted every 7 years.
The type of paint to be used should be specified by the designer in the design process and should consider exactly what is going to be stored in the tank and how often the tank will be emptied for maintenance.
In some cases, it’s possible that other forms of protection may be applied either before or after the painting, such as galvanic protection, electrochemical protection, or other methods.
Storage Tank – Document Review
Managing the construction of a storage tank is a difficult task, it makes sense that many people are happy to finish up and go home once the tank is standing but this isn’t the end. One last task that needs to be completed is a document. At each step of the process various documents, such as material certificates, weld procedures, ITP’s and NDT reports should be produced, these documents are what highlight the tasks that were undertaken and to what standards they were completed to, by reviewing these documents and making sure they comply with the original design future headaches can be avoided.
Imagine a situation where the tank has been constructed, the job has been finished and everybody has gone home, the on site team is getting ready to fill the tank up (or maybe has already filled it up) and only now does somebody realize the steel plate provided by the supplier was 5mm thick when the supplier specified 10mm, this sort of thing is all too common and can create major hazards and expenses for tank owners. This is why documents need to be provided throughout the construction process and need to be reviewed as the job goes ahead, and once again during the close out stage of the job.
Rapallo has found itself needing to fix many issues that could have easily been avoided had an appropriate document review been performed at an appropriate time, or through some other type of mismanagement. In many cases it’s cheaper and easier to engage an expert up front to ensure the process is carried out properly the first-time round and avoid the errors altogether.
