API 579 FFS Assessments of Storage Tanks with Uneven Support

Introduction

Storage tanks play a crucial role in supply chain management in industries such as oil and gas, chemicals, and water storage. However, in permafrost areas, the design and maintenance of these tanks become more challenging due to the uneven and unstable foundation caused by the thawing and freezing of the permafrost. Permafrost is a permanently frozen layer of soil or rock which acts as the natural foundation for many infrastructures in northern Canada and Alaska. The gradual thawing of permafrost can result in uneven settlement, leading to foundation instability, which can cause significant challenges for storage tanks built in these regions.

Fitness-for-service (FFS) is a method to evaluate the structural integrity and fitness of various types of equipment, such as storage tanks, to continue to operate safely and effectively. In the context of storage tanks in permafrost areas, FFS assessments can be crucial to ensure the safe and reliable operation of these tanks. In this article, a real-world scenario of a storage tank built in a permafrost area will be described, highlighting the challenges of foundation instability and how API 579/ASME FFS-1 assessment and finite element analysis were used to address the challenges.

Description of the Problem

In a tank farm located in a permafrost area in Northern Canada, eleven atmospheric storage tanks were installed on rig mats. Visual inspection revealed gaps between the mats and floors of several tanks, as shown in Figures 1-3. The following is the design information for the tanks in the subject tank farm.

• The floors, shells, and roofs were made of CSA G40.21-44W material.
• The nozzles, flanges, and repads were made of ASTM A106B, ASTM A105N, and ASTM A36 carbon steel.
• The tanks were constructed in accordance with API 650 modified.
• The tanks were designed for hydrostatic pressure, 16 oz internal pressure, and a 0.4 oz vacuum.

Laser scan inspection of the tanks during the operation showed that changes in tank liquid levels resulted in tank movement/inclination. This movement can be described by the displacement of the roof tip, as shown in Figure 4. The displacement of the roof tip was characterized as follows:

• Based on the laser scans, midpoints of the tank floor and tank rooftop were established.
• For each tank, the floor midpoint was projected vertically to the same level at the roof tip and served as a reference point for vertical position.
• The difference in positions between the roof tip and the reference point formed a displacement vector, which started at the reference point and ended at the roof tip.
• In polar coordinates, the deflection vector was characterized by the deflection magnitude (distance between the reference point/vertical position of the roof tip) and its direction.
• The direction was measured by a vector angle with respect to the North direction.