Tweet

May 1, 2001 12:00 PM

Design Tests

In the summer of 1999, the tank was taken out of service for repairs. The annular ring of the tank bottom was to be replaced, and the entire bottom and the lower portion of the tank shell were to be painted with a protective coating. Colonial determined that these tank repairs could be performed without removing the internal floating roof.

“The floating roof has more than 100 pontoon end caps linking its structural frame, and the floating roof was inspected after the tank was taken out of service to repair the steel bottom. There was not a single crack or failure of any type in any of the pontoon end caps.”

Colonial specified that the deck leg attachment to the pontoon end cap be more durable than API 650 Appendix H required, Ferry concludes.

Results of another case study on an internal floating roof come from Kenneth Erdmann, engineering manager of Matrix Service Inc, Tulsa, Oklahoma. The case study looks at a full-scale puncture test on the internal steel pontoon-floating roof. The floating roof consists of a single steel deck surrounded by a pontoon. The pontoon ring is divided into several compartments by radial bulkheads.

API 650 Appendix H requires any component of the floating roof in contact with liquid or vapor to be a minimum 3/16 inch thick, Erdmann notes. Steel not in contact with the liquid or vapor must be a minimum 0.095 inch thick.

A common regulatory instrument that prompts industry to use floating roofs is the Code of Federal Regulations, Title 40, Part 60, sub-part K, Ka, and Kb, he says. Sub-part K initially was enacted in June 1973 and regulates the use of floating roofs in tanks that contain 40,000 gallons or more of product with a true vapor pressure equal to or greater than 1.5 pounds per square inch atmosphere but no greater than 11.1 psia.

Sub-part Ka was introduced in May 1978. It requires either an external pontoon or double deck floating roof, a fixed roof with an internal floating roof cover, or a vapor recovery system that reduces emission by at least 95%. The floating roofs are required to have continuous closure devices around their circumference. External floating roofs must have a double rim seal.

Sub-part Kb came into effect in July 1984. It applies to tanks from 20,000 gallons to 40,000 gallons that contain product with a true vapor pressure from 4 psia to 11.1 psia. Sub-part Kb is similar to sub-part Ka, with a few additions.

API 650 Appendix H describes the types of loads that internal floating roofs must be designed to withstand. All floating roofs must be capable of supporting the weight of two people (500 pounds) placed in one sq-ft area anywhere on the floating roof. All internal roofs are required to remain buoyant while supporting twice their dead weight.

The legs of internal floating roofs must support the roof's dead weight plus 12.5 pounds per square foot of live load unless means of draining accumulated liquid off the roof is available.

In addition, each type of floating roof must be capable of remaining buoyant in various punctured conditions. Bulkheaded, double-deck, and hybrid floating roofs must be capable of floating with any two compartments punctured. Pontoon floating roofs must remain afloat with the center deck and any two adjacent pontoon compartments punctured. “In many cases, these punctured compartment design requirements will be the controlling load condition that drives the design of the floating roof,” Erdmann says.

Two factors are critical to the ability of a pontoon-floating roof to withstand the punctured condition specified in API 650. First, the pontoon ring must be capable of supporting the loads being applied to it without being overstressed. Secondly, in addition to supporting the applied loads, the floating roof's deflection must be limited to prevent the product surface from entering other sealed pontoons, resulting in eventual flooding and sinking of the floating roof.

In the punctured condition, the pontoon-floating roof will deflect in three basic ways, Erdmann says. First, the center deck will deflect. It is no longer uniformly supported but is hanging from the pontoon ring around the perimeter. Secondly, the pontoon ring will list to one side as a result of uneven support when two adjacent pontoon compartments are punctured. Thirdly, the pontoon ring will undergo a deformation as a result of no longer having uniform buoyancy support.

Errors in the shape of the punctured deck will not affect the vertical loads applied to the pontoon ring, which must still support the weight of the deck, regardless of the deck's shape. The shape of the deck will affect the radial load that is applied to the pontoon, but this will primarily result in changes to the structural status of the pontoon and will have minimal effect on the vertical deflections of the pontoon ring.

“To properly design a pontoon floating roof, the various types of deflections must be closely modeled,” Erdmann says. “Errors in the calculation of these deflections will directly affect the accuracy of determining the surface level of the product with respect to the pontoon ring and pontoon manways. Since the only remaining buoyancy in the floating roof is in the pontoon ring, errors in the deflected shape of this ring could result in unacceptable product heights when in the punctured condition.”

Erdmann evaluates several methods of analyzing pontoon roof deflection. He compares these analysis techniques with the case study on a 123-ft-diameter pontoon-floating roof placed in the punctured condition.