Chevron Floating Roof Support

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					Chevron Floating Roof Support

  Providing Safe Working Conditions
          Under Floating Roofs
       Types of Roof Collapse
• Roof Spiral
  – this will result in the entire roof coming down
  – the strength and condition of the legs have no effect on
    this type of collapse
  – this is the most dangerous collapse mode
• Vertical Collapse
  – will not result in complete roof collapse unless this
    initiates spiral mode above
  – typically will result in excessive distortion in a local
    area not complete collapse to the tank bottom
    Sources Of Complete Roof
            Collapse
• Roof Spiral
  – Roof anti-rotation device may not exist or may fail.
  – Lateral support provided by a modern rim seal is
    adequate to resist lateral movement. If the seal is
    removed without adding temporary lateral restraint, the
    roof can move laterally. The lateral movement alone is
    not usually enough to cause problems beyond local leg
    failures. However, it may initial rotational movement
    which is almost always the source of complete roof
    collapse.
  Sources of Local Leg Failures
• Vertical Collapse
  – The flexibility of most floating roofs allow them to
    withstand local leg failures without total collapse of the
    roof.
  – Landing floating roofs which are holding water or
    product on the deck is the most common source of leg
    failure.
  – A properly landed roof with corroded or damaged legs
    is also susceptible to collapse.
    Prevention of Roof Spiraling
• Prevent Rotational Movement
   – The Euler buckling mode of a floating roof leg determines its
     strength. Without restraint to movement at the deck, the allowable
     load of the leg is greatly decreased regardless of its condition or
     size.
   – The forces required to prevent rotation are not excessive. Once the
     roof begins to rotate the loading on back up systems are much
     harder to quantify and design for. Properly engineered systems can
     insure the spiral mode is avoided.
• Prevent Lateral Movement
   – Lateral movement can be easily controlled by blocking the rim.
     This limits potential movement of the roof to maintain stability.
    Prevention of Vertical Roof
             Collapse
• Be sure roof is not landed with water or
  product on the deck.
• Open the deck emergency drain to insure
  the landed roof does not accumulate water.
• Inspect corrosion of legs and leg sleeves.
• Inspect plumness and alignment of legs.
 Industry Standard Roof Support
• Insure rotational and lateral roof support.
   –  Unloaded roofs will not completely collapse when properly
     restrained against rotation or lateral movement.
   – The restraint must be in good working order. If this cannot be
     verified, backup anti-rotation shall be used.
   – Rim seals which are removed shall be replaced by rim blocks or
     other lateral restraints.
• Inspect the condition of each leg.
   – Test leg thickness. UT or use a hammer test.
   – Verify plumbness and proper landing of legs. Insure legs are not
     under designed.
   – Prevent loads from developing on the roof deck.
   – Provide temporary support to questionable legs.
                           Summary
• Floating roof supports are designed very similar to fixed roof columns.
  Fixed roof collapse is virtually non-existent because the fixed roof’s
  attachment to the shell limits any movement at the top of the columns.
  If floating roofs are similarly restrained against this movement they
  will be highly reliable.
• Local buckling or collapse of legs can be sustained by most floating
  roofs if anti-rotations and lateral support is present. Legs which are
  susceptible to local collapse can easily be identified for corrective
  action. Landed roofs should never be holding water or product on the
  deck.
• Back up systems should directly address the sources of collapse.
  Systems which are designed to support the roof after a failure mode is
  initiated are difficult to design as the loading during this failure cannot
  be easily predicted.