Chevron Floating Roof Support

1
Chevron Floating Roof Support

• Providing Safe Working Conditions Under Floating
  Roofs

2
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

3
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.

4
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.

5
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.

6
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.

7
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.

8
Summary

• Floating roof supports are designed very similar
  to fixed roof columns. Fixed roof collapse is
  virtually non-existent because the fixed roofs
  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.