Others
There are a number of other methods used, such as flange bolt supports,
various type of pipe hangers and other specialty type supports, however the
first two categories account statistically for better than 95% of support points
on a typical offshore structure.
There are a number of other methods used, such as flange bolt supports,
various type of pipe hangers and other specialty type supports, however the
first two categories account statistically for better than 95% of support points
on a typical offshore structure.
The industry has long been aware of the problem, but has failed to appreciate the true causes; this is evidenced by some of the solutions that have been implemented to stop the problem which have actually accelerated the problem.
Rubber Pads & Liners
As previously stated, it was thought that the metal-to-metal contact was the
main problem, hence if this were eliminated the problem would also be
eliminated. The use of rubber pads of some type has been and is still thought
to solve the problem. Not true (Fig. 8.) In fact, rubber pads under pipes do a
wonderful job of reducing the life of the pipe. The crevice that was formed
without the rubber pad is mild in comparison to the new crevice, which now
has the ability to actually suck water in (by capillary action). Not only is it
better at getting water in, it is better at holding it, since air circulation and
natural evaporation is eliminated. The situation is further worsened by the
length of the crevice which allows an oxygen concentration gradient to go
from full natural concentration to anaerobic in a few centimeters.
As previously stated, it was thought that the metal-to-metal contact was the
main problem, hence if this were eliminated the problem would also be
eliminated. The use of rubber pads of some type has been and is still thought
to solve the problem. Not true (Fig. 8.) In fact, rubber pads under pipes do a
wonderful job of reducing the life of the pipe. The crevice that was formed
without the rubber pad is mild in comparison to the new crevice, which now
has the ability to actually suck water in (by capillary action). Not only is it
better at getting water in, it is better at holding it, since air circulation and
natural evaporation is eliminated. The situation is further worsened by the
length of the crevice which allows an oxygen concentration gradient to go
from full natural concentration to anaerobic in a few centimeters.
Fiberglass Pads
Contoured pads attached to the pipe at support points (Fig. 9.) Obviously another attempt to eliminate metal to metal contact. This is better than the rubber pads but still allows a crevice to be formed at the pipe surface.
Contoured pads attached to the pipe at support points (Fig. 9.) Obviously another attempt to eliminate metal to metal contact. This is better than the rubber pads but still allows a crevice to be formed at the pipe surface.
Welded Supports
The welded support is a viable solution. However it adds significant cost to a typical project both in terms of construction and inspection. In some situations it would be undesirable to make so many external longitudinal welds to a pressured piping system.
The welded support is a viable solution. However it adds significant cost to a typical project both in terms of construction and inspection. In some situations it would be undesirable to make so many external longitudinal welds to a pressured piping system.
The Optimal Solution
Clearly the solution to this problem must address the root causes of the problem, and should have certain features that make application practical.
The important features of one successful solution that is in widespread use throughout the offshore community are as follows:
1. The crevices at the pipe surface and the ability to trap and hold water in contact with the pipe surface must be eliminated.
2. As a secondary concern, metal-to-metal contact should be eliminated.
3. The solution should allow easy maintenance and inspection of the pipe at the support point.
4. The system must provide complete support to the piping system.
5. The system will ideally be non size-specific.
6. Must be applicable to new construction and retrofits, and should
require no hot work to install.
7. Must be cost effective.
The half round, high strength thermo-plastic rod (Fig. 10), meets all of the above requirements. The half round configuration minimizes the crevice at the pipe and allows no water accumulation. The standoff provided allows easy inspection and maintenance at the support. The metal-to-metal contact is eliminated, and if used with an insulated bolt (Fig.11), the pipe can be totally isolated from the support structure. The low cost material has been selected and configured to optimize compressive strength while exhibiting very low creep, excellent u.v. stability is also a material feature. The material can be deployed as a continuous dressing to the top of a pipe support beam (Fig.12) , or can be integrated with a stabilizing U-bolt (Fig.13). Either way allows cold work installation for new construction or retrofit applications.
When using U-bolts it is important to apply a polyolefin sleeve over the shank of the bolt. This reduces the risk of cracking the paint film around the pipe as the bolt is torqued down. The olefins provide the right combination of hardness an durability to protect the pipe paint but avoid setting up a capillary crevice around the circumference of the pipe.
Clearly the solution to this problem must address the root causes of the problem, and should have certain features that make application practical.
The important features of one successful solution that is in widespread use throughout the offshore community are as follows:
1. The crevices at the pipe surface and the ability to trap and hold water in contact with the pipe surface must be eliminated.
2. As a secondary concern, metal-to-metal contact should be eliminated.
3. The solution should allow easy maintenance and inspection of the pipe at the support point.
4. The system must provide complete support to the piping system.
5. The system will ideally be non size-specific.
6. Must be applicable to new construction and retrofits, and should
require no hot work to install.
7. Must be cost effective.
The half round, high strength thermo-plastic rod (Fig. 10), meets all of the above requirements. The half round configuration minimizes the crevice at the pipe and allows no water accumulation. The standoff provided allows easy inspection and maintenance at the support. The metal-to-metal contact is eliminated, and if used with an insulated bolt (Fig.11), the pipe can be totally isolated from the support structure. The low cost material has been selected and configured to optimize compressive strength while exhibiting very low creep, excellent u.v. stability is also a material feature. The material can be deployed as a continuous dressing to the top of a pipe support beam (Fig.12) , or can be integrated with a stabilizing U-bolt (Fig.13). Either way allows cold work installation for new construction or retrofit applications.
When using U-bolts it is important to apply a polyolefin sleeve over the shank of the bolt. This reduces the risk of cracking the paint film around the pipe as the bolt is torqued down. The olefins provide the right combination of hardness an durability to protect the pipe paint but avoid setting up a capillary crevice around the circumference of the pipe.
A failure analysis based on ASME Sec-VIII div-2 was done to evaluate ‘Plastic-Collapse’ & ‘Buckling analysis’ using the elastic & elastic-plastic stress methods, by considering the static loads & dynamic loads in combination that act on the 60” pipe header of the flare system. The results were analyzed for the location of buckling failure that occurred during the 1st Failure recorded. The expansion stresses in the pipe header were twice as much higher than that those due to the normal operating temperature loads and were greatest at the time of the failure, when compared to normal operating temperatures. It was found that the failure in the pipe header characterized as the inward plastic collapse could be attribute to 'Buckling' failure mechanism and that may have been caused by multiple static loads creating the pre-stressed state combined with the dynamic load that triggers the actual failure
The overall vibration characteristics of the 60" flare header piping system, based on the first 15 modes, suggested concerns regarding the presence of maxima / minima and ‘Point of Inflections’ on the 60” section of the main header in the vicinity of Node-3383, which is location of actual failure that occurred during the failure incident. The current configuration of the sulfur lateral branch was also found to provide restraint to the 60" header, such that it is believed to impose unfavorable modes characteristics at the location of actual failure that occurred during the failure incident.
The mode characteristics thus acting on the old (0.375" thickness) header when combined with the high D/t ratio would result in header bending. Under bending, when the cylinder is very long, the flattening of the cross-section leads to large reduction of the effective section modulus of the 60" header, and the instability occurs as a single transverse wave (Buckle) on the compression side of the shell. This phenomenon was attributed to the failure of the 60” sections of flare header piping system. Decreasing the D/t ratio is the most effective way to counter this failure mechanism.
Forwarded from Piping Stress Analysis (PSA Group)
TANK SETTLEMENT: