—— Riser Arrangement Options and Trade-Offs ——
The first factor of significant impact is whether the riser is made up of a single hard-connected Pipe system or multiple segments: A hard-connected single pipe riser is made up of lengths of pipe that are welded, hard bolted or otherwise locked together. A Multiple segment riser is made up of smaller lengths of pipe that are interconnected with expansion joints or couplings that allow relative motion. Where a single pipe system is used, the riser will expand and contract as a unit. Forces can be transferred up and down its length as desired and all expansions or contraction will be additive and will affect the total length of the riser system. A riser made up of multiple pipe segments joined be expansive couplings will not behave as a unit. Each segment will expand and contract and move independently. Forces cannot be transferred through the couplings, but will have to be absorbed in the segment in which they are generated.
The second key factor is whether the riser is anchored versus non-anchored: Anchoring the system will "fix" the anchorage point. Each pipe segment in the riser can have at most one anchorage point, but need not have any. To prevent expansion couplings from collapsing and becoming damaged, most multiple segment risers (as described above) will have an independent anchor on each segment. There are typically two benefits to anchoring, the first is that overall motion of the riser is limited (simplifying interfacing with it) and second, significant variations in weight can be absorbed by the anchor (and properly located, not transferred into the pipe itself as added stress).
Expansion or Contraction: If the system has a high operating temperature relative to the support structure, it will expand. If the temperature is low, it will contract. This can impact the desired location for the anchor and the isolator selection as indicated below.
Restrained spring isolators can be pre-adjusted and will, if properly applied, protect the riser from the excessive forces that would be generated by unrestrained coils (necessary to support the water) when the riser is empty. When using Restrained coils, it is critical that the riser system be designed such that in service, deflection on these coils always increases and never decreases. See also below.
Bottom Anchored systems will, if hot, grow from the bottom. If using non-restrained support coils above the anchor, these can be pre-loaded through the pipe against the anchor during installation and no significant issues are likely to result except that it will try to lift the anchor off the floor. If the coils are restrained coils however, uplift loads will be eliminated from the anchor, but in service the pipe will grow and since the isolators will be "locked out" by the restraint hardware and damage would result. This is not a good combination.
On the other hand, if the system is a cold system, the opposite would be true. The non-restrained coils would behave as above with no significant change. Restrained coils would now work as, with the drop in temperature, the shrinkage in the pipe would cause the coils to compress and come off of the restraint bolt, thereby picking up additional load.
The exact opposite is true of a Top anchored system. These work well for hot systems with restrained coils, but should not be used for cold systems with restrained coils.
