Conceptual design of submerged pipeline in reservoir

#1

SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=177906
Please click the source URL above to read the entire web page. Posted below is a snippet.

QUESTION
I’m undertaking a concept design/preliminary costing for a submerged pipeline approx 5mi long in a reservoir. Location is California. The pipeline will be conveying freshwater (non-drinking), peak flows of up to 2400cfs and under pressure (not sure how much pressure yet).

My questions are:

  1. Is PVC or HDPE pipe suitable? Do these pipe types come in rolls, or lengths?
  2. Does the pipe need to be anchored, or just weighed down? If anchored, what method? I assume bottom is mostly mud/silt.
  3. Are there any standards/guidelines for this type of construction?

Any guidance appreciated.

REPLIES

rconner
Subaqueous pipe, and maybe particularly plastic subaqueous pipe as framed in the original post, is an interesting field. Plastic pipes have quite low “bulk density” and while touted as an advantage it it is my further understanding some hdpe pipes have negative buoyancy and will essentially float, even full in water. The negative buoyancy would be greater in salt water, and per Archimedes much greater (considering buoyancy on the the entire pipe system including weights) in an even denser fluid-acting surround that might be present in some bodies of water (such as liquid concrete, muck, or seismic liquefied soil?) Further, while interior surfaces of such pipes may not necessarily be super smooth, the exterior surfaces usually are.

Plastic pipe is from manufacturing, experience and/or practical matters also some limited in size. I really can’t imagine how e.g. one could possibly do a demanding underwater job (at least successfully) for a two thousand four hundred cubic feet per second flow quantity with a “pvc” pipeline? Likewise, I’m rough guessing that it might take in parallel something like FIFTEEN OR TWENTY of even the very largest hdpe pipelines available (assuming they were acceptable in all other respects for such installation?) to come close to handling that magnitude of flow volume at reasonable flow velocity for plastic pipes!

That being said, in my opinion some underwater pipe installation and (perhaps more importantly) subsequent trouble-free performance of underwater pipelines is a very rigorous application perhaps/arguably worthy of the very strongest, toughest pipe available. To get a feel for some extreme demands on some large underwater pipelines in even rather shallow water, and in some cases not too long after original design/installation, you might want to read e.g. the ASCE paper, “Marine Outfall Performance II: Stabilization and Case Studies” etc… (I noticed there were case studies of many underwater pipelines in this paper, including an 800mm O.D. x 700mm I.D. hdpe line that had been installed in the 1980’s near Cape Town South Africa to replace an old, shorter line that had been in service over 50 years. In a very short time after installation of this new pipeline (and even before the line was ”commissioned”), storms reportedly caused “heavy damage and dislocation” on the plastic line. The 50mm wall thickness of the pipe was reportedly severely gouged and abraded, and completely abraded through in multiple locations. This apparently required salvage of what they could and then a new installation of the line. A couple years after the line was finally commissioned a ship anchor apparently broke a diffuser on the line again, and four years after that another significant storm hit and broke the pipe severely again. The author states, “Many of the star weights had slipped and come together and in some cases had acted as wheels, allowing the pipe to wander back and forth under the strong wave surge.” I guess a subsequent incident relayed by the author pretty well summed up the experience to that date (as the breaks exposed the populated shore to raw sewage), “The public was outraged at the double debacle. In a Cape Town wharf pub, a lead member of the CED had a beer bottle thrown at his head by a fellow member of the local dive club.” Apparently after seven or eight years of this, the line was apparently installed a third time but in a trench “carved through the shale bottom, and then concreted over.” Also, “Tragically, one of the contractor’s divers was killed in 1992…” before the line was commissioned for a third time in 1993, it appears about ten years after this all started.

Some closer to home, I am aware of a significant 36” hdpe line that was laid across a rather shallow bay in Florida a few hours from where I live just a very few years ago (reportedly also by an experienced marine contractor). I understand this line, that I think was shorter than your job being a little less than a mile long, was initially required to be trenched, and also was very frequently concrete-weighted. I have heard third-hand this line also somehow encountered some weight slippage, I guess resulting in a sort of bunching up of weights from their original assembled positions. For whatever reasons also (I believe there may have been much time$?-consuming debate as to who/what exactly was responsible), a significant extent of this line also ended up being too shallow. Apparently also there was additionally at some point a realization or fear that the inside diameter of this newly line might not be adequate for (future?) flows (this is also something that should be very carefully considered proably best upfront, as the plastic pipe inside diameter used for such purposes can be quite small for nominal pipe sizes), as I eventually even noticed a budget issue request apparently once contemplated to replace at least some of the new line at:

http://www.leg.state.fl.us/data/Publications/2001/Senate/reports/budget_issues/SENReq937FY0001.htm

I don’t think the replacement larger line was ever laid at the estimated additional high cost, however, some sort of regulatory allowance and a new contract may have eventually been awarded for a protective concrete encasement or sort of carapace somehow installed over the too-shallow length. While I have not followed this closely since, I think maybe quite a bit of money and time later (and I’m not sure that the extra cost was borne by the original contractor nor his sub on the original installation), this job may have been eventually completed, in that case also probably some years after it started. I’m also not sure that (perhaps substantial in some cases?) effects of thermal expansion/contraction fluctuations on e.g. long lengths of welded pipelines, particularly maybe plastic lines not well backfilled on the bottom of a body of water are real well understood either. At least without anchorage specifically designed for mechanical thermal stresses/significant movements, I would think axial bucking forces due to increases in temperature could be rather large for even plastic pipes, and due to curve geometry effects even small axial growth might tend to cause significant lateral (or even vertical?) bucking movement, unless same is effectively restrained. Conversely, thermal contraction due to cooling might tend to raise an initially taut line (i.e. straighten out/lift a vertical sag laid straight in plan view?) The former expansion action/behavior I mention could be similar to the effect of the old engineering riddle, “If I had a string stretched tightly for tens of thousands of kilometers around the earth, and then I extended it with a short piece of string only a couple meters in length, how much would it extend above the earth?” (I guess the answer is all but the chubbiest of us could probably crawl under it anywhere on earth!)

I do not write this to diminish the utility of the specified/suggested pipe nor those who have thus trusted it (no pipe material is invulnerable, and I suspect there are quite probably many more claimed successes than problems with even the plastics); however, I just wanted to make the point that this can be very rough business with some sobering though perhaps some non-obvious realities for even the very strongest/most gouge resistant pipes conventionally used for such purposes.

Additionally, I think some of this sort of work is sometimes done with very heavy equipment and rip-rap etc. working/rubbing around the pipes, and in conditions with zero visibility of what is actually going on beneath the surface. Over the years when I have been associated with marine pipeline work, I have even heard from some contractors in many situations that instead of long welded strings of pipe, individual shorter or segmented pipe assemblies can be easier or maybe even critically needed in some areas to get them installed properly where they are intended to go (I suspect examples could include strong winds or currents and/or unstable trenches that fill more rapidly than most lay folks might realize).

Just a couple other comments – while I guess it could be argued that conditions in a “reservoir” might be kinder than say that on the Cape of Good Hope, I don’t know enough of your perhaps inland application to know that there might not be some significant storms, cyclonic or tornado, marine traffic, erosion, or seismic activity to challenge the location and integrity of even your line(s) over many years. According to Al gore and his ilk some of these sorts of events are even increasing and not necessarily “Acts of God”?!!

If you have gotten this far reading this or other references, you might realize also that an ability to dependably repair a broken or damaged marine pipeline in place might also be an admirable consideration for at least some Engineers/Owners in upfront material selection (in this regard, I wonder if it is difficult to get a proper fusion job on a pipeline underwater?) Finally, I’m not even exactly sure how one can safely make an assumption that even some underwater pumped pipelines will always be totally full of water, maybe particularly if when installed they really end up looking some like “Nessie”, instead of a profile view straight line on paper!

While I probably have not addressed all issues in even this very long post, in summary I think this can be a complicated business, that is thus perhaps/arguably not for the faint of heart, pocketbook, nor experience.

StephenA
I was the Site Engineer later Site Agent on the Cape Town Outfall mentioned above by rconner. The pipe failed in a storm that had 21m waves approaching the coast. That was about a 1 in 50 year storm. The pipeline was installed on a rock seabed. The final replacement was buried in the seabed with divers trenching the rock using an underwater excavator.

Since then I have installed many HDPE pipelines in trenches in the sea without any problems. Diffusers on any Outfall need special consideration regardless of the material.

Most HDPE outfalls are installed by the Float and Flood method and this needs to catered for in the design as air in the pipe during operation could bring the pipe to the surface. This happened in Richards Bay on a paper mill Outfall where the pumps were too high and cavitating. The newspaper reports were that the locals thought that it was a Russian submarine.

HDPE pipe has a high thermal expansion but also creeps and loses strength at elevated temperatures (above about 40 degrees C) but as with all things it is “horses for courses” and HDPE is the most suitable materials for certain cases.

Sizewise HDPE / HPPE is now made in up to 2000mm diameter and has been towed to Brazil from Norway in around 500m lengths.