Will appreciate comments and views on the following:

Background Info:
===============

1. An offshore production facility (complex), consisting of several platforms has been in

operation for the last 8 months. Among the platforms in this complex are a Central

Processing Platform (CPP) and a bridge linked WHRP Platform (WHRP).

2. The bridge is pinned at one end (CPP side) and is sliding on the end (WHRP side).

3. Our inhouse stress analysis methodology/criteria requires all lines on the bridge to be

stress analysed because of externally imposed movement (bridge movements).The methodology

also requires all hydrocarbon lines to be stress anlysed andthe utility lines, according to the temperatures/line sizes.

4. All the required stress analysis was successfully completed (using Caesar II) during detailed design stage and whatever supports and recommendations implemented, or at least we thought.

5. During a routine inspection service recently, one of our site personnel noticed that almost all the piping expansion loops on the bridge has been restrained severly from movement. On the expansion loops itself, two directional restraints (inverted goal post) have been used. This prevented the expansion loops from functioning properly and there was a cause for concern. We were concerned over the possibility that as a result of the expansion loops beign restrained, the piping could have been overestressed.

6. We checked the design recommendation with what was installed and found that there were some differences between what was actually recommended and what was insatlled at site. There differences include piping routing and the types of restraints used. (the "as-builts" were nto that accurate either.

7. We did a as-built survey and we captured all the details we wanted to do the stress analysis, at least for the main lines runnign across the bridge.

8. After completing the as-built activity, we realised there were no two convenient anchor points located on the either side of the bridge for each line to allow us to quickly model it into Caesar II to get a stress result (using a two-point anchor system).

9. Some lines were anchored on one side with the other running well into the platform facility, until an equipment nozzle anchor is met.

10. For lines (like item 9 above) that run well into platform facility, to our horror, there were many branch lines to it, not to mention sub-branch lines.


The Question/Problem
====================

1. IF we recall, the main issue was the stress condition of the lines on the bridge, which had their expansion loop freedom restrained.

2. To stress every single branch lines/subrnach lines (process hydrocarbon) again will be an intolerably expensive affair.

3. Question: With the fact that all lines have been stress analysed earlier durign detailed esign stage, could we just drop all the branch lines regardless of their size and just stress analysis the main line alone?

4. Question (Related to Item 3): If we could drop the branch lines, is there a general criteria used in the industry? Could we drop all branch lines that are smaller than half the size of the main line (process hydrocarbon and utiltiy lines)?

5. Question (Related to ITem 3): OR should we model the node at the branch connections and limit the displacement such that the branch lines are not affected by excessive movement on the main line? What will be the acceptable discplacement criteria in this case with respect to pipe sizes, service, class rating?

6. Question: For a moment consider a hypothetical case whereby we have dropped all branch lines and we are just left with the main line across the bridge and it is anchored at one end (CPP) and on the other side of the bridge (WHRP) there is none. In this case, how will we able to model this line with just one anchor point?

7. Question (Related to Item 6): Do we need to assume a what is called a Virtual/MAthematical Anchor point? What is the criteria for the selection/modelling of a virtual anchor point in Caesar II? Can we assume that if a main line 4" connects to a line that is at least twice its size, we can assume the t-point as a virtual anchor? Are there any other similar guidelines in the oil and gas industry?


8. Can the same approach be used for the branch and sub-branch lines?

9. Once we have used a virtual/mathematical anchor in the model, do we impose any restraint/limitations to this virtual anchor such that any forces, movements generated at this vuirtual anchor does not get transfered to the other segment of line?

10. General Question: Lastly, has Caesar II any provisions to take into consideration the stress analysis modelling of post contruction work? Note that the as-built dimensions we input will be based on a "stressed/expanded" pipe, not new condition/unexpanded condition.


A million thanks for anyone could throw some light on the above.

I guess I'm somewhat confused. If you've already got the Caesar models from the detail design stage, why can't you simply update them with the changes you've found and rerun them?

As far as most of your questions go, I don't think there are any hard and fast rules for any of them, that's what engineering judgement is for - to determine reasonable boundary points so that you model enough of the system to insure code compliance. One thing for sure, you are going to have to have all your degrees of freedom accounted for on both sides of the loops, whether by full anchors, or by combinations of supports, limit stops, and guides.

Spring has sprung out on the ocean!!! (I Like Mr. Kleins signature it has a lot of truth to it!)

Well the way I see it you have only tough days ahead on this problem...

1) Branch Piping can be evaluated separately assuming its Moment of inertia is 25% or so of the header, but note I said it can be evaluated separately, not ignored all together. Evaluation of the branches by visual review is usually easily done using maximum displacements from the header. Stick a linear restraint (X,Y,and Z) on the branch far enough from the header so that you can say that its is isolated from the rest of the run. That way you only have a short segment of each branch to deal with

2) Cut loose the loop restraints.

3) Buy steel to provide linear restraints X,Y,and Z where the main header enters the slip jt platform.

Otherwise ignoring the problems may work in the short run, but eventually fatigue damage will probably show up. When it does the owner may hire an outside consultant who will discover what you have and then say, "The design firm of record knew about this and did nothing about it!"

First, thanks for the response. I'm a mechanical engineer looking after piping aspects too. Work for a oil and gas operator here and we have engaged a consultant to help us with the design work. However, the consultant is not able to give me any good answers for the questions I had on this issue, so I decided to give this web site a try. The piping drawings, stresses etc carried out by this consultant during detailed design stage are in shambles.

Ed, this answer your question, the previous Caesar model does not at all represent the true as-built pipework on the bridge. The two anchor points used in the model does not exist at all on either side of the bridge on most of the cases. The piping routing does not match either.

John, my initial thoughts were along the same line too, but I was just guessing. I'll see if I could influence our consultant to consider this method.

Thanks once again to both of you.

I would be seriously concerned as to why the
construction is so different from the analysis!.
Especially considering it is an offshore petrochem
instalation.
I would also be wary of using your consultant again, considering his documentation/work is in disarray.
I would personally bring in a new consultant to analise the existing pipe configuration for
damage/overstressing, with an eye to resolving
the new configuration with the minimum of change.

I am supprised that you have that level of miscommunication/error, especially on a platform.
I thought those problems were only evident in our low-tech industry.
I would also be wary of any advise given to you by so-called Stress Analysists.
In my very limited experience (and it is limited)
i have been astounded by the lack of basic engineering knowledge that some "experts" have.
Just read some of the posts on this forumn.
The likes of Mr Luff/Breen/Klien are rare.

Often times during projects where the schedule requirements over-ride everything, including common-sense disconnects occur in the design process.

However in the end all disconnects and variations should be reconciled, otherwise undesirable things may happen.

Also another factor in our economy of today, is everybody wants the cheapest rock bottom price, often times ignoring quality of the final product, the resulting efforts are usually never good... there is cheap and then there is too cheap!

If the differences that exist between the analysis model vs what’s has actually been built are as great as you have stated, then you really have no analysis and therefore do not comply with the intent of the B31.3 code. para. 319.4.1 & 319.4.2.

A field survey should be commissioned, and analysis based on the survey should be done.

Finally any re-design based on the analysis should occur followed by field construction and field inspection of the finished work for compliance to the final design.

Or... sit back and see what happens!!!! The allowable stress for wave displacements will be quite low and if we assume the current system does not meet these low limits a fatigue type failure some time from now may occur. When it occurs it will be a crack and an associated leak.

A few points to put things into perspective for those not directly involved in the offshore industry:

Bridges between offshore platforms are often approx 100m long and carry a range of utility and process lines, I have experience of line sizes up to DN500 (20”) and up to 900lb.

The deflections of one platform relative to each other in a design case storm are often in the order of plus and minus 300mm. Structurally one end of the bridge is pinned to stop it falling into the sea and the other end is guided.

As you could imagine these platform deflections often govern the expansion loop sizes. The stresses induced from these deflections are expansion and fatigue stresses.

As for small bore piping… irrespective of size, if a small bore line is connected to a header that has large relative deflections it is unlikely to survive. Keep in mind that just because it has not failed yet this is no cause for comfort. There could be a fatigue on its way or it could fail in storm conditions.

The fatigue analysis of such systems can be complex as there is very high number of cycles and the deflections and induced stresses are proportional to wave height. The wave height/deflections are often given as a probabilistic distribution against number of cycles. This is another topic all of its own.

Nigel well said or as Adm Hyman Rickover said on building SSN Nautilus "The devil is in the details"

Cheers &

We have designed similar brige connected lines for offshore platforms and follow the guidelines as below.

Generally, for the bridge lines, one side is fixed at one platform and other side is allowed to slide freely as per bridge movement and the first anchor or restraint shall be suitably located in second platform to allow for free movement.( inherent flexibility in the line without any expansion loops in the bridge). Also, the bridge movement shall be checked for both positive and negative directions and the expansion stresses shall be checked for worst condition.

Generally the fatigue bridge deflection shall be furnished by Naval architect depending on the wave heights and cycles and in the absence of the data, it is safe to to assume 60% of the operating bridge movement as fatigue movement. The fatigue analysis can be done using the Caesar fatigue option considering only bridge deflection.

In your case, I feel if it is possible to introduce new anchor on the second platform which can allow free bridge movement, without affecting the flexibility of the lines on the second platform, it is the best option. In such case you can remove the faulty anchor on the bridge after the expansion loop.

However a careful study shall be made before introducing the new anchor. Since I am available in KL/Singapore, if possible, I can have a look at your system and suggest some remedial actions.

You can contact me at krsrini@pacific.net.sg

regards,

K.R.Srinivasan.

A quote from K.R.Srinivasan “Also, the bridge movement shall be checked for both positive and negative directions and the expansion stresses shall be checked for worst condition.”

Bridge movements are a displacement stress and as with temperature effects the complete range must be considered. With an expansion stress range, the stresses from ambient to low temperature are added to the stresses from ambient to high temperature. Similarly with bridge movements the stresses from positive and negative movements must be added together.

I am intrigued with your 60% rule. I have not come across this one before. What do you base your number of cycles on with this?

60% of the displacement is for Fatiuge what about the other 40%????...


The actual fatiuge range is 100% of both plus and minus displacements as pointed out... usually the load case is setup to subtract the + from the - displacement for the range....

I second Mr. Marsh's question on this 60% stuff!!!!

I have some questions on the details of bridge piping design.

1. I assume most bridge designs have (at least) one loop on the bridge to absorb the platform deflections. Have designs involving flexible pipe or ball joints been analyzed and implemented?

2. How is the +/- movement on the sliding end of the platform best handled? Extra long shoes? Rolling guides?

Last year I had to design a bridge with +/- 44" of differential movement. At the time, I had no idea what was "standard". I essentially re-invented the wheel.

John

I believe that this “60% rule”, in all but an exceptional case, is quite conservative if the total number of cycles is used. The problem is that the number of cycles is very high and usually in excess of the 2,000,000 limit currently in B31.3. Note that a typical platform natural period is on the order of 5 seconds, hence the high number of cycles.

This fatigue case is caused by platform deflections induced by wave action on the platform legs. In the life of the facility it will experience a large range of wave sizes with the largest waves in storm conditions. The problem is if you took the peak deflection from storm conditions and applied it to the full number of cycles (say 10^8) you would be excessively conservative. In reality the number of cycles experienced with anywhere near this load would be less than 10^3 cycles. The problem is that a large proportion of the fatigue damage can be caused at a displacement of 1/3 full displacement due to the high number of cycles.

If you know enough about the distribution of loading (deflections in this case) one can calculate the cumulative damage across the range of loads. In fact DNV RP-C203 is often used to this ends.

Given the detail of information required to perform this type of analysis it would be good to have a rule of thumb to use in the early stages of a project, hence my interest in this “60% rule”. Is anyone out there using such a rule of thumb or know of its origin?

All the numbers above are typical only and vary depending of conditions, location, the type of facility, etc are only quoted as example only and should not used.


Richard

Be careful with design of rolling support elements as in a marine environment these will be high maintenance items and after a few years would be unlikely to work.

For utility lines I have seen hoses used quite often. The hose is installed in a long U shape. In process lines I have never seen any sort of expansion joint or flexible.

You will be happy to know the next version of B31.3 will allow an extension out to the endurance limit in essence. We changed this specifically with offshore applications in mind.... so stay tuned and pick it up when it comes out!!!!

Once it becomes published the smaller cycles can then be equated to n equivalent cycles per the current rules.

Nigel,

I only ask because the client wanted to know what other design options were considered. They specifically asked about flexible pipe. Since flexible pipe is so expensive per foot, that pretty much killed it there. Another problem using flexible pipe involves keeping the flexible portion out of the "splash zone". For an equivalent 16" OD pipe, the minimum bend radius would be on the order of 13 feet. Technically the platforms were not high enough above mean sea level to accommodate the flexible pipe hanging down AND stay out of the 25' maximum wave height. Also, rollers were not used. Opted instead for guided shoes.

With regards to the fatigue analysis, I used an approximation based on data provided for storm deflections and everyday deflections. I seem to remember there being a pretty comprehensive write up in the CAESAR manual, but I may be thinking of another reference. The difficult part of the fatigue analysis was devising a deterministic analysis of a stochastic system. Finding the worst case of wave height, wave direction and frequency and then using that for each "segment" of the fatigue analysis. Not to mention extrapolating beyond the published stresses.

All in all, a very interesting and enlightening exercise.

Have you considered modelling the bridge together with the piping.. I know it would be quite a large model but would sure give a more realistic picture.

Gokhan