Repair of Square Hollow Sections 1

[12345abc6ttyui67]

Hi,

I have a 250x250x10 square hollow section (SHS) horizontal beam where the entire top wall has corroded over a length of approx. 200 / 300mm, e.g. the cross section now resembles a U rather than a square.

I am looking at potential repair options. The beam is on an offshore oil platform, so welding is not really a solution the Client wants.

Further, the existing loads in the beam are 'unknown' at this stage (I am trying to get hold of the original design report, but it's proving difficult). I therefore want to try and approach this to make the repair full strength, or "as strong as the original section". If this is impractical, I will need to do an analysis of the structure (which will be time consuming) just to find the loads in this beam, in order to apply a (potentially more practical) "non full strength" repair. By the end of the analysis, I of course might discover I need it to be full strength anyway!

So, I see my possible repair options as:
1) Welding a new cover plate to replace the top wall (least preferred by Client due to Offshore restrictions on 'hot work')
2) Bolting a new cover plate to replace the top wall (not preferred as introducing new holes / corrosion paths into the box, reducing section strength at bolt holes, hard to maintain seal to prevent corrosion inside the SHS, likely many bolts required to develop full strength)
3) Bonding a cover plate to the top wall (e.g. Belzona 1111 Super Metal as the bonding agent) (not preferred as no data on Belzona shear strength or fatigue life)
4) A composite wrap (e.g. FurmaWrap or Belzona SuperWrap) (Client has basically said No to this option right off the bat)
5) Any other options you kind ladies and gents may know?

Thanks.

 

[oldestguy]

I suspects the loads on this "beam" are rather light, but some truss effect may be needed for other purposes. Anyhow, have you looked at filling each of these tubes with something that will take over the loads? Such a material might be light weight cement grout filled with stainless steel needles or fiberglass needles. How about a flow able fiberglass with it also reinforced with needles of some sort? Sure would do for the life of one of these rigs maybe? Something that can be pumped and then will set up and is corrosion resistant is what I envision.

 

[12345abc6ttyui67]

SAIL3 - I can achieve a seal with Belzona fairly easily if it is deemed 'not load bearing' - e.g. if we implemented one of the other strengthening options as well. I've used this pretty frequently in this respect and not had any issues with it. The Client actually suggested just putting some thin plate over the hole with Mastick sealant all round - I don't feel this would be robust enough.

You are right, there are plenty of work procedures which will *allow* welding, however it is still the 'non preferred' option for most Clients. I'll have some pretty tough questions to answers if I propose welding without have exhausted other possibilities. I'll need to prepare a cost / benefit analysis comparing the (many) Structural benefits of welding vs. the additional scope (e.g. hot work habitats, fire watch, addition into platform shutdown plan etc.). None of this is impossible but it will depend on the pros / cons and comparison with 'cold work' solutions.

I've seen some Clients implement cold work solutions which were more than 10 times as costly than an alternative hot work solution.

oldestguy - yes, filling the SHS with grout is also another option. I do not have much (any) experience with this though, so need to do some research on it. I also suspect the construction team also won't have any experience implementing it either (they are used to spanners and hammers), but it's certainly worth investigating further. The obvious concern I have is the lack of shear studs or similar inside the SHS, which I suspect will restrict the lack of composite action which is generated, and I suppose means the tension capacity will not be increased. Perhaps this isn't an issue... Need to read some more about it.

 

[oldestguy]

On injecting some structural useful material inside the tubes, I'd contact grouting and specialty contractors that make repairs,etc. structurally even for foundations especially. I'd also suspect other parts of the overall facility corrode and need some form of extending their life without arc or gas welding and this general procedure may turn out to do a number of fixes. I'd bet nationwide outfits, such as those who advertise in Engineering News Record would be one source.