13 replies to this topic

Hello,

I am currently undertaking my design project and am doign the plant facilities layout task, for an offshore natural gas prcessig plant. I was wondering if anyone could recommend any books or so specific to offshore plant layout. Also I am having considerable trouble finding the size of compressor, i have calculated the power requirement and have looked at manufacturer websites for possble sizes but cannot find any is there a particular way to determine the size?

Thank You very much for your help


John Smith

[Art Montemayor]

John:

There is no specific book that addresses equipment layout on an offshore platform - at least not to my knowledge.

Supply us with a listing of all your major equipment and I will furnish you with the locations (by level) of each of them on a real, existing offshore platform. That's the easy way to go about the problem.

The way a new offshore project is undertaken is that it is handed over to a knowledgeable and experienced contractor who has done this before. It is never given to someone such as yourself, who has no idea of where to put what and why. Therefore, if I tell you where the equipment should go you will be embarking from the same level as a real-life engineering contractor. Fair is fair.

What "size" of compressor are you referring to? - a process size or a physical, mechanical size? The latter is difficult to obtain unless you seriously want to purchase a compressor. I would suggest you estimate the physical size if that is what you need.

Initial spearation
two phase separator
three phase separator

Gas Sweetening
Absorption column
stripping column

Gas Dehydration
Absorption column
stripping column

Dew point control
turboexpander

Condensate stabilisaion
Distillation column

Waste Water treatment
Hydrocyclone
stripping column

Ancillary units
numerous heat exchangers, flash vessels, pumps, compressors etc...

from what i have found out is that the plant facilities are put together in 'modules' or 'building blocks' and that the layout follows somewhat this rule of thumb: accomodation, control rooms, utilities, process with firewalls and blast walls in between.

I was referring to its physical size, i was looking for an estimation but have been unlucky so far as to find one.

Thank you so much for your help

John

[gvdlans]

Reasoning behind your rule of thumb is to have largest separation between vulnerable locations (accommodation, control room) and the high hazard locations (e.g. sweetening unit, production wells). So within the process units there is also an order from low hazard --> high hazard. Instead of fire and blast walls you preferably have separation by distance, e.g. by having two or more bridge linked platforms. This is more inherently safe than having blast/fire walls. Note that at Piper Alpha the fire wall between the gas compression and separation modules was destroyed during the initial explosion.

A (real) example of a layout is as follows:

Accommodation (incl. control room, Firewater pump)-bridge-water treatment platform (incl. 2nd firewater pump, instrument air compressor)-bridge-manifold platform-bridge-process platform (including separation, sweetening, dehydration)-bridge-gas compression platform. Flare platform is also bridge linked to the process platform, perpendicular to the other platforms.

Bridge length is typically 50 m, but could be subject to consequence modelling and risk assessment

I understand your reasoning for the bridged platforms, however, wouldn't this be extremely expensive as it would require a number of separate platforms and thus supports etc...

Also where would i locate the flare tower, its unerstandable to lacate it as far as possible from the accomodation areas and downstream of the prevailing wind conditions but also wouldn't it have to be away from the process units also?

Thanks

[gvdlans]

I understand your reasoning for the bridged platforms, however, wouldn't this be extremely expensive as it would require a number of separate platforms and thus supports etc...

This is indeed a consideration and this depends a.o. on water depth. Note that an accident can also be expensive...

As so often, there is not a single good solution. That's also why you see so many different configurations, even within the same country.

Also where would i locate the flare tower, its unerstandable to lacate it as far as possible from the accomodation areas and downstream of the prevailing wind conditions but also wouldn't it have to be away from the process units also?

Thanks

As I wrote I would preferably locate the flare perpendicular to the other platforms (and to the prevailing wind direction), so that it has a low likelihood to act as an ignition source, but also that flare gases are not blown towards the platforms in case of a flame-out situation. Other issue is that you don't want the flame to be blown towards the other platforms for heat radiation reasons.

[Qalander (Chem)]

Many Good Points indicated by gvdlans.

However I believe Human Lives are most precious asset and

even at any cost Fire and Life safety requires top most priority.

Although not normally envisaged, but heavy petroleum gases close to LPG etc. may be real danger for 'Flash Back' case in very low winds or dull weather conditions

or when flare's flame finds these.
Hope this helps
Best regards
Qalander

[astro]

Offshore plant / platform layout is an involved and complex discipline.

By design, there is a need to space plant closer together offshore by virtue of space restrictions. As a result, there is a greater need for active & passive fire protection and overpressure blast walls. This makes a cookie cutter approach to plant layout that you may have come across for onshore plant, very difficult to apply. In simple terms, you have the freedom to squeeze plant closer together (to a point) by offsetting this with hardware protection.

Greater consideration of loss risk and cost/benefit is needed during design. You should also look to the statutory authority for guidance as they may provide useful direction. The other port of call is whether you're looking to engage a surveying company. I know that DNV has an array of offshore standards and recommended practices, the one that comes to mind for your purposes is:
DNV Offshore Standard DNV-OS-A101 Safety Principles and Arrangements.
From memory, I was able to locate this on the web and access it freely. The only shortcoming is that the pdf is security protected so that you can't print out. Of course, you can address this through payment but you'll achieve your ends by obtaining access to the content. Again, they do not offer a prescriptive approach to design. It's more a case of principles and guiding criteria that offers flexibility in the end result.

There is an equivalent US outfit, the American Bureau of Shipping, that also has a set of standards for offshore facilities. Not sure whether accessibility is as open as DNV.

A couple of other points with offshore layout.

1. Topside weight is a key cost consideration and heavy or light equipment can have an impact on layout. Not my strong suit but I'm thinking structural design consideration for point / distributed loads.

2. Escape route planning and the provision of temporary refuges in the event of a nasty incident is critical. This explains why there is a human limit to which you can squeeze plant together.

3. Design for installation and maintenance has to be considered. You will need to address how you get plant into position and then how you get it out, if required for maintenance reasons.

I recognise that there is probably more in the detail above than you probably want to deal with but the least you can do is discuss these matters in your assessment submission and show that your considering issues in key areas. In the "real world" this is the point where you would engage specialist to fill the gaps in your knowledge.

[gvdlans]

Good post from Astro!

...and then in the real world nobody would make such a layout on his own, it would be a multi-disciplinary job.

You can also check-out the Norsok (Norwegian) standard S-001: http://www.standard..../1052/S-001.pdf

its a fixed platform (steel jacket) at a depth of 155m. specification states that only one platform is allowed. a production rate of 420 MMscfd natural gas with a feed of 570 MMscfd.

what is skids? (as in skid mounting?)

Thanks

hello,

what are the typical size of platforms? i have been looking for a platform size just to get an idea of how big they are and haven't managed to find anything.

would a platform of 30m x 45m be too small?


also can the flare stack be bigger than the platform itself (even if the radiation emitted is at reasonably safe level for humans) or must the platform be larger than the flare stack?

[gvdlans]

There is no typical size...

Platforms vary from tiny BP wellhead platform to very large Zadco Upper Zakum Central Complex.

30 x 45 m seems very small looking at the number of units you want to put on it. Maybe you have a lot of elevation levels?

I don't understand your question about the flare stack. Do you mean higher i.s.o. bigger?

yeah i thought it was very small, but that does not include room for expansion. i think i will look into it again.

i meant the height of the flare stack. it is an angle boomed flare stack at a height of 60m. what i mean to say (sorry for not being clear) was that is it ok if the flare stack is larger in height than the lentgh of the platform (just considering in terms of magnitude of platform and flare stack length/height).

we have been given an example size of 30m x 80m does that seem reasonable?

thank you