When to consider flexibles?

The simple answer is “always” and “as early as possible”.

So to explain that first, why shouldn’t we always consider flexibles during concept select? It maybe clear straight away that they aren’t the way to go, but at least it would be considered and rejected. Too often it’s not rejected, it just not considered and then either the opportunity is lost or not fully obtained.

Whilst at TPFMC I worked numerous projects where they’d won the EPCIC for the flexible installation, but all the FEED work (by others I should add) was done on rigid pipelines. This can cause quite a few issues that may become operational challenging and could leave people with a bad perception of flexibles. However, if FEED was performed on flexibles these issues would have been engineered out at the time.

The potential issues that may arise from a late change to flexibles are:

• Change in roughness leading to higher DP and liquid hold-up profiles.
• Reduced maximum design temperature and FWHTs that have increased after a change in WGR assumption from subsurface.
• Higher minimum design temperatures causing issues for well restarts because of JT across the choke.
• Depressurisation limits for the flexible resulting in longer blowdown times and hence shorter no touch times.

However, there are some real advantages to using flexibles and when I would be seriously considering them is:

• Pipeline size is limited by EVR and not by pressure drop; or
• Sour/corrosive operation is leading to CRA material selection; or
• Frequent pigging is required for corrosion management; and
• Line size is 16” or smaller; and
• Pipeline length is less than 30km

Another optimisation of the flexible is that they don’t have to be standard size. It doesn’t need to be 6” or 8” or 10”. If 6.5” is what you need, then 6.5” is what you use. They also come with a bit of “free insulation” (~25 W/m2-K) without adding additional insulation layers, so you have a nice cool down time naturally. Both of these things can actually be quite useful in shaving a bit off the CAPEX.

I think the future for flexibles is going to be exciting too, TPFMC have now also successfully installed the first “heated flexible” which I think is going to really open up some deepwater fields with wax and hydrate issues that before we classed as "too difficult".

Anyway, I’ve babbled on enough!