It is subject to what kind of process and system. If few length around elbow are un-buried and straight portion is fully buried, there will be no floating problem. Also, if the thermal expansion from buried straight pipe is minor, I do not see any disadvantage. However, I would like you to check;

-is it coated pipe or not ?
-make sure there is no frost heave effect
-is 500 mm too shallow to undertake live load ?
-does elbow portion have enough flexibility to absorb thermal expansion from buried straight pipe ?
-check if the straight portion is enough against buoyancy force
-do not put directly contacted support type at elbow portion, if pipe is coated

Long time years ago, I had involved in a trouble shooting which had leakage problem in pipeline from Jetty to storage tank farm. Some portion was buried and other quite long portion was laid on concrete trench. The pipe laid in trench was lift-up frequently because of buoyancy. Every support point had perforation on pipe and leakage problem because the contact point between pipe(P.E coated) and the steel support on concrete sleeper had a pin-hole.
My conclusion was ; once P.E coated pipe has a small pin-hole or hair crack, the corrosion of metal piping may be easily accelerated by differential oxygen density as “Fretting Corrosion and Crevice Corrosion”. I thought pin-hole or hair crack resulted from dynamic loading during floating and vibrating during pumping.

Hope to hear more opinion from members.