Weight of the pipe vs. weight of water it replaces and weight of the soil above the pipe. you are an engineer, go for it.

I have used the following method to check a pipe for flotation.
The calculation is usually done for a one foot section of pipe.
Forces up equal to the volume of the pipe X the unit weight of water.
Forces down equal the weight of the pipe plus the weight of the prism of
soil over the pipe. For the soil below the water table use a unit weight of
the dry unit of the soil minus the unit weight of the water.( if the pipe is
empty ie. no water flowing in it, the weight of the water in the pipe can not
be counted as part of the force down.)  
To keep the pipe from floating the force down needs to exceed the  force
up. It is good practice to add a safety factor to this calculation.

I agree with AHSmith's approach, except the resisting force should use the SATURATED unit weight minus the unit weight of water. I suppose if the flooding happened immediately, reaching around the pipe before the overlying soil got saturated then maybe use the normal (not dry) unit weight.

Carl

Thanks for the responses. I wasn't sure if I should consider it as dunking it into a tub of water or taking the volume of water in the voids of the soil and using the weight of this for the bouyant force.