Flow assurance Definition
Flow assurance is contined ability of subsea field to produce hydrocarbons efficiently thought the whole field life. the most critical task during deep water energy production because of the high pressures and low temperature (~4 degree Celsius) involved. The financial loss from production interruption or asset damage due to flow assurance mishap can be astronomical.

As flow moves from the reservoir to the topside process facility the temperature decreases. At the reservoir, the temperature is more than 100 C. At the seabed the ambient temperature is 4C and the fluid will start to cool down. Cooling down causes flow assurance problems such as:

Hydrate Fromation
Deposition of waxes

Mitigation of flow assurance problems is othen accomplissed by:

Thermal Insulation
Chemical injection
Heating

 

As the flow (mixture of oil, gas and water) moves from the reservoir to the process facilities the pressure decreases since:

a) the well fluids move upwards in the production tubing of the well. The larger the distance from the reservoir to the sea bed the larger the pressure drop (distance a, image 1). The distance a is in the order of magnitude of 2000-6000 m.

b) the well fluid passes through the wellhead, X-mas tree, the manifold and through the flowlines

c) from the upward flow in the riser and into the process facilities.

In the design of the system the pressure loss is taken into account during the hydraulic design. The input are the fluid properties, reservoir pressure and the flowline parameters. With input we can calculate the pressure through the system and check wheather the proposed production system will allow the oil to flow to the process plant and consider what should be planted for the pressure maintenance requirements,

Possible ways to boost the pressure are:

  1. water or gas injection
  2. well gas injection
  3. downhole pumping
  4. seabed pressure boosting (multiphase pumps, separation and then boosting, subsea separation and injection)
  5. riser lift

Problems associated with pressure loss

The dissolved gas in the flow of oil will bubble out of the oil. Thus, the flow becomes multiphase (liquide and gas). Mulitphase is undisirable because it us unstable and pressure boosting can only be applied with expensive and unreliable new technology such as mulriphase pumps or subsea seperation.

How can we maintain the pressure of the reservoir?

Reservoir pressure can be maintaned over the life of the field by water or gas injection. The injection well location and depth is selected such that the optimum result is accomplished