Page 73 - Handbook Of Multiphase Flow Assurance

68 4. Hydraulic and thermal analysis

Introduction

Hydraulic analysis is used to calculate and optimize pressure loss, holdup accumulation,
vibration, water hammer and surge exceeding normal operating parameters during steady
and transient flow operations. Sensitivity to the amounts of produced water should include
an assumption for planned and deferred water injection for pressure support, based on reser-
voir simulation production profiles.
Thermal analysis predicts the temperature changes in the produced fluid and in the pro-
ducton system as heat is lost to ambient cooling or to JT expansion in multiphase flow or
gained from JT heating in supercritical (dense phase) fluids. Thermal analysis in wells is also
related to mechanical integrity if annular spaces are filled with fluids with low compressibility.
Both thermal and hydraulic analysis are performed simultaneously, coupled through location
in the system. Fluid properties depend both on temperature and pressure, and proper fluid char-
acteristics and phase transitions rely on accurate calculation of both temperature and pressure.
Hydraulic analysis results are supplied to operability design analysis which aims to ensure
that temperature, pressure and flow are within normal operating limits for the production
system at any stage in field development life.
Flow lines from trees to manifolds and from manifolds to hubs are usually routed pre-
dominantly uphill to minimize pressure losses due to terrain liquids holdup in low spots, in
order to maximize overall fluid recovery by limiting backpressure on wells, and also to avoid
slugging induced by liquid accumulation in low spots and to enable uniform subsea chemical
distribution in produced fluid, and to reduce fatigue from cyclic slug impacts on pipe elbows
at bends and crossings.
Detailed field layout should ensure that both production and chemical injection systems
can operate with acceptable pressure drop and chemical stability in subsurface, subsea, top-
sides and export systems conditions.
Single line tieback concepts are economically attractive and may be considered for early
production systems if reliable ways to mitigate the risk of solids restriction are in place.

Hydraulic restrictions boundaries and management

Besides the regular solid blockages which derive from a phase transition such as solid
crystallization of normal paraffins as wax there also may be hydraulic restrictions in the
pipelines. Such hydraulic restrictions may occur both with and without phase changes,
merely due to accumulation of a gas or a liquid in a pipe due to variations in geometry or
due to evaporation or condensation. Such examples may include water or liquid holdup in
a low spot creating resistance to flow, a viscous emulsion accumulation in a process vessel
reducing its effective volume or a vapor lock at the intake of a centrifugal pump breaking
fluid continuity. Each may restrict or interrupt flow. Additional cases of restrictions pertain
to fluids flowing outside their normal operating conditions. Such examples include pro-
duction chemicals slack flow which may cause a blockage in a chemical delivery umbilical
tube. Slack flow is a generalization term which means the fluid flows at a pressure below
its vapor pressure or bubble point. This may occur in late life production in deep water
operations when produced fluid has a lower density than the production chemical injected
at the subsea tree, and the flowing wellhead pressure is lower than the hydrostatic pressure