desire to do so disappeared. The reason is to lower the capital expenditures
(CAPEXs), thus improving the economic feasibility of producing deeper water reservoirs. Today’s offshore oil and gas production facilities are located
on the seabed. The only evidence of offshore energy production would possibly be a floating vessel, known as floating production and storage offloading
(FPSO) vessel, used to transport the hydrocarbons to an onshore facility.
There are massive oil and gas
reserves located in ultra-deepwater
(depths greater than 10,000 ft.). Equally impressive to the billions of barrels
of oil located in ultra-deepwater is that
these reservoirs have shut-in pressures of 15,000 psi. The fundamental
engineering challenges facing today’s
ultra-deepwater oil and gas production reside under a new engineering
discipline, the subsea engineer. There
is uniqueness to the harsh underwater location that manifests itself as a
corrosive environment with external
pressures of 5,000 psi and internal
pressures reaching 15,000 psi. These
underwater facilities (equipment) must
have a useful service life of 30 years to
be cost-effective despite being subject to huge producing well uncertainties (flow composition and pressures). Of even greater importance is that
subsea production facilities must completely protect the environment.
SUBSEA OIL AND GAS PRODUC TION EQUIPMENT
The major cost for producing an ultra-deepwater reservoir is the drilling operation. At this stage it is important to note that oil and gas reservoirs are not subsurface cavities filled with crude oil. Instead a reservoir is
a subsurface formation of permeable rock containing hydrocarbons. Ultra-deepwater drilling is accomplished using dynamically positioned (DP)
drill ships enabled through global positioning systems (GPS). The drilling
operation is known as upstream oil and gas engineering. Offshore drilling operations are occurring everyday where the drill string (thick walled
piping with a bit attached at the end) travels 2 miles to reach the mud line
and then another 3 miles to the reservoir. Roughly speaking, this is equivalent to stretching a guitar string the length of a football field and twisting
one end to perform drilling. Once the well is established, the engineering
operations necessary to produce oil and gas are known as downstream
engineering. This phase of subsea oil and gas production focuses on the
design and installation of the subsea production system (Figure 1). The
primary issues facing downstream engineers include equipment reliability,
connectors (connections between subsystems) and multiphase flow (flow
comprising oil, gas, water and sand).
Subsea hydrocarbon production facilities are separated into two primary
categories, subsea (Figure 1) and topside (above the water). To describe an
underwater production facility, we will begin at the wellhead. The wellhead
is a pipe with a flange put in place by upstream operations. This pipe (well
casing) travel into the earth’s crust to penetrate the reservoir. The well casing within the reservoir is perforated to allow flow from the high-pressure
reservoir. Bolted to the well casing flange is the so-called Christmas Tree
(XTree) (Figure 2, see the engineered system below the word “Well”).
XTrees can be either horizontal or vertical
and allow both access and control of the
reservoir. These two functions are needed
to guarantee production flow (flow assurance) over the life of the well while producing the reservoir in a manner that does not
collapse its porous rock composition.
The product flows from the XTree to a
subsea manifold via a jumper (Figure 2).
The jumper is a pipe designed to with-
stand sand erosion and product corrosion.
All jumpers have U-like shapes enabling
stiffness/flexibility and function [ 3].
Owing to the capital expenditures as-
sociated with ultra-deepwater systems,
it is mandatory for each subsea produc-
tion site to have multiple wells (Figure
2). Each well has its dedicated XTree and
jumper, and is producing hydrocarbons.
A subsea manifold gathers these indi-
vidual productions into a central place.
Subsea manifolds are not small. In fact,
subsea manifolds can reach dimensions
of up to 75ft. x 40ft. x 20ft., having huge
weights that match their dimensions. A
subsea manifold serves many functions.
For example, the manifold allows access
to the flow hydrocarbons to inject chemicals for flow assurance issues. Manifolds
provide the primary location for pipeline
cleaning using pipeline inspection gauges
(PIGs). However, the primary function of
the subsea manifold is to combine flow
hydrocarbons from multiple wells and
send the resulting aggregate through a
single pipeline to the so-called tieback.
The manifold is connected to the pipeline
FIGURE 2 Subsea production system
with pipeline and jumpers.