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Offshore reservoirs are being developed inenvironments where water depths are
approaching 2000 m and well products
may contain aggressive chemical proper-
ties. Semisubmersibles; floating produc-
tion, storage, and offloading (FPSO) facili-ties; tension-leg platforms (TLP’s); and
spars are favored configurations in deep
water. While economically more viable
than jackets, the rapid evolution of theseconcepts introduces challenging problems
in design of marine-riser systems as well as
in design of station-keeping (mooring) sys-tems. New marine-riser concepts have been
developed to cope with the problems of operation in deeper water.
MARINE RISERS
A marine riser is a pressure link that trans-
ports the well fluid, or one or more of its
components, from the seafloor to a primaryprocessing platform on the water’s surface.
From an engineering-design viewpoint, the
external forces considered during design of
a riser include both static (depth) and
dynamic (currents and waves) water pres-sure. The motions of the platform under
consideration dictate the displacements
imposed at the water’s surface. These
motions are critical in determining the
stresses experienced by the riser along itslength. The ability of a riser system to
accommodate platform motion depends on
both the riser configuration and the mate-
rial from which the riser is constructed.
PLATFORM MOTIONS
The maximum excursions of a platform
vary with water depth, platform hull shape
and mooring system, and environmentalconditions. Typically, FPSO’s exhibit watch-circle radii of up to 30% of the water depth.
Semisubmersibles and spars may move by
20 and 15%, respectively. TLP’s have a
watch circle equal to approximately 10% of
their installed water depth. The maximum
horizontal offsets of platforms are an
approximately constant fraction of the
water depth because they largely depend onthe restoring force supplied by the platform
mooring system.
The heave component of platform
motion is relatively depth independent,
with the typical maximum design valueapproximately ±15 m for FPSO’s and semi-
submersibles in harsh environments.
Platform heave motions become a smallerfraction of water depth as depth increases.
This trait is advantageous because relativeheave frequently dictates riser design. In
contrast, TLP’s and spars exhibit heave
motions that are an order of magnitude
smaller, approximately ±1.5 m.
FLEXIBLE RISERS
Flexible risers are constructed from pipe
with a layered profile. They deform easily in
bending but are stiff in response to tension,
torsion, and internal pressure. The structure
of flexible pipe requires complex manufac-turing techniques, and its cost is very high.
Traditionally, flexible risers have been
used in shallow and moderate water depths
with floating platforms. The displacements
of these platforms are large relative to waterdepth, and flexible risers have the advan-
tage of being compliant while still fulfilling
their function. Fig. 1 shows typical config-
urations of flexible risers, including the
simple catenary and the lazy and steep vari-ations of the ‘S’ and wave catenaries.
Flexible pipe is used in the deepest field
developments, and the advent of new profiles
in the pressure-resisting layers will extendthe boundaries. However, use of flexible pipein ultradeep water is restricted by the capa-
bilities of the pipe to withstand high external
pressures. Small-diameter pipe is used at the
greater depths because it is capable of with-
standing high external hydrostatic pressure.To prevent the layered flexible pipe from
buckling, devices that restrict the bend
radius are applied. Use of flexible pipe in
conjunction with harsh well products can
result in gas permeation between the layersof the pipe. This can cause corrosion of the
armor layers, and the gas pockets can leadto failure of the flexible pipe. Also, the
compliancy of flexible risers eliminates the
need for heave-compensation or tension-
ing devices.
RIGID RISERS
Traditionally, rigid risers are vertical lengthsof pipe constructed from steel. Rigid risers
are used on steel-jacket and concrete gravi-
ty-based structures, compliant towers,
TLP’s, and spar platforms.
For compliant platforms, stress joints areused at the base of rigid risers (where bend-
ing stresses aregreatest) to prevent failure of the steel at that point. The material of choice
for these joints frequently is titanium.The high axial stiffness of rigid risers
introduces the need for heave compensa-
tion, and their use is limited by the stroke
amplitude of the compensating mecha-
nism. When a rigid riser is not supportedlaterally at intermediate points along its
length, tensioning devices enable it to resist
the loads of waves and currents. Tensioning
can be mechanical, where the riser attaches
to a compliant platform; in the form of buoyancy modules attached along a riser’s
length; or a combination of the two.
The ability of rigid risers to accommo-
date horizontal platform motion increases
with water depth. As the length of riserincreases, the riser develops additional
bending flexibility.
HYBRID RISERS
As Fig. 2 shows, hybrid risers use a combi-
nation of two riser concepts. The lower por-tion of the riser consists of either a single
rigid pipe or a large-diameter vertical cylinder
housing a bundle of smaller production
tubes. These rigid sections extend from eithera flex or stress joint at the seafloor. The dis-
tance from the top of this rigid section to theplatform is spanned by flexible pipe.
The rigid section of the riser is construct-
ed with self-buoyancy, reducing the weight
carried by the platform and facilitating rapidconnection and disconnection. Flexible
jumpers complete the link to the platform,
giving the riser inherent compliancy with
the motions of the vessel. Therefore, the
hybrid-riser system is a flexible-riser config-uration connected to the top of a rigid riser.
The maximum motion that a hybrid con-figuration can accommodate is controlled
DEEPWATER-RISER TECHNOLOGY
This article is a synopsis of paper SPE
50140,“Deepwater-Riser Technology,”
B.A. Carter, SPE, Hamersley Iron, and
B.F. Ronalds, SPE, U. of Western
Australia, originally presented at the
1998 SPE Asia Pacific Oil and Gas
Conference and Exhibition, Perth, Australia, 12–14 October.
O F F S H O R E D E V E L O P M E N T
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by the depth below the surface of the tran-
sition from flexible pipe to the rigid verticalsection. A hybrid riser can be configured to
withstand the same vessel motions as an
isolated flexible riser. The length of flexible
pipe can be increased (and bend restrictors
incorporated) to help the system withstandlarge vessel motions.
The hybrid riser has a small footprint on
the seafloor and, overall, may require short-
er lengths of pipe to transfer well products
to the surface than steel catenary risers(SCR’s) or flexible risers. The hybrid con-
figuration has potential weaknesses
(including permeation of gases between the
layers of the flexible pipe) associated withflexible risers. However, with the top of the
rigid section within the air-diving limit,
maintenance and component changeout iseasier than for deepwater flexible risers.
A disadvantage of the hybrid riser is that
the size of the production tubing in the
rigid section of riser is restricted by the
maximum available diameter of the flexibleriser. Use of larger rigid pipes requires a
manifold at the top of the rigid section.
SCR’S
These risers consist of pipe (either reeled or
segments welded end to end) hung from aplatform to the seafloor in a near-catenary
shape. SCR applications have been limited
to import and export functions.
Only one catenary configuration has
been used. The simple catenary relies onthe self-buoyancy of the pipe for its config-
uration. Double catenaries have been stud-
ied (steep- and lazy-wave configurations)
and are assisted in attaining their shape byexternal buoyancy elements. These config-urations are similar to the flexible-riser
arrangements used on floating platforms.
A potential advantage of the simple- and
lazy-wave-catenary configurations is that
the pipe lies parallel to the seafloor anddoes not necessarily require installation of a
riser base. The steep-wave-catenary
arrangement meets the seafloor nearly per-
pendicularly and requires a riser base.Therefore, the steep configuration may find
application as a production riser, while the
simple and lazy configurations may be suit-ed to export or import functions.
Flex/Stress Joints. As a platform under-
goes an of fset at the water’s surface, theshape of the catenary and, consequently,
the stresses along the pipe change. The con-
nection of an SCR to a platform requires a
joint capable of accommodating angle
changes at the top of the SCR caused byvariations in platform position. Flex joints
have been used and are attached to a plat-
form at an angle of between 10 and 20° to
the vertical. A low angle to the vertical min-
imizes tension in the riser and influenceswhether vortex-induced vibrations occur. A
higher angle to the vertical reduces the
maximum bending moments experienced
in the lower sections of the riser.
Touchdown Point (TDP). The TDP is the
critical point on simple and lazy catenaries
(i.e., the hot spot) for fatigue failure. Vessel
motions in the plane of a catenary have agreater effect on the stresses in the riser thando vessel motions out of the plane.
Furthermore, second-order (or slow-drift)
motions have the largest influence on the
fatigue of an SCR. The nature of SCR’s is such
that the TDP is essentially a touchdownregion rather than a single point. Vessel
motions cause the location of the TDP to
move as the length of the riser on the seafloor
increases (an offset in the slack direction) or
decreases (an offset in the taut direction).If used for export and import functions, the
large backtensions encountered in someSCR’s can require laying of a significant length
Fig. 1—Typical catenary riser configurations.
Fig. 2—Hybrid-riser configuration.
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of pipe on the seafloor (past the TDP) before
a change in pipe direction can be accommo-
dated. Alternatively, a riser base can be incor-
porated to take these loads. The problem of the constant abrasion experienced by the pipe
in this touchdown region has been solved by
use of bonded-rubber coatings.
Water Depths. Because of the bending and
the rigidity of steel pipe, in water depths
less than 300 m, flexible pipe of fers the
optimum solution for risers to floating plat-
forms. In depths between 300 and 2000 m,
SCR’s may be a feasible alternative.
TITANIUM RISERS
The search for new materials for use in
marine risers led to development of titani-
um and its alloys for use in offshore envi-ronments. The drilling riser on the Heidrun
TLP was constructed with titanium andinstalled in 1995. Other uses of titanium
include components of fire-water and bal-
last systems and heat-exchanger piping.
Properties. Titanium’s density and elastic
modulus are approximately one-half those
values for steel. Titanium alloys exhibit
high yield and tensile strengths. The fatigue
life of the welds of these alloys is shorter
than for the parent metal; however, it is still
greater than that for steel. Excessive wear
was observed when the steel drillstringcame into contact with the titanium drilling
riser. In this case, the problem was over-
come with an internal rubber liner.
Titanium alloys exhibit excellent corro-sion, erosion, and cavitation characteristics.
Titanium alloys are cathodic with respect to
steel, so galvanic protection is required for
steel with titanium-alloy interfaces. Also,
exposure to chlorides and galvanic poten-tials greater than 800 mV leads to forma-
tion of brittle hydrides.
Future Developments. The trend to use
titanium in positions of high stress and
fatigue, such as at riser bases, may lead toapplications of titanium in other critical
areas, including the TDP of SCR’s.Extending this concept further, production
risers constructed entirely from titanium,
either rigid risers or titanium catenary ris-ers, also are feasible.
GENERAL DISCUSSION
Flexible risers have been used extensively
in moderate water depths. At greater
depths, use of flexible pipe is restricted to
low-diameter pipe. In deeper water, riser
configurations can be developed with
greater flexibility that are better able toaccommodate horizontal and vertical
motions of a platform at the water surface.
While rigid and catenary configurations
develop horizontal flexibility, their flexibili-ty in the vertical direction is critical to their
design. Depending on the material used,
minimum depths are required for the riser
to withstand heave motions of a platform.
Hybrid risers and SCR’s are feasibleoptions in deep water, and the envelopes of
their applicability are growing as confidence
in the technology grows. Their use is being
found to be feasible for shallow as well as
deep water. These alternatives, along withthe introduction of flexible, strong, and
durable materials like titanium, are emerg-
ing as competitive options across a widerange of marine-riser applications, both
technologically and economically.
Please read the full-length paper for
additional detail, illustrations, and ref-
erences. The paper from which the
synopsis has been taken has not been
peer reviewed.
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