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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.