Riser Desgn

PART IV: Riser Engineering

Part IV

Riser Engineering

Chapter 22 Design of Deepwater Risers

22.1 Description of a Riser System

22.1.1 General

A riser system is essentially conductor pipes connecting floaters on the surface and the wellheads at the seabed. There are essentially two kinds of risers, namely rigid risers and flexible risers. A hybrid riser is the combination of these two.

The riser system must be arranged so that the external loading is kept within acceptable limits with regard to: - Stress and sectional forces - VIV and suppression - Wave fatigue - Interference

The riser should be as short as possible in order to reduce material and installation costs, but it must have sufficient flexibility to allow for large excursions of the floater.

22.1.2 System Descriptions

The riser system of a production unit is to perform multiple functions, both in the drilling and production phases. The functions performed by a riser system include: - Productiodinjection - Exporthmport or circulate fluids - Drilling - Completion & workover A typical riser system is mainly composed of: - Conduit (riser body) - - Components - Auxiliary

Interface with floater and wellhead

402 Part IV Riser Engineering

22.1.3 Flexible Riser Global Configuration

Flexible risers can be installed in a number of different configurations. Riser configuration design shall be performed according to the production requirement and site-specific environmental conditions. Static analysis shall be carried out to determine the configuration. The following basis shall be taken into account while determining the riser configuration: - Global behavior and geometry - - Cross sectional properties - Means of support - Material - costs

Structural integrity, rigidity and continuity

The six main configurations for flexible risers are shown in Figure 22.1. Configuration design drivers include a number of factors such as water depth, host vessel access / hang-off location, field layout such as number and type of risers and mooring layout, and in particular environmental data and the host vessel motion characteristics.

Free Hanging Catenary

Lazy Wave Steep Wave

Lazy-S Steep4 Pliant Wave@

Figure 22.1 Flexible riser configurations.

- Free Hanging Catenary This is the simplest configuration for a flexible riser. It is also the cheapest to install because it requires minimal subsea infrastructure, and ease of installation. However a free hanging catenary is exposed to severe loading due to vessel motions. The riser is simply lifted off or lowered down on the seabed. A free hanging catenary under high vessel motions is likely to suffer from compression buckling at the riser touch down point and tensile armor wire ‘birdcaging’. In deeper water the top tension is large due to the long riser length supported.

Chapter 22 Design of Deepwater Riser 403

- Law wave and steep wave In the wave type, buoyancy and weight are added along a longer length of the riser, to decouple the vessel motions from the touch down point of the riser. Lazy waves are preferred to steep waves because they require minimal subsea infrastructure. However lazy waves are prone to configuration alterations if the internal pipe fluid density changes during the riser lifetime. On the other hand, steep wave risers require a subsea base and subsea bend stiffener, and yet are able to maintain their configuration even if the riser fluid density changes.

Buoyancy modules are made of syntactic foam which has the desirable property of low water absorption. The buoyancy modules need to be clamped tightly to the riser to avoid any slippage which could alter the riser configuration and induce high stress in the armor wires. On the other hand the clamping arrangement should not cause any significant damage to the external sheath of the riser as this might cause water ingress into the annulus. Buoyancy modules tend to lose buoyancy over time, and wave configurations are inherently designed to accommodate up to a 10% loss of buoyancy.

- L a q S and steep S In the lazy S and steep S riser configuration there is a subsea buoy, either a fixed buoy, which is fixed to a structure at the seabed or a buoyant buoy, which is positioned by e.g. chains. The addition of the buoy removes the problem with the TDP, as described above. The subsea buoy absorbs the tension variation induced by the floater and the TDP has only small variation in tension if any.

‘S’ configurations are considered only if catenary and wave configurations are not suitable for a particular field. This is primarily due to the complex installation required. A lazy-S configuration requires a mid-water arch, tether and tether base, while a steep4 requires a buoy and subsea bend stiffener. The riser response is driven by the buoy hydrodynamics and complex modeling is required due to the large inertial forces in action. In case of large vessel motions a lazy-S might still result in compression problems at the riser touchdown, leaving a steep4 as a possible alternative.

- Pliant wave The pliant wave configuration is almost like the steep wave configuration where a subsea anchor controls the TDP, i.e. the tension in the riser is transferred to the anchor and not to the TDP. The pliant wave has the additional benefit that it is tied back to the well located beneath the floater. This makes well intervention possible without an additional vessel.

This configuration is able to accommodate a wide range of bore fluid densities and vessel motions without causing any significant change in configuration and inducing high stress in the pipe structure. Due to the complex subsea installation that is required, it would be required only if a simple catenary, lazy wave or steep wave configurations are not viable.


22.1.4 Component Descriptions

The components of a riser system must be strong enough to withstand high tension and bending moments, and have enough flexibility to resist fatigue, yet be as light as practicable to minimize tensioning and floatation requirements, A short description of the most common riser components and auxiliary components (such as end fittings and bend stifferners) is given below.

- Riser joints A riser joint is constructed of seamless pipe with mechanical connectors welded on the ends. For drilling risers, choke and kill lines are attached to the riser by extended flanges of the connector. The riser can be run in a manner similar to drill pipes by stabbing one stalk at a time into the string and tightening the connector.

- Buoyancy modules Buoyancy modules can be attached to the riser to decrease the tension required at the surface. These modules may be thin-walled air cans or fabricated syntactic foam modules that are strapped to the riser. These buoyancy modules require careful design and the material for their construction needs to be selected appropriately so as to ensure that they have a long-term resistance to water absorption.

- Bend Stiffeners and Bellmouths One of the critical areas of a flexible riser is the top part of the riser just before the hang-off arrangement. This area is prone to over-bending and hence an ancillary device is incorporated into the design to increase the stiffness of the riser and prevent over-bending of the riser beyond its allowable bend radius. The two devices used for this application are bend stiffeners and bellmouths. Figure 22.2 illustrates a schematic of both devices. Flexible pipe manufacturers tend to have a preference for one or the other device, yet bend stiffeners are known to provide a better performance in applications with high motion vessels. Bend stiffeners also provide a moment transition between the riser and its rigid end connection. The ancillary devices are designed separately from the pipe cross section analysis, and specialized software is used for this purpose. Global loads from the flexible riser analysis are used as input to the ancillary device design.

Bend stiffeners are normally made of polyurethane material and their shape is designed to provide a gradual stiffening to the riser as it enters the hang-off location. The bend stiffener polyurethane material is itself anchored in a steel collar for load transfer. Bend stiffeners are sometimes utilized subsea, such as in steep-S or steep-wave applications to provide support to the riser at its subsea end connection, and to prevent over-bending at this location. Design issues for bend stiffeners include polyurethane fatigue and creep characteristics. Figure 22.3 shows an example of a bend stiffener. It is to be noted that bend stiffeners longer than 20 ft have been manufactured and are in operation in offshore applications.

Bellmouths are steel components that provide the same function as bend stiffeners, Le. to prevent over-bending of the riser at its end termination topsides. The curved surface of a


bellmouth is fabricated under strict tolerances to prevent any kinks on the surface that might cause stress concentrations, and damage to the pipe external sheath.

Figure 22.2 Schematic of bend stiffener (left) and bellmouth (right).

Figure 22.3 Example of a bend stiffener.

- Bending Restricter This is normally located at the bottom and top connections. The purpose is to provide additional resistance to over-bending of the riser at critical points (such as the ends of the riser, where the stiffness is increased to infinity).

Bend restrictors are designed to limit bending on static pipelines. They are made of a hard plastic material and typically used at wellhead tie-ins and at riser bases to restrain the riser tension, bending and shear loads. Bend restrictors provide mechanical locking to prevent over-bending. Figure 22.4 illustrates a bend restrictor used at the end termination of a flexible pipe subsea.


End fitting

Figure 23.4 Bend restrictor.

22.1.5 Catenary and Top Tensioned Risers

In shallow water it has been practice to use top tensioned risers, but as design for larger water depth is accounted the need for new design practice has increased, see Figure 22.5. The ordinary Top Tensioned riser is very sensitive to the heave movements due to wave and current loads because the rotation at the top and bottom connections is limited. The heave movement also requires top tension equipment to compensate for the lack of tension. If the top tension is reduced it will cause larger bending moment along the riser especially if the riser is located an environment with strong current. If the effective tension becomes negative (Le. compression) then Euler buckling will occur.

CURRRENT WAVE

TO PREVENT

MOMENT INDUCED BY

II CATENARY RISER

TOPTENSIONED 1 RISER

Figure 22.5 Top Tensions Risers and Steel Catenary Risers and Their Components.

The catenary riser is self compensated for the heave movement, i.e. the riser is lifted of or lowered on the seabed. The catenary riser still need a ball joint to allow for rotation induced by waves, current and vessel motion, at the upper end connection.


The catenary riser is sensitive to environmental loads, i.e. wave and current due to the normally low effective tension in the riser. The fatigue damage induced by Vortex Induced Vibration (VIV) can be fatal to the riser. Use of the VIV suppression devices such as helical strakes and fairing can reduce the vibrations to a reasonable level.

22.2 Riser Analysis Tools

Various analysis tools are available for riser design, examples of these are: 0 General purpose finite element programs: ABAQUS, ANSYS, etc; 0 Riser Analysis Tools: Flexcom, Orcaflex, Riflex, etc; 0 Riser VIV Analysis Tools: Shear7, VIVA, VIVANA, CFD based programs; 0 Coupled motion analysis programs: HARP, etc; 0 Riser Installation Analysis Tools: OFFPIPE, Orcaflex, Pipelay, etc.

Riser analysis tools are special purpose programs for analyses of flexible risers, catenary risers, top tensioned risers and other slender structures, such as mooring lines and pipelines. The most important features for the finite element modeling are listed below:

Beam or bar element based on small strain theory. Description of non-linear material properties. Unlimited rotation and translation in 3D space. Stiffness contribution from material properties as well as geometric stiffness. Allowing varying cross-sectional properties.

Riser analyses Typical analyses are for instance:

- Strength analysis; - Fatigue Analysis; - VIV Analysis; - Interference Analysis .

The results from the finite element analysis are listed below: - Nodal point co-ordinates; - Curvature at nodal points; - Axial forces, bending moment, shear forces and torsion.

Time domain analvsis and freauencv domain analvsis


The purpose of the analysis is to determine the influence of support vessel motion and direct wave induced loads on the system. The results from the frequency domain analysis are the systems eigenfrequencies and eigenvectors. The results from the time-domain analysis are time series of a selected limited response parameters, such as stress, strain and bending moment.

The results from the above analyses are stored in separate files for subsequent post processing, such as plots or calculation. Some of the more interesting output is listed below:

Plots - System geometry; - Force variation along lines; - Pipe wall forces; - - Response time series; - Vessel motion transfer function; -

Geometry during variation of parameters;

Animation of the dynamic behavior of the complete system including support vessel and exciting waves.

0 Tables - Support forces; - Pipe wall forces; - Velocities and accelerations from wave and vessel motion time series;

Statistical time series analysis, estimation of spectral densities

22.3 Steel Catenary Riser for Deepwater Environments

22.3.1 Design Codes

Riser maximum equivalent stresses during extreme storm conditions are limited to 80% yield stress. 100% yield stress is acceptable during abnormal conditions such as a mooring line or tether failure. This approach has been adopted on other (vertically tensioned) riser systems and is in line with API RP 2RD and the ASME Boiler Code.

Higher stress allowables are particular interest at the Touch Down Point (TDP) where stresses are largely displacement controlled. Whilst this offers some scope to the designer to address extreme storm response, caution must be exercised. Designing with higher utilization may lead to an unacceptable fatigue life and the validity of assuming that TDP response is displacement controlled is not always correct. This is particularly true where low-tension levels are observed near the TDP.


22.3.2 Analysis Parameters

Hydrodynamic Loads There are uncertainties related to vortex-induced vibration (VIV). If the stresses are above the endurance limit of the material then fatigue may take place. In addition, VIV may result in drag amplification that may result in increased stresses. Finally the hydrodynamic interaction between risers may result in riser crashing loads which must be considered. VIV suppression has been used on most SCR’s.

Material Properties The steel material to be used in deepwater SCR’s offshore is likely to be steel of API grade X65 or above. The main uncertainty lies in the effect of welding combined with plastic strain (reeling and laying). Until validated S-N curves (Stress range versus Number of cycles to failure curves) are available, SCR design has to be based on conservative assumptions which may limit the use and complicate installation.

Soil Interaction In most deepwater fields, relatively loose clay is-found on the seabed. The pipe will sink into this clay and might be buried over time. The exact behavior of the soil is not known. The soil uplift and sideways resistances are hence important aspects. It is important to properly model riser-soil interaction effects.

Extreme Storm The primary objective of the extreme storm analysis is to define basic geometry and assess acceptability of response. A large number of analyses need to be conducted when optimizing a steel catenary riser. The approach is highly iterative in order to ensure that the response is optimized for all combinations of load and vessel offset.

22.3.3 Soil-Riser Interaction

When a pipe is placed on soil and subjected to oscillatory motion, there is complex interaction between pipe movements, penetration into the soil and soil resistance. At the touch down point (TDP) region of the riser, transverse (out-of-plane) motions will occur as a consequence of oscillatory forces caused by transverse wave acting on the free hanging part of the riser.

A proper description of the pipe-soil interaction is therefore important for the accuracy in calculation of riser fatigue damage. Depending upon the stiffness and friction of the seafloor, out-of-plane bending stresses will be more or less concentrated in the TDP region when the riser is subjected to oscillatory motion.

In riser response analysis tools, the pipe-soil interaction is commonly modeled by use of friction coefficients (sliding resistance) and linear springs (elastic soil stiffness). However, these parameters must be selected carefully in order to properly represent the complex pipe- soil interaction.


During small and moderate wave loading (the seastates contributing most to the fatigue damage) the riser TDP response in the lateral direction is very small (in the order of 0.2 pipe diameters). This will cause the riser to dig into the top sand soil layer and create its own trench. This effect will gradually decrease as the riser gets closer to the underlying stiff clay soil, where very limited penetration is expected. The width of this trench will typically be 2-3 pipe diameters, which leaves space within the trench for the pipe to move without hitting the trench edges. During a storm build-up, the trench will gradually disappear as a result of larger riser motions in addition to natural back fill. For the extreme strength analysis, the pipe-soil interaction is found to be of minor importance even if higher lateral soil resistance is mobilized.

22.3.4 Pipe Buckling Collapse under Extreme Conditions

Within the industry, there are considerable differences between recommended methods for sizing riser pipe for resistance to collapse and propagation buckling in deepwater particularly for low D/t ratios. Existing formulations are based on empirical data, which attempt to account for variations in material properties and pipe imperfections. Application of these codes to deepwater applications provides scatter of results. Additionally, the effects of tension and bending (dynamic and static) are uncertain, depending on the nature of the loading condition.

22.3.5 Vortex Induced Vibration Analysis

The VIV analyses of the SCR could follow two different approaches: using SHEAR7 (MIT, 1995 and 1996). As the VIV- and wave-induced fatigue damage is established independently, results from both calculations must be combined to get the total distribution of fatigue damage for the SCR’s. The areas where significant wave-induced fatigue damage occurs are very distinct. The VIV-induced fatigue damage occurs more evenly distributed (according to the larger variations in mode shapes and their superposition). The total fatigue damage is then obtained by a simple sum of the two contributions. The fact that VIV- and wave-induced response will be more or less perpendicular to each other is conservatively not accounted for (“hot-spots” are assumed to coincide).

22.4 Stresses and Service Life of Flexible Pipes

Calculation of ultimate capacity may be performed with good accuracy by tools estimating the average layer stress. All the available flexible pipe analysis tools, including the manufacturers design programs calculate the average stresses in each layer.

Service life prediction on the other hand requires detail knowledge of the mechanism leading to failure. The manufacturers have established estimation methods based on theory and test results. These analysis methods must be calibrated for each manufacturer, each wire geometry and type of pipe (Le. additional hoop spirals). The advantage with such empirical methods is that residual stresses from manufacturing, actual tolerance on wire geometry, etc are present in the tests and hence incorporated in the analysis. The problem is that design optimization is hardly possible and independent verification is impossible.

LPrtveit and Bjarum (1995) has found that by combining detailed knowledge of flexible pipes with state of the art non-linear FEM programs it is possible to develop an analysis tool that can

Chapter 22 Design of Deepwater Riser 41 1

predict the stresses sufficiently accurately to provide input to service life prediction. SeaFlex has recently developed a second-generation analysis tool, PREFLEX, for analysis of flexible pipes. PREFLEX is based on the general non-linear FEM program MARC. PREFLEX can model each wire with a mesh sufficiently detailed to calculate local hot spot stresses.

Examples of attractive features of PREFLEX are: - -

Virtually no modeling limitations. End fitting areas, damaged pipe etc., can be modeled. Service life predictions based on a minimum of test results. PREFLEX can accurately calculate the stresses and small-scale tests of the wires may hence be used to define the capacity. The previous analyses tools required results from hll-scale test for service life prediction. Analyses have shown that the use of simplified analysis tools based on average stresses in the layer may recommend the use of hoop spirals where local stresses are very high. One example is use of a rectangular back-up spiral as an additional hoop strength layer.

-

22.5 Drilling and Workover Risers

Deepwater drilling and workover is presently performed with jointed steel risers. The vessels and equipment have been upgraded to work in a water depth down to more than 1700 m. In deepwater and harsh environment the challenges related to operation are large due to use of buoyancy, fairings etc. The drilling contractors are presently building new vessels and upgrading existing vessel to meet the deepwater requirement. Smedvig and Navion have contracted a new drillship MST ODIN to be rented by Statoil. The vessel is fully equipped for drilling in 2500m water-depth. Drilling in even deeper water is planned. The technology status is, however, presently limited to approximately 2500 m.

Two of the critical items for deepwater drilling are riser weight and riser control. In order to reduce the riser weight, alternative materials are considered. SeaFlex and Raufoss have completed a JIP project related to composite risers. At the Heidrun TLP a titanium drilling- riser has been installed. One composite drilling joint has been qualification tested and is ready for offshore trial in the Gulf of Mexico.

22.6 References

1. API RP 2RD, (1998) “Recommended Practice for Design of Risers for Floating Production Systems and TLP’s”, First Edition, 1998.

2. DNV (1998) “VISFLOW Users Manual”, Det Norske Veritas 1998. 3. Hatton, S.A., and Willis, N., (1998) “Steel Catenary Riser for Deepwater Environments-

STRIDE”, Offshore Technology Conference 1998. 4. Hibbitt, Karlsson & Sorensen (1998), “ABAQUS, Ver. 5.8”. 5 . Langner, C.G., and Bharat C.S., (1997) “Code Conflicts for High Pressure Flowlines and

Steel Catenary Risers”, OTC’97.


6. Lratveit, S.A. and Bjerum, R., (1995) “Second Generation Analysis Tool for Flexible Pipes”, MarinFlex 95.

7. Lund, K.M., Jensen, P., Karunakaran, D. and Hake, K.H., (1998) “A Steel Catenary Riser Concept for Statfjord C”, OMAE’98.

8. Marine Computational Services (MCS), (1994) “FLEXCOM3D, Version 3.1.1”. 9. MIT, (1995) “SHEAR7 Program Theoretical Manual”, Department of Ocean Engineering,

Massachusetts Institute of Technology. 10. MIT, (1996) “User Guide for SHEAR7, Version 2.0”, Department of Ocean Engineering,

Massachusetts Institute of Technology. 1 1. SINTEF (1998) “RIFLEX- Flexible Riser System Analysis Program- User Manual”,

Marintek and SINTEF Division of structures and concrete report-STF70 F952 18.

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