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Aker Riser Systems AS

FIXED BASE HYBRID RISER IN ALUMINIUM

A SUMMARY DESCRIPTION

Aker Riser Systems AS Hydro Aluminium Maritime AS

Aker Riser Systems.doc/2/EB 228.04.00

Preface

Current concepts for hybrid risers have been based on using foam buoyancy. Theconcept developed by Aker Riser Systems AS is based on using aluminium conduitpipes, which replace the foam buoyancy and the steel tether. They also make a fixedriser base connection possible which in turn makes it possible to perform the completetow-out and installation in one operation. This has a profound impact on cost. The maincost drivers are foam buoyancy and offshore operations. ARS’ concept minimise thesecosts. This concept reduces the cost of a hybrid riser by 30% to 40%. Thisdocument explains and documents this cost advantage with simple traceable costmodels, which permit direct comparisons of concepts. It is also seen that this conclusionis insensitive to the underlying unit rates.



1. Introduction

A hybrid riser consists of a low cost submerged metallic riser tower fromwhich short flexible risers extend to connect to a floating structure. The towertension is supplied by a soft tank which is open at the bottom to ensure thatthe internal and the external pressure in equilibrium. This ensures that thetank is safe against implosion. The soft tank provides low cost buoyancy forsuspending the riser. When the riser is suspended from a surface vesselsuperstructure must be provided in addition to closed tank buoyancy. In thiscase the riser load will also have an effect on stability.

The riser tower is installed independently of the floating surface structure.Riser towers may be installed in the traditional way using a spider or it maybe assembled inshore and towed to location where they are upended andinstalled. The installation method selected will depend on project specificconditions. This discussion addresses a tow-out installation, which is likely tobe the most economical in the majority of cases. For the tow out the towercross section needs to be near neutrally buoyant. This buoyancy is in currentconcepts provided by encasing the tower section in plastic foam.

The riser tower in current concepts incorporates a central steel tether, whichanchors the buoyancy tank to a riser base. The tether is surrounded by anumber of risers, which are tied to it at suitable intervals to prevent vortex-induced vibrations. The production risers will contain hydrocarbons underpressure and at increased temperature, which will cause them to expand.This expansion is very significant and may exceed one meter for a 1000 mriser.

In current concepts the tower is connected to the preinstalled base with alatch and a flex-element or a taper-joint. The risers are connected to thepipelines with flexible jumpers or expansion spool type flexible metallic pipeconfigurations.

Apart from the flexible jumpers to the surface structure the major cost driversfor a hybrid riser are:

• Tow out and upending of the tower.• Foam buoyancy.• Flexible connections to base and incoming pipelines including

components and installation.

How these aspects are handled in the design will govern the cost.

2. The fixed base concept in aluminium

Aker Riser Systems AS have developed an alternative riser towerconfiguration, which is seen to be far more cost efficient than previousconcepts. The significant features of this alternative are shown in the figure



overleaf. The traditional central tether is replaced by a number of guide tubesin aluminium. One centralguide tube is surroundedby a number of guide tubesaround its circumference.The production risers andhigh pressure service risersare steel pipes locatedwithin the aluminium guidetubes. For low pressure oilexport not containing carbondioxide the aluminium tubesmay be used directly asrisers. When the steel risersconfined in the conduitexpand they will buckle ina helix. Due to the confiningpipe the deformation will be restricted and only modest deformation inducedelastic stresses will result. A worked example is included in Appendix C.

The guide tubes will be prevented from imploding by being internallypressurised with air. To ensure that an accidental depressurisation shall notlead to implosion the guide tubes will be vented from the bottom to a levelwhere the external pressure is sufficient that it exceeds the internal pressurenecessary to prevent implosion.



This configuration has several advantages:

• The conduit tubes act as tethers.• The conduit tubes provide buoyancy during tow out and installation.• The conduit tubes confine the risers.• Risers and conduit tubes have a fixed connection to the base and

the buoyancy tank.

The fixed base in turn has several advantages:

• Latch connectors can be avoided.• Costly pipeline connections can be avoided.• Conventional pull-in porches with guide posts or funnels can be

used.• Conventional field proven ROV equipment for pipeline pull-in can be

used.• The riser base can be installed together with the riser tower.

The following sections address each aspect of this design in sequence.

3. Material

The material needs to be of high strength to minimise weight. Aluminium ofalloy 6082 will therefore be adopted. This has a yield strength of 270 MPa.Conventional aluminium welds have the weakness that the welds are under-matched. Continuous extrusion over a die also produces weak fusion zones.This impacts on both strength and ductility. There are two avenues forovercoming this issue.

Within DEMO 2000 it has been proposed to develop friction stir welding forwelding of longitudinal pipe seams and girth welds. With heat treatment andageing the full material strength can be developed. The other alternative is toextrude a pierced ingot over a free-standing die and produce pipes withdouble upsets. This technology has been developed for steel pipe and couldbe extended to aluminium pipe.

4. Corrosion

As aluminium and carbon steel is combined in a pipe in pipe solutioncorrosion protection measures may be required both on the outside andinside the conduit pipe. In Appendix B the relevant corrosion aspects arediscussed and corrosion protection solutions are presented.



5. Installation

For tow-out and installation it is necessary to have a tower cross sectionwhich is near neutrally buoyant. Typically, seven air filled conduit tubes willprovide the necessary buoyancy providing that they are of aluminium. Theeighth conduit tube will be water filled during tow-out and installation and mayalso be used to accommodate umbilicals. This eighth conduit will act as akeel keeping the tower stable in the water.

The concrete base and the water filled buoyancy tank will act as clumpweights during tow out and installation, which is performed in one operation.This approach provides a safer and more economical approach than othermethods. The clump weights provide tension in the riser during tow-out andinstallation. With clump weights the depth of the sub-surface tow can beadjusted to suit the prevailing conditions such as available draft inshore anddepth of wave action for the offshore tow. The upending, which is a reversibleprocess, is performed by a controlled paying out of the towline.

A

B

C

A

C

B



In the vertical position the tank is suspended from a heave compensator andlowered until the anchor reaches the sea bottom. By avoiding pre-installationof the riser a substantial cost saving is achieved.

6. Flow assurance

The pipe-in-pipe configuration lends itself to warm water circulation wherethis is required. As the riser pipe only is in contact with the aluminium conduitpipe at one point around its circumference a modest thickness of insulationaround the riser pipe will have a great effect on heat loss. Air convection willreduce the insulating effect of the air filled annulus. Allowing for this a 10”riser with a 1” cover of syntactic foam placed in a 20” conduit pipe will have athermal conductivity of k = 2.7 W/m2 0K.

A 10” riser with a 5” cover of syntactic foam will have k = 1.6 W/m2 0Kproviding there is absolutely no circulation of water in gapes along theinterfaces of the blocks of syntactic foam. Any flow of water will be driven byconvection. This will have at least as great an impact as convection of air.

Another alternative is to fill the annulus with a gel or a mineral oil forinsulation. This would lead to k = 1.1 W/m2 0K. An example based oninformation from Block 17 in Angola is given in Appendix D. Thisdemonstrates that such gels have great potential providing assurance can beachieved that they will not degrade.

7. Cost

The advantage of using aluminium conduit pipes lies partly in that a fixedbase provides for simpler and less costly pipeline pull-in and connection, asingle operation tow-out and installation and a simple low cost connection tothe base. The other aspect is that costly foam buoyancy is replaced with aninexpensive aluminium conduit pipe, which also serves as a tether.

The efficiency of using steel and aluminium can be compared by calculatingthe cost of supporting an applied force over a unit distance. This may beexpressed as the cost of one unit area of metal with a thickness inverselyproportional to the yield strength of the metal. When the cost of providingbuoyancy for floating applications, is added to the material cost This can beexpressed as:

I = (M?+B(?-1))/s y

Where:M is the price of the metal per kgB is the price of the buoyancy per kg? is the density of the materials y is the yield strength of the material



With steel at a cost of NOK 6 per kg and aluminium at a cost of NOK 40 perkg we obtain:

Buoyancy Unit rate Steel AluminiumSyntactic foam >1000 m 240 NOK/kg 4.32 NOK/lMPa 1.91 NOK/lMPaFoam buoyancy<1000 m 100 NOK/kg 1.87 NOK/lMPa 1.03 NOK/lMPaTank buoyancy 10 NOK/kg 0.30NOK/lMPa 0.46 NOK/lMPa

Strength to weight ratio 400/8= 50 270/2.7= 100

The effective cost including compensating buoyancy per kg of aluminium isseen to be about half that for steel for tower buoyancy. The reason for this isthat the cost of buoyancy considerably exceeds the cost of the material. If thecost of welding is included at a rate of NOK 30 per kg for steel and NOK 90per kg for aluminium we get the following result:

Buoyancy Unit rate Steel AluminiumSyntactic foam >1000 m 240 NOK/kg 4.80 NOK/lMPa 2.41 NOK/lMPaFoam buoyancy<1000 m 100 NOK/kg 2.35 NOK/lMPa 1.53 NOK/lMPaTank buoyancy 10 NOK/kg 0.78NOK/lMPa 0.96 NOK/lMPa

From the above it is also evident that syntactic foam is a formidable costdriver and that the advantage of using aluminium in place of steel willincrease dramatically with depth. To illustrate this in more detail the cost ofthree different tower configurations for two different depths have beenanalysed in Appendix A. The following results were obtained:

Depth: 1500 m 2 500 mAluminium conduits 79 020 NOK/m 155 635 NOK/mSteel conduits 189 825 NOK/m 324 135 NOK/mFoam buoyancy 143 670 NOK/m 252 260 NOK/m

The advantage of the aluminium concept is clearly demonstrated. Thecalculations also demonstrate that the cost ratio is influenced by the payloadand the required buoyancy tank size. This in turn means that the current hasan influence. Another aspect is that the above result is insensitive to theassumed unit rates. Even if the adopted unit rate of aluminium shouldincrease by 50% the foam buoyancy solution would still be 40% moreexpensive.

These results are in turn reflected in the comparative costs for a hybrid risergiven below:



Cost element Steel concept Aluminium conceptRiser base 7 000 kNOK 7 000 kNOKRiser base connection 8 000 kNOKRiser tower 141000 kNOK 73 000 kNOKBuoyancy tank 16 000 kNOK 16 000 kNOKInstallation 104 000 kNOK 68 000 kNOKAssembly, testing and engineering 30 000 kNOK 30 000 kNOKHybrid riser total cost 303 000 kNOK 194 000 kNOKJumpers 80 000 kNOK 80 000 kNOK

This solution which is based on the use of aluminium to avoid the need offoam buoyancy and to accommodate a fixed base which in turn makes tow-out and installation in one operation feasible is seen to be extremely costeffective. The principal cost reduction is seen to arise from avoiding the useof foam buoyancy in the riser section. The two first tables show that whetherthe comparisons are based on material prices or as welded prices the costcomparison between the two alternatives are not altered significantly. Theother significant cost item is tow-out and installation. Although clearly projectdependant, there will obviously accrue a cost saving due to the singleoperation tow-out and installation. The above conclusion is seen to beinsensitive to the underlying unit rate assumptions.



APPENDIX A – Cost analysis of tower section



Scenario A:

Depth: 1500 mTank top: 100 mRisers: 7 Ø 9 5/8”Umbilical conduit: 1 (Includes gas lift risers.)Payload: 130 kg/m (umbilicals and gas lift risers.)Internal pressure in risers: 250 barYield strength of steel: 400 MPaYield strength of aluminium: 270 MPaMinimum tension at base: 6 000 kNAs welded steel: 30 NOK/mAs welded aluminium: 90 NOK/mBuoyancy (100m): 100 NOK/kgBuoyancy (1000m): 240 NOK/kgAverage buoyancy cost: (900x(100+240)/2+500x240)/1400 =

195 NOK/kg

For tow out, upending and installation the riser must be neutrally buoyant.

Net tank tension includes tension at base plus weight of oil in risers:

T = 6 000 + (Π/4)0.222x1400x9.81x7x0.82 = 9 000 kN

Riser pipe:

External diameter: 244.5 mmWall thickness: 12.7 mmCorrosion allowance:1 mm

Required wall thickness: t = 1+1.5x14x219.1/(2x400) = 11.3mm < 12.7 mm

Wall thickness including rolling tolerance: 13.7 mm

Case A1 - Aluminium conduit pipes:

External diameter: 480 mmWall thickness: 18 mmThermal insulation on riser pipe: 1”

Required wall thickness: t = 1.5x14x445/(2x270) = 17.3 mm

Wall thickness including extrusion tolerance: 18.1 mm

Axial stress in conduit pipe:

σ = 9x106/(Πx462x18x8) + 14x444/(4x18) = 129 MPa



Exposed diameter allowing 200 mm for compact flange:

Ø(exposed) = 480x3 + 2x200 = 1840 mm

Weight of 1meter tower section:

Riser pipe: (Π/4)(2.4452-2.1712)10x7.84x7 = 545 kgConduit pipe: (Π/4)(4.802-4.452)10x2.7x7 = 483 kgPlate connection: (Π/4)(2.042)0.181x2.7/10 = 16 kgInsulation: (Π/4)(2.952-2.452)10x0.5x7 = 74 kgPayload: Umbilicals and gas lift risers 130 kgTotal 1248 kg

Buoyancy (Π/4)(4.802)10x7 = 1267 kg

Cost: 499x90 + 545x30 + 74x240 = 79 020 NOK/m

Case A2 - Steel conduit pipes:

External diameter: 480 mmWall thickness: 12.7 mmThermal insulation on riser pipe: 1”

Required wall thickness: t = 1+1.5x14x455/(2x400) = 12.9 mm



σ = 9x106/(Πx467x12.7x8) + 14x445/(4x12.7) = 183 MPa


Ø(exposed) = 480x3 + 2x200 = 1840 mm





Buoyancy (Π/4)(4.802)10x7 = 1267 kg

Cost: 1686x30 + 74x240 + (1890-1267)195 = 189 825 NOK/m

Case A3 - Without conduit pipes:

Tether diameter: Ø 457.2 mmWall thickness: 34.9 mmCorrosion allowance: 3 mm

σ = 9x106/(Πx422.3x31.9) + 14x387.4/(4x31.9) = 255 MPa


Riser pipe: (Π/4)(2.4452-2.1712)10x7.84x7 = 545 kgTether (Π/4)(4.5922-3.8742)10x7.84 = 374 kgPlate connection: (Π/4)(2.042)0.137x7.84/10 = 35 kgPayload: Umbilicals and gas lift risers 130 kgTotal 1084 kg

Buoyancy, riser: (Π/4)(2.452)10x7 = 330 kgBuoyancy, tether: (Π/4)(4.5722)10 = 164 kg

Cost: 954x30 + (1084-494)195 = 143 670 NOK/m

Scenario B:

Depth: 2500 m



Tank top: 100 mRisers: 7 Ø 9 5/8”Umbilical conduit: 1 (Includes gas lift risers.)Payload: 130 kg/m (umbilicals and gas lift risers.)Internal pressure in risers: 250 barYield strength of steel: 400 MPaYield strength of aluminium: 270 MPaMinimum tension at base: 16 000 kNAs welded steel: 30 NOK/mAs welded aluminium: 90 NOK/mBuoyancy (100m): 100 NOK/kgBuoyancy (1000m): 240 NOK/kgAverage buoyancy cost: (900x(100+240)/2+1500x240)/2400 =

215 NOK/kg

For tow out, upending and installation the riser must be neutrally buoyant.

Net tank tension includes tension at base plus weight of oil in risers:

T = 16 000 + (Π/4)0.222x2400x9.81x7x0.82 = 21 000 kN

Riser pipe:

External diameter: 244.5 mmWall thickness: 12.7 mmCorrosion allowance:1 mm

Required wall thickness: t = 1+1.5x14x219.1/(2x400) = 11.3mm < 12.7 mm


Case B1 - Aluminium conduit pipes:


Required wall thickness: t = 1.5x24x470/(2x270) = 31.3 mm

Wall thickness including extrusion tolerance: 32.1 mm


σ = 21x106/(Πx498x32x8) + 24x466/(4x32) = 140 MPa




Ø(exposed) = 530x3 + 2x200 = 1990 mm



Buoyancy (Π/4)(5.302)10x7 = 1544 kg

Cost: 946x90 + 561x30 + 74x240 + 167x215 = 155 635 NOK/m

Case A2 - Steel conduit pipes:


Required wall thickness: t = 1+1.5x24x482/(2x400) = 22.7 mm

Wall thickness including rolling tolerance: 25 mm


σ = 21x106/(Πx506x24x8) + 24x482/(4x24) = 189 MPa


Ø(exposed) = 530x3 + 2x200 = 1990 mm


Riser pipe: (Π/4)(2.4452-2.1712)10x7.84x7 = 545 kgConduit pipe: (Π/4)(5.322-4.822)10x7.84x7 = 2185 kgPlate connection: (Π/4)(2.042)0.137x7.84/10 = 35 kgInsulation: (Π/4)(2.952-2.452)10x0.5x7 = 74 kgPayload: Umbilicals and gas lift risers 130 kg



Total 2969 kg

Buoyancy (Π/4)(5.302)10x7 = 1544 kg

Cost: 2765x30 + 74x240 + (2969-1544)215 = 324 135 NOK/m

Case B3 - Without conduit pipes:

Tether diameter: Ø 700 mmWall thickness: 60 mmCorrosion allowance: 3 mm

σ = 21x106/(Πx640x57) + 24x580/(4x57) = 244 MPa


Riser pipe: (Π/4)(2.4452-2.1712)10x7.84x7 = 545 kgTether (Π/4)(7.022-5.82)10x7.84 = 963 kgPlate connection: (Π/4)(2.042)0.137x7.84/10 = 35 kgPayload: Umbilicals and gas lift risers 130 kgTotal 1673 kg

Buoyancy, riser: (Π/4)(2.452)10x7 = 330 kgBuoyancy, tether: (Π/4)(72)10 = 385 kg

Cost: 1543x30 + (1673-715)215 = 252 260 NOK/m



APPENDIX B – Corrosion protection



Outside conduit pipeThe riser tower will be electrically isolated from the floating steel structure byflexible jumpers and from the steel pipeline by a concrete riser base. As seenfrom the figure 1, showing corrosion potential between different materials inseawater, the actual aluminium alloy (AlMgSi, 6082) is active compared tocarbon steel (St. 52). Taken in account the huge area of aluminium incomparison with the modest area of carbon steel, which also will be coated,the structurally integrity of the aluminium conduit should not be harmed bygalvanic corrosion effects. Nevertheless, sacrificial anodes may be furnishedin order to ensure against any unexpected corrosion effects. Decision onsolution to be selected will be taken when results from the proposed testingare available.

Inside conduit pipeThere will be contact between the internal carbon steel pipe and thealuminium conduit pipe, as the internal pipe will buckle in a helix due to itsexpansion. Condensation water must be expected to form on the insidesurface of the aluminium conduit pipe, as this surface will be cold relative tothe warm internal pipe. As is shown in figure 2, the corrosion potentialbetween aluminium and steel for this case be different to the conditions inseawater discussed above. In this latter case the carbon steel will be theactive element and may suffer galvanic corrosion when in contact withaluminium. It is seen that the difference in galvanic potential is fairly lowwhich implies relatively low corrosion currents and activity. However,appropriate corrosion protection measures must be taken and one of thefollowing solutions or combinations of them should be applied:

• Due to process reasons the internal pipe in many cases will be furnishedwith an isolation material which simultaneously also will act as a corrosionprotection material. Mechanical damage is not expected to be a problem,as the material will have sufficient robustness and thickness that dentsand gouges will not penetrate to the carbon steel surface.

• To obtain aluminium to aluminium contact surfaces and avoid anygalvanic corrosion effects thermally sprayed aluminium (TSA) could beapplied to the internal carbon steel pipe. For such an application TSA isfeasible due to its high resistance to mechanical damage.

• Spacer rings in an insulating material could be installed at regularintervals inside the conduit to prevent metal contact between the pipes.



Figure 1

Galvanic series in sea-water at 10 °CThe materials are ranked after average corrosion the last 10 days ofexposure. The range within which the potential varied in the significant part ofthe exposure is shown for each material.

Reference: “Korrosjonsbestandigematerialer i prosess og sjøvannssytemer”,Sintef reprot; STF34,187116, 1987-10-22.



Figure 2

Galvanic series in water with pH=6

On aluminium, tin lead the potential measurements are done after a passivelayer has been formed.

Reference: “Aluminium-Tashenbuch”, 15. Auflage.



APPENDIX C – Restrained riser expansion



This worked example of spiral buckling is an extension of case A2 inappendix A. The following additional data apply:

Total length: 1400 mOperational temperature: 840 CWater temperature: 40 CInternal pressure: 200 bar

Relative extension:

Temperature: 1400x10x10-6x80 = 1.120 mEnd cap effect: (1-2x0.3)x20x4802x1400/((5302-4802)200000) = 0.256 mCurvature: Πx0.8x5/360 = 0.034 mTotal: = 1.410 m

This extension is related to the buckling length by the following equation:

11 2

22

2

2

2

2

−Π++••••Π+

••Π=

lD

lEAIE

lAI s

aa

ss

s

sε

Where:

ε is the average nominal restraint induced strain equalling the totalextension divided by the total strain.

Is is the moment of inertia of the riser pipe cross sectionEs is the modulus of elasticity of the riser pipe cross sectionAs is the area of the riser pipe cross sectionIa is the moment of inertia of the guide tube cross sectionEa is the modulus of elasticity of the guide tube cross sectionAa is the area of the guide tube cross sectionDs is the diameter of the axis of the spiral formed by the centreline of

the riserL is the wavelength of the spiral

Solving the above table gives a wave length l = 17.05 m. The resulting axialcompressive force is:

MNl

IEP ss 923.02

2

=Π= og σ = 91 MPa

In the aluminium guide pipe the resulting tensile stress increment will only be23 Mpa.



APPENDIX D – FLOW CONTROL



The following example is based on data from the Dhalia field in Block 17, Angola. Thisfield has subsea wells with long flow lines where passive flow assurance is a primaryconcern.

Flowline length: 10 000 m

Flow rate (assumed): 100 kg/s (υ=75 Cp)10 000 sm3/day58 000 bopd

Well head pressure: 100 bar

Well head temperature: 45 0C

Ambient temperature in sea water: 5 0C

Hydrate formation temperature: 15 0C

The above data gives a steady state temperature at the foot of the riser of 35 0C.The riser is 10” with a 20” conduit pipe. Petroleum gel has the followingproperties:

k = 0.14 W/m2 0C U = 1.75 W/m2 0Cρ = 830 kg/m3

Cp = 2200 J/kg 0C

k/(ρ Cp) = 77x10-9 m/s

And foam has:

k = 0.21 W/m2 0Cρ = 900 kg/m3

Cp = 800 J/kg 0C

k/(ρ Cp) = 292x10-9 m/s

It is seen that paraffin gel (kerosene gel) has less than one third of thediffuseitivity of foam insulation. The calculation overleaf shows that for atemperature differential of 30 0C convection will not occur providing the viscosityof the gel exceeds 2Cp. This is comfortably achievable. It thus appears thatparaffin gel has very attractive properties for this application.

The calculation assumes the following values:



Oil:ρ = 900 kg/m3

Cp = 1800 J/kg 0C

Water:ρ = 1000 kg/m3

Cp = 4200 J/kg 0C

For oil, water and gel ρ Cp has conservatively been assumed equal to1.8x106 J/m3 0C. The results of the calculation is shown in the second graphoverleaf. It is seen that 35 hours may elapse before hydrate formationoccurs.

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