Aker Riser Systems AS FIXED BASE HYBRID RISER IN ALUMINIUM A SUMMARY DESCRIPTION

Transcription

1 FIXED BASE HYBRID RISER IN ALUMINIUM A SUMMARY DESCRIPTION

2 Preface Current concepts for hybrid risers have been based on using foam buoyancy. The concept developed by Aker Riser Systems AS is based on using aluminium conduit pipes, which replace the foam buoyancy and the steel tether. They also make a fixed riser base connection possible which in turn makes it possible to perform the complete tow-out and installation in one operation. This has a profound impact on cost. The main cost drivers are foam buoyancy and offshore operations. ARS concept minimise these costs. This concept reduces the cost of a hybrid riser by 30% to 40%. This document explains and documents this cost advantage with simple traceable cost models, which permit direct comparisons of concepts. It is also seen that this conclusion is insensitive to the underlying unit rates. Aker Riser Systems.doc/2/EB 2

3 1. Introduction A hybrid riser consists of a low cost submerged metallic riser tower from which short flexible risers extend to connect to a floating structure. The tower tension is supplied by a soft tank which is open at the bottom to ensure that the internal and the external pressure in equilibrium. This ensures that the tank is safe against implosion. The soft tank provides low cost buoyancy for suspending the riser. When the riser is suspended from a surface vessel superstructure must be provided in addition to closed tank buoyancy. In this case 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 may be assembled inshore and towed to location where they are upended and installed. The installation method selected will depend on project specific conditions. This discussion addresses a tow-out installation, which is likely to be the most economical in the majority of cases. For the tow out the tower cross section needs to be near neutrally buoyant. This buoyancy is in current concepts provided by encasing the tower section in plastic foam. The riser tower in current concepts incorporates a central steel tether, which anchors the buoyancy tank to a riser base. The tether is surrounded by a number of risers, which are tied to it at suitable intervals to prevent vortexinduced vibrations. The production risers will contain hydrocarbons under pressure and at increased temperature, which will cause them to expand. This expansion is very significant and may exceed one meter for a 1000 m riser. In current concepts the tower is connected to the preinstalled base with a latch and a flex-element or a taper-joint. The risers are connected to the pipelines with flexible jumpers or expansion spool type flexible metallic pipe configurations. Apart from the flexible jumpers to the surface structure the major cost drivers for 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 tower configuration, which is seen to be far more cost efficient than previous concepts. The significant features of this alternative are shown in the figure Aker Riser Systems.doc/3/EB 3

4 overleaf. The traditional central tether is replaced by a number of guide tubes in aluminium. One central guide tube is surrounded by a number of guide tubes around its circumference. The production risers and high pressure service risers are steel pipes located within the aluminium guide tubes. For low pressure oil export not containing carbon dioxide the aluminium tubes may be used directly as risers. When the steel risers confined in the conduit expand they will buckle in a helix. Due to the confining pipe the deformation will be restricted and only modest deformation induced elastic stresses will result. A worked example is included in Appendix C. The guide tubes will be prevented from imploding by being internally pressurised with air. To ensure that an accidental depressurisation shall not lead to implosion the guide tubes will be vented from the bottom to a level where the external pressure is sufficient that it exceeds the internal pressure necessary to prevent implosion. Aker Riser Systems.doc/4/EB 4

5 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 of alloy 6082 will therefore be adopted. This has a yield strength of 270 MPa. Conventional aluminium welds have the weakness that the welds are undermatched. Continuous extrusion over a die also produces weak fusion zones. This impacts on both strength and ductility. There are two avenues for overcoming this issue. Within DEMO 2000 it has been proposed to develop friction stir welding for welding of longitudinal pipe seams and girth welds. With heat treatment and ageing the full material strength can be developed. The other alternative is to extrude a pierced ingot over a free-standing die and produce pipes with double upsets. This technology has been developed for steel pipe and could be extended to aluminium pipe. 4. Corrosion As aluminium and carbon steel is combined in a pipe in pipe solution corrosion protection measures may be required both on the outside and inside the conduit pipe. In Appendix B the relevant corrosion aspects are discussed and corrosion protection solutions are presented. Aker Riser Systems.doc/5/EB 5

6 5. Installation For tow-out and installation it is necessary to have a tower cross section which is near neutrally buoyant. Typically, seven air filled conduit tubes will provide the necessary buoyancy providing that they are of aluminium. The eighth conduit tube will be water filled during tow-out and installation and may also be used to accommodate umbilicals. This eighth conduit will act as a keel keeping the tower stable in the water. A C B B A C The concrete base and the water filled buoyancy tank will act as clump weights during tow out and installation, which is performed in one operation. This approach provides a safer and more economical approach than other methods. The clump weights provide tension in the riser during tow-out and installation. With clump weights the depth of the sub-surface tow can be adjusted to suit the prevailing conditions such as available draft inshore and depth of wave action for the offshore tow. The upending, which is a reversible process, is performed by a controlled paying out of the towline. Aker Riser Systems.doc/6/EB 6

7 In the vertical position the tank is suspended from a heave compensator and lowered until the anchor reaches the sea bottom. By avoiding pre-installation of the riser a substantial cost saving is achieved. 6. Flow assurance The pipe-in-pipe configuration lends itself to warm water circulation where this is required. As the riser pipe only is in contact with the aluminium conduit pipe at one point around its circumference a modest thickness of insulation around the riser pipe will have a great effect on heat loss. Air convection will reduce 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 a thermal conductivity of k = 2.7 W/m 2 0 K. A 10 riser with a 5 cover of syntactic foam will have k = 1.6 W/m 2 0 K providing there is absolutely no circulation of water in gapes along the interfaces of the blocks of syntactic foam. Any flow of water will be driven by convection. 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 for insulation. This would lead to k = 1.1 W/m 2 0 K. An example based on information from Block 17 in Angola is given in Appendix D. This demonstrates that such gels have great potential providing assurance can be achieved that they will not degrade. 7. Cost The advantage of using aluminium conduit pipes lies partly in that a fixed base provides for simpler and less costly pipeline pull-in and connection, a single operation tow-out and installation and a simple low cost connection to the base. The other aspect is that costly foam buoyancy is replaced with an inexpensive aluminium conduit pipe, which also serves as a tether. The efficiency of using steel and aluminium can be compared by calculating the cost of supporting an applied force over a unit distance. This may be expressed as the cost of one unit area of metal with a thickness inversely proportional to the yield strength of the metal. When the cost of providing buoyancy for floating applications, is added to the material cost This can be expressed as: I = (M?+B(?-1))/s y Where: M is the price of the metal per kg B is the price of the buoyancy per kg? is the density of the material s y is the yield strength of the material Aker Riser Systems.doc/7/EB 7

8 With steel at a cost of NOK 6 per kg and aluminium at a cost of NOK 40 per kg we obtain: Buoyancy Unit rate Steel Aluminium Syntactic foam >1000 m 240 NOK/kg 4.32 NOK/lMPa 1.91 NOK/lMPa Foam buoyancy<1000 m 100 NOK/kg 1.87 NOK/lMPa 1.03 NOK/lMPa Tank buoyancy 10 NOK/kg 0.30NOK/lMPa 0.46 NOK/lMPa Strength to weight ratio 400/8= /2.7= 100 The effective cost including compensating buoyancy per kg of aluminium is seen to be about half that for steel for tower buoyancy. The reason for this is that the cost of buoyancy considerably exceeds the cost of the material. If the cost of welding is included at a rate of NOK 30 per kg for steel and NOK 90 per kg for aluminium we get the following result: Buoyancy Unit rate Steel Aluminium Syntactic foam >1000 m 240 NOK/kg 4.80 NOK/lMPa 2.41 NOK/lMPa Foam buoyancy<1000 m 100 NOK/kg 2.35 NOK/lMPa 1.53 NOK/lMPa Tank buoyancy 10 NOK/kg 0.78NOK/lMPa 0.96 NOK/lMPa From the above it is also evident that syntactic foam is a formidable cost driver and that the advantage of using aluminium in place of steel will increase dramatically with depth. To illustrate this in more detail the cost of three different tower configurations for two different depths have been analysed in Appendix A. The following results were obtained: Depth: 1500 m m Aluminium conduits NOK/m NOK/m Steel conduits NOK/m NOK/m Foam buoyancy NOK/m NOK/m The advantage of the aluminium concept is clearly demonstrated. The calculations also demonstrate that the cost ratio is influenced by the payload and the required buoyancy tank size. This in turn means that the current has an influence. Another aspect is that the above result is insensitive to the assumed unit rates. Even if the adopted unit rate of aluminium should increase by 50% the foam buoyancy solution would still be 40% more expensive. These results are in turn reflected in the comparative costs for a hybrid riser given below: Aker Riser Systems.doc/8/EB 8

9 Cost element Steel concept Aluminium concept Riser base knok knok Riser base connection knok Riser tower knok knok Buoyancy tank knok knok Installation knok knok Assembly, testing and engineering knok knok Hybrid riser total cost knok knok Jumpers knok knok This solution which is based on the use of aluminium to avoid the need of foam buoyancy and to accommodate a fixed base which in turn makes towout and installation in one operation feasible is seen to be extremely cost effective. The principal cost reduction is seen to arise from avoiding the use of foam buoyancy in the riser section. The two first tables show that whether the comparisons are based on material prices or as welded prices the cost comparison between the two alternatives are not altered significantly. The other significant cost item is tow-out and installation. Although clearly project dependant, there will obviously accrue a cost saving due to the single operation tow-out and installation. The above conclusion is seen to be insensitive to the underlying unit rate assumptions. Aker Riser Systems.doc/9/EB 9

10 APPENDIX A Cost analysis of tower section Aker Riser Systems.doc/10/EB 10

11 Scenario A: Depth: 1500 m Tank top: 100 m Risers: 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 bar Yield strength of steel: 400 MPa Yield strength of aluminium: 270 MPa Minimum tension at base: kn As welded steel: 30 NOK/m As welded aluminium: 90 NOK/m Buoyancy (100m): 100 NOK/kg Buoyancy (1000m): 240 NOK/kg Average buoyancy cost: (900x( )/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 = (Π/4) x1400x9.81x7x0.82 = kn Riser pipe: External diameter: mm Wall thickness: 12.7 mm Corrosion 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 mm Wall thickness: 18 mm Thermal 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 = 9x10 6 /(Πx462x18x8) + 14x444/(4x18) = 129 MPa Aker Riser Systems.doc/11/EB 11

12 Exposed diameter allowing 200 mm for compact flange: Ø(exposed) = 480x3 + 2x200 = 1840 mm Weight of 1meter tower section: Riser pipe: (Π/4)( )10x7.84x7 = 545 kg Conduit pipe: (Π/4)( )10x2.7x7 = 483 kg Plate connection: (Π/4)( )0.181x2.7/10 = 16 kg Insulation: (Π/4)( )10x0.5x7 = 74 kg Payload: Umbilicals and gas lift risers 130 kg Total 1248 kg Buoyancy (Π/4)( )10x7 = 1267 kg Cost: 499x x x240 = NOK/m Case A2 - Steel conduit pipes: External diameter: 480 mm Wall thickness: 12.7 mm Thermal insulation on riser pipe: 1 Required wall thickness: t = 1+1.5x14x455/(2x400) = 12.9 mm Wall thickness including rolling tolerance: 13.7 mm Axial stress in conduit pipe: σ = 9x10 6 /(Πx467x12.7x8) + 14x445/(4x12.7) = 183 MPa Exposed diameter allowing 200 mm for compact flange: Ø(exposed) = 480x3 + 2x200 = 1840 mm Aker Riser Systems.doc/12/EB 12

13 Weight of 1meter tower section: Riser pipe: (Π/4)( )10x7.84x7 = 545 kg Conduit pipe: (Π/4)( )10x7.84x7 = 1106 kg Plate connection: (Π/4)( )0.137x7.84/10 = 35 kg Insulation: (Π/4)( )10x0.5x7 = 74 kg Payload: Umbilicals and gas lift risers 130 kg Total 1890 kg Buoyancy (Π/4)( )10x7 = 1267 kg Cost: 1686x x240 + ( )195 = NOK/m Case A3 - Without conduit pipes: Tether diameter: Wall thickness: Corrosion allowance: Ø mm 34.9 mm 3 mm σ = 9x10 6 /(Πx422.3x31.9) + 14x387.4/(4x31.9) = 255 MPa Weight of 1meter tower section: Riser pipe: (Π/4)( )10x7.84x7 = 545 kg Tether (Π/4)( )10x7.84 = 374 kg Plate connection: (Π/4)( )0.137x7.84/10 = 35 kg Payload: Umbilicals and gas lift risers 130 kg Total 1084 kg Buoyancy, riser: (Π/4)( )10x7 = 330 kg Buoyancy, tether: (Π/4)( )10 = 164 kg Cost: 954x30 + ( )195 = NOK/m Scenario B: Depth: 2500 m Aker Riser Systems.doc/13/EB 13

14 Tank top: 100 m Risers: 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 bar Yield strength of steel: 400 MPa Yield strength of aluminium: 270 MPa Minimum tension at base: kn As welded steel: 30 NOK/m As welded aluminium: 90 NOK/m Buoyancy (100m): 100 NOK/kg Buoyancy (1000m): 240 NOK/kg Average buoyancy cost: (900x( )/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 = (Π/4) x2400x9.81x7x0.82 = kn Riser pipe: External diameter: mm Wall thickness: 12.7 mm Corrosion 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 B1 - Aluminium conduit pipes: External diameter: 530 mm Wall thickness: 32 mm Thermal insulation on riser pipe: 1 Required wall thickness: t = 1.5x24x470/(2x270) = 31.3 mm Wall thickness including extrusion tolerance: 32.1 mm Axial stress in conduit pipe: σ = 21x10 6 /(Πx498x32x8) + 24x466/(4x32) = 140 MPa Exposed diameter allowing 200 mm for compact flange: Aker Riser Systems.doc/14/EB 14

15 Ø(exposed) = 530x3 + 2x200 = 1990 mm Weight of 1meter tower section: Riser pipe: (Π/4)( )10x7.84x7 = 545 kg Conduit pipe: (Π/4)( )10x2.7x7 = 946 kg Plate connection: (Π/4)( )0.181x2.7/10 = 16 kg Insulation: (Π/4)( )10x0.5x7 = 74 kg Payload: Umbilicals and gas lift risers 130 kg Total 1711 kg Buoyancy (Π/4)( )10x7 = 1544 kg Cost: 946x x x x215 = NOK/m Case A2 - Steel conduit pipes: External diameter: 530 mm Wall thickness: 24 mm Thermal insulation on riser pipe: 1 Required wall thickness: t = 1+1.5x24x482/(2x400) = 22.7 mm Wall thickness including rolling tolerance: 25 mm Axial stress in conduit pipe: σ = 21x10 6 /(Πx506x24x8) + 24x482/(4x24) = 189 MPa Exposed diameter allowing 200 mm for compact flange: Ø(exposed) = 530x3 + 2x200 = 1990 mm Weight of 1meter tower section: Riser pipe: (Π/4)( )10x7.84x7 = 545 kg Conduit pipe: (Π/4)( )10x7.84x7 = 2185 kg Plate connection: (Π/4)( )0.137x7.84/10 = 35 kg Insulation: (Π/4)( )10x0.5x7 = 74 kg Payload: Umbilicals and gas lift risers 130 kg Aker Riser Systems.doc/15/EB 15

16 Total 2969 kg Buoyancy (Π/4)( )10x7 = 1544 kg Cost: 2765x x240 + ( )215 = NOK/m Case B3 - Without conduit pipes: Tether diameter: Wall thickness: Corrosion allowance: Ø 700 mm 60 mm 3 mm σ = 21x10 6 /(Πx640x57) + 24x580/(4x57) = 244 MPa Weight of 1meter tower section: Riser pipe: (Π/4)( )10x7.84x7 = 545 kg Tether (Π/4)( )10x7.84 = 963 kg Plate connection: (Π/4)( )0.137x7.84/10 = 35 kg Payload: Umbilicals and gas lift risers 130 kg Total 1673 kg Buoyancy, riser: (Π/4)( )10x7 = 330 kg Buoyancy, tether: (Π/4)(7 2 )10 = 385 kg Cost: 1543x30 + ( )215 = NOK/m Aker Riser Systems.doc/16/EB 16

17 APPENDIX B Corrosion protection Aker Riser Systems.doc/17/EB 17

18 Outside conduit pipe The riser tower will be electrically isolated from the floating steel structure by flexible jumpers and from the steel pipeline by a concrete riser base. As seen from the figure 1, showing corrosion potential between different materials in seawater, the actual aluminium alloy (AlMgSi, 6082) is active compared to carbon steel (St. 52). Taken in account the huge area of aluminium in comparison with the modest area of carbon steel, which also will be coated, the structurally integrity of the aluminium conduit should not be harmed by galvanic corrosion effects. Nevertheless, sacrificial anodes may be furnished in order to ensure against any unexpected corrosion effects. Decision on solution to be selected will be taken when results from the proposed testing are available. Inside conduit pipe There will be contact between the internal carbon steel pipe and the aluminium conduit pipe, as the internal pipe will buckle in a helix due to its expansion. Condensation water must be expected to form on the inside surface of the aluminium conduit pipe, as this surface will be cold relative to the warm internal pipe. As is shown in figure 2, the corrosion potential between aluminium and steel for this case be different to the conditions in seawater discussed above. In this latter case the carbon steel will be the active element and may suffer galvanic corrosion when in contact with aluminium. It is seen that the difference in galvanic potential is fairly low which implies relatively low corrosion currents and activity. However, appropriate corrosion protection measures must be taken and one of the following solutions or combinations of them should be applied: Due to process reasons the internal pipe in many cases will be furnished with an isolation material which simultaneously also will act as a corrosion protection material. Mechanical damage is not expected to be a problem, as the material will have sufficient robustness and thickness that dents and gouges will not penetrate to the carbon steel surface. To obtain aluminium to aluminium contact surfaces and avoid any galvanic corrosion effects thermally sprayed aluminium (TSA) could be applied to the internal carbon steel pipe. For such an application TSA is feasible due to its high resistance to mechanical damage. Spacer rings in an insulating material could be installed at regular intervals inside the conduit to prevent metal contact between the pipes. Aker Riser Systems.doc/18/EB 18

19 Figure 1 Galvanic series in sea-water at 10 C The materials are ranked after average corrosion the last 10 days of exposure. The range within which the potential varied in the significant part of the exposure is shown for each material. Reference: Korrosjonsbestandigematerialer i prosess og sjøvannssytemer, Sintef reprot; STF34,187116, Aker Riser Systems.doc/19/EB 19

20 Figure 2 Galvanic series in water with ph=6 On aluminium, tin lead the potential measurements are done after a passive layer has been formed. Reference: Aluminium-Tashenbuch, 15. Auflage. Aker Riser Systems.doc/20/EB 20

21 APPENDIX C Restrained riser expansion Aker Riser Systems.doc/21/EB 21

22 This worked example of spiral buckling is an extension of case A2 in appendix A. The following additional data apply: Total length: Operational temperature: Water temperature: Internal pressure: 1400 m 84 0 C 4 0 C 200 bar Relative extension: Temperature: 1400x10x10-6 x80 = m End cap effect: (1-2x0.3)x20x480 2 x1400/(( )200000) = m Curvature: Πx0.8x5/360 = m Total: = m This extension is related to the buckling length by the following equation: Π ε = A Where: 2 2 I s Π + 2 s l Aa 2 Es I s Π D Ea l l 2 s 1 ε I s E s A s I a E a A a D s L is the average nominal restraint induced strain equalling the total extension divided by the total strain. is the moment of inertia of the riser pipe cross section is the modulus of elasticity of the riser pipe cross section is the area of the riser pipe cross section is the moment of inertia of the guide tube cross section is the modulus of elasticity of the guide tube cross section is the area of the guide tube cross section is the diameter of the axis of the spiral formed by the centreline of the riser is the wavelength of the spiral Solving the above table gives a wave length l = m. The resulting axial compressive force is: 2 Π EsI s P = = MN og σ = 91 MPa 2 l 23 Mpa. In the aluminium guide pipe the resulting tensile stress increment will only be Aker Riser Systems.doc/22/EB 22

23 APPENDIX D FLOW CONTROL Aker Riser Systems.doc/23/EB 23

24 The following example is based on data from the Dhalia field in Block 17, Angola. This field has subsea wells with long flow lines where passive flow assurance is a primary concern. Flowline length: m Flow rate (assumed): 100 kg/s (υ=75 Cp) sm 3 /day bopd Well head pressure: Well head temperature: 100 bar 45 0 C Ambient temperature in sea water: 5 0 C Hydrate formation temperature: 15 0 C The above data gives a steady state temperature at the foot of the riser of 35 0 C. The riser is 10 with a 20 conduit pipe. Petroleum gel has the following properties: And foam has: k = 0.14 W/m 2 0 C U = 1.75 W/m 2 0 C ρ = 830 kg/m 3 C p = 2200 J/kg 0 C k/(ρ C p ) = 77x10-9 m/s k = 0.21 W/m 2 ρ = 900 kg/m 3 C p = 800 J/kg 0 C 0 C k/(ρ C p ) = 292x10-9 m/s It is seen that paraffin gel (kerosene gel) has less than one third of the diffuseitivity of foam insulation. The calculation overleaf shows that for a temperature differential of 30 0 C convection will not occur providing the viscosity of the gel exceeds 2Cp. This is comfortably achievable. It thus appears that paraffin gel has very attractive properties for this application. The calculation assumes the following values: Aker Riser Systems.doc/24/EB 24

25 Oil: ρ = 900 kg/m 3 C p = 1800 J/kg 0 C Water: ρ = 1000 kg/m 3 C p = 4200 J/kg 0 C For oil, water and gel ρ C p has conservatively been assumed equal to 1.8x10 6 J/m 3 0 C. The results of the calculation is shown in the second graph overleaf. It is seen that 35 hours may elapse before hydrate formation occurs. Aker Riser Systems.doc/25/EB 25

26 Aker Riser Systems.doc/26/EB 26