Risers For Mooring FPSO

1. System descriptiona. Complete system for high production field (150.000 bpd) 1 FPSO 16 Riser Systems (production and injection) consisting of: 16 Jumpers from the FPSO to Buoys 16 Buoys 32 Tethers from Buoys to Sea Floor, called Down-Tethers or T1-2 16 Suction Anchors (2 Tethers in each one) 16 SCRs from Buoy to Sea Floor Therefore, 1 Individual Riser System consists of: 1 Jumper, 1 Buoy, 2 Tethers from Buoy to Sea Floor, 1 Suction Anchor, and 1 SCR. No Mooring Lines: 16 Tethers from FPSO to Buoys for station keeping purposes, called Up-Tethers or T3b. Top view with radial lengths (XXX sketch XXX) Spreaded buoys as a taught mooring system Angle between buoys: 22.5 degreesc. Lateral view with horizontal and vertical lengths (XXX sketch XXX) Water depth: 3,000 m FPSO to Buoy Horizontal distance: 270 m Vertical distance: 200 m Buoy to TDP of SCR Horizontal distance: 1,424 m Vertical distance: 2,800 m Buoy to Suction Anchor Horizontal distance: 342 m Vertical distance: 2,800 md. Spacing (XXX sketch XXX) In the FPSO: Horizontal spacing between Jumpers: 2.5 m > 5*OD Horizontal spacing between Up-Tethers in the FPSO: 2.5 m (to maintain symmetry) Vertical spacing between Riser and Up-Tether of the same Riser System: 5 m In the Buoy: Spacing between Down-Tethers: 3 m In the Suction Anchor: Spacing between Down-Tethers: 3 m (to maintain symmetry)e. Advantages FPSO: Any: new or conversion Turret: no needed Payload of the Riser system: reduced (only support a part of the weight of the jumper) Riser Systems: Jumpers: Reduced length SCRs: Maximum length Mooring System: Reduced to one Tether between each Buoy and the FPSO Therefore, Main Advantage = Availability and Cost effectivenessf. Parameters of the System FPSO: L = 310 m, B = 48 m, H = 24 m, DWT = (XXX Check XXX) Jumpers: OD = 424 mm, ID = 254 mm, W = 420 kg/m SCRs: OD = 458 mm (including coating), ID = 254 mm, W = 244 Kg/m Buoys: L = 18.1 m, D = 7 m, W = 204 tonne Down-Tethers: OD = 139 mm, W = 12.1 kg/m Up-Tethers: OD = 201 mm, W = 21.9 kg/mg. Parameters of the System Reasons: FPSO based on Production Storage Capacity (7 days) Jumpers and SCR: Diameter based on production and injection Thickness based on loads Buoys based on buoyancy Tethers based on loadsh. Tethers table

2. Extreme conditions designa. Environmental loads: Wave: 100-years 10-year wave Current: 100-years 10-years Design Matrix 100-years wave & 10-years current 10-years wave & 100-years currentb. Wave loads Jonswap spectrum 100-years wave: Hs = 7.6 m Tp = 12.7 s 10-years wave: Hs = 7.4 m Tp = 12.4 sc. Current loadsDepth (m)10-years current (m/s)100-years current (m/s)

01.651.85

501.261.4

1001.251.4

2001.091.2

3000.830.9

4000.740.8

5000.730.8

6000.60.65

8000.60.65

10000.60.6

12000.60.6

15000.460.5

30000.460.5

d. Extreme Conditions Design Matrix (XXX sketch repetition XXX)The fails are considered in the worst tether depending on the direction of the environmental loads, in the table is indicated the tether which fails in each case.Angle of environmental conditionsIntact1 tether fail2 tethers failAll tethers fail

0-T3-9T3-8&9All T3-i

45-T3-11T3-10&11All T3-i

90-T3-12T3-12&13All T3-i

135-T3-15T3-15&16All T3-i

180-T3-16T3-1&16All T3-i

Orcaflex Modelling Procedure1. Obtain significant parameters to establish pilot model in Orcaflex from the optimumized parameters for one COBRA riser from existing reference academic paper. [See ANNEX II]2. Preliminary calculations of the diameter necessary based on the production (see ANNEX II-mathcad) and of the thickness based on the code DNV-F202 as above paragraph 2.2.1. Therefore, the optimized parameters from the paper can be used,3. Preliminary calculations for one COBRA riser (see ANNEX II-mathcad)4. Perform catenary calculations for initial flexible Jumper and SCR attributions.5. Perform trigonometrical calculations for initial tensioned tethers condition.6. Buoyancy mass calculation (see ANNEX II-mathcad). In addition, the moment of inertia of the buoy has been calculated, simple cylindrical shape is applied.7. Set the connection points between the FPSO and the jumpers based on a spacing of 5 times the outer diameter of the jumper to provide sufficient clearance, mitigating encroachment between lines at both splash zone and beneath sea surface.8. Set the initial location of buoyancy modules based on the eight-line mooring theory because the problem has 16 risers, and trigonometrical calculations. From plan view, Buoy 1 is arranged into angle of 22.5 degrees respect to horizontal axis. The incremental angle between each buoys is 22.5 degrees throughout the final buoy, 16. Buoy 4, 5, 12 and 13 have the same angle of 90 degrees with the horizontal. 9. Set the initial position of the Touch Down Points (TDP) of the SCR based on trigonometrical calculations.10. Set the location of PLETs of SCRs (Fixed point) by adding 900 m (equal to 30% of the water depth) to the TDP and trigonometrical calculations.11. Set the connection between the buoys and the tethers location by 3m clearance space between each of them and trigonometrical calculations based on symmetrical geometry.12. Set the location of the anchoring points (Fixed point) of the tethers based on trigonometrical calculations.13. After these steps, the first problem from tether crossing had occurred because the tethers had an azimuth angle approaching to the vessel. The azimuth angle of the tethers was changed iteratively to resolve that problem. Finally, that the maximum angle to avoid crossing 4 degrees was taken into account.14. However, from previous study as of the reference, the COBRA risers was concluded that they perform very well in their function to decouple the motion perfectly between FPSO and SCRs. Therefore, it can presumed that they do not have any influence in the mooring and the FPSO was free to move.15. It is necessary to change the concept. The additional tether is considered to anchor in order to restrict the movement of FPSO from moving away from desire position. This new tether is designed to be added between FPSO and each directional buoyancy module. The final parameters are in ANNEX II.

ResultsWe have investigated both Extreme 100-year conditions to determine design values for the system and Normal Operating Condition to determine the typical response of the system.Results for Extreme Conditions Offsets for FPSO and BUOYS in ULS and ALS Buoy Depth Variation FPSO Forces Buoy Forces Strength analysis: Jumper, Tethers, SCR

FPSO Offset in Extreme Conditions (100-year) without Tether Failure

Without failure, the maximum FPSO offset in extreme conditions (100-year) is 530m which is equivalent to 18% of the water depth (3000m). This is significantly higher than the typical industry-accepted value of 10% water depth, however, this is under extreme conditions and station-keeping is still maintained without failure, during normal operation the system offset could be expected at 135m (4.5% water depth). Under extreme conditions the majority of force is applied to the riser system and up-tethers on the side of current and wave direction and the tethers on the opposite side undergo a significant loss of tension, this is considered acceptable as stresses are within design limitations and no line-clashing is observed.

FPSO Offset in Extreme Conditions With all the Up-Tethers Fail

With up-tether failure, the maximum FPSO offset in extreme conditions is 718m, equivalent to 24% of the water depth (3000m). This is significantly higher than the typical industry-accepted value of 10% water depth, however, in this very unlikely instance the flexible jumpers alone still maintain station-keeping of the vessel without failure. This can be considered an additional safety factor in the mooring of the vessel. Under extreme conditions the majority of force is applied to the flexible jumpers on the side of current and wave direction and the tethers on the opposite side undergo a loss of tension and reduced minimum bending radius, this is considered acceptable as stresses are within design limitations and no line-clashing is observed.

Buoy Offset in Extreme Conditions without Tether Failure

The chart below shows the identity of buoys at their respective positions around the FPSO:

The figure below shows the buoy offset in extreme conditions (100-year) without tether failure. The green boxes indicate the buoy with maximum offset at the angle of incoming current and wave forces (i.e. 0, 45, 90 etc.). The maximum offset is observed at 600m. Again, considering that this is extreme 100-year conditions no failure is observed at this point.

BUOY Offset in Extreme Conditions with all the Up-Tethers FailThe figure below shows the buoy offset in extreme conditions (100-year) with failure of all tethers. Here the maximum offset is observed at 665m. It should be noted that under normal conditions the offset of the buoy would be significantly lower. Again, considering that this is extreme 100-year conditions no failure is observed at this point.

BUOY Depth in Extreme Conditions

The vertical depth of the buoy will also change in extreme conditions as follows: Equilibrium depth = 200 m Without Failure Minimum depth = 170 m (-30 m respect original depth) B1 & B2 in 0 degree and B8 & B9 in 180 degree Maximum depth = 294 m (+94 m respect original depth) B12 & B13 in 90 degree or B4 & B5 in 270 degree With all the Up-Tethers Fail Minimum depth = 175 m (-25 m respect original depth) Maximum depth = 338 m (+138 m respect original depth)

TDP Offset in Extreme Conditions Without Failure Minimum arc length = 2,874 m Maximum arc length = 3,665 m Difference = 791 m With all the Up-Tethers Fail Minimum depth = 2,854 m Maximum depth = 3,645 m Difference = 791 m Both, in 90 degree, in SCR S12 and S13

FPSO Results For Extreme condition Without Failure Maximum tether forces = 6,000 kN Maximum jumper forces = 1,200 kN With all the Up-Tethers Fail Maximum jumper forces = 3,900 kN

Time History for FPSO Results Maximum tether forces in tethers T3-12 in 90 degree

Maximum jumper forces in jumper J12 in 90 degree

BUOY Results Without Failure Maximum jumper forces = 500 kN Maximum SCR forces = 4,200 kN Maximum up-tether forces = 6,000 kN (vs. Tether Breaking Load = 11,000 kN) Maximum down-tether forces = 3,000 kN (vs. Tether Breaking Load = 5,000 kN) With all the Up-Tethers Fail Maximum jumper forces = 2,900 kN Maximum SCR forces = 4,200 kN

e. Time History for BUOY Results Maximum SCR forces in SCRs S12 in 90 degree

Maximum up-tether forces in T3-12 in 90 degree

Maximum down-tether forces in T2-12 in 90 degree

f. JUMPER Results Without Failure Maximum effective tension = 1,200 kN Minimum bending radius = 290 m With all the Up-Tethers Fail Maximum effective tension = 3,900 kN Minimum bending radius = 410 mg. SCR Results Without Failure Maximum tension = 4,200 kN Minimum bending radius = 131 m Maximum Von Mises Stress = 290.7 MPa (vs the Yield Strength X65 448 MPa) With all the Up-Tethers Fail Maximum tension = 4,200 kN Minimum bending radius = 76 m Maximum Von Mises Stress = 429.6 MPa (vs the Yield Strength X65 448 MPa)h. Time History for SCR Maximum Von Mises Stress in SCRs S12 in 90 degree

i. TETHER Results Down-Tether Breaking Load = 5,000 kN Up-Tether Breaking Load = 11,000 kN Without Failure Up-Tethers, Maximum Tension = 6,000 kN Down-Tethers, Maximum tension = 3,000 kN With all the Up-Tethers Fail Down-Tethers, Maximum tension = 2,800 kN

Clashing No clashing was gotten from the simulations done.

Fatigue Analysisj. Basic Scatter DiagramWave numberHeight Hs (m)Period Tz (s)Time %

W11.51140

W221350

W33178

W4724.52

Operational life = 25 years = 219,000 hours Basic Scatter diagram in hoursWave numberHeight Hs (m)Period Tz (s)Time %Hours (25 Years)

W11.5114087,600

W221350109,500

W3317817,520

W4724.524,380

k. Fatigue Life matrix The basic scatter diagram has been applied in 0, 45, 90, 135, and 180 degrees.

ScenarioDegreeWaves

F10W1, W2, W3, and W4

F245W1, W2, W3, and W4

F390W1, W2, W3, and W4

F4135W1, W2, W3, and W4

F5180W1, W2, W3, and W4

To consider the worst scenario, the fatigue life has been calculated considering each scenario separately. The parameters for the tether T-N curve are: K = 1000 m = 3 (conservative taking into account the manufacturer assume 5)

l. ResultsElementNameFatigue life (years)

JumperJi (i = 1, , 16)> 10,000,000 except hang-off point> 1,000,000 at hang-off point

SCRSi (i = 1, , 16)> 10,000 except hang-off and TDP> 1,000 in hanging (stiffeners are needed)> 100 in TDP (stiffeners are needed)

Down-TethersT1-i (i = 1, , 16)T2-i (i = 1, , 16)>10,000

Up-TethersT3-i (i = 1, , 16)> 9,000

m. Worst scenario The shortest fatigue life occurs in the F3 scenario, this is 90 degree, and in the following points. In the Jumpers J12 and J13 In the SCRs S12 and S13 in the TDP In the Down-Tethers T1-12, T1-13, T2-12, and T3-13 In the Up-Tethers T3-12 and T3-13

n. Graphs Jumper

SCR

Down-Tethers

Up-Tethers

Normal operationIn normal operating conditions the maximum FPSO offset in is significantly less than in extreme conditions at 135m, equivalent to 4.5% of the water depth (3000m).

FPSO Offset Time History The figure below shows the X-offset time history response of the FPSO in normal conditions with 0 current and wave direction:

The figure below shows the Y-offset time history response of the FPSO in normal conditions with 0 current and wave direction:

BUOY Offset in Normal Conditions

The figure below shows the buoy offset in normal conditions. The green boxes indicate the buoy with maximum offset at the angle of incoming current and wave forces (i.e. 0, 45, 90 etc.). The maximum offset is observed at 148m.

BUOY Depth Equilibrium depth = 200 m Minimum depth = 180 m (-20 m respect original depth) B1 & B2 in 0 degree and B8 & B9 in 180 degree Maximum depth = 240 m (+40 m respect original depth) B12 & B13 in 90 degree or B4 & B5 in 270 degreeTDP Offset in Extreme Conditions Minimum arc length = 3,064 m Maximum arc length = 3,174 m Difference = 110 m Both, in 90 degree, in SCR S12 and S13FPSO Results Maximum tether forces = 300 kN (vs. Tether Breaking Load = 5,000 kN) Maximum jumper forces = 1,000 kN

Time History for FPSO Results Maximum tether forces in tethers T3-1 in normal condition.

Maximum jumper forces in jumper J1normal condition

BUOY Results Maximum jumper forces = 500 kN Maximum SCR forces = 3,000 kN Maximum up-tether forces = 300 kN (vs. Tether Breaking Load = 5,000 kN) Maximum down-tether forces = 900 kN (vs. Tether Breaking Load = 5,000 kN)Time History for BUOY Results Maximum SCR forces in SCRs S1 in normal condition

Maximum up-tether forces in T3-1 in normal condition

Maximum down-tether forces in T1-1 in normal condition

JUMPER Results Maximum effective tension = 1,000 kN Minimum bending radius = 530 m

Time History for SCR Maximum Von Mises Stress in SCRs S1 in normal condition

3. Validation of resultsa. Result from COBRA riser paper Although the data from the paper are from 1,500 m water depth and now the water depth is 3,000 m, it is a good reference to compare results. The result from the paper are shown in the following tableConceptValueUnit

SCR maximum tension1,687kN

SCR Von Mises maximum stress413.6Mpa

Jumper maximum tension at FPSO1,315kN

Jumper maximum tension at Buoy412kN

Down-Tether maximum tension529kN

b. Summary of results in extreme conditionsConceptValueUnit





Down-Tether maximum tension3,000kN

c. Conclusion The comparison of both sets of results as shown in the following seems to be correlatives with the difference in the water depth.

ConceptValue fromPaper in1,500 mValue fromOrcaflex in3,000 mUnit

SCR maximum tension1,6874,200kN

SCR Von Mises maximum stress413.6290.7Mpa

Jumper maximum tension at FPSO1,3151,200kN



Strength Analysis/ValidationThe strength analysis was performed for considering the following states for ULS and ALS:- normal case which refers to most frequent wind, current and wave in 0o heading- extreme case with the worst condition which is wind, current and wave at 900 heading for 100-year and 10-year return periods

The results obtained are represented and compared with the results of a paper about COBRA riser which has some similarities which this concept

Normal Case Extreme Case ULS ULS ALS JumperMaximum effective tension (kN) Minimum bending radius (m) 1,000 530 1,200 3,900 290 410

Tether:FPSO to BuoyMaximum Tension (kN)

Buoy to SeabedMaximum Tension (kN)

300

1,000

6,000 -

2,200 1,400

SCRMaximum tension (kN)Minimum bending radius (m) Maximum wall tension (kN)Max. Von Misses Stress (MPa) 3,000 600 3,000 158 4,200 4,200 131 76 4,000 4,000 290.7 430

As presented in the table xxx above, the tethers connected from the buoy to the FPSO have a maximum tension of 2,300kN in the ULS. With a safety factor of 0.95 according to DNV-OS-E301 of the minimum breaking load of 4905kN, the maximum tension in the tethers is 0.49 of the characteristic strength of the tether. The SCR Von Mises utilization for extreme case ULS is 0.66 while for ALS is 0.98. This implies that for ALS the SCR doesnt exceed its specified minimum yield stress since the maximum along the SCR length is 430 MPa, which occurs at the sag bend of the SCR. The figure below shows the Von Mises Stress across the length of the SCR from the top end connection at the buoy to the touch down point for normal condition.

Von Mises stress along riser length From Top end to TDPResult from COBRA riser paper:Although the data from the paper are for 1,500 m water depth as opposed to our case of 3,000 m water depth, it is a good reference to compare results. The ALS result from the strength analyses as obtained in the paper for 100-year waves and 10-year currents extreme conditions are shown in the following table:

ConceptValueUnit





Down-Tether maximum tension535kN

Comparison of paper result and the current study

The Von Mises utilization from the paper is 0.8 while that of this work is 0.98. Although it is considerably higher, it is never the less within the characteristic strength of the SCR.As expected, the top tension of the SCR is higher considering the increased water depth i.e from 1500m in the paper to 3000m as in this study.

Further StudyWe have also investigated the effect of the failure of down-tethers considering single failure and failure of both tethers. The results showed that with a single tether failure the system stayed in position and with both failing the buoyancy module rose to the waters surface whilst the integrity of the system remained intact

3. Conclusion The comparison of both sets of results as shown in the following seems to be correlatives with the difference in the water depth.

ConceptValue fromPaper in1,500 mValue fromOrcaflex in3,000 mUnit

SCR maximum tension1,6874,200kN

SCR Von Mises maximum stress413.6290.7Mpa

Jumper maximum tension at FPSO1,3151,200kN



Under the expected normal operating conditions and environmental loads in Brazils Santos Basin the system has been shown to be stable and effective at vessel station-keeping with small FPSO and buoy excursions and no clashing.

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Risers For Mooring FPSO