omae 1051

Proceedings of OMAE’0120th International Conference on Offshore Mechanics and Arctic Engineering

June 3-8, 2001, Rio de Janeiro, Brazil

OMAE2001/OFT-1051

STEEL HYBRID RISER FOR WATER DEPTHS UP TO 3000 METERS

Alejandro AnduezaSercon Consulting Company

Segen F. EstefenCOPPE/Federal University of Rio de Janeiro

Renato Marques da SilvaASME member

Proceedings of OMAE’01 20th International Conference on Offshore Mechanics and Arctic Engineering

Rio de Janeiro, Brazil, June 3-8, 2001

OMAE2001/OFT-1051

ABSTRACTCurrently, the Brazilian oil company, Petrobras, has

around 17.3 billions barrels of oil reserves. More than half ofthem under more than 1000 meters of water depth. In addition,60% of the offshore exploratory concessions are also deeperthan 1000 meters and half of these concessions at more than2000 meters. Therefore, the most probably future Brazilian oildiscoveries should be located offshore in ultra-deep waters.Beyond 2000 meters of water depth flexible riser technology isso far not proved. Therefore, the new challenge is to developan ultra-deep water hybrid riser technology in order to allowthe completion over the present limits.

This paper shows the technical feasibility of a hybrid riserconfiguration for a typical deepwater field in Campos Basin,Offshore Brazil. The study was applied for two conditions ofwater depth, 1800 and 3000 meters. The results for bothconfigurations are presented in this paper. The riser isattached to a turret at bow of a FPSO (Floating ProductionStorage & Offloading) vessel. The results indicate anattractive alternative design concept because of the highcompliance and total elimination of TDP (Touch Down Point).

The analysis methodology uses the Finite Element Methodthrough computer codes MSC/Patran and Abaqus. Thehydrodynamic loads are computed through programmingusing PCL (Patran Command Language).

The strength of the pipe under bending, tension andexternal pressure is considered through the standard DNV OS-F101 [1], proposed specifically for the analysis of submarinepipelines.

IntroductionThe steel hybrid riser concept [2,3,4,5] is composed by a

SCR (Steel Catenary Riser) connected to a SSVR (Self-StandingVertical Riser) through a goose neck spool piece, as shown inFigure 1. The same typical configuration was used either for1800 and 3000 meters for different design parameters. Theconnection of the two rigid risers is provided by a typical flex-joint through a standard pull-in procedure. The SSVR requires asubsurface buoy at the top to keep its vertical stability. Inaddition, the SCR is attached at the base of a bow turret of atypical FPSO built on the hull of a former VLCC (Very LargeCrude Carrier). At the seabed the connection is modeled as ahinge, however, a taper joint or a flexjoint could be consideredbased on a further local analysis not included in this paper.

One of the main advantages of such configuration is that iteliminates entirely riser soil interaction, therefore, any TDP(Touch Down Point) design constraints. Another importantadvantage is that the concept is made completely of steel pipes,which gives more flexibility in terms of construction andprocurement.

The proposed configurations in this work were definedthrough the analysis of the results of preliminary studies ofseveral simulations, as indicated below.

System Specification for 1800 m of water depth:• WD (Water depth) = 1800 meters• Offset distance = 950m• Pipe Outer Diameter = 12.75 in• SCR Pipe wall thickness = 1.125 in• SSVR Pipe wall thickness = 1.312 in• Pipe material: API 5L X65• SCR length = 1700 meters• SSVR length = 800 meters

1 Copyright © 2001 by ASME

• Buoy upthrust = 1470 kN• Flex joint connecting SCR top and FPSO• Flex joint connecting SCR and SSVR• Operational offset: 10% of water depth• Accidental offset: 15% of water depth• VLCC ship with turret systemSystem Specification for 3000 m of water depth:• WD (Water depth) = 3000 meters• Offset distance = 1150m• Pipe Outer Diameter = 12.75 in• SCR Pipe wall thickness = 1.125 in• SSVR Pipe wall thickness = 1.312 in• Pipe material: API 5L X65• SCR length = 2370 meters• SSVR length = 1500 meters• Buoy upthrust = 3139 kN• Flex joint connecting SCR top and FPSO• Flex joint connecting SCR and SSVR• Operational offset: 10% of water depth• Accidental offset: 15% of water depth• VLCC ship with turret system

Meteorological DataTypical meteocean data [6] from Campos Basin were used

for the analysis. The critical loading cases were analyzedconsidering 10 years return period for the current profile and 100years return period for the waves. Table 1 shows the designvalues adopted. The current presented in Table 1 correspondsto the value at the sea surface.

Table 1: Current and Wave data

ReturnPeriod(years)

Wave Height (m)Current on

Surface (m/s)

10 ---------- 1.78

100 14.60 ----------

Compliance AnalysisSeveral loading cases simulating the motion of the FPSO

according to environmental conditions were run for bothconfigurations in order to verify the compliance of the system.The required top tension at the SCR top connection is alsodetermined. Table 2 presents a list of all the verified cases for1800 m of water depth. Table 3 presents the verified cases for awater depth of 3000m. Cases 1 and 1A analyze the neutral(nominal) configuration for both water depths. Cases 2, 3, 2Aand 3A analyze the near and far conditions for normal operation.The heave values presented in Tables 2 and 3 were computedbased on the maximum expected values for a return period of 100years, taking into account all motions of the FPSO and maximumtide amplitude. Finally, cases 4, 5, 4A and 5A analyze theaccidental condition that simulates the offset achieved when

lines of the mooring system break. It should be stressed that theheave motion is the most severe for typical steel catenary riserdesign and it could induce undesirable fatigue at the TDPcausing an increasing of the top connection angle. Theconfiguration studied in this paper overcomes these problems.

Table 2: Deviations from neutral configurationfor 1800 meters of water depth

Case Position Offset (m) Heave (m)

1 Neutral 0 0

2 Near +180 -20

3 Far -180 +20

4 Near* +270 0

5 Far* -270 0* Accidental operation

Table 3: Deviations from neutral configurationfor 3000 meters of water depth

Case Position Offset (m) Heave (m)

1A Neutral 0 0

2A Near +300 -20

3A Far -300 +20

4A Near* +450 0

5A Far* -450 0* Accidental operation

Response for Wave and CurrentThe riser behavior was analyzed based on wave and current

loads [7] specified in Table 1. The current velocity shown inTable 1 corresponds to the value at the sea surface. Theanalysis considered the current variation according to the waterdepth. The Cases 6 and 6A in Table 4 were considered the mostcritical conditions for normal operation, in spite of the stresseshave reached less than 60% of pipe yield stress. This analysissimulates the worst expected environmental condition with areturn period of 100 years.

Table 4: Deviations from neutral configuration andenvironmental loads for 1800m and 3000 m

Case Position Offset HeaveWave

+Current

6 Near +180 -20 Table 1

6A Near +300 -20 Table 1

Finite Element ProcedureThe analyses were performed using Abaqus commercial

package. Beam elements with cubic interpolation and pipe


sections have been employed. The analyses were non-linearstatic considering large displacements.

The first step of the riser analysis is to set up the system atthe neutral configuration, according to the self-weight. Theneutral configuration depends basically on the buoy upthrustthat is being used and the geometry specification. Figures 2 and3 present the graphs of the upthrust buoy force versus thevertical displacement of the top at the SSVR for bothconfigurations. These figures are used to specify the requiredbuoy force to maintain the system in a stable configuration.

After the model set up at the nominal configuration, all theverification cases proposed in Tables 2 and 3 could be easilyanalyzed imposing the corresponding boundary conditions.

ResultsThe first practical result of the riser analysis is the

specification of the required buoy upthrust to maintain stablethe system configuration. The specified buoy forces for theproposed systems are 1472 kN for 1800m of water depth and3139 kN for 3000m. The top tensions required at the top of theSCR are 1432 kN for 1800m and 2420 kN for 3000m.

Tables 5 and 6 present the angles at the riser ends for theneutral configuration and both far and near positions of theFPSO for extreme intact operational conditions for 1800m and3000m, respectively.

Table 5: Predicted angles for normal operationfor 1800m of water depth

Normal Operation

Position NeutralNear

+180 mFar

-180 mTop of SCR 7.5° 5.5º 10.1º

Bottom of SCR 58.8º 67.9º 47.3º

Top of SSVR 9.1° 6.6° 12.3°

Table 6: Predicted angles for normal operation for 3000m of water depth

Normal Operation

Position NeutralNear

+300 mFar

-300 mTop of SCR 5.7° 3.7º 8.2º

Bottom of SCR 62.3º 73.2º 47.4º

Top of SSVR 5.1° 3.2° 7.5°

Tables 7 and 8 present the maximum expected riser anglesconsidering FPSO far and near position for accidental operationfor both configurations.

Table 7: Angles for accidental operation for 1800m ofwater depth

Accidental OperationPosition Near

+270 mFar

-270 mTop of SCR 4.6° 11.6°

Bottom of SCR 70.7° 40.4°

Top of SSVR 5.5° 14.1°

Table 8: Angles for accidental operation for 3000m ofwater depth

Accidental OperationPosition Near

+450 mFar

-450 mTop of SCR 2.8° 9.8°


Top of SSVR 2.5° 8.9°

The maximum expected variations for the two flexjoints ofthe system is presented in Table 9 for 1800m and in Table 10 for3000m of water depth.

Table 9: Maximum variations of flex-joints angles for 1800m of water depth

OperationPositionNormal Accidental

Top of SCR+pitch max. 10.0° 13.2°

Top of SCR 4.6° 7.0°


Figures 4, 5 and 6 show the longitudinal stresses over theentire riser for intact operational conditions for cases 1, 2 and 3,respectively, corresponding to 1800m of water depth. Figures 9,10 and 11 show the same results for cases 1A, 2A and 3Acorresponding to 3000m of water depth. Figures 7 and 12 showthe longitudinal stresses for accidental condition, cases 4 and4A. Finally, Figures 8 and 13 show the longitudinal stresses fornormal operational condition, cases 6 and 6A, accounting forhydrodynamic loads.


Table 10: Maximum variations of flexjoint angles for 3000m of water depth

OperationPositionNormal Emergency

Top of SCR + max. pitch 9.9° 13.2°

Top of SCR 4.5° 7.0°


The maximum stresses and strains occur at the sag bendregion as expected. Tables 11 and 12 present the results for theseveral cases for normal and accidental operational conditions,respectively.

Table 11: Maximum longitudinal stress (MPa) andstrain for normal operation

Case Position Stress Strain (x10-3

)

1 Sag bend -158/173 -0.76/0.84

2 Sag bend -225/236 -1.09/1.14

3 Sag bend -108/130 -0.52/0.63

6 Sag bend -243/252 -1.17/1.22

1A Sag bend -145/163 -0.70/0.79

2A Sag bend -233/244 -1.13/1.18

3A Sag bend -89/116 -0.43/0.56

6A Sag bend -234/245 -1.13/1.18

Table 12: Maximum longitudinal stress (MPa) andstrain for accidental condition

Case Position Stress Strain (x10-3

)

4 Sag bend -266/275 -1.29/1.33

5 Sag bend -90.3/115 -0.44/0.56

4A Sag bend -297/305 -1.44/-1.47

5A Sag bend -70/102 -0.34/0.49

Pipe Limit StrengthThe riser pipe is submitted to external pressure, axial

tension and bending moment simultaneously. The standardDNV OS-F101 proposed specifically for the design of submarinepipelines was used to account for these effects. The limitstrength curves for all the selected pipes are presented in Figure14. The equation (1) below represents the curves in Figure 14.

1

8.0

=

⋅

+

mSC

c

e

c

d

pp

γγγεε

ε

………………………..……(1)

Verification According DNV OS-F101The maximum predicted longitudinal compressive strain

was 0.129% for the accidental condition established in case 4 for1800 m of water depth. The result for 3000 m of water depth,according to the analysis of case 4A, is a compressive strain of0.144%. According to the curves in Figure 14, thesecompressive strains satisfy the requirements of maximumexternal pressure for both pipes.

The selected pipes had the wall thickness increased to thepresent values in order to avoid buckling propagation. Thiseffect was verified through Melosh equation (2) and alsothrough DNV standard procedures. A safety factor of 1.33 wasconsidered.

5.226

⋅

⋅⋅=D

tSyp p …………………………..……(2)

ConclusionsThe results presented in this work show the feasibility and

excellent behavior of hybrid riser configurations attached to aFPSO for the environmental conditions of Campos Basin, Brazil

The system presented compliance up to 15% of offset ofthe production unit with reasonable level of stresses.

The proposed riser is not sensitive to the heave movementof the production unit. The imposed variation of ±20 m for theFPSO vertical position produced a stress variation lower than ±3MPa. This result indicates a potential good performance inrelation to the riser fatigue strength.

The results obtained for the required top tension indicatethat the proposed configuration, using steel pipes, could beemployed for 3000 m of water depth. The specified top tensionof 3139 kN is close to the capacity of the present ships. In orderto overcome higher values for the top tension, it could beevaluated the possibility of using titanium as an alternativematerial.

Another important aspect for the design of a riser system isthe understanding of the influence of VIV on the fatigue life.Therefore, the evaluation of VIV effects on the fatigue life of theriser is recommended.

NOMENCLATUREOD – Outer diameterSCR – Steel Catenary RiserSSVR – Self-standing Vertical RiserTDP – Touch Down Point


PCL – Patran Command LanguageWD – Water depthVLCC – Very Large Crude Carrier10 Y – Ten years of return period100 Y – One hundred years of return periodSy – Yield strengthVIV – Vortex Induced Vibration

εd – Design compressive strain

εc – Collapse compressive strain

pp – Propagation pressure

pc – Collapse pressure

γε – Safety strain factor

γSC – Safety class resistance factor

γm – Safety material resistance factor

REFERENCES[1] DNV OS-F101, “Submarine Pipeline Systems”, 2000.[2] Quiniou, V., Pionetti, F.R., Hatton, S. (1999), “SHR’ewd – A

New Hybrid Riser for Deep and Ultra-Deep Waters”, DOT99.[3] Hibbert, M. (1999), “Deepwater Opportunities in Angola”,

Report of a SUT Evening Meetings, London, UK.[4] Legras, J.L., Traube, D. (1999), “New Concept of Export Line

for Deepwater Fields”, 99-IFR 22.[5] Andueza A. and Estefen S. F., (2000) “Configuration Study for

a Hybrid Riser for Ultradeep Waters”, Congress onShipbuilding, Construction and Maritime Transportation, Riode Janeiro.

[6] Petrobras Technical Specification, I-ET-3000.00-1000-941-PPC-001, “Metocean Data”.

[7] DNV, “Rules for the Design and Inspection of OffshoreStructures”, Appendix B, Loads, 1977.


Figure 1: Concept Configuration for the Steel Hybrid Riser


Top tension (N)

Buoy force (N)

Figure 2: Buoy/Top tension forces x Vertical displacement of vertical riser for 1800 m of water depth

Top tension (N)

Buoy force (N)

Figure 3: Buoy/Top tension forces x Vertical displacement of vertical riser for 3000 m of water depth


Figure 4: Nominal configuration (case 1), normal operation – 1800m

Figure 5: Configuration for near position (case 2), normal operation – 1800m


Figure 6: Configuration for far position (case 3), normal operation – 1800m

Figure 7: Configuration for near position (case 4), accidental condition – 1800m


Figure 8: Configuration for near position (case 6), normal operation with hydrodynamic loads – 1800m

Figure 9: Nominal configuration (case 1A), normal operation – 3000m


Figure 10: Configuration for near position (case 2A), normal operation – 3000m

Figure 11: Configuration for far position (case 3A), normal operation – 3000m


Figure 12: Configuration for near position (case 4A), accidental condition – 3000m

Figure 13: Configuration for near position (case 6A), normal operation with hydrodynamic loads – 3000m


Figure 14: Limit strength curves according to DNV OS-F101 for API X65 ( [pressure]=Mpa )


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