otc-8605-ms.pdf

OTe 8605

Qualification of a Flexible Riser for Deepwater Dynamic Applications

N.-J. Rishej Nielsen, NKT Cables, Frank W. Grealish, MCS International, Tim O'Sullivan, MCS International.

Copyright 1998, Offshore Technology Conference

This paper was prepared for presentation at the 1998 Offshore Technology Conferenceheld in Houston, Texas, 4-7 May 1998.

This paper was selected for presentation by the OTC Program Committee following reviewof information contained in an abstract submitted by the author(s), Contents of the paper,as presented, have not been reviewed by the Offshore Technology Conference and aresubject to correction by the author(s), The material, as presented, does not necessarilyreflect any position of the Offshore Technology Conference or its officers. Electronicreproduction, distribution, or storege of any part of this paper for commercial purposeswithout the written consent of the Offshore Technology Conference is prohibited.Permission to reproduce in print is restricted to an abstract of not more than 300 words;illustrations may not be copied. The abstract must contain conspicuous acknowledgment ofwhere and by whom the paper was presented.

AbstractAs the offshore oil industry continues to move into deeperwaters, there is an increasing requirement for thedevelopment and qualification of technologies to enable thisexpansion into deep water. Most of these developments arecurrently based on semi-sub or tanker based floatingproduction systems, which use flexible risers for production,export and injection services. The continuing qualificationof flexible risers for higher pressures/temperatures, deeperwater depths and more severe load conditions is critical tothe future success of deep water field developments.

This paper describes the qualification of a lO-inchinternal diameter, 4000 psi flexible riser for dynamicapplications in the severe Northern North Sea environmentalconditions. Results from static and dynamic prototype testsare presented and compared with analytical results. Thetests performed on the riser include collapse, axial tension,burst and dynamic bending fatigue tests.

The philosophies and methodologies used in thequalification process are discussed and compared with therequirements and guidelines in the most recent standards forflexible pipe, namely API Spec 17J [I] and API RP 17B [2],

IntroductionIn recent years floating production systems (FPS/FPSO)have being increasingly used for the development ofoffshore oil and gas fields, in particular for small reservoirfields in shallow waters and fields in deep waters from300m water depth and greater. A key component of FPStechnology is the riser systems, which have beenpredominantly unbonded flexible risers. Flexible pipe isalso used widely for flowline systems, in particular wellheadand manifold tie-backS to the FPS systems.

To meet the increased demand for this project the flexiblepipe industry is expanding manufacturing capacity. In

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1996, NKT commenced production of flexible pipe fromtheir new factory in Kalundborg, Denmark. The NKT pipedesign is largely based on the designs developed andqualified by Furakawa [3, 4, 5], which uses the unique C-shaped wires for the pressure armour layer. Furakawa haspreviously performed extensive qualification tests on thispipe design, including both static and dynamic tests.

In 1997, NKT commenced a qualification programme toverify their pipe. The qualification programme is intendedto verify the pipe design, design methodologies andmanufacturing procedures. This paper describes the'qualification programme, in particular related to a highpressure 10-inch ID pipe for deep water severe environmentapplications.

Qualification Programme ObjectivesThe overall objective of conducting a qualificationprogramme for flexible risers is to verify and documentquality, reliability and safety of the pipe products to provethat they are suitable for use according to the specifieddesign requirements. To fully qualify a dynamic riser, dueattention must be given to the applied design philosophy,long term performance characteristics of the materials (inparticular polymers), fabrication means and methods, aswell as the conduction of relevant material and performancetests.

The present riser qualification programme is part of aType Approval Certification carried out by Bureau Veritasaccording to API Spec 17J [I], including verification ofcalculation' tools, review of design rules, investigation ofmaterial dossier and fabrication methods, acceptance ofQAJQC procedures, as well as undertaking performancetests to correlate with predictions. The riser test programmeconsists of:

- Hydrostatic burst pressure test- Hydrostatic collapse test- Axial tension test- Bending stiffness test- Crush strength test- Dynamic fatigue test (in-plane bending and rotary

bending testing)

These tests are performed in accordance with the guidelinesgiven in the draft API 17B, Second Edition [2].

2 N.-J. R. NIELSEN, F.W. GREALISH. T. O'SULLIVAN OTC 8605

Also, testing is performed to establish data to be used in fatigue prediction models, including:

- S-N curves of tensile armour wires - Performance characteristics of pressure armour wires (C-

profiles) - Wear characteristics of individual layers - Friction coefficients between the individual layers

The methodology for developing the in-plane dynamic fatigue test programme is largely based on the recommendations in the draft of API RP 17B, Second Edition [2] with the primary objective to verify the structural integrity and fatigue characteristics of the flexible pipe, including end fitting and bend stiffener, under simulated operational conditions. Figure 1 presents a flowchart showing the development of the test regime, from the initial pipe and riser design to the final test programme. This identifies the tools required at each stage in the process. The critical philosophies in developing the test programme into the Test Acceleration Tool and Test Programme Definition Tool are as follows; with these philosophies being largely supplementary to the guidelines given in the draft RP [2]:

1. The total number of test cycles to be used is two million, with these representing the two million cycles of highest loading from the 25 year design life (total number of cycles in the 25 years is approximately 100 million cycles).

2. Six load cases are used in the test programme, with Load Case 1 using one million cycles approximately and the remaining Load Case 2 to 6 using a fiuther one million cycles. The pipe is designed such that the stresses in Load Case 1 are below the Goodman Line in the Haigh diagram for the armour wire. Therefore, it is representative to use the one million cycles in Load Case 1 to simulate all load cycles which induce lower fatigue loading on the pipe, these being the vast majority of cycles that the pipe will experience. The load cycles simulated by Load Case 2 to 6 are therefore the only cycles which will induce fatigue damage in the armour wires.

3. A dynamic fatigue test may be used to qualify a particular design or to verify the fatigue analysis procedures. In the fust case, the actual in-service operational loads over the design life are simulated. No failure of the pipe should occur, as the required factor of safety on fatigue life is 10 and therefore the test programme should only use up to 10% of the total fatigue capacity of the pipe. In the second case the pipe

equal to 90% of the capacity and thereby qualify both the pipe design and fatigue analysis methodology. At the end of the two million cycles the pipe will be subjected to a hydrotest at 1.5 times design pressure to c o n f i the structural integrity of the pipe. After completion of the initial two million cycles the pipe may then be cycled to failure.

Riser Description Pipe Design. The main specification of the riser in the qualification programme is as follows:

Size - 10-inch ID (254 mm) Bore - Rough Design Pressure - 4,000 psi (27.6 MPa) Design Temperature - 95OC Water Depth - Minimum 5OOm Design Life - 25 years Application - Extreme environment

dynamic applications

The selected riser design comprises the main structural components: Carcass, internal pressure sheath, pressure armour, cross wound tensile armour and outer sheath, resulting in an outer pipe diameter of 357mm (14 inches). Details of the cross section design are presented in Table 1 and a schematic is presented in Fig. 2.

Riser System. One aspect of the strategy taken for the qualification programme is to perform the detailed design of a riser configuration for the selected application and use this as a basis for the dynamic test. The design of the riser system was undertaken by MCS International. Based on previous experience and an assessment of the relevant advantages and disadvantages of the potential riser configurations, a lazy wave configuration is selected for this application.

The riser design configuration is shown in Figure 3. The main advantages of the lazy wave configuration are ease of installation (critical to deep water applications) and the compliance in the riser system. Relatively large vessel offsets (up to %30% of the water depth) can be absorbed by this configuration, reducing the design requirements for the mooring system. The main concern with the lazy wave configuration is the stability of the riser in transverse currents, when large out-of-plane motions of the riser can be experienced. This increases the spacing required between adjacent risers. To ensure the transverse stability of the riser, its drag diameter to apparent weight ratio (DdAW) is reduced to 3.5 m2/tonne (empty pipe). A DdAW value approaching 3.0 will be relatively stable, while values above 5.0 would be subject to large motions.

is cycled to failure, soas to verify the fatigue analysis procedures (and also design and manufacturing Design Results. The load cases for the design are selected procedures). The philosophy taken for the PMI dynamic based on the guidelines given in the draft API RP 17B fatigue test is in between these two extremes. The fatigue Second Edition [2]. This standard was recently developed test programme is derived to induce damage in the pipe in a joint industry project led by MCS International. A

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OTC 8605 QUALIFICATION OF A FLEXIBLE RISER FOR DEEPWATER DYNAMIC APPLICATIONS 3

summary of the main results from the extreme load case analyses is presented in Table 2. The results in Table 2 are from regular wave (10-2/year probability) analyses using the MCS International 3D nonlinear finite element program Flexcom3D [7]. The range of vessel offsets considered for the design (* 1 1 Om) covers most expected FPS applications in this water depth (500m).

The next stage in the riser design is to perform a global fatigue analysis, which is based on use of regular wave load cases. The load cases are selected from the seastate scatter diagram for Northern North Sea conditions. Seven cases in both Near and Far load directions are identified and are presented in Table 3. The number of occurrences specified for each wave class is based on the 25 year design life. A summary of the results from the global fatig re analyses is presented in Table 4. Fatigue calculations b, sed on these global results, and using the methodology desc ibed later in this paper, give a fatigue life for the riser of 543 years. This meets the design requirement to achieve a factor of safety of 10 on the design life [l], i.e. a fatigue life greater than 250 years.

Static Qualification Tests The draft M I FU' 17B [2] lists a variety of static tests which may be carried out to verify the capability of flexible pipes. Burst, axial tension and hydrostatic collapse tests are classified in M I FU' 17B as standard prototype tests (Class I tests), i.e. tests which are most commonly used.

The static qualification programme undertaken by NKT uses Class I tests as a basis, supplemented by bending stiffness and crush strength tests. A brief description will be presented in the following of burst and axial tension performance tests which have been carried out on pipes with an internal diameter ranging from 2.5-inch to 10-inch.

These pipes are equipped with instrumentation to obtain as much information from the testing as practically possible. Normally, the following data are recorded:

- Internal pressure - Temperature of pipe - Elongation of pipe using two independent measuring

inethods - Change of pipe circumference - Rotation of free end of pipe sample - Strain in selected outer tensile armour wires - Strain in essential structural end fitting components

The qualification testing is carried out in two steps. First a hydrostatic pressure test at 1.5 times the design pressure is performed over a 24 hours period to document the integrity of pipe and end fitting structure. Thereafter, the burst or tension test is conducted, including proper conditioning of the pipe sample by pressure cycling between zero and the design pressure up to 20 times. The certifying authority is provided with predictions of the test results prior to conduction of the performance test. A comparison between predictions and obtained test results is presented in Table 5 showing good and consistent agreement. Following the testing the pipe samples are dissected.

Dynamic Qualification Tests General. Two dynamic fatigue prototype tests are being performed in this qualification programme. The main test is the in-plane fatigue test of the 10-inch ID riser which is described in detail in this paper. Furthermore, a rotary bending fatigue test is performed on a 6-inch ID riser. Summary details of this test are given in the following section.

Rotary Bending Fatigue Test. Rotary bending fatigue testing is a commonly used method for determining the fatigue life of flexible pipes [5,6]. In May 1997 a rotary bending fatigue test was commenced at NKT using a 9 m long 6-inch ID flexible riser pressurised to the design pressure of 276 bar (4,000 psi). Refer to Fig. 4 for a description of the test rig and to Table 1 for details of the pipe cross section. The objective of performing this fatigue test is to compare the dynamic behaviour of the 6-inch riser with test results from similar testing [5]. Furthermore, the test is suitable for calibration of fatigue prediction models owing to the relatively simple test condition where a constant pipe curvature can be used during the entire test period. Also, investigation concerning the integrity of the individual layers at end of testing, in particular the anti-wear layers, will be of unique value.

Methodology. The testing is focused on determining the behaviour of the pipe itself and is considered to represent the conditions of a flexible pipe in the riser sag bend area. The advantage of this test method as compared to in-plane bending test is the relative simple test set-up as well as the possibility of carrying out an accelerated test.

The test is performed by imposing the pipe a prescribed curvature and rotating it around the neutral axis of the pipe. For the present test a bending radius of 20 m was adopted. Consequently, each rotation subjects the pipe to bending loads around the entire circumference which ultimately may result in fatigue failure of the pipe, e.g. in the tensile annour wires. It is expected that the inner layer of the tensile annour wires will fail in fatigue first, as this layer experiences slightly larger cyclic stresses than the outer tensile layer.

Hydraulic oil was used as test fluid. The pipe was equipped with a pressure gauge and a temperature probe inside the pipe bore as well as on the outer sheath. The number of rotations were monitored using two independent methods, i.e. a meter based upon the induction principle and a mechanical counter. The temperature of the pipe is governed by the rotational speed, contact pressure and friction between the layers. The test was carried out whilst maintaining an internal pressure close to the design pressure and limiting the internal temperature to about 50C.

Results. Extrapolation of test results [5] indicated that the pipe should fail at approximately 1 million cycles. However, numerical analysis showed this to be a lower limit and that the pipe most probably would not fail until having been subjected to at least 3 million cycles, depending upon the interlayer contact pressure and friction. At 1.3 million cycles the pipe was subjected to a hydrostatic pressure test at 1.5 times the design pressure, i.e. 414 bar (6,000 psi), to document the pipe integrity. At about 3.3 million cycles the

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4 N.-J. R. NIELSEN, F.W. GREALISH, T. O'SULLIVAN OTC 8605

noise level and temperature of the pipe increased significantly indicating that failure had been initiated. No leakage was yet obtained at the time of writing. Once fatigue failure of the pipe is achieved, the pipe will be dissected and results presented.

In-plane Bending Fatigue. The in-plane bending fatigue test commenced in January 1998, with the objective of qualifying an 10-inch ID riser with a design pressure of 276 bar (4,000 psi) for dynamic applications at water depths of at least 500m. The prototype pipe will be subjected to two million cycles over a four month test period. The riser is fitted with a bend stiffener at one end fitting. Figure 5 shows the 10-inch ID riser sample in the fatigue test rig.

The qualification test programme has been established on the basis of load cases taken from a field development located in the Northern North Sea at 500m water depth comprising of a turret moored tanker based floating production system. The fatigue testing shall document that the riser performance correlates with the predictions corresponding to at least 25 years design life. The following sub-sections describe the test set-up, methodology and fatigue analysis results.

Test Set-Up. The dynamic prototype test is being performed at the PMI Industries facility in Cleveland, Ohio. A schematic of the rig is shown in Fig. 5. The pipe sample is f ~ e d at the base at an angle of up to 10" from the vertical. The top of the pipe sample is connected to an upper termination mounting assembly. The top assembly is supported by an A-frame. Hydraulic power and a solid state proportional control valve, driven by a function generator, produces a smooth sinusoidal motion of the A-frame structure, providing rotation of the riser about its base support. The A-frame carries a pair of suspended cylinders which are gimbal-mounted to the top end of the pipe sample. These tensioning cylinders are connected to two large accumulators that maintain near constant tension in the pipe. The main specifications for the test rig capacity are as follows:

Maximum Tension - 70 tomes Maximum Rotation - k25" Max. Internal Working Pres. - 345 bar (5,000 psi) Typical Cycling Rate - 10 cpm at k 5" rotation

Test Methodology. The methodology for the determination of the load case matrix for the dynamic prototype test is largely based on the global analyses referred to above. The selection of the load case matrix for the prototype test is made in such a way as to reflect the behaviour of the riser in expected service conditions. This means that the angles and tensions selected for the prototype test load case matrix should represent the angles and tensions which the riser would be expected to experience during the course of its life.

Table 6 presents the load case matrix for the prototype test in terms of cyclic angles and applied tensions. The constant tension of 70 tonnes represents the upper limit of

the test rig's capacity. While the maximum tension experienced by the riser from the global fatigue analyses is reasonably close to the capacity of the test rig, the tensions experienced by the riser in the lower sea states are close to 50% of the capacity. However, the maximum tension capacity of the rig is used for all test load cases to provide for a more stringent test programme. The cyclic angles presented in Table 6 are derived from the angles presented in Table 4 from the global fatigue analyses.

The maximum angle variation specified for this test is 15". This includes a static 5" initial offset thereby giving a cyclic rotation of - 10" to +20. Note that due to this limit on the maximum rig angle load cases 6 and 7 are effectively grouped together for the fatigue analyses.

To better represent the riser in actual service conditions and to provide a method for controlling the peak curvatures a bend stiffener is designed for the riser connection to the base of the test rig. The bend stiffener is designed based on the maximum tensions and angles the riser experiences when subjected to the environmental loading described in Table 3.

The prototype test is designed so that the pipe will incur 90% damage after completion of the full test regime. This level of damage is significant for a prototype test and is selected primarily to impose more stringent limits for the qualification programme. This test regime is primarily achieved by reducing the size of the bend stiffener, until pipe curvatures and wire stresses are obtained which result in the target damage in the armour wires being achieved.

Fatigue Analysis. The input data into the fatigue analyses for the armour wires of the riser consists of pipe curvatures, tensions and the pressure exerted on the tensile armour wires from the intermediate polymer liner due to the design internal fluid pressure of 27.6 MPa. The methodology employed to predict the stresses in the tensile armour wires of the pipe accounts for pipe bending, pipe tension and inter layer friction. The fatigue analysis procedure is based on the methodology presented by Fuku et al [5 ] . Fatigue test data for the armour wires of the test pipe are available as a S-N curve, from which to calculate cycles to failure as a function of stress. Once the stresses in the armour wires are calculated these data are used to calculate the respective number of cycles to failure of the wire. From this the percentage damage is obtained, using a Palmgren-Miner summation procedure.

To generate the required input for calculating the armour wire stresses a Flexcom3D model of the riser and bend stiffener in the test rig is used. For each load case presented in Table 6 a Flexcom3D analysis is carried out to obtain the peak curvatures in the pipe. Table 7 presents these curvatures along with the results from the fatigue analyses in terms of number of cycles to failure. In addition, the % damage is given as the number of cycles in 25 years (from the sea state discritisation) divided by the number of cycles to failure. Note the cumulative damage from all load cases is 89.75%.

The cumulative damage presented in Table 7 is obtained after several iterations on the bend stiffener design. By


altering the bend stiffener design the peak curvatures in the pipe increased accordingly. Therefore, the required armour wire damage is achieved with the load case matrix specified in Table 6 by iterating on the bend stiffener dimensions until the cumulative damage of all the load cases amounted to 90%.

Conclusions The following are the main conclusions from the qualification test programme described in this paper:

1. The qualification programme for the NKT pipe design is described, including both static and dynamic prototype tests. The programme is based on the guidelines given in the new draft API RP 17B and represents the most recent implementation of these guidelines.

2. The methodologies and philosophies used in the test programme are presented and are largely supplementary to the guidelines in the draft RP. The presented dynamic test philosophy will result in the qualification of both pipe design and fatigue analysis procedures.

The prototype tests presented in this paper will qualify the NKT pipe for deep water extreme environment dynamic riser applications. Results from the dynamic fatigue tests will be compared with results from the fatigue analysis described in this paper, and will be presented in a future paper.

Acknowledgements The authors would like to acknowledge the contributions to this paper of Henk Kastelein (Consultant). The authors also thank their respective companies for permission to publish this paper, namely NKT Cables and MCS International.

References 1. API Spec 175: "Specification for Unbonded Flexible Pipe",

First Edition, December 1996. 2. API RP 17B: "Recommended Practice for Flexible Pipe",

Draft Second Edition, Prepared in an MCS Led JIP, MCS Doc. No. 2-1 -4-029lRP01, Rev. 05, May, 1997.

3. Oliveira, J.G., Goto, Y. and Okamoto, T., 'Theoretical and Methodological Approaches to Flexible Pipe Design and Application", Offshore Technology Conference, Paper No. 502 1, Houston, May 1985.

4. Goto, Y., Okamoto, T., Araki, M. and Fuku, T.: "Analytical Study of the Mechanical Strength of Flexible Pipe", Journal of w s h o r e Mechanics and Arctic Engineering, Vol. 109, pp. 249-253, August 1987.

5. Fuku, T., Ishii, K., Tada, H., and Matsui, Y.: "Fatigue Properties and Analysis of Flexible Rise?', Offshore Technology Conference (May 1992), OTC 6876.

6. Out, J.M.M.: "On the Prediction of Endurance Strength of Flexible Pipe", Offshore Technology Conference (May 1989), OTC 6165.

7. Flexwm3D hogram Manuals: "3D Finite Element Time Domain Analysis Software Package", MCS International, Version 4.1, September 1997.


PA1 1 - 8 mm Thickness PA1 1 - 9 mm Thickness Pressure Armour Carbon Steel - 10 mm C-Profile Carbon Steel - 12 mm C-Profile

1st Tensile Armour Carbon Steel - 3 x 7.5 mm Carbon Steel - 5 x 12.5 mm

2nd Tensile Armour Carbon Steel - 3 x 7.5 mm Carbon Steel - 5 x 12.5 mm


Notes: 1) Predictions are based upon actual material properties. 2) Test ongoing at time of writing.

8 N.-J. R. NIELSEN. F.W. GREALISH. T. O'SULLIVAN OTC 8605

L Fbater Data

Bend St~f iner Deslgn Tool

- - -

Prehmary Stf iner Des~gn

I Fatigue

Analysis Data

Desrgn Tool

Bend St~ffener for Prototype

Flerrble Prpe Desrgn Tool

Prelm~nary P ~ p e Deslgn

Statrc Analysis

Statr Configuration Design

Prototype Confibwration .-

Design

Selectnn Data

Prototype Service Life Prediction

Definition Tool

Conditions

Final Pipe Design

i i

I 4 Prototype

Test Functnnal Requuements

Test Programme

Fig. 1 Flowhart for Definition of Test Progtamme for Dynamic Prototype Test (21.


Fig. 2 - Built-up of 10-inch Flexible Pipe for Qualification Testing

Turret FPSO I

Mudline Dist.butd B~~~~ \\ Rr Ba

Section 1

I Flg. 3 Schedc of Lazy Wme Riser Configuration.


Fig. 4 - Schematic of Rotary Bending Test Rig

10-inch Qualification

L Fig. 5 - PMI's In-Plane Dynamic Fatigue Test Rig

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