- Email: [email protected]
Testing and structural integrity of flexible pipes
Testing and structural integrity of flexible pipes P. J. Cocks Dunlop Armaline Limited, Grimsby, UK
Tests developed to determine the properties and demonstrate the structural integrity of high pressure flexible pipes are described, and typical results are given. Keywords: flexible pipes, structural integrity, testing High pressure flexible pipes are increasingly being used in the offshore production of crude oil and gas. They are used as short, easy-to-install, flexible tie-ins between rigid steel pipes and manifolds, templates, etc.; as long flowlines in their own right; and as fully dynamic risers for fixed and floating production systems. When compared with the more traditional steel pipe, flexibles are not well understood by many operators or specifiers who, with few exceptions, have limited but increasing experience of their use. It is important, therefore, that the operating and installation requirements of the flexible pipe are clearly defined in order to enable materials to be selected correctly and the flexible pipe to be designed properly. High quality assurance at all stages is also vital. Structural integrity begins and ends with quality, with materials, design, manufacturing and installation all being vital components in ensuring that the product meets the operator's requirements. Once manufactured, testing of the product becomes vital in order to prove conclusively the properties of the pipe. This paper describes a comprehensive series of tests covering both materials and the finished product in order to demonstrate the long life structural integrity of the product.
Materials The materials of construction are vitally important and must exhibit a number of critical characteristics which will remain acceptable not only in a new pipe but also through to the end of the pipe design life. The materials must be resistant to the fluids being carried at the design temperatures and pressures. They must have low gas permeation properties and be resistant to the destructive nature of explosive decompression. Also vitally important, but often overlooked, the materials must remain flexible over the specified service period. Traditionally, nylon has been used as the pressurecontaining member in flexible pipes. However, others entering the market found that this had certain limitations at the higher temperature end of the operating scale, particularly in terms of heat ageing. PVDF is a material that performs well at the higher temperature but is expensive and relatively stiff. As a result of the limitations This paper was originally presented at a meeting on 'Flexible risers' held 9 January 1989at UniversityCollege London, UK. 0141-0296/89/040217-06/$03.00 q:) 1989 Butterworth & Co (Publishers) Ltd
of thermoplastic material, two companies entered the market with products based on bonded elastomer. In at least one case the elastomer was specially developed to have excellent resistance to sour oils and gases and good high temperature characteristics. In a similar way, flexible pipe suppliers continue to investigate all other materials used in the pipe structure. For the stainless steel interlock liner, there has been a move to 316L for the standard material instead of the previously used 304, and even more resistant materials such as duplex and 904 are frequently being used. For the strength-bearing member, flat carbon steel is used in the traditional unbonded pipe, whereas encapsulated high strength cables are used in the bonded type. The encapsulation gives additional corrosion protection to the reinforcement. The choice of outer cover material is also important; it must resist the external environment and withstand the wear and abrasion to which a pipe can be subjected, both during installation and service.
Product design Once selected, the chosen materials must be incorporated into a pipe structure that will withstand the often arduous demands made on it. Over the years two types of pipe structure have emerged. The initial entrant was an 'unbonded' pipe and latterly this has been joined by the 'bonded' pipe. Most people will by now be familiar with the two design concepts, but in simple terms the 'unbonded' pipe is constructed from a series of materials built one upon another, but not adhered together, whereas the 'bonded' type is built up in layers (but normally from different materials), until the pipe structure is complete. Then it is vulcanized such that the wall is completely bonded together. Both types have proved themselves capable of meeting the physical demands of subsea operations, although there are significant differences between the performances of the two types, the relevance of which must be judged against the operating requirements of each individual pipe application.
Testing For over 30 years experience has been gained in testing materials and flexible hoses and pipes for the offshore oil and gas industry. The background, resources and equip-
Eng. Struct. 1989, Vol. 1 1, October
217
Testing and structural integrity of flexible pipes. P.J. Cocks ment to carry out a wide range of tests to ensure product integrity are now available. Tests carried out on full pipes include: • hydrostatic burst tests • tensile tests • hydrostatic collapse tests • bending stiffness tests • in-plane and rotational fatigue tests • multi-load dynamic tests • permeation and decompression tests Many other tests such as fire tests are also routinely carried out but not described in this paper. Tests are conducted over a range of pressures and temperatures with liquids and gases. The tests are normally witnessed by a certifying authority or representatives of operating companies, many of whom have been, and continue to be, involved in sponsoring ongoing test work. The following sections briefly describe some of the above tests and illustrate typical results obtained.
employed between burst strength and recommended working pressure, although this has been reduced to 2:1 in some of the recently issued flexible pipe guidelines. Short pipes are normally tested in a straight configuration, with the larger flowlines tested on large diameter reels. The pipes are hydrostatically pressurized with test pressure verified by a dead weight tester. The pressure can be maintained for extended periods, and 24 hour proof tests at 1.5 times working pressure are a routine part of the normal quality process. Any length change of the pipe during the pressure test is measured using linear transducers attached to the pipe. Although elongations will vary with different pipe designs, currently all pipes (particularly the bonded type) are very stable, with elongations well below 0.5%. Table I lists burst pressures for typical bonded pipes. Pipes with higher burst pressure capability are also routinely supplied. The results of burst tests carried out across the full product range clearly demonstrate that the failure pressure can be accurately calculated, and that the pipes meet this basic criterion.
Hydrostatic proof and burst test This is a basic test aimed at demonstrating the design capability of the pipe and the integrity of the steel end fitting (see Figure 1). A safety factor of 3:1 is normally
Tensile test The second basic parameters that need to be established are the tensile capabilities of the pipe. Dunlop, for example, has in-house a tensile testing machine capable of applying loads up to 200 tonnes (see Figure 2). The machine will test 2-12 in. bore flexible pipes, either pressurized or unpressurized, at temperatures up to at least 130°C. The pipe can be loaded into the machine either straight or with each end fitting inclined at up to 15 ° from the horizontal. The tensile load is applied to the pipe specimen by means of a hydraulic cylinder and power pack unit. These are controlled by means of a remotely mounted electrical console. When subjected to tensile loading, bonded pipes fail by collapse of the stainless steel interlock tube. The inward collapse of the tube is termed 'snap through buckle' and is caused by the load-bearing armour cables creating a 'chinese fingers' effect and applying an inward radial force onto the interlock. Therefore by knowing the properties of the interlock and its dimensions and profile, it is possible to predict accurately the tensile load necessary to cause collapse. Table 2 details the test machine specification, while Table 3 lists some recent test results on bonded pipe. An interesting point to note from these tests is that when the interlock collapses in a pressurized pipe, pressure is still maintained within the bore of the pipe.
Table I Bore (in.) 2.5 3 4 6 8 Figure I
218
Hydrostatic proof and burst testing rig
Eng. Struct. 1989, Vol. 11, O c t o b e r
Burst pressures for typical bonded pipes Actual burst pressure (psi)
Calculated burst pressure (psi)
24 500 16 000 17 000 13 300 12 000
22 267 15 390 16 350 12 000 10 580
All failures occurred within the pipe structure
Testing and structural integrity of flexible pipes: P.J. Cocks Table 2
Specification of tensile test machine
Number of pipes per test Pipe length (ft) Pipe bore (in.) Fittings Maximum load (tonnes) Maximum stroke (in.) Cycling rate Waveforms available Pressure capability (psi) Pressurizing fluid Measurements taken
1 off 15 2-12 To suit rig 200 42 Variable Sine, triangular or square 50 000 Oil Pipe length/pressure/ temperature/load
Hydrostatic collapse test The interlock is also the means by which collapse under external hydrostatic load is resisted. In the hydrostatic collapse test (Figure 3), a pipe is mounted inside a pressure vessel with the pipe outside diameter sealed and the inside diameter open to atmosphere. The pressure vessel is filled with water and pressurized to levels equivalent to the required depth below sea level. Recent test results are given in Table 4. In this table, the designation 'flexsteel' indicates 316L stainless steel, interlocked flexible pipe. 'Duplex steel' signifies that the stainless steel interlock pipe was produced from Sanicro SAF2205 duplex steel. As would be expected, no benefit is obtained from the material change; it is only the profile and dimensions of the pipe that matter.
Bending stiffness tests Figure 2 Tensile testing rig
A fourth basic test is to establish the pipe bending stiffness. Two test methods have been employed to measure pipe bending stiffness: beam bend and centre lift. The beam bend test has been found to be the most reliable.
of
Beam bend method. The sample is mounted in low friction wheeled trolleys, and special level brackets are attached to the end fittings. A power tirfor draws the two ends together in a controlled manner. Recent test results are detailed in Table 5. Having applied a number of basic property tests and demonstrated an ability to calculate them accurately, one has done little to really prove the ability of the pipe to withstand combined loading over long periods, which is the reality of what the pipes will see in service. It is also necessary to determine the effect of rapid decompression and ageing on the pipe. Equipment for fatigue testing has
Figure 3 Hydrostatic collapse test rig Table 3 Tensile test results on bonded pipe Bore size (in.)
Actual tensile strength (tonnes)
Calculated tensile strength (tonnes)
Temperature
Pressure
(oc)
(psi)
4 6 8 8 8 11.5
55 79 89 93 95 146
55.0 57.0 81.0 81.0 81.0 130.8
120 110 Amb. Amb. 110 Amb.
5 0130 4 000 Amb. Amb. 3 000 Amb.
Eng. Struct. 1 9 8 9 , Vol. 11, O c t o b e r
219
Testing and structural integrity of flexible pipes: P.J. Cocks Table 4
Hydrostatic collapse test results
Bore
Sample description
Flex steel strip size
(in.)
(mm)
6 7~ 8 8 8 8 8 11.5
Flexsteel Flexsteel Flexsteel Duplex Steel Flexsteel Flexsteel Flexsteel Flexsteel
40 40 40 40 55 55 72 55
x x x x x x x x
Calculated collapse pressure (psi)
Actual collapse pressure (psi)
896 486 438 438 872 1244 2159 360
920 640 630 590 985 1195 1950 390
1.0 1.2 1.2 1.2 1.4 1.6 2.0 1.4
Figure 4 Table 5
Size
8 in. 3 in. 3 in. 8in. 8in. 8 in. 8 in. 8 in.
x 8.3 x 3.25 x 3.25 x7.3 x7.3 x 7.3 x 7.3 x 7.3
m m m m m m m m
Pressure (psi)
Bending radius (m)
Bending stiffness (kN m 2)
0 0 5 000 0 500 1 000 1 500 2 000
1.89 0.80 0.84 1.78 1.78 1.78 1.78 1.78
60.9 37.5 79.3 87.3 141.5 209.4 243.4 276.8
recently been developed which allows tension, torsion and bending loads to be simultaneously applied to the pipe for the required number of cycles. More details of these tests are given below.
Flex fatigue test The test machine comprises a steel framework which supports two oscillating arms. The 3.6 m long pipes to be tested are attached to the arms and to the fixed framework. The arms are oscillated at 20 cycles per minute, causing the long hoses to flex in the vertical plane to their minimum bending radius. The pipe on test can be pressurized or unpressurized as required. The flexing pattern is intricate. The centre of the pipe is repeatedly stressed to minimum bend radius. Tension is repeatedly applied to the upper end fitting and stress, normal to the end fitting axis, is applied to the lower end fitting. The initial mode of failure for bonded pipes was fretting corrosion of the interlock tube, compared with reinforcement fatigue and wear in the unbonded type. Through careful design of the tube geometry, fretting corrosion can be minimized and product life greatly extended. Similarly, the addition of lubricant can increase the life of the unbonded product. With the bonded pipe, it should be noted that cable strength members are completely encapsulated in elastomeric materials and do no fret or chafe against each other. They therefore remain virtually unaffected by this fatigue test.
Rotational fatigue test Rotation tests are performed on pipes positioned in an
220
Rotational fatigue test rig
Bending stiffness test results
Eng. Struct. 1989, Vol. 11, October
arc where the pipe is rotated about its axis, such that compression and tension occurs sequentially in the pipe structure (see Figure 4). The pipe can be pressurized internally at up to 10 000 psi and the rotation rate is fixed at 10 revs per minute. This is a most severe test, and 7 000 000 cycles (without failure) have been achieved with bonded pipes, with 5 000 psi internal pressure. The all-bonded flexible structure allows this test to proceed without heat being generated through internal friction of the plies, whereas external cooling is needed for the unbonded type. The test demonstrates that a pipe which is inherently flexible will have a long life expectancy under severely dynamic conditions.
Dynamic test regime The most comprehensive fatigue test, however, is the simulation of the actual riser motion and loads. Comprehensive riser analysis forms the basis on which to plan the dynamic pipe test regime. Each test is designed to verify that high pressure flexible pipes are able to withstand static and dynamic forces imposed during installation and operation. Generally, the test stress or pressure employed is three times the actual predicted working condition.
Dynamic pipe test. The dynamic test rig for flexible pipes (Figure 5), simultaneously or singularly, applies bending, torsional, axial and internal pressure stresses. The functional specifications for the rig are given in Table 6. Pipes with a nominal bore range of 2-36 in. and nominal lengths of 20-40 ft may be tested. Dynamic pipe test procedure. The dynamic test rig is versatile, and realistic test programmes can be compiled to evaluate individual riser systems. A typical programme for an 8 in. riser located in the North Sea for operation of 3 000 psi was as follows. Prior to fitting into the dynamic test rig, the pipe was proof pressure tested for 16 hours at 4 500 psi and checked for leakage or pipe distortion. Regime 1 Applied cyclical tensile load Cyclical rate Applied torsion Internal pressure
6-10 tonnes 6 per minute + 6° 1 000 psi
Testing and structural integrity of flexible pipes: P.J. Cocks
Dynamic test rig
Figure 5
Table 6
Functional specifications for dynamic pipe test rig
Axial load
Load magnitude (tons) Application frequency (cycles/min) Displacement magnitude (ft)
+ 60 O-6 0-6 (nominally + 3ft
about static mean) End bending moment
Load magnitude (ft tons) + 45 Application frequency (cycles/min) 0-6 Displacement magnitude (°) 0-50 (NB: Either end of the rig may be operated independently) Rotation
Application frequency (cycles/min)
PAPA apparatus
0-12
Figure 6
+ 45 0-6 0-360
Pipe ageing and permeating apparatus (PAPA)
Torsion
Load magnitude (ft tons) Application frequency (cycles/min) Displacement magnitude (ft) Pressurization
Vacuum (in, Hg) Pressure (Ibs f/in 2)
0-28 0-15 000 (gauge)
The pipe was maintained at 6 tonnes tensile load minimum, but at a rate of 6 cycles per minute the tensile load was increased to 10 tonnes. Simultaneously 6 ° torsion was applied alternately clockwise and anti-clockwise. Maximum tensile load and maximum torsion were coincident. The pipe was subjected to 40000 cycles without failure or movement of the end fitting. Proof pressure testing for 30 minutes at 4 500 psi gave satisfactory results. The pipe was then subjected to a second series of tests.
Regime 2 Applied Cyclical Applied Internal
cyclical tensile load rate torsion pressure
4-25 tonnes 3 per minute + 7° 1 000 psi
The pipe was subjected to 40 000 cycles as above without failure. Proof pressure testing for 16 hours at 4 500 psi gave satisfactory results.
•
This test regime is designed to determine the effects of ageing and particularly its effect on gas permeation, resistance to rapid decompression and, by re-testing on the previously described rigs, any other changes in pipe properties as a result of the material ageing. The pipe to be tested is filled with the test fluid and then mounted in an oscillating frame (see Figure 6). The oscillating frame ensures that gases and liquids remain mixed while under test. Typically a pipe would be filled with crude oil from a known source, water, carbon dioxide, hydrogen sulphide and nitrogen. The whole assembly is then immersed into a controlled temperature water bath in order to simulate external environmental temperatures and the internal temperature and pressure raised to the test temperature and pressure. Pipes are then normally left for a period of several weeks in order to allow the pipe to become saturated with oil and gas prior to starting any specific tests. Permeation rates can be measured during this initial period and throughout the test. Once conditions have stabilized, the pipes are normally subjected to repeated decompressions at rates of up to .- 1 000 psi per minute. Typically these would be done at the rate of one per week, but higher rates are possible. Both bonded and unbonded pipes have been tested in this rig, with the longest duration test being 27 months on a bonded pipe. In this case, the test temperature was
Eng. Struct. 1989, Vol. 11, October
221
Testing and structural integrity of flexible pipes. P.J. Cocks 105°C. Observations during and after the test showed that the pipe permeation rates decreased to a very low level after 4 to 6 weeks and remained at that level for the remainder of the test. The pipe withstood approximately 200 decompressions without adverse effects and, most importantly, the pipe remained flexible at the end of the test. No corrosive attack on any of the metal components was detected. The PAPA test rig has confirmed many of the factors observed when testing base materials and confirms the importance of correct material selection and design if the product is to have long life.
Conclusions By correct material selection and good design, flexible pipes can be produced to meet the demands of the offshore oil and gas industries. These pipes must be compre-
222
Eng. Struct. 1989, Vol. 11, October
hensively tested, in as near realistic conditions as possible, in order to demonstrate their capabilities prior to being put into service. The leading manufacturers have developed complex and comprehensive test programmes and equipment in order to prove the structural integrity of their products, both as built and after prolonged exposure to operating conditions, The confidence resulting from these test regimes is now being borne out by many successful applications of high pressure flexible pipes. However, it remains vitally important that operators should fully specify the maximum conditions to which the pipes are expected to be exposed. Installations must be carried out correctly and, finally, the operating condition must remain within the specified condition. By these means flexible pipes will provide an increasingly utilized tool to enable safe and economic extraction of oil and gas from beneath the seabed.
Tests developed to determine the properties and demonstrate the structural integrity of high pressure flexible pipes are described, and typical results are given. Keywords: flexible pipes, structural integrity, testing High pressure flexible pipes are increasingly being used in the offshore production of crude oil and gas. They are used as short, easy-to-install, flexible tie-ins between rigid steel pipes and manifolds, templates, etc.; as long flowlines in their own right; and as fully dynamic risers for fixed and floating production systems. When compared with the more traditional steel pipe, flexibles are not well understood by many operators or specifiers who, with few exceptions, have limited but increasing experience of their use. It is important, therefore, that the operating and installation requirements of the flexible pipe are clearly defined in order to enable materials to be selected correctly and the flexible pipe to be designed properly. High quality assurance at all stages is also vital. Structural integrity begins and ends with quality, with materials, design, manufacturing and installation all being vital components in ensuring that the product meets the operator's requirements. Once manufactured, testing of the product becomes vital in order to prove conclusively the properties of the pipe. This paper describes a comprehensive series of tests covering both materials and the finished product in order to demonstrate the long life structural integrity of the product.
Materials The materials of construction are vitally important and must exhibit a number of critical characteristics which will remain acceptable not only in a new pipe but also through to the end of the pipe design life. The materials must be resistant to the fluids being carried at the design temperatures and pressures. They must have low gas permeation properties and be resistant to the destructive nature of explosive decompression. Also vitally important, but often overlooked, the materials must remain flexible over the specified service period. Traditionally, nylon has been used as the pressurecontaining member in flexible pipes. However, others entering the market found that this had certain limitations at the higher temperature end of the operating scale, particularly in terms of heat ageing. PVDF is a material that performs well at the higher temperature but is expensive and relatively stiff. As a result of the limitations This paper was originally presented at a meeting on 'Flexible risers' held 9 January 1989at UniversityCollege London, UK. 0141-0296/89/040217-06/$03.00 q:) 1989 Butterworth & Co (Publishers) Ltd
of thermoplastic material, two companies entered the market with products based on bonded elastomer. In at least one case the elastomer was specially developed to have excellent resistance to sour oils and gases and good high temperature characteristics. In a similar way, flexible pipe suppliers continue to investigate all other materials used in the pipe structure. For the stainless steel interlock liner, there has been a move to 316L for the standard material instead of the previously used 304, and even more resistant materials such as duplex and 904 are frequently being used. For the strength-bearing member, flat carbon steel is used in the traditional unbonded pipe, whereas encapsulated high strength cables are used in the bonded type. The encapsulation gives additional corrosion protection to the reinforcement. The choice of outer cover material is also important; it must resist the external environment and withstand the wear and abrasion to which a pipe can be subjected, both during installation and service.
Product design Once selected, the chosen materials must be incorporated into a pipe structure that will withstand the often arduous demands made on it. Over the years two types of pipe structure have emerged. The initial entrant was an 'unbonded' pipe and latterly this has been joined by the 'bonded' pipe. Most people will by now be familiar with the two design concepts, but in simple terms the 'unbonded' pipe is constructed from a series of materials built one upon another, but not adhered together, whereas the 'bonded' type is built up in layers (but normally from different materials), until the pipe structure is complete. Then it is vulcanized such that the wall is completely bonded together. Both types have proved themselves capable of meeting the physical demands of subsea operations, although there are significant differences between the performances of the two types, the relevance of which must be judged against the operating requirements of each individual pipe application.
Testing For over 30 years experience has been gained in testing materials and flexible hoses and pipes for the offshore oil and gas industry. The background, resources and equip-
Eng. Struct. 1989, Vol. 1 1, October
217
Testing and structural integrity of flexible pipes. P.J. Cocks ment to carry out a wide range of tests to ensure product integrity are now available. Tests carried out on full pipes include: • hydrostatic burst tests • tensile tests • hydrostatic collapse tests • bending stiffness tests • in-plane and rotational fatigue tests • multi-load dynamic tests • permeation and decompression tests Many other tests such as fire tests are also routinely carried out but not described in this paper. Tests are conducted over a range of pressures and temperatures with liquids and gases. The tests are normally witnessed by a certifying authority or representatives of operating companies, many of whom have been, and continue to be, involved in sponsoring ongoing test work. The following sections briefly describe some of the above tests and illustrate typical results obtained.
employed between burst strength and recommended working pressure, although this has been reduced to 2:1 in some of the recently issued flexible pipe guidelines. Short pipes are normally tested in a straight configuration, with the larger flowlines tested on large diameter reels. The pipes are hydrostatically pressurized with test pressure verified by a dead weight tester. The pressure can be maintained for extended periods, and 24 hour proof tests at 1.5 times working pressure are a routine part of the normal quality process. Any length change of the pipe during the pressure test is measured using linear transducers attached to the pipe. Although elongations will vary with different pipe designs, currently all pipes (particularly the bonded type) are very stable, with elongations well below 0.5%. Table I lists burst pressures for typical bonded pipes. Pipes with higher burst pressure capability are also routinely supplied. The results of burst tests carried out across the full product range clearly demonstrate that the failure pressure can be accurately calculated, and that the pipes meet this basic criterion.
Hydrostatic proof and burst test This is a basic test aimed at demonstrating the design capability of the pipe and the integrity of the steel end fitting (see Figure 1). A safety factor of 3:1 is normally
Tensile test The second basic parameters that need to be established are the tensile capabilities of the pipe. Dunlop, for example, has in-house a tensile testing machine capable of applying loads up to 200 tonnes (see Figure 2). The machine will test 2-12 in. bore flexible pipes, either pressurized or unpressurized, at temperatures up to at least 130°C. The pipe can be loaded into the machine either straight or with each end fitting inclined at up to 15 ° from the horizontal. The tensile load is applied to the pipe specimen by means of a hydraulic cylinder and power pack unit. These are controlled by means of a remotely mounted electrical console. When subjected to tensile loading, bonded pipes fail by collapse of the stainless steel interlock tube. The inward collapse of the tube is termed 'snap through buckle' and is caused by the load-bearing armour cables creating a 'chinese fingers' effect and applying an inward radial force onto the interlock. Therefore by knowing the properties of the interlock and its dimensions and profile, it is possible to predict accurately the tensile load necessary to cause collapse. Table 2 details the test machine specification, while Table 3 lists some recent test results on bonded pipe. An interesting point to note from these tests is that when the interlock collapses in a pressurized pipe, pressure is still maintained within the bore of the pipe.
Table I Bore (in.) 2.5 3 4 6 8 Figure I
218
Hydrostatic proof and burst testing rig
Eng. Struct. 1989, Vol. 11, O c t o b e r
Burst pressures for typical bonded pipes Actual burst pressure (psi)
Calculated burst pressure (psi)
24 500 16 000 17 000 13 300 12 000
22 267 15 390 16 350 12 000 10 580
All failures occurred within the pipe structure
Testing and structural integrity of flexible pipes: P.J. Cocks Table 2
Specification of tensile test machine
Number of pipes per test Pipe length (ft) Pipe bore (in.) Fittings Maximum load (tonnes) Maximum stroke (in.) Cycling rate Waveforms available Pressure capability (psi) Pressurizing fluid Measurements taken
1 off 15 2-12 To suit rig 200 42 Variable Sine, triangular or square 50 000 Oil Pipe length/pressure/ temperature/load
Hydrostatic collapse test The interlock is also the means by which collapse under external hydrostatic load is resisted. In the hydrostatic collapse test (Figure 3), a pipe is mounted inside a pressure vessel with the pipe outside diameter sealed and the inside diameter open to atmosphere. The pressure vessel is filled with water and pressurized to levels equivalent to the required depth below sea level. Recent test results are given in Table 4. In this table, the designation 'flexsteel' indicates 316L stainless steel, interlocked flexible pipe. 'Duplex steel' signifies that the stainless steel interlock pipe was produced from Sanicro SAF2205 duplex steel. As would be expected, no benefit is obtained from the material change; it is only the profile and dimensions of the pipe that matter.
Bending stiffness tests Figure 2 Tensile testing rig
A fourth basic test is to establish the pipe bending stiffness. Two test methods have been employed to measure pipe bending stiffness: beam bend and centre lift. The beam bend test has been found to be the most reliable.
of
Beam bend method. The sample is mounted in low friction wheeled trolleys, and special level brackets are attached to the end fittings. A power tirfor draws the two ends together in a controlled manner. Recent test results are detailed in Table 5. Having applied a number of basic property tests and demonstrated an ability to calculate them accurately, one has done little to really prove the ability of the pipe to withstand combined loading over long periods, which is the reality of what the pipes will see in service. It is also necessary to determine the effect of rapid decompression and ageing on the pipe. Equipment for fatigue testing has
Figure 3 Hydrostatic collapse test rig Table 3 Tensile test results on bonded pipe Bore size (in.)
Actual tensile strength (tonnes)
Calculated tensile strength (tonnes)
Temperature
Pressure
(oc)
(psi)
4 6 8 8 8 11.5
55 79 89 93 95 146
55.0 57.0 81.0 81.0 81.0 130.8
120 110 Amb. Amb. 110 Amb.
5 0130 4 000 Amb. Amb. 3 000 Amb.
Eng. Struct. 1 9 8 9 , Vol. 11, O c t o b e r
219
Testing and structural integrity of flexible pipes: P.J. Cocks Table 4
Hydrostatic collapse test results
Bore
Sample description
Flex steel strip size
(in.)
(mm)
6 7~ 8 8 8 8 8 11.5
Flexsteel Flexsteel Flexsteel Duplex Steel Flexsteel Flexsteel Flexsteel Flexsteel
40 40 40 40 55 55 72 55
x x x x x x x x
Calculated collapse pressure (psi)
Actual collapse pressure (psi)
896 486 438 438 872 1244 2159 360
920 640 630 590 985 1195 1950 390
1.0 1.2 1.2 1.2 1.4 1.6 2.0 1.4
Figure 4 Table 5
Size
8 in. 3 in. 3 in. 8in. 8in. 8 in. 8 in. 8 in.
x 8.3 x 3.25 x 3.25 x7.3 x7.3 x 7.3 x 7.3 x 7.3
m m m m m m m m
Pressure (psi)
Bending radius (m)
Bending stiffness (kN m 2)
0 0 5 000 0 500 1 000 1 500 2 000
1.89 0.80 0.84 1.78 1.78 1.78 1.78 1.78
60.9 37.5 79.3 87.3 141.5 209.4 243.4 276.8
recently been developed which allows tension, torsion and bending loads to be simultaneously applied to the pipe for the required number of cycles. More details of these tests are given below.
Flex fatigue test The test machine comprises a steel framework which supports two oscillating arms. The 3.6 m long pipes to be tested are attached to the arms and to the fixed framework. The arms are oscillated at 20 cycles per minute, causing the long hoses to flex in the vertical plane to their minimum bending radius. The pipe on test can be pressurized or unpressurized as required. The flexing pattern is intricate. The centre of the pipe is repeatedly stressed to minimum bend radius. Tension is repeatedly applied to the upper end fitting and stress, normal to the end fitting axis, is applied to the lower end fitting. The initial mode of failure for bonded pipes was fretting corrosion of the interlock tube, compared with reinforcement fatigue and wear in the unbonded type. Through careful design of the tube geometry, fretting corrosion can be minimized and product life greatly extended. Similarly, the addition of lubricant can increase the life of the unbonded product. With the bonded pipe, it should be noted that cable strength members are completely encapsulated in elastomeric materials and do no fret or chafe against each other. They therefore remain virtually unaffected by this fatigue test.
Rotational fatigue test Rotation tests are performed on pipes positioned in an
220
Rotational fatigue test rig
Bending stiffness test results
Eng. Struct. 1989, Vol. 11, October
arc where the pipe is rotated about its axis, such that compression and tension occurs sequentially in the pipe structure (see Figure 4). The pipe can be pressurized internally at up to 10 000 psi and the rotation rate is fixed at 10 revs per minute. This is a most severe test, and 7 000 000 cycles (without failure) have been achieved with bonded pipes, with 5 000 psi internal pressure. The all-bonded flexible structure allows this test to proceed without heat being generated through internal friction of the plies, whereas external cooling is needed for the unbonded type. The test demonstrates that a pipe which is inherently flexible will have a long life expectancy under severely dynamic conditions.
Dynamic test regime The most comprehensive fatigue test, however, is the simulation of the actual riser motion and loads. Comprehensive riser analysis forms the basis on which to plan the dynamic pipe test regime. Each test is designed to verify that high pressure flexible pipes are able to withstand static and dynamic forces imposed during installation and operation. Generally, the test stress or pressure employed is three times the actual predicted working condition.
Dynamic pipe test. The dynamic test rig for flexible pipes (Figure 5), simultaneously or singularly, applies bending, torsional, axial and internal pressure stresses. The functional specifications for the rig are given in Table 6. Pipes with a nominal bore range of 2-36 in. and nominal lengths of 20-40 ft may be tested. Dynamic pipe test procedure. The dynamic test rig is versatile, and realistic test programmes can be compiled to evaluate individual riser systems. A typical programme for an 8 in. riser located in the North Sea for operation of 3 000 psi was as follows. Prior to fitting into the dynamic test rig, the pipe was proof pressure tested for 16 hours at 4 500 psi and checked for leakage or pipe distortion. Regime 1 Applied cyclical tensile load Cyclical rate Applied torsion Internal pressure
6-10 tonnes 6 per minute + 6° 1 000 psi
Testing and structural integrity of flexible pipes: P.J. Cocks
Dynamic test rig
Figure 5
Table 6
Functional specifications for dynamic pipe test rig
Axial load
Load magnitude (tons) Application frequency (cycles/min) Displacement magnitude (ft)
+ 60 O-6 0-6 (nominally + 3ft
about static mean) End bending moment
Load magnitude (ft tons) + 45 Application frequency (cycles/min) 0-6 Displacement magnitude (°) 0-50 (NB: Either end of the rig may be operated independently) Rotation
Application frequency (cycles/min)
PAPA apparatus
0-12
Figure 6
+ 45 0-6 0-360
Pipe ageing and permeating apparatus (PAPA)
Torsion
Load magnitude (ft tons) Application frequency (cycles/min) Displacement magnitude (ft) Pressurization
Vacuum (in, Hg) Pressure (Ibs f/in 2)
0-28 0-15 000 (gauge)
The pipe was maintained at 6 tonnes tensile load minimum, but at a rate of 6 cycles per minute the tensile load was increased to 10 tonnes. Simultaneously 6 ° torsion was applied alternately clockwise and anti-clockwise. Maximum tensile load and maximum torsion were coincident. The pipe was subjected to 40000 cycles without failure or movement of the end fitting. Proof pressure testing for 30 minutes at 4 500 psi gave satisfactory results. The pipe was then subjected to a second series of tests.
Regime 2 Applied Cyclical Applied Internal
cyclical tensile load rate torsion pressure
4-25 tonnes 3 per minute + 7° 1 000 psi
The pipe was subjected to 40 000 cycles as above without failure. Proof pressure testing for 16 hours at 4 500 psi gave satisfactory results.
•
This test regime is designed to determine the effects of ageing and particularly its effect on gas permeation, resistance to rapid decompression and, by re-testing on the previously described rigs, any other changes in pipe properties as a result of the material ageing. The pipe to be tested is filled with the test fluid and then mounted in an oscillating frame (see Figure 6). The oscillating frame ensures that gases and liquids remain mixed while under test. Typically a pipe would be filled with crude oil from a known source, water, carbon dioxide, hydrogen sulphide and nitrogen. The whole assembly is then immersed into a controlled temperature water bath in order to simulate external environmental temperatures and the internal temperature and pressure raised to the test temperature and pressure. Pipes are then normally left for a period of several weeks in order to allow the pipe to become saturated with oil and gas prior to starting any specific tests. Permeation rates can be measured during this initial period and throughout the test. Once conditions have stabilized, the pipes are normally subjected to repeated decompressions at rates of up to .- 1 000 psi per minute. Typically these would be done at the rate of one per week, but higher rates are possible. Both bonded and unbonded pipes have been tested in this rig, with the longest duration test being 27 months on a bonded pipe. In this case, the test temperature was
Eng. Struct. 1989, Vol. 11, October
221
Testing and structural integrity of flexible pipes. P.J. Cocks 105°C. Observations during and after the test showed that the pipe permeation rates decreased to a very low level after 4 to 6 weeks and remained at that level for the remainder of the test. The pipe withstood approximately 200 decompressions without adverse effects and, most importantly, the pipe remained flexible at the end of the test. No corrosive attack on any of the metal components was detected. The PAPA test rig has confirmed many of the factors observed when testing base materials and confirms the importance of correct material selection and design if the product is to have long life.
Conclusions By correct material selection and good design, flexible pipes can be produced to meet the demands of the offshore oil and gas industries. These pipes must be compre-
222
Eng. Struct. 1989, Vol. 11, October
hensively tested, in as near realistic conditions as possible, in order to demonstrate their capabilities prior to being put into service. The leading manufacturers have developed complex and comprehensive test programmes and equipment in order to prove the structural integrity of their products, both as built and after prolonged exposure to operating conditions, The confidence resulting from these test regimes is now being borne out by many successful applications of high pressure flexible pipes. However, it remains vitally important that operators should fully specify the maximum conditions to which the pipes are expected to be exposed. Installations must be carried out correctly and, finally, the operating condition must remain within the specified condition. By these means flexible pipes will provide an increasingly utilized tool to enable safe and economic extraction of oil and gas from beneath the seabed.