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Experimental study of marine pipelines on unstable and liquefied seabed
Coastal Engineering 50 (2003) 1 – 17 www.elsevier.com/locate/coastaleng
Experimental study of marine pipelines on unstable and liquefied seabed T.C. Teh a, A.C. Palmer a,*, J.S. Damgaard b a
Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, UK b HR Wallingford Ltd., Howbery Park, Wallingford, OX10 8BA, UK Received 9 August 2002; accepted 17 July 2003
Abstract Experimental investigation was carried out in a wave flume to study the stability of marine pipelines on mobile and liquefied seabed. A wide range of different pipeline specific gravities and wave condition were examined. The results showed that for the given soil the seabed response is governed mainly by liquefaction and the pipeline behaviour on unstable seabed is strongly dependent on its specific gravity. D 2003 Elsevier B.V. All rights reserved. Keywords: Pipeline; Liquefaction; Seabed instability; Waves
1. Introduction The objective of this research is to understand the behaviour of a marine pipeline on a seabed that is mobile and liquefied, and ultimately to devise a rational design method. It is motivated by questions about on bottom stability of marine pipeline design. The conventional process of stability design assumes that the seabed itself is stable and considers the incipient movement of a pipeline across a stable seabed. There is substantial field evidence that this model can be incomplete for erodible seabed under severe wave condition (Palmer, 1996; Damgaard and * Corresponding author. Tel.: +44-1223-332718; fax: +441223-339713. E-mail addresses: [email protected] (T.C. Teh), [email protected] (A.C. Palmer), [email protected] (J.S. Damgaard). 0378-3839/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-3839(03)00066-8
Palmer, 2001). Surveys of the Rankine gas pipeline before and after hurricanes Ilona and Orson plainly showed that the seabed had moved but the pipeline had not, and a study of the nearby Harriet pipeline confirmed that the seabed becomes unstable long before the extreme design conditions for the pipeline are reached. Those examples happen to be off the coast of Australia, but the conclusion is a general one. Failure of marine structures has been observed to be linked to wave-induced instability of soil deposits leading up to liquefaction (Lee and Focht, 1975; de Groot and Meijers, 1992; and others). Sumer et al. (1999) investigated the stability of pipelines on liquefied seabed. General seabed mobility and sediment transport has been studied extensively (see e.g. Fredsoe and Deigaard, 1992). An important parameter for seabed mobility is the Shields parameter h = s/[(qs q)gd50], where s is the bed shear stress, g is the gravitational acceler-
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T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
Fig. 1. Wave flume layout.
ation, qs and q are the sediment grains and water densities, and D50 is the median grain size of the sediment by mass. Sleath (1994, 1998), Sassa and Sekiguchi (1999), Sassa et al. (2001), and others have investigated the broader problem of mobility of the seabed. Sleath (1994) suggested a parameter S =qUox/(qs q)g (where Uo is the amplitude of the oscillatory flow just above the boundary layer
at the bed and x is wave frequency). S can be interpreted as a ratio between the horizontal pressure gradient force on an element of soil and its submerged unit weight. Under certain conditions, seabed mobilization is not limited to a thin mobile layer at surface but extends to a thick mobile layer, and Sleath showed that the thickness of the layer is strongly dependent on S.
Fig. 2. Pore pressure transducer set up.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
2. Experiment design 2.1. Experimental set up A series of experiments were carried out in a HR Wallingford wave flume (Fig. 1). The wave flume is approximately 50 m long, 1.2 m deep and 1 m wide. It is equipped with a piston wave generator at one end and a 1:10 slope flint beach at the other end to dissipate the incident wave energy and minimise the reflection. The wave generator is capable of producing a wide range of wave frequency and wave height. A silt test section with 1.6 m long, 1 m wide and 0.3 m thick seabed was built next to the flume side window, so that the behaviour of the pipe and the bed could be observed. A wooden false floor 1 m long and 1 m wide was placed level with the top of the bed at both ends of the test section in order to allow the waves to propagate on the false floor. Two wooden ramps with a slope of 1:5 were placed at the ends of the false floor, to ensure a smooth transition for the travelling waves from 0.75 m water depth to 0.45 m
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over the test section. The ramps fitted close to the side-walls of the flume and were impermeable. In order to have a fully saturated silt bed, silt was poured slowly into the water filled soil basin. So as to restore the condition of the silt bed to the same state before each test, the silt bed was flushed with high pressure water jet before a test and left to settle for 18 h. Soil samples were carefully taken from the top layer of silt bed before and after a test to determine the soil density using a thin hollow cylinder and two plates. The average silt density was 1946 kg/m3 before a test and 2008 kg/m3 after a test. The density of liquefied soil was between 1730 and 1860 kg/m3, which was determined from the behaviour of the floating pipe as a hydrometer. The water pressures were measured directly in the test section at five different soil depths by using 175 mBar Druck PDCR 810 pressure transducers. The pressure transducers were placed on a tripod stand. A filter cap with a 10-Am nylon filter was screwed to the stand to keep silt particles away from the transducer. The transducers were screwed to the stand
Fig. 3. Sketch of pipeline model set up.
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Table 1 Silt physical properties Property
Value
Units
Method of measurement
D10 D50 D60 D90 Uniformity coefficient, Cu Specific gravity, Gs Minimum void ratio, emin Maximum void ratio, emax Friction angle at critical state, /
10 33 47 120 4.7
Am Am Am Am –
AccuSizer AccuSizer AccuSizer AccuSizer –
2.71
–
0.39
–
1.18
–
BS 1377:1975 Clause 2.6.2 ASTM Standards: D4253 (04.08) ASTM Standards: D4254 (04.08) Triaxial test
36
j
780 780 780 780
machine machine machine machine
inside the water to ensure that there was no air trapped between the transducers and the filter caps. The stand was removed from the silt bed after every test to clean the filters and to re-fit the transducers to the stand. The stand was placed back onto the tank base through silt agitation with high-pressure water jet. The details of the transducers set up are shown in Fig. 2. In a real sea, the pipeline may be anchored at discrete intervals, but the intervals will typically be separated by relatively large distances so the middle section can move horizontally and vertically. It can also roll to a certain degree. The specific gravity of a real pipeline may range from 1.1 to 2. The pipeline was modelled by a length of PVC tube with sealed ends, suspended so that it could move freely horizontally and vertically but restrained against rotation in
plan, and free from the side-walls. The sketch of pipeline model set up is shown in Fig. 3. The pipeline model was 0.97 m long, and consisted of a 75 mm outer tube and a 25 mm inner tube. Different pipeline specific gravity from about 1.1 to 2.1 was achieved by slotting combinations of additional weights into the inner tube and outer tube. The specific gravity of the pipeline is defined as a ratio of the averaged density of the pipeline to water density. Both tubes were filled with water. Marks were made on the pipeline sidewall to follow rotations. The displacement was measured by using a video recorder through the flume window glass with gridlines. The profile of the wave was measured by wave gauge mounted vertically in the water above the test section. Before each test, the water level was adjusted to a required height. The water temperature was measured using a thermometer. It was between 7 and 9 jC for all tests. The velocities at different heights near to the bed were measured using an Acoustic Doppler Velocimeter (ADV). The transducer stand was placed about 0.4 m from test section wall and the pipeline was placed about 0.6 m from another test section wall. A wave gauge was placed above the transducer stand. The wave elevation, pore pressure and displacements of pipeline were measured during a test. 2.2. Soil properties Limestone silt was used as the seabed material. It is a very fine but non-cohesive material,
Fig. 4. Particle size distribution.
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Table 2 Experiment programmes Experiment
Test
Date
Wave height (m)
Soil depth (m)
Initial density (kg/m3)
Shield parameter
TA (Pipeline specific gravity 1.11)
01** 02 03 04 05*,^ 05a 05b 05c 05d 05e 05f 06** 07 07a 07b 07c 07d 07f 01 01a 01b 01c 01 01a 01b 01c 01*,^ 01a 02 02a 02b 02c 02d 02f 02g 02f 01 01a 01b 01 01a 01b 01 01a 01b 02 02a 02b 01 01a 01b
20/11/01 27/11/01 28/11/01 29/11/01 3/12/01
0.220 0.220 0.150 0.095 0.050 0.065 0.080 0.080 0.095 0.150 0.220 0.220 0.065 0.150 0.050 0.095 0.150 0.220 0.080 0.095 0.150 0.220 0.065 0.095 0.150 0.220 0.095 0.150 0.025 0.050 0.065 0.065 0.095 0.095 0.150 0.220 0.095 0.150 0.220 0.150 0.095 0.220 0.150 0.220 0.095 0.220 0.150 0.095 0.220 0.150 0.095
0.260 0.250 0.250 0.250 0.255
1947 1948 1950 1920 1919
0.240 0.285
2020 1963
0.280
1940
0.240
2000
0.270
1919
0.265
1919
0.283
1933
0.270
1955
0.270
1955
0.290
1928
0.300
1955
0.391 0.391 0.233 0.111 0.047 0.057 0.073 0.073 0.111 0.233 0.391 0.391 0.057 0.233 0.047 0.111 0.233 0.391 0.073 0.111 0.233 0.391 0.057 0.111 0.233 0.391 0.111 0.233 0.011 0.047 0.057 0.057 0.111 0.111 0.233 0.391 0.111 0.233 0.391 0.233 0.111 0.391 0.233 0.391 0.111 0.391 0.233 0.111 0.391 0.233 0.111 (continued
TB (Pipeline specific gravity 1.37)
TC (Pipeline specific gravity 1.23)
TD (Pipeline specific gravity 1.53)
TE (Pipeline specific gravity 2.08)
TF (Pipeline specific gravity 1.73)
TG (Pipeline specific gravity 1.86)
TH (Pipeline specific gravity 1.94)
5/12/01 7/12/01
6/12/01
4/12/01
26/11/01 30/11/01
9/1/02
10/1/02
11/1/02
18/1/02
15/1/02
Liquefaction Yes Yes Yes Yes No No Yes Yes Yes Partial Partial Partial Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Partial Partial Partial Partial Yes Yes No No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes on next page)
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Table 2 (continued) Experiment
Test
Date
Wave height (m)
Soil depth (m)
Initial density (kg/m3)
Shield parameter
Liquefaction
TH (Pipeline specific gravity 1.94)
02 02b 02c 01 01a 01b
17/1/02
0.150 0.220 0.095 0.220 0.150 0.095
0.300
1950
0.295
1985
0.233 0.391 0.111 0.391 0.233 0.111
Yes Yes Yes Yes Yes Yes
TI (Pipeline specific gravity 2.03)
16/1/02
a, b, c,. . . denotes test was continued from previous test. Partial liquefaction refers to liquefaction front advancement until an intermediate soil depth. * Denotes soil consolidation was 60 h; otherwise, soil consolidation is 18 h. ** Denotes test for unrestrained pipe; otherwise the test from TA to test TI are for pipe with two degrees of movement freedom. ^ Denotes that the pipe is initially buried; otherwise, the pipe is sitting on the bed.
which is essential for the pore pressure to build up. The physical silt properties is summarised in Table 1. Fig. 4 is the particle size distribution curve. 2.3. Test programmes The piston wave maker generated regular waves with a wave period of 1.25 s and with a range of wave heights from 0.05 to 0.22 m without breaking. The tests were mostly run for about 15 min and some for 30 min at sampling rate 0.05 s. Pipelines with 75 mm diameter and different specific gravity were tested on a fresh and loose silt bed. Most tests were run with the support frame that allows translation that prevents rotation in plan, but few tests had a completely free pipeline without any frame. The experiment programmes are summarised in Table 2.
3. Experimental results 3.1. Liquefied seabed response to progressive waves The critical Shields parameter hcr was calculated using the algebraic version of the Shields curve in Soulsby (1997). The value of hcr for all the tests is about 0.15. A summary of the test conditions and the liquefaction occurrences can be found in Table 2. For a fresh and loose seabed, liquefaction was observed for waves as low as 0.065 m (test number 7). This corresponds to a h value of 0.057 and a Sleath number of 0.018. In other words, the bed liquefied
well before initiation of motion. This is in conflict with the theoretical magnitude assessments put forward by Damgaard and Palmer (2001) and may indicate that the resistance to liquefaction is much smaller in a saturated, under-consolidated silt than in the soils from which the publicly available and widely used data on earthquake-induced liquefaction originate. In some of the dense bed tests the soil liquefied to an intermediate depth, just as Tzang (1998) observed in his silt bed and Sassa and Sekiguchi (1999) observed in their sand bed. Figs. 5, 6, 11 and 12 show the typical excess pore pressure measurement response to the regular waves in our experiment. The changes in pore pressure from hydrostatic pressure value measured at different soil depth, z, is the excess pore pressure. z is measured from the mud line, upwards positive. Figs. 6 –9 refer to a same test. The pressure near the bed agrees well with the linear wave theory. Fig. 5 shows a test without liquefaction. The loose silt bed was subjected to a wave height 0.05 m. No bed movement was observed. The pore pressure did not build up. The pore pressure oscillation within the non-liquefied bed was less than the pressure above the bed. The pore pressure oscillation was apparently the same within the non-liquefied bed. Fig. 6 shows a test where liquefaction occurred. The loose silt bed was subjected to a wave height 0.095 m. The pore pressure built up to the initial overburden pressure (rvo V = (qb q)gz) until the vertical effective stress became zero, where rvo V is the initial overburden pressure or the initial vertical effective stress and qb is the bulk density of soil. The
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Fig. 5. Excess pore pressure measurements in Test TA05.
development of build up pore pressure at different depth is more clearly seen in Fig. 7. The liquefaction depth advancing downward was observed, which is consistent with Sassa and Sekiguchi’s (1999) observation. At time = 35 s, a layer of 0.04 m thick was liquefied. The thickness of liquefied layer increased to 0.085 m at time = 40 s and to 0.185 m at time = 60 s. From visual observation, the whole bed moved, indicating the liquefied layer advanced downward until to the flume base. Fig. 7 shows the development of excess pore pressure at different soil depth. The built up excess pore pressures at depths z = 0.085 m and z = 0.185 m were the same from time 15 to 30 s as shown in Fig. 7 because the pore pressure build up rate was the same at these depths during this time as shown in Fig. 8. Fig. 8 also shows that the build up pore pressure rate started to become slower when
liquefaction front almost advanced downward to the depth z = 0.085 m. This explains why the build up pore pressure rate is different with time in Fig. 7. Fig. 7 shows that the increment of pore pressure was the same at three measured depths z = 0.04, 0.085 and 0.185 m from time 15 to 25 s but the increment of pore pressure at z = 0.085 and 0.185 m was greater than z = 0.04 m from time 25 to 30 s because liquefaction front almost reached the soil depth z = 0.04 m at about 30 s. At the following time, the rate of build up pore pressure was greater at deeper soil as the liquefaction front propagating downwards. The change in wave-induced oscillation of pore pressure within the bed is shown in Fig. 9. The pore pressure oscillation in the bed, P, is normalised with the steady state of near bed pressure, Po. The term ‘‘steady state’’ used here refers to a state
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Fig. 6. Excess pore pressure measurements in Test TA04.
where the value of a measured parameter becomes constant after many wave cycles. The soil depth, z, is normalised with the wave length, L. L is found
to be 2.10 m using linear wave theory. Fig. 9 shows that the oscillation of near bed pressure, P/ Po, at z/L = 0 m increased to unity at time = 25 s,
Fig. 7. Development of excess pore pressure in Test TA04.
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Fig. 8. Wave elevation and fluid pressure/excess pore pressure measurements in Test TA04.
corresponding to the increment of wave height as shown in Fig. 8. The soil bed had yet been liquefied from time 10 to 19 s as shown in Fig.
Fig. 9. Double amplitude of oscillatory of excess pore pressure with time in Test TA04.
7. The pore pressure oscillation during this time was less than the oscillation of near bed pressure and apparently the same at any measured depth (Fig. 9). The remarkable rate of pore pressure oscillation increment started from the top soil layer, and then followed by lower layer, until reaching a steady state value. This is consistent with the observation of liquefaction depth advancing downward. Pore pressure oscillation was observed to reach a maximum value when the build up pore pressure reached a maximum value. For example, the pore pressure oscillation was observed to reach a steady state at time = 35 s at z/L = 0.019 (Fig. 9) when liquefaction occurred at the same time, 35 s, at same soil depth, z = 0.04 m (Fig. 7). The steady state value within the liquefied soil at any depth was same and was slightly larger than the near bed pressure oscillation. This may be because the liquefied soil that acted like a dense fluid
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generated an additional pressure that was superimposed onto the pressure generated by the water above the liquefied soil. During liquefaction, the movement of upper silt layer was observed after a few wave cycles under critical wave condition. The upper layer moved forward and backward over the bed. The liquefied layer surface also oscillated vertically in a wavy motion. The mobile layer then gradually advanced progressively downward until the whole bed was in motion. It was observed that that soil element forward and backward motion decreased with depth. The top layer moved at a larger displacement compared to the bottom layer. Fig. 10 shows the oscillation of liquefied soil at the surface that is normalised with wave height and soil depth. The soil was stable for Sleath parameter S less than 0.018. The soil surface started to oscillate at S = 0.018. The liquefied soil oscillation normalised with soil depth, HLS/d, increased with S. The ratio of soil surface oscillation to wave oscillation, HLS/H, increased with S and then it decreased after a certain S value. Fig. 11 shows a test with longer test duration where liquefaction occurred. The loose silt bed was subjected to a wave height 0.08 m. We observed pore pressure dissipation started at z = 0.215 m during the progressive waves loading and after the wave cessation. No bed movement was observed at this depth during the pore pressure dissipation, indicating that the partially liquefied bed was stable. At the other soil depths, the build up of pore pressure remained constant and the soil was observed in motion. We hypothesise that the excess pore pressure would
dissipate if our test duration had been long enough or our soil test section depth shallow enough, as observed by Sumer et al. (1999). If the excess pore pressure in the whole bed dissipates, the bed will become denser and thus may not liable to liquefaction in subsequent group of waves. Sassa and Sekiguchi (1999) and Sumer et al. (1999) discussed the subsequent series of waves effect on the seabed behaviour. The pore pressure oscillation during the pore pressure dissipation phase was less than the pore pressure oscillation within the liquefied bed. The pore pressure oscillation is worth investigation in detail in future, since there appears to be a strong link between the oscillation and liquefaction process. Fig. 12 shows a test with increasing wave heights. The pore pressure built up to the initial overburden pressure. An interesting phenomena was observed at soil depth z = 0.2 m at time = 1050 s. There was a sudden increase of pore pressure to the value of initial overburden pressure after the pore pressure remained constant for while. This may be known as liquefaction transitional process. Sumer et al. (1999) also observed a similar regime. 3.2. Pipeline behaviour on liquefied seabed 3.2.1. Instability phases The instability process of pipelines resting on loose bed was observed using a video camera. Two instability mechanisms were identified in our experiment, depending on the specific gravity of the pipelines. Firstly, for a light pipeline, the pipeline is light enough for the pipeline to become unstable first
Fig. 10. Oscillation of liquefied soil surface.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
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Fig. 11. Excess pore pressure measurements in Test TB01.
before the seabed does. Secondly, for a heavy pipeline, the pipeline is heavy enough for the seabed to become unstable first before the pipeline does. The heavier pipeline sinks through the liquefied seabed faster than it can be carried sideways. Both the phenomena are subjected to a constant wave height in the tests. For an unrestrained and light pipeline, the instability phases are illustrated in Fig. 13. The pipeline is initially resting on a loose bed at time t0. When the hydrodynamic force acting on the pipeline is larger than the soil resistance acting on the pipeline, it starts to move at time t1. It then rocks slightly forward and backward from time t2 to t3, inducing a scour. When the flow reverses, the pipeline slides down from top position to bottom position of the scour at time t4. It then starts to roll again from bottom to the top
position of the scour at time t5. At time t6, it slides down to the bottom and the processes repeat until liquefaction occurs. The pipeline moves with the soil in an elliptical path under oscillatory flow, with only a slight rotation as shown at time t7. The pipeline moved at a same frequency as the oscillatory flow but might move with a phase lag as described in Section 3.2.3. The instability phases of a light pipe before liquefaction are quite similar to Gao et al. (2002) observation in their restrained pipe (i.e. can move freely in vertical and horizontal directions but restricted from rolling) resting on a non-liquefied sand bed in a U-shaped oscillatory flow tunnel. They observed the pipe rocking after the onset of sand scour and followed by the pipe breakout. Fig. 14 shows the instability phases of a heavy pipeline. At time t0, the pipeline is initially resting on
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Fig. 12. Excess pore pressure measurements in Test TD02.
Fig. 13. Instability phases of a light pipeline.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
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Fig. 14. Instability phases of a heavy pipeline.
a loose bed. The wave-induced hydrodynamic force is not large enough to move the heavy pipeline; however the force is large enough to liquefy the seabed. Thus, the pipeline is more stable than the seabed at time t1. Then the pipeline starts to rock slightly forward and backward; and at the same time, it sinks into the moving liquefied bed as shown at time t2 and t3. The pipeline sinks to a certain depth inside the moving liquefied seabed at time t4. The described instability phases may represent a real movement of a long section anchored pipeline in a real sea that may roll (though its rotation will be constrained by the torsional stiffness). The instability
phases in our experiment demonstrate that the conventional approach to design a pipeline may be relevant for a pipeline before liquefaction occurs, but not for a pipeline after liquefaction. 3.2.2. Sinking depth Fig. 15 shows the pipeline embedment after liquefaction for a pipeline of specific gravity 1.11. The initial and final depth from pipeline bottom to surface layer is normalised with pipeline diameter. If the bed liquefies, then whatever the wave condition the pipeline sinks to a depth that depends only on its specific gravity. The final position of the pipeline
Fig. 15. Specific gravity of 1.11 pipeline embedment due to different wave loading.
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depends on the liquefied soil parameter and the pipeline specific gravity, but not on the wave conditions, which is consistent with Sumer et al. (1999) observation. Fig. 16 shows the sinking depth of a wide range of specific gravity of pipelines and compares to Sumer et al.’s (1999) results. The on-bottom pipes of different specific gravity in present study were subjected to a series of wave heights as summarised in Table 2. The final position of pipe was the same after being subjected to different wave heights in a same test. Due to the pipeline weight and loose bed, some initial embedment always existed. The initial positions for different specific gravity were different because of the different weight of the pipelines and the different soil density in each test. The initial pipeline embedment is not a factor to determine the final floating pipeline embedment as shown in Figs. 15 and 16. The pipelines were observed to sink or float when the seabed started to liquefy. No sinking of a pipeline with any specific gravity was observed in the small wave condition where the seabed was stable. In Test TA05, liquefaction took place far field from a buried pipe with specific gravity 1.11 but not around the pipe, and thus the pipe did not float to surface. In Test TD01, the buried pipe with specific
gravity of 1.53 was found to be floated to a depth that was similar to a sinking depth of an unburied pipeline. This may suggest that the presence of pipe may sometime give resistance for the soil liquefaction. Sumer et al. (1999) observed that their buried pipe with specific gravity more than 1.36 did not float to surface but sank into the bed as shown in Fig. 16. Our experimental results shows that for specific gravity pipeline less than the 1.8, the pipeline embedded partially after the test where liquefaction occurred. For specific gravity more than 1.8, the whole pipeline sank into the liquefied bed until it hit the stable soil as observed through the flume side window. The onbottom 2.08 specific gravity pipeline stopped sinking at small distance of about 0.04 pipe diameter from the flume base. There is reason to suggest that a specific gravity higher than 2.08 will sink to a same depth as observed for the 2.08 specific gravity or to the flume base. Sumer et al. (1999) observed that their very heavy buried pipe with specific gravity from 3.1 to 8.9, the pipe stopped sinking at a distance of about 1 diameter pipe from flume bottom. This difference between the present result and Sumer et al.’s (1999) result may due to different soil properties and experiment set up. However, the results of narrow range of specific gravity pipeline in the
Fig. 16. Sinking/floatation depth of different specific gravity pipeline.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
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Fig. 17. Normalized pipeline semi-horizontal displacement, ap, with pipeline diameter, D, for different pipeline specific gravity under different wave loading.
present study and the Sumer et al.’s (1999) observation on higher specific gravity conclude that the sinking depth of a light floating pipeline was not
influenced by the pipeline initial position but the sinking depth of heavy sinking pipeline depends on initial position.
Fig. 18. Normalized pipeline semi-horizontal displacement, ap, with water particle semi-horizontal displacement, aw, for different pipeline specific gravity under different wave loading.
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3.2.3. Horizontal movement No pipeline movement was observed in small wave condition where the seabed was stable. Fig. 17 shows the movement of light pipelines on a liquefied seabed and heavy pipelines within the liquefied seabed. The pipeline semi-horizontal displacement, ap, is normalised with pipeline diameter, D. The displacement is clearly shown in Fig. 17 at different wave heights, H, for pipeline specific gravity ranged from 1.1 to 2.1. The heavier pipeline moved and displaced at a smaller distance compared to a lighter pipeline. A higher wave height caused the pipeline to displace at a larger distance compared to a lower wave height with a same wave period and a same water depth. Fig. 18 demonstrates plots the ratio between the horizontal displacement of the pipeline and the water (at bed level), for different wave conditions and as a function of different pipeline specific gravity. The slope in the region of specific gravity more than 1.8 is higher than in the region of specific gravity less than 1.8. This is because the pipeline moved inside the liquefied soil for the region of specific gravity more than 1.8. For a pipeline with specific gravity less than 1.8, it moved on the liquefied soil in an orbital motion. The pipeline and water displacements are not in phase when the amplitude ratio is less than 1.
4. Conclusion The behaviour of marine pipelines on unstable and liquefied seabed was investigated by using HR Wallingford wave flume. The principal findings may be summarized as follows. (a) The loose silt bed was liquefied due to the build up pore pressure reaching the initial overburden pressure when the wave condition exceeded a critical value of Shields parameter 0.057 and Sleath parameter 0.018. (b) The build up pore pressure rate before liquefaction was the same at any soil depth but became slower when the liquefaction front advanced downward nearer to the soil depth compared to a deeper soil depth. (c) The pore pressure oscillation within non-liquefied silt was less than the pressure oscillation above the bed; however the pore pressure oscillation
(d)
(e)
(f)
(g) (h)
within the liquefied silt bed was slightly larger than the pressure oscillation above the bed. The build up of pore pressure started to dissipate from bottom layer during the wave loading and after cessation of the wave loading. A light pipeline becomes unstable first before the seabed does. A heavy pipeline is heavy enough for the seabed to become unstable first. A long pipeline may roll. The pipeline sinking or floating depth in liquefied seabed depends on the pipeline specific gravity and liquefied soil parameter, but not on the wave condition. The pipeline initial position influenced the final embedment of heavy pipeline but not on the light floating pipeline. The horizontal movement depends on the wave condition and the pipeline specific gravity. A theoretical investigation to explain the pipeline behaviour on liquefied seabed is now in progress. The experimental results and theoretical analysis findings will be used to formulate an improved pipeline design guideline.
Acknowledgements This study was partially funded by the Commission of the European Union Directorate General XII within the Fifth Framework Programme with specific programme: ‘‘Energy, Environment and Sustainable Development’’ under contract EVK3-CT-2000-00038, Liquefaction Around Marine Structures (LIMAS). Andrew Palmer is supported by the Jafar Foundation. The authors are grateful for this support, and thank John Sleath, Malcolm Bolton and Scott Dunn for helpful discussions. References Damgaard, J.S., Palmer, A.C., 2001. Pipeline stability on mobile and liquefied seabed: a discussion of magnitudes and engineering implications. Proceedings of 20th Conference on Offshore Mechanics and Artic Engineering, OMAE’01, Rio de Janeiro, Brazil. de Groot, M.B., Meijers, P., 1992. Liquefaction of trench fill around a pipeline in the sea bed. BOSS 92: Behaviour of Offshore Structures. BPP Technical Services, London, pp. 1333 – 1344. Fredsoe, J., Deigaard, R., 1992. Mechanics of Coastal Sediment Transport, World Scientific, Singapore.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17 Gao, F.P., Gu, X.Y., Jeng, D.S., Teo, H.T., 2002. An experimental study for wave-induced instability of pipelines: the breakout of pipelines. Applied Ocean Research 24, 83 – 90. Lee, K.L., Focht, J.A., 1975. Liquefaction potential at Ekofisk tank in North Sea. Journal of the Geotechnical Engineering Division, ASCE 101 (GT1), 1 – 18. Palmer, A.C., 1996. A flaw in the conventional approach to stability design of pipelines. Proceedings, Offshore Pipeline Technology Conference, Amsterdam. Sassa, S., Sekiguchi, H., 1999. Wave-induced liquefaction of beds of sand in a centrifuge. Geotechnique 49 (5), 621 – 638. Sassa, S., Sekiguchi, H., Miyamoto, J., 2001. Analysis of progressive liquefaction as a moving-boundary problem. Geotechnique 51 (10), 847 – 857.
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Sleath, J.F.A., 1994. Sediment transport in oscillatory flow. In: Belorgey, M., Rajaona, R.D., Sleath, J.F.A. (Eds.), Sediment Transport Mechanisms in Coastal Environments and Rivers. World Scientific Singapore. Sleath, J.F.A., 1998. Depth of erosion under storm conditions. Proceedings of the 26th Conference Coastal Engineering. ASCE, New York, pp. 2968 – 2979. Soulsby, R., 1997. Dynamics of Marine Sands, Thomas Telford, London. Sumer, B.M., Fredsoe, J., Christensen, S., Lind, M.T., 1999. Sinking/Floatation of pipelilnes and other objects in liquefied soil under waves. Coastal Engineering 38, 53 – 90. Tzang, S.Y., 1998. Unfluidized soil responses of a silty seabed to monochromatic waves. Coastal Engineering 35, 283 – 301.
Experimental study of marine pipelines on unstable and liquefied seabed T.C. Teh a, A.C. Palmer a,*, J.S. Damgaard b a
Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, UK b HR Wallingford Ltd., Howbery Park, Wallingford, OX10 8BA, UK Received 9 August 2002; accepted 17 July 2003
Abstract Experimental investigation was carried out in a wave flume to study the stability of marine pipelines on mobile and liquefied seabed. A wide range of different pipeline specific gravities and wave condition were examined. The results showed that for the given soil the seabed response is governed mainly by liquefaction and the pipeline behaviour on unstable seabed is strongly dependent on its specific gravity. D 2003 Elsevier B.V. All rights reserved. Keywords: Pipeline; Liquefaction; Seabed instability; Waves
1. Introduction The objective of this research is to understand the behaviour of a marine pipeline on a seabed that is mobile and liquefied, and ultimately to devise a rational design method. It is motivated by questions about on bottom stability of marine pipeline design. The conventional process of stability design assumes that the seabed itself is stable and considers the incipient movement of a pipeline across a stable seabed. There is substantial field evidence that this model can be incomplete for erodible seabed under severe wave condition (Palmer, 1996; Damgaard and * Corresponding author. Tel.: +44-1223-332718; fax: +441223-339713. E-mail addresses: [email protected] (T.C. Teh), [email protected] (A.C. Palmer), [email protected] (J.S. Damgaard). 0378-3839/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-3839(03)00066-8
Palmer, 2001). Surveys of the Rankine gas pipeline before and after hurricanes Ilona and Orson plainly showed that the seabed had moved but the pipeline had not, and a study of the nearby Harriet pipeline confirmed that the seabed becomes unstable long before the extreme design conditions for the pipeline are reached. Those examples happen to be off the coast of Australia, but the conclusion is a general one. Failure of marine structures has been observed to be linked to wave-induced instability of soil deposits leading up to liquefaction (Lee and Focht, 1975; de Groot and Meijers, 1992; and others). Sumer et al. (1999) investigated the stability of pipelines on liquefied seabed. General seabed mobility and sediment transport has been studied extensively (see e.g. Fredsoe and Deigaard, 1992). An important parameter for seabed mobility is the Shields parameter h = s/[(qs q)gd50], where s is the bed shear stress, g is the gravitational acceler-
2
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
Fig. 1. Wave flume layout.
ation, qs and q are the sediment grains and water densities, and D50 is the median grain size of the sediment by mass. Sleath (1994, 1998), Sassa and Sekiguchi (1999), Sassa et al. (2001), and others have investigated the broader problem of mobility of the seabed. Sleath (1994) suggested a parameter S =qUox/(qs q)g (where Uo is the amplitude of the oscillatory flow just above the boundary layer
at the bed and x is wave frequency). S can be interpreted as a ratio between the horizontal pressure gradient force on an element of soil and its submerged unit weight. Under certain conditions, seabed mobilization is not limited to a thin mobile layer at surface but extends to a thick mobile layer, and Sleath showed that the thickness of the layer is strongly dependent on S.
Fig. 2. Pore pressure transducer set up.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
2. Experiment design 2.1. Experimental set up A series of experiments were carried out in a HR Wallingford wave flume (Fig. 1). The wave flume is approximately 50 m long, 1.2 m deep and 1 m wide. It is equipped with a piston wave generator at one end and a 1:10 slope flint beach at the other end to dissipate the incident wave energy and minimise the reflection. The wave generator is capable of producing a wide range of wave frequency and wave height. A silt test section with 1.6 m long, 1 m wide and 0.3 m thick seabed was built next to the flume side window, so that the behaviour of the pipe and the bed could be observed. A wooden false floor 1 m long and 1 m wide was placed level with the top of the bed at both ends of the test section in order to allow the waves to propagate on the false floor. Two wooden ramps with a slope of 1:5 were placed at the ends of the false floor, to ensure a smooth transition for the travelling waves from 0.75 m water depth to 0.45 m
3
over the test section. The ramps fitted close to the side-walls of the flume and were impermeable. In order to have a fully saturated silt bed, silt was poured slowly into the water filled soil basin. So as to restore the condition of the silt bed to the same state before each test, the silt bed was flushed with high pressure water jet before a test and left to settle for 18 h. Soil samples were carefully taken from the top layer of silt bed before and after a test to determine the soil density using a thin hollow cylinder and two plates. The average silt density was 1946 kg/m3 before a test and 2008 kg/m3 after a test. The density of liquefied soil was between 1730 and 1860 kg/m3, which was determined from the behaviour of the floating pipe as a hydrometer. The water pressures were measured directly in the test section at five different soil depths by using 175 mBar Druck PDCR 810 pressure transducers. The pressure transducers were placed on a tripod stand. A filter cap with a 10-Am nylon filter was screwed to the stand to keep silt particles away from the transducer. The transducers were screwed to the stand
Fig. 3. Sketch of pipeline model set up.
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Table 1 Silt physical properties Property
Value
Units
Method of measurement
D10 D50 D60 D90 Uniformity coefficient, Cu Specific gravity, Gs Minimum void ratio, emin Maximum void ratio, emax Friction angle at critical state, /
10 33 47 120 4.7
Am Am Am Am –
AccuSizer AccuSizer AccuSizer AccuSizer –
2.71
–
0.39
–
1.18
–
BS 1377:1975 Clause 2.6.2 ASTM Standards: D4253 (04.08) ASTM Standards: D4254 (04.08) Triaxial test
36
j
780 780 780 780
machine machine machine machine
inside the water to ensure that there was no air trapped between the transducers and the filter caps. The stand was removed from the silt bed after every test to clean the filters and to re-fit the transducers to the stand. The stand was placed back onto the tank base through silt agitation with high-pressure water jet. The details of the transducers set up are shown in Fig. 2. In a real sea, the pipeline may be anchored at discrete intervals, but the intervals will typically be separated by relatively large distances so the middle section can move horizontally and vertically. It can also roll to a certain degree. The specific gravity of a real pipeline may range from 1.1 to 2. The pipeline was modelled by a length of PVC tube with sealed ends, suspended so that it could move freely horizontally and vertically but restrained against rotation in
plan, and free from the side-walls. The sketch of pipeline model set up is shown in Fig. 3. The pipeline model was 0.97 m long, and consisted of a 75 mm outer tube and a 25 mm inner tube. Different pipeline specific gravity from about 1.1 to 2.1 was achieved by slotting combinations of additional weights into the inner tube and outer tube. The specific gravity of the pipeline is defined as a ratio of the averaged density of the pipeline to water density. Both tubes were filled with water. Marks were made on the pipeline sidewall to follow rotations. The displacement was measured by using a video recorder through the flume window glass with gridlines. The profile of the wave was measured by wave gauge mounted vertically in the water above the test section. Before each test, the water level was adjusted to a required height. The water temperature was measured using a thermometer. It was between 7 and 9 jC for all tests. The velocities at different heights near to the bed were measured using an Acoustic Doppler Velocimeter (ADV). The transducer stand was placed about 0.4 m from test section wall and the pipeline was placed about 0.6 m from another test section wall. A wave gauge was placed above the transducer stand. The wave elevation, pore pressure and displacements of pipeline were measured during a test. 2.2. Soil properties Limestone silt was used as the seabed material. It is a very fine but non-cohesive material,
Fig. 4. Particle size distribution.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
5
Table 2 Experiment programmes Experiment
Test
Date
Wave height (m)
Soil depth (m)
Initial density (kg/m3)
Shield parameter
TA (Pipeline specific gravity 1.11)
01** 02 03 04 05*,^ 05a 05b 05c 05d 05e 05f 06** 07 07a 07b 07c 07d 07f 01 01a 01b 01c 01 01a 01b 01c 01*,^ 01a 02 02a 02b 02c 02d 02f 02g 02f 01 01a 01b 01 01a 01b 01 01a 01b 02 02a 02b 01 01a 01b
20/11/01 27/11/01 28/11/01 29/11/01 3/12/01
0.220 0.220 0.150 0.095 0.050 0.065 0.080 0.080 0.095 0.150 0.220 0.220 0.065 0.150 0.050 0.095 0.150 0.220 0.080 0.095 0.150 0.220 0.065 0.095 0.150 0.220 0.095 0.150 0.025 0.050 0.065 0.065 0.095 0.095 0.150 0.220 0.095 0.150 0.220 0.150 0.095 0.220 0.150 0.220 0.095 0.220 0.150 0.095 0.220 0.150 0.095
0.260 0.250 0.250 0.250 0.255
1947 1948 1950 1920 1919
0.240 0.285
2020 1963
0.280
1940
0.240
2000
0.270
1919
0.265
1919
0.283
1933
0.270
1955
0.270
1955
0.290
1928
0.300
1955
0.391 0.391 0.233 0.111 0.047 0.057 0.073 0.073 0.111 0.233 0.391 0.391 0.057 0.233 0.047 0.111 0.233 0.391 0.073 0.111 0.233 0.391 0.057 0.111 0.233 0.391 0.111 0.233 0.011 0.047 0.057 0.057 0.111 0.111 0.233 0.391 0.111 0.233 0.391 0.233 0.111 0.391 0.233 0.391 0.111 0.391 0.233 0.111 0.391 0.233 0.111 (continued
TB (Pipeline specific gravity 1.37)
TC (Pipeline specific gravity 1.23)
TD (Pipeline specific gravity 1.53)
TE (Pipeline specific gravity 2.08)
TF (Pipeline specific gravity 1.73)
TG (Pipeline specific gravity 1.86)
TH (Pipeline specific gravity 1.94)
5/12/01 7/12/01
6/12/01
4/12/01
26/11/01 30/11/01
9/1/02
10/1/02
11/1/02
18/1/02
15/1/02
Liquefaction Yes Yes Yes Yes No No Yes Yes Yes Partial Partial Partial Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Partial Partial Partial Partial Yes Yes No No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes on next page)
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Table 2 (continued) Experiment
Test
Date
Wave height (m)
Soil depth (m)
Initial density (kg/m3)
Shield parameter
Liquefaction
TH (Pipeline specific gravity 1.94)
02 02b 02c 01 01a 01b
17/1/02
0.150 0.220 0.095 0.220 0.150 0.095
0.300
1950
0.295
1985
0.233 0.391 0.111 0.391 0.233 0.111
Yes Yes Yes Yes Yes Yes
TI (Pipeline specific gravity 2.03)
16/1/02
a, b, c,. . . denotes test was continued from previous test. Partial liquefaction refers to liquefaction front advancement until an intermediate soil depth. * Denotes soil consolidation was 60 h; otherwise, soil consolidation is 18 h. ** Denotes test for unrestrained pipe; otherwise the test from TA to test TI are for pipe with two degrees of movement freedom. ^ Denotes that the pipe is initially buried; otherwise, the pipe is sitting on the bed.
which is essential for the pore pressure to build up. The physical silt properties is summarised in Table 1. Fig. 4 is the particle size distribution curve. 2.3. Test programmes The piston wave maker generated regular waves with a wave period of 1.25 s and with a range of wave heights from 0.05 to 0.22 m without breaking. The tests were mostly run for about 15 min and some for 30 min at sampling rate 0.05 s. Pipelines with 75 mm diameter and different specific gravity were tested on a fresh and loose silt bed. Most tests were run with the support frame that allows translation that prevents rotation in plan, but few tests had a completely free pipeline without any frame. The experiment programmes are summarised in Table 2.
3. Experimental results 3.1. Liquefied seabed response to progressive waves The critical Shields parameter hcr was calculated using the algebraic version of the Shields curve in Soulsby (1997). The value of hcr for all the tests is about 0.15. A summary of the test conditions and the liquefaction occurrences can be found in Table 2. For a fresh and loose seabed, liquefaction was observed for waves as low as 0.065 m (test number 7). This corresponds to a h value of 0.057 and a Sleath number of 0.018. In other words, the bed liquefied
well before initiation of motion. This is in conflict with the theoretical magnitude assessments put forward by Damgaard and Palmer (2001) and may indicate that the resistance to liquefaction is much smaller in a saturated, under-consolidated silt than in the soils from which the publicly available and widely used data on earthquake-induced liquefaction originate. In some of the dense bed tests the soil liquefied to an intermediate depth, just as Tzang (1998) observed in his silt bed and Sassa and Sekiguchi (1999) observed in their sand bed. Figs. 5, 6, 11 and 12 show the typical excess pore pressure measurement response to the regular waves in our experiment. The changes in pore pressure from hydrostatic pressure value measured at different soil depth, z, is the excess pore pressure. z is measured from the mud line, upwards positive. Figs. 6 –9 refer to a same test. The pressure near the bed agrees well with the linear wave theory. Fig. 5 shows a test without liquefaction. The loose silt bed was subjected to a wave height 0.05 m. No bed movement was observed. The pore pressure did not build up. The pore pressure oscillation within the non-liquefied bed was less than the pressure above the bed. The pore pressure oscillation was apparently the same within the non-liquefied bed. Fig. 6 shows a test where liquefaction occurred. The loose silt bed was subjected to a wave height 0.095 m. The pore pressure built up to the initial overburden pressure (rvo V = (qb q)gz) until the vertical effective stress became zero, where rvo V is the initial overburden pressure or the initial vertical effective stress and qb is the bulk density of soil. The
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
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Fig. 5. Excess pore pressure measurements in Test TA05.
development of build up pore pressure at different depth is more clearly seen in Fig. 7. The liquefaction depth advancing downward was observed, which is consistent with Sassa and Sekiguchi’s (1999) observation. At time = 35 s, a layer of 0.04 m thick was liquefied. The thickness of liquefied layer increased to 0.085 m at time = 40 s and to 0.185 m at time = 60 s. From visual observation, the whole bed moved, indicating the liquefied layer advanced downward until to the flume base. Fig. 7 shows the development of excess pore pressure at different soil depth. The built up excess pore pressures at depths z = 0.085 m and z = 0.185 m were the same from time 15 to 30 s as shown in Fig. 7 because the pore pressure build up rate was the same at these depths during this time as shown in Fig. 8. Fig. 8 also shows that the build up pore pressure rate started to become slower when
liquefaction front almost advanced downward to the depth z = 0.085 m. This explains why the build up pore pressure rate is different with time in Fig. 7. Fig. 7 shows that the increment of pore pressure was the same at three measured depths z = 0.04, 0.085 and 0.185 m from time 15 to 25 s but the increment of pore pressure at z = 0.085 and 0.185 m was greater than z = 0.04 m from time 25 to 30 s because liquefaction front almost reached the soil depth z = 0.04 m at about 30 s. At the following time, the rate of build up pore pressure was greater at deeper soil as the liquefaction front propagating downwards. The change in wave-induced oscillation of pore pressure within the bed is shown in Fig. 9. The pore pressure oscillation in the bed, P, is normalised with the steady state of near bed pressure, Po. The term ‘‘steady state’’ used here refers to a state
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T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
Fig. 6. Excess pore pressure measurements in Test TA04.
where the value of a measured parameter becomes constant after many wave cycles. The soil depth, z, is normalised with the wave length, L. L is found
to be 2.10 m using linear wave theory. Fig. 9 shows that the oscillation of near bed pressure, P/ Po, at z/L = 0 m increased to unity at time = 25 s,
Fig. 7. Development of excess pore pressure in Test TA04.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
9
Fig. 8. Wave elevation and fluid pressure/excess pore pressure measurements in Test TA04.
corresponding to the increment of wave height as shown in Fig. 8. The soil bed had yet been liquefied from time 10 to 19 s as shown in Fig.
Fig. 9. Double amplitude of oscillatory of excess pore pressure with time in Test TA04.
7. The pore pressure oscillation during this time was less than the oscillation of near bed pressure and apparently the same at any measured depth (Fig. 9). The remarkable rate of pore pressure oscillation increment started from the top soil layer, and then followed by lower layer, until reaching a steady state value. This is consistent with the observation of liquefaction depth advancing downward. Pore pressure oscillation was observed to reach a maximum value when the build up pore pressure reached a maximum value. For example, the pore pressure oscillation was observed to reach a steady state at time = 35 s at z/L = 0.019 (Fig. 9) when liquefaction occurred at the same time, 35 s, at same soil depth, z = 0.04 m (Fig. 7). The steady state value within the liquefied soil at any depth was same and was slightly larger than the near bed pressure oscillation. This may be because the liquefied soil that acted like a dense fluid
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T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
generated an additional pressure that was superimposed onto the pressure generated by the water above the liquefied soil. During liquefaction, the movement of upper silt layer was observed after a few wave cycles under critical wave condition. The upper layer moved forward and backward over the bed. The liquefied layer surface also oscillated vertically in a wavy motion. The mobile layer then gradually advanced progressively downward until the whole bed was in motion. It was observed that that soil element forward and backward motion decreased with depth. The top layer moved at a larger displacement compared to the bottom layer. Fig. 10 shows the oscillation of liquefied soil at the surface that is normalised with wave height and soil depth. The soil was stable for Sleath parameter S less than 0.018. The soil surface started to oscillate at S = 0.018. The liquefied soil oscillation normalised with soil depth, HLS/d, increased with S. The ratio of soil surface oscillation to wave oscillation, HLS/H, increased with S and then it decreased after a certain S value. Fig. 11 shows a test with longer test duration where liquefaction occurred. The loose silt bed was subjected to a wave height 0.08 m. We observed pore pressure dissipation started at z = 0.215 m during the progressive waves loading and after the wave cessation. No bed movement was observed at this depth during the pore pressure dissipation, indicating that the partially liquefied bed was stable. At the other soil depths, the build up of pore pressure remained constant and the soil was observed in motion. We hypothesise that the excess pore pressure would
dissipate if our test duration had been long enough or our soil test section depth shallow enough, as observed by Sumer et al. (1999). If the excess pore pressure in the whole bed dissipates, the bed will become denser and thus may not liable to liquefaction in subsequent group of waves. Sassa and Sekiguchi (1999) and Sumer et al. (1999) discussed the subsequent series of waves effect on the seabed behaviour. The pore pressure oscillation during the pore pressure dissipation phase was less than the pore pressure oscillation within the liquefied bed. The pore pressure oscillation is worth investigation in detail in future, since there appears to be a strong link between the oscillation and liquefaction process. Fig. 12 shows a test with increasing wave heights. The pore pressure built up to the initial overburden pressure. An interesting phenomena was observed at soil depth z = 0.2 m at time = 1050 s. There was a sudden increase of pore pressure to the value of initial overburden pressure after the pore pressure remained constant for while. This may be known as liquefaction transitional process. Sumer et al. (1999) also observed a similar regime. 3.2. Pipeline behaviour on liquefied seabed 3.2.1. Instability phases The instability process of pipelines resting on loose bed was observed using a video camera. Two instability mechanisms were identified in our experiment, depending on the specific gravity of the pipelines. Firstly, for a light pipeline, the pipeline is light enough for the pipeline to become unstable first
Fig. 10. Oscillation of liquefied soil surface.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
11
Fig. 11. Excess pore pressure measurements in Test TB01.
before the seabed does. Secondly, for a heavy pipeline, the pipeline is heavy enough for the seabed to become unstable first before the pipeline does. The heavier pipeline sinks through the liquefied seabed faster than it can be carried sideways. Both the phenomena are subjected to a constant wave height in the tests. For an unrestrained and light pipeline, the instability phases are illustrated in Fig. 13. The pipeline is initially resting on a loose bed at time t0. When the hydrodynamic force acting on the pipeline is larger than the soil resistance acting on the pipeline, it starts to move at time t1. It then rocks slightly forward and backward from time t2 to t3, inducing a scour. When the flow reverses, the pipeline slides down from top position to bottom position of the scour at time t4. It then starts to roll again from bottom to the top
position of the scour at time t5. At time t6, it slides down to the bottom and the processes repeat until liquefaction occurs. The pipeline moves with the soil in an elliptical path under oscillatory flow, with only a slight rotation as shown at time t7. The pipeline moved at a same frequency as the oscillatory flow but might move with a phase lag as described in Section 3.2.3. The instability phases of a light pipe before liquefaction are quite similar to Gao et al. (2002) observation in their restrained pipe (i.e. can move freely in vertical and horizontal directions but restricted from rolling) resting on a non-liquefied sand bed in a U-shaped oscillatory flow tunnel. They observed the pipe rocking after the onset of sand scour and followed by the pipe breakout. Fig. 14 shows the instability phases of a heavy pipeline. At time t0, the pipeline is initially resting on
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T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
Fig. 12. Excess pore pressure measurements in Test TD02.
Fig. 13. Instability phases of a light pipeline.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
13
Fig. 14. Instability phases of a heavy pipeline.
a loose bed. The wave-induced hydrodynamic force is not large enough to move the heavy pipeline; however the force is large enough to liquefy the seabed. Thus, the pipeline is more stable than the seabed at time t1. Then the pipeline starts to rock slightly forward and backward; and at the same time, it sinks into the moving liquefied bed as shown at time t2 and t3. The pipeline sinks to a certain depth inside the moving liquefied seabed at time t4. The described instability phases may represent a real movement of a long section anchored pipeline in a real sea that may roll (though its rotation will be constrained by the torsional stiffness). The instability
phases in our experiment demonstrate that the conventional approach to design a pipeline may be relevant for a pipeline before liquefaction occurs, but not for a pipeline after liquefaction. 3.2.2. Sinking depth Fig. 15 shows the pipeline embedment after liquefaction for a pipeline of specific gravity 1.11. The initial and final depth from pipeline bottom to surface layer is normalised with pipeline diameter. If the bed liquefies, then whatever the wave condition the pipeline sinks to a depth that depends only on its specific gravity. The final position of the pipeline
Fig. 15. Specific gravity of 1.11 pipeline embedment due to different wave loading.
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T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
depends on the liquefied soil parameter and the pipeline specific gravity, but not on the wave conditions, which is consistent with Sumer et al. (1999) observation. Fig. 16 shows the sinking depth of a wide range of specific gravity of pipelines and compares to Sumer et al.’s (1999) results. The on-bottom pipes of different specific gravity in present study were subjected to a series of wave heights as summarised in Table 2. The final position of pipe was the same after being subjected to different wave heights in a same test. Due to the pipeline weight and loose bed, some initial embedment always existed. The initial positions for different specific gravity were different because of the different weight of the pipelines and the different soil density in each test. The initial pipeline embedment is not a factor to determine the final floating pipeline embedment as shown in Figs. 15 and 16. The pipelines were observed to sink or float when the seabed started to liquefy. No sinking of a pipeline with any specific gravity was observed in the small wave condition where the seabed was stable. In Test TA05, liquefaction took place far field from a buried pipe with specific gravity 1.11 but not around the pipe, and thus the pipe did not float to surface. In Test TD01, the buried pipe with specific
gravity of 1.53 was found to be floated to a depth that was similar to a sinking depth of an unburied pipeline. This may suggest that the presence of pipe may sometime give resistance for the soil liquefaction. Sumer et al. (1999) observed that their buried pipe with specific gravity more than 1.36 did not float to surface but sank into the bed as shown in Fig. 16. Our experimental results shows that for specific gravity pipeline less than the 1.8, the pipeline embedded partially after the test where liquefaction occurred. For specific gravity more than 1.8, the whole pipeline sank into the liquefied bed until it hit the stable soil as observed through the flume side window. The onbottom 2.08 specific gravity pipeline stopped sinking at small distance of about 0.04 pipe diameter from the flume base. There is reason to suggest that a specific gravity higher than 2.08 will sink to a same depth as observed for the 2.08 specific gravity or to the flume base. Sumer et al. (1999) observed that their very heavy buried pipe with specific gravity from 3.1 to 8.9, the pipe stopped sinking at a distance of about 1 diameter pipe from flume bottom. This difference between the present result and Sumer et al.’s (1999) result may due to different soil properties and experiment set up. However, the results of narrow range of specific gravity pipeline in the
Fig. 16. Sinking/floatation depth of different specific gravity pipeline.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17
15
Fig. 17. Normalized pipeline semi-horizontal displacement, ap, with pipeline diameter, D, for different pipeline specific gravity under different wave loading.
present study and the Sumer et al.’s (1999) observation on higher specific gravity conclude that the sinking depth of a light floating pipeline was not
influenced by the pipeline initial position but the sinking depth of heavy sinking pipeline depends on initial position.
Fig. 18. Normalized pipeline semi-horizontal displacement, ap, with water particle semi-horizontal displacement, aw, for different pipeline specific gravity under different wave loading.
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3.2.3. Horizontal movement No pipeline movement was observed in small wave condition where the seabed was stable. Fig. 17 shows the movement of light pipelines on a liquefied seabed and heavy pipelines within the liquefied seabed. The pipeline semi-horizontal displacement, ap, is normalised with pipeline diameter, D. The displacement is clearly shown in Fig. 17 at different wave heights, H, for pipeline specific gravity ranged from 1.1 to 2.1. The heavier pipeline moved and displaced at a smaller distance compared to a lighter pipeline. A higher wave height caused the pipeline to displace at a larger distance compared to a lower wave height with a same wave period and a same water depth. Fig. 18 demonstrates plots the ratio between the horizontal displacement of the pipeline and the water (at bed level), for different wave conditions and as a function of different pipeline specific gravity. The slope in the region of specific gravity more than 1.8 is higher than in the region of specific gravity less than 1.8. This is because the pipeline moved inside the liquefied soil for the region of specific gravity more than 1.8. For a pipeline with specific gravity less than 1.8, it moved on the liquefied soil in an orbital motion. The pipeline and water displacements are not in phase when the amplitude ratio is less than 1.
4. Conclusion The behaviour of marine pipelines on unstable and liquefied seabed was investigated by using HR Wallingford wave flume. The principal findings may be summarized as follows. (a) The loose silt bed was liquefied due to the build up pore pressure reaching the initial overburden pressure when the wave condition exceeded a critical value of Shields parameter 0.057 and Sleath parameter 0.018. (b) The build up pore pressure rate before liquefaction was the same at any soil depth but became slower when the liquefaction front advanced downward nearer to the soil depth compared to a deeper soil depth. (c) The pore pressure oscillation within non-liquefied silt was less than the pressure oscillation above the bed; however the pore pressure oscillation
(d)
(e)
(f)
(g) (h)
within the liquefied silt bed was slightly larger than the pressure oscillation above the bed. The build up of pore pressure started to dissipate from bottom layer during the wave loading and after cessation of the wave loading. A light pipeline becomes unstable first before the seabed does. A heavy pipeline is heavy enough for the seabed to become unstable first. A long pipeline may roll. The pipeline sinking or floating depth in liquefied seabed depends on the pipeline specific gravity and liquefied soil parameter, but not on the wave condition. The pipeline initial position influenced the final embedment of heavy pipeline but not on the light floating pipeline. The horizontal movement depends on the wave condition and the pipeline specific gravity. A theoretical investigation to explain the pipeline behaviour on liquefied seabed is now in progress. The experimental results and theoretical analysis findings will be used to formulate an improved pipeline design guideline.
Acknowledgements This study was partially funded by the Commission of the European Union Directorate General XII within the Fifth Framework Programme with specific programme: ‘‘Energy, Environment and Sustainable Development’’ under contract EVK3-CT-2000-00038, Liquefaction Around Marine Structures (LIMAS). Andrew Palmer is supported by the Jafar Foundation. The authors are grateful for this support, and thank John Sleath, Malcolm Bolton and Scott Dunn for helpful discussions. References Damgaard, J.S., Palmer, A.C., 2001. Pipeline stability on mobile and liquefied seabed: a discussion of magnitudes and engineering implications. Proceedings of 20th Conference on Offshore Mechanics and Artic Engineering, OMAE’01, Rio de Janeiro, Brazil. de Groot, M.B., Meijers, P., 1992. Liquefaction of trench fill around a pipeline in the sea bed. BOSS 92: Behaviour of Offshore Structures. BPP Technical Services, London, pp. 1333 – 1344. Fredsoe, J., Deigaard, R., 1992. Mechanics of Coastal Sediment Transport, World Scientific, Singapore.
T.C. Teh et al. / Coastal Engineering 50 (2003) 1–17 Gao, F.P., Gu, X.Y., Jeng, D.S., Teo, H.T., 2002. An experimental study for wave-induced instability of pipelines: the breakout of pipelines. Applied Ocean Research 24, 83 – 90. Lee, K.L., Focht, J.A., 1975. Liquefaction potential at Ekofisk tank in North Sea. Journal of the Geotechnical Engineering Division, ASCE 101 (GT1), 1 – 18. Palmer, A.C., 1996. A flaw in the conventional approach to stability design of pipelines. Proceedings, Offshore Pipeline Technology Conference, Amsterdam. Sassa, S., Sekiguchi, H., 1999. Wave-induced liquefaction of beds of sand in a centrifuge. Geotechnique 49 (5), 621 – 638. Sassa, S., Sekiguchi, H., Miyamoto, J., 2001. Analysis of progressive liquefaction as a moving-boundary problem. Geotechnique 51 (10), 847 – 857.
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Sleath, J.F.A., 1994. Sediment transport in oscillatory flow. In: Belorgey, M., Rajaona, R.D., Sleath, J.F.A. (Eds.), Sediment Transport Mechanisms in Coastal Environments and Rivers. World Scientific Singapore. Sleath, J.F.A., 1998. Depth of erosion under storm conditions. Proceedings of the 26th Conference Coastal Engineering. ASCE, New York, pp. 2968 – 2979. Soulsby, R., 1997. Dynamics of Marine Sands, Thomas Telford, London. Sumer, B.M., Fredsoe, J., Christensen, S., Lind, M.T., 1999. Sinking/Floatation of pipelilnes and other objects in liquefied soil under waves. Coastal Engineering 38, 53 – 90. Tzang, S.Y., 1998. Unfluidized soil responses of a silty seabed to monochromatic waves. Coastal Engineering 35, 283 – 301.