Wave pressures and uplift forces on and scour around submarine pipeline in clayey soil

doi:10.1016/S0029-8018(02)00022-7

Ocean Engineering

Volume 30, Issue 2, February 2003, Pages 271-295

https://doi.org/10.1016/S0029-8018(02)00022-7 Get rights and content

Abstract

Experimental investigations are carried out on wave-induced pressures and uplift forces on a submarine pipeline (exposed, half buried and fully buried) in clayey soil of different consistency index both in regular and random waves. A study on scour under the pipeline resting on the clay bed is also carried out. It is found that the uplift force can be reduced by about 70%, if the pipeline is just buried in clay soil. The equilibrium scour depth below the pipeline is estimated as 42% of the pipe diameter for consistency index of 0.17 and is 34% of the pipe diameter for consistency index of 0.23. The results of the present investigations are compared with the results on sandy soil by Cheng and Liu (Appl. Ocean Res., 8(1986) 22) to acknowledge the benefit of cohesive soil in reducing the high pore pressure on buried pipeline compared to cohesionless soil.

Introduction

The search for oil under the deep-seabed has led into extensive theoretical and experimental investigations in the field of offshore oil exploration and exploitation. The discovery of large deposits of oil and gas in many deep-water offshore regions has resulted in the construction of large production and drilling platforms. Since the 1940s, the tendency to build the fixed offshore structures in deep waters has progressed steadily. Once the production platforms are erected in the sea, the next immediate task is to transport the oil and natural gas from the production site to onshore terminal facilities. Submarine pipeline offers an efficient mode of transportation of oil and gas continuously from offshore to onshore.

Though the pipeline appears as a simple structure, interaction of submarine pipeline with waves is a complex phenomenon. Many foundations of water front structures built on sandy soils have suffered extensive damage due to scour. Even the foundations built in clayey soil beds are not free from scour effects. There are many thick under-consolidated marine clayey deposits located in several coastal areas and the scouring of foundations in such soft cohesive sediments can be quite significant. Scour under the pipeline results in spanning and vibration and vortex induced loading. If the natural frequency of the pipe is closer to the frequency of waves, then the pipeline vibration magnifies, which results in failures of submarine pipelines. The review of existing literature reveals that studies on wave-induced pressure, uplift force on submarine pipeline especially in cohesive soils is in the developing stage as compared to the studies in sandy soils. Therefore, the present study has been carried out to investigate the magnitude of wave-induced pressure, uplift force on and scour around submarine pipeline in soft marine clayey soil. One of the important questions for selection of burial of a submarine pipeline is ‘WHAT IS THE OPTIMUM DEPTH OF BURIAL FOR ACHIEVING SUFFICIENT STABILITY AGAINST FLOATATION?’ This question can be solved, if the uplift force for different depths of burial in cohesive soil is known. Theoretical estimation of the uplift force in cohesive soil is cumbersome. For such situations, experimental investigation is the only option available. This is one of the main motivations for the present study. The effect of consistency index on uplift forces and wave pressures are also not known clearly and this aspect is also considered in the present investigation.

Section snippets

Literature review

In the last few decades, greater attention is being focused by many researchers in the field of hydrodynamics to understand the physics of the unsteady fluid flow phenomenon around the submerged pipelines. Knowledge about hydrodynamic aspects of submarine pipelines placed near the seabed boundary (both above and below the seabed) and subjected to the action of regular and random waves is increasing. Garrison et al. (1975) have conducted experiments to predict the wave forces on a horizontal

Test facility

The present experimental investigations are carried out in a 30 m long, 2.0 m wide and 1.7 m deep wave flume in the Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, India. The piston type wave maker is used for generation of regular and random waves. The wave height and period to be generated are controlled by a personal computer. The other end of the flume is provided with a rubble mound absorber to effectively absorb the incident waves. The details of the

General

Both for regular and random waves, the peak pressures and maximum uplift forces were obtained by using threshold crossing analysis. DHI wave synthesiser software package was used to perform the time domain analyses. The wave pressures and forces are normalised by using appropriate input parameters. The discussion is provided for both regular and random wave studies in the following sections.

Effect of relative water depth, d/L on the wave pressure

In the field, the pipeline has to cross-different water depths, wave height and periods. It is essential

Conclusions

A comprehensive investigation on the wave induced pressures and uplift forces by regular and random waves on a pipeline resting on the seabed as well as buried at different relative burial depths were carried out using physical model studies. The present experimental results are compared with the experimental and theoretical results from exiting literature. The important conclusions arrived from this study are:

1.
Normalised wave induced pressures on the submarine pipeline decreased by about

Acknowledgements

The authors wish to express their gratitude to the Head, Department of Ocean Engineering, Indian Institute of Technology, Madras, for providing the necessary facilities for carrying out this work. Thanks to the appropriate authorities of IIT Madras for necessary infrastructure for carrying out this study.

References (17)

W Magda
Wave-induced uplift force on a submarine pipeline buried in a compressible seabed
Ocean Engineering
(1997)
H.J Mitchener et al.
Erosion of mud/sand mixtures
Coastal Engineering
(1996)
S.E.J Spierenburg
Wave-induced pore pressures around submarine pipeline
Coastal Engineering
(1986)
H.D Cheng et al.
Seepage force on a pipeline buried in a poroelastic seabed under wave loading
Applied Ocean Research
(1986)
L Dormieux et al.
Effective stress response of a plane seabed under wave loading
Geotechnique
(1988)
C.J Garrison et al.
Wave forces on bottom mounted large diameter cylinder
Journal of Waterways, Harbours and Coastal Engineering, ASCE
(1975)
N Jothisankar et al.
Wave forces on large offshore pipelines
Ocean Engineering
(1985)

There are more references available in the full text version of this article.

Cited by (32)

Numerical investigate of the effects of surface roughness on local scour around pipeline under steady current condition
2023, Ocean Engineering
A two-dimensional numerical model was developed to investigate the effects of the surface roughness on local scour around pipeline based on Naiver-Stokes equations. Both suspended load and bedload transport are considered for the sand motion equation. Present numerical model is verified accurately for simulating local scour around pipeline. The numerical results show that the surface roughness has a significant effect on the process of local scour, i.e. forces, Shields parameter and scour depth. For rough pipeline, the average value of the drag force coefficient on the pipeline, the RMS value of the lift coefficient, the scour depth and the scour width all decrease rapidly with increasing roughness and then change slowly.
Numerical and experimental modeling of wave-induced forces on submarine pipeline buried in the soil of different engineering properties
2023, Ocean Engineering
In the present paper, the wave-induced forces on the submarine pipeline buried in porous soil having different properties are studied. The mathematical problem is solved using the higher-order boundary element method, and the numerical results are validated with experimental results obtained through rigorous model tests. Four different types of Kuwaiti marine soil: Al-Khiran, Al-Koot, Sabiya, and Shuaiba are considered in the present study. The effect of burial depth of the submarine pipeline in the soil seabed, incident wave period and wave height, and hydraulic conductivity of the soils on the wave forces acting on the submarine pipeline is analyzed in a detailed manner. The study reveals that for moderate burial depth of the pipeline in the soil bed, the wave force acting on the submarine pipeline can be reduced up to 50% for soil having less hydraulic conductivity. The measured wave forces and numerical predictions are matching well for a wide range of wave, soil and structural parameters.
Experimental study of soil responses around a pipeline in a sandy seabed under wave-current load
2023, Applied Ocean Research
Wave and current coexist widely in the oceans. Ever-changing wave characteristics, such as height and length, have a significant influence on the dynamic response within the seabed soil due to the presence of currents. Some aspects of wave, seabed and pipeline interactions have been widely investigated over past years, incorporating both experimental and numerical approaches. However, previous studies on seabed dynamic response around pipelines have mainly focused on cases with wave loading. In this experimental laboratory study, the current loading and diverse backfill conditions which commonly occur far offshore are discussed in depth. It is found that when subjected to the combined wave and current loads, the response of the seabed around a pipeline is more complex than that subjected to pure wave loading as current velocity and propagation direction would significantly change the wave characteristics as well as seabed response. Most notably when comparing with the pure wave loading, the pore pressure amplitude and its attenuation rate within the seabed increases when current and wave travel in the same direction. This could increase the possibility of liquefaction around the pipeline. When current travels in opposing direction to the wave, the effect of current on the decrease of pore pressure and its attenuation rate is more significant than that of the co-current actions. This means that the counter-current will reduce the probability of soil liquefaction.
Wave force reduction due to burial of submarine pipeline: A probability based analysis and comparison with DNVGL-RP-F109
2022, Ocean Engineering
Horizontal and vertical wave forces were assessed on submarine pipeline for different burial depths in soils of four different hydraulic conductivity in random waves of wide range of wave height and periods. Probability analysis is carried out for 99.99% and 98% probability of non-exceedance of vertical and horizontal wave forces on the pipeline. The force reduction due to burial of the pipelines are compared with the recommendations by DNVGL-RP-F109. The effect of changing the wave height, wave period and hydraulic conductivity on the wave forces and its change due to different burial depths are revealed. The wave force values and the quantitative differences due to 99.99% and 98% probability of non-exceedance can be used for risk-based design of buried pipelines. The recommended value of wave force reduction factor for different relative burial depth of the pipeline by DNVGL-RP-F109 is much smaller than the value of wave force reduction factor from the present study. Strong scope exists to revise this code for the betterment of submarine pipeline design engineers. The knowhow revealed in this study will be useful for optimal design of buried submarine pipelines.
The propagation of wave scour along the spanwise direction of submarine pipelines in case of clear-water regime
2021, Coastal Engineering
The local sediment motion around cylindrical structures laid on erodible seabed occurs with wave and/or current effects and causes significant stability problems. In this study, the development of the scour hole along the spanwise direction of the pipeline and the scour propagation velocity were investigated experimentally. The time dependent measurements beneath the test pipe were performed in a large wave channel. In the experiments, the measurements were recorded by using ultrasonic equipment. Since the coarse bed materials were used, all of the experiments were carried out in the clear-water scour regime condition, differently than live-bed data in literature. The experimental results reveal that the scour propagation along the spanwise direction of the submarine pipe is not relevant with Shields parameter under clear-water condition; however, it is inversely related to the Keulegan-Carpenter number. By analyzing the findings of the study, a new empirical relation was established to estimate the non-dimensional scour propagation velocity based on the Keulegan-Carpenter number along the spanwise direction of submarine pipe, in case of wave effect. The statistical significance of the equation was demonstrated by the correlation coefficient and scatter index.
Effect of pipeline-seabed gaps on the vertical forces of a pipeline induced by submarine slide impact
2021, Ocean Engineering
Citation Excerpt :
Offshore pipelines often cross through undulating seabeds (e.g. submarine escarpments, canyons), causing suspension of some pipeline sections (Jeanjean et al., 2005; Xiu et al., 2016). Moreover, due to the complex pipeline-soil-water interactions, initially ‘on bottom’ seafloor pipelines may be suspended at different distances above the seabed (Vijaya kumar et al., 2003; Cathie et al., 2005) due to scour of the underlying soil. Despite this, the influence of the suspended distance between the pipeline and seabed (termed gap henceforth) on the slide-pipeline interaction forces is rarely considered.
In recent years, the interaction between submarine slide runout and offshore pipelines has received extensive attention, based on the need to protect pipelines crossing regions susceptible to submarine slides. The interaction force between the slide material and a pipeline may be resolved into horizontal and vertical components, but with most attention to date focusing on the former while the latter has not been studied thoroughly. This paper reports the results from a series of multiphase flow (slide material and the ambient seawater) numerical simulations using a computational fluid dynamics (CFD) approach aimed at investigating the vertical forces during submarine slide flow around a pipeline at different gap ratios (pipeline-seabed gap normalized by pipeline diameter) from 0.08 to 10.0 and at Reynolds numbers ranging from 0.38 to 267. The oscillatory characteristics and magnitude of the vertical forces for different gap ratios are discussed. The effects of the gap ratios on the geotechnical and inertial components of the slide-pipeline vertical forces are analyzed systematically using a hybrid geotechnical-fluid mechanics framework. From the results of the CFD simulations, a modified hybrid geotechnical-fluid mechanics method to estimate the slide-pipeline vertical forces is developed considering the effects of pipeline-seabed gaps.

View all citing articles on Scopus

View full text

Wave pressures and uplift forces on and scour around submarine pipeline in clayey soil

Abstract

Introduction

Section snippets

Literature review

Test facility

General

Effect of relative water depth, d/L on the wave pressure

Conclusions

Acknowledgements

Ocean Engineering

Coastal Engineering

Coastal Engineering

Seepage force on a pipeline buried in a poroelastic seabed under wave loading

Applied Ocean Research

Effective stress response of a plane seabed under wave loading

Geotechnique

Wave forces on bottom mounted large diameter cylinder

Journal of Waterways, Harbours and Coastal Engineering, ASCE

Wave forces on large offshore pipelines

Ocean Engineering