Optimum design of On-Bottom stability of offshore pipelines

Interest on offshore energy resources has been increasing globally, especially in Brazil, India, China, and other developing countries. The marine world in 2030 will be almost unrecognizable owing to the rise of emerging countries as mentioned, new consumer classes and resource demand (Fang et al., 2013). As development activities for energy demand move to deepwater, many offshore pipelines to transport oil and gas will be installed. The clay soil on seabed below installed pipelines ranges from very soft to very stiff soil. This paper deals with soft and very soft clay soil and the pipelines will be embeded further into the soft soil. The exact behavior between a pipeline and soil is not known. The heaved soil and resistance forces are hence important aspects. It is important to properly model pipe-soil interaction effects (Bai and Bai, 2005). Pipe embedment depth by pipesoil interaction has become critical design parameter to design offshore pipeline such as free span and thermal expansion as well as on-bottom stability. On-bottom stability analysis that a pipeline maintains stability on seabed against hydrodynamic load of wave and current has to be considered with a relevant pipe-soil interaction analysis. The pipelines are under the combined loads such as bending, axial force, and external pressure during installation due to the dynamic vessel motion. At TDP, the seabed disturbance will occur by the dynamic pipe-soil interaction. A typical s-lay configuration and applied pipeline loading during installation is presented in Fig. 1 . Studies on pipe-soil interaction and assessing penetration depth have been progressing lately. Verley and Lund (1995) developed pipe-soil interaction models on clay soils considering penetration effects of pipe subjected to oscillatory forces in waves based on model test data and this formula was adapted in DNV-RP-F109. White and Cheuk (2008) studied soil resistance on seabed pipelines during large cycles of the lateral movement for realistic soil behaviors. Randolph and White (2008b) and Merifield et al. (2009) present pushed-in-place (PIP), taking into account of combining the vertical and horizontal loading using a heaved soil model. Before these studies, the model based on wished-in-place (WIP) considering relations only between the vertical load and normalized pipe penetration was used. The WIP and PIP models are illustrated in Fig. 2 .

Fig 1.Typical s-lay configuration and applied pipeline loading during installation.

 The environmental loads on the embedded pipelines are reduced on soft clay. If an exact initial penetration depth can be calculated, an optimized required submerged weight can be derived easily. Currently, no methodology has developed considering simultaneously both of the dynamic pipeline installation and the pipe embedment at TDP. Thus a new proposed procedure was developed in this study. The details of the procedure are described in the next chapter.

In this study, the recent trend for the on-bottom stability analysis was investigated and a case study was performed using the proposed new procedure. Analysis results between seabed penetration depths with dynamic effect and without dynamic effect were compared.

Fig 2. Schematic of wished-in-place (WIP) and pushed-in-place (PIP).

A NEW PROCEDURE OF ON-BOTTOM STABILITY ANALYSIS

Fig. 3 shows the proposed new procedure for the on-bottom stability with dynamic effect of offshore pipeline installation. The analysis procedures are made up three steps such as global pipelay analysis, simulation of dynamic local pipeline embedment at the TDP, and modified on-bottom stability design. Details on each step are described in the subchapters. The time histories of horizontal displacement and vertical soil resistance at TDP are calculated through global pipelay analysis using pipe properties, environmental data, and others. The time histories of horizontal displacement and vertical soil resistance were used for input of local pipe-soil interaction analysis with a finite element method (FEM). The maximum value among pipe embedment depths by local dynamic analysis was used for an initial penetration depth, the modified on-bottom stability analysis based on DNV-RP-F109. The calculation result based on design codes considering the dynamic penetration depth from FEA can be used for an optimum on-bottom stability design.

Fig 3.Proposed new procedure for on-bottom stability analysis.

- Global installation analysis

The commercial pipeline installation programs are based on FEA and calculated the stress, strain, touchdown length, departure angle, and soil reaction force at TDP. Installation method of the s-lay was used for the present study. General procedure and details of the global analysis are summarized in Fig. 4 . The DNV-OS-F101 (DNV, 2012) for design criteria was used to check the allowable stress and strain. The OFFPIPE software as an analysis tool was used in this study. The background of OFFPIPE software could be referred to OFFPIPE manual (OFFPIPE, 2013). The environmental input data include waves, currents, water depth, and soil shear strength, submerged weight of soil, and friction coefficient. The lay vessel information incorporates hull dimension, locations of rollers and tensioners, configuration of stinger, and vessel response amplitude operator (RAO). The installation analysis was divided into two steps: static and dynamic analyses. Important parameters in this analysis are summarized in Table 1 . Results by static analysis were checked with design criteria and then dynamic analysis was performed. Time histories of horizontal displacement and vertical soil resistance were obtained by the dynamic pipelay analysis.

Tabel 1.Global installation analysis parameters from OFFPIPE.

Fig 4.Global installation analysis of offshore pipelines.

- Local analysis at TDP

The purpose of the local analysis is to estimate initial pipe embedment depth considering the dynamic loads from global pipelay analysis. The procedure of local analysis is illustrated in Fig. 5 . The most important information to obtain pipe embedment depth by local analysis is soil data of seabed. Data such as shear strength, Young’s modulus, and Poisson’s ratio for soft clay should be entered exactly though in-situ testing. Plastic properties such as sensitivity and soil ductility are also important. There are FEA tools such as Abaqus and ANSYS for the pipe-soil interactions. Elastic perfectly plastic soil models including Mohr-Coulomb, Tresca, and von-Mises criteria and elasto-plastic soil model including modified Cam clay are used for many geo-engineering problems. Recently, researches on pipe-soil interaction behavior have been conducted actively and many constitute equations which are expressed mathematically about stress-strain relation, initial yield strength, hardening, and softening on soil behavior are being developed. For example, Tresca criteria with the strain-softening was used for the pipe-soil interaction analysis and research work including tests was actively performed by Einav and Randolph (2005), Zhou and Randolph (2007), Wang et al. (2009), Chatterjee et al. (2010) and many others.

Fig 5.Local analysis at TDP of offshore pipelines.

The empirical expression of shear strength for tresca constitutive model with exponential strain softening relationship is as follow (Zhou and Randolph, 2007; Wang et al., 2009).

• Su0= Intact undraind shear strength, ( =Sum+k⋅z);
• k= Strength gradient;
• z= Soil depth;
• δrem= Fully remoulded strength ratio, ( 1/=St);
• St= Sensitivity;
• ξ= Accumulated absolute plastic shear strain;
• ξ95= Cumulative absolute shear strain required to cause 95% reduction.

- Modified on-bottom stability analysis

The design procedure of modified on-bottom stability is presented in Fig. 6 . General environmental data for each condition are selected as follows (Guo et al., 2004).

• Installation condition: empty pipes with 1-yearperiod environment
• Operation condition: product filled pipes with 10-yearsor 100-yearsperiod environment

Fig 6. Modified on-bottom stability analysis of offshore pipelines.

A design of on-bottom stability was performed according to following. Firstly, environmental criteria for the 1-year, 10- years and 100- years years conditions, including water depth, wave spectrum, current characteristics, and soil properties were defined. Hydrodynamic coefficients: drag (C _D ), lift (C _L ), and inertia (C _I ) which are adjusted with Reynolds numbers, Keulegan-Carpenter numbers, ratio of wave and the steady current, and embedment were determined. Hydrodynamic forces, typically, drag (F _D ), lift (F _L ), and inertia (F _I ) was calculated. Lastly, static force balance at time step increments was performed to assess the onbottom stability. A concrete coating thickness for the worst combination of lift, drag, and inertial force was calculated (Guo et al., 2004). Above mentioned various forces are illustrated in Fig. 7 . In this study, on-bottom stability analyses were performed based on the modified analysis and DNV-RP-F109. The details on this design code are explained in chapter 3 of DNV-RP-F109.

Fig 7. Schematic of forces acting on offshore pipelines.

 Reference : http://kpubs.org/article/articleMain.kpubs?articleANo=E1JSE6_2013_v5n4_598