Elsevier

Ocean Engineering

Volume 38, Issue 7, May 2011, Pages 888-899
Ocean Engineering

Centrifuge modelling of SCR vertical motion at touchdown zone

https://doi.org/10.1016/j.oceaneng.2010.12.002Get rights and content

Abstract

Steel catenary risers (SCR) connect seabed pipelines and flow lines to floating structures used for oil and gas production in deep waters. Waves and currents induce motions of the structure and the risers. The repeated motions of the risers at the touchdown zone in turn induce loads on the seabed soil and might eventually cause fatigue damage to the risers. The analysis of riser fatigue damage is heavily dependent on the soil model. Soil behaviour at touchdown zone such as soil remoulding, stiffness degradation and deformation of the seabed at the touchdown zone further complicate the accurate assessment of riser fatigue damage, which is currently not appropriately quantified in existing design methods. This paper presents centrifuge model tests simulating the repeated vertical movement of a length of riser on clay seabed with increasing undrained shear strength with depth. During the tests, the pipe was subject to cyclic motion over fixed vertical displacement amplitude from an invert embedment of 0.5–3.5 pipe diameters into the soil. The test results show a significant progressive degradation of soil strength and diminution of excess pore water pressure with increasing number of riser penetration/uplift cycle. In view of the different types of environment loadings experienced by floating platforms and various soil conditions, tests were also conducted to investigate the effect of soil strength, riser displacement rate and loading mode on riser–soil interaction during repetitive penetration/uplift motion of the riser.

Introduction

With increasing demands for energy worldwide, much of the oil and gas reserves in relatively shallow waters offshore have been exploited. As a result, deepwater oil and gas exploration activities have expanded significantly in recent years. Flexible floating production systems (FPS) such as semi-submersibles, Floating Production Storage and Offloading vessels (FPSO) and SPARs are often utilized to carry out deepwater exploration and production. To enable access of the production system to the pipelines on the seabed, risers are connected to the subsea pipelines from the floating structure. As water depth increases, the choice of economical offshore platforms becomes fewer, which in turn affects the types of risers that can be used. The introduction of compliant floating systems for offshore hydrocarbon production has led to new designs for the riser pipes.

Steel catenary risers (SCR), which are steel pipes suspended from the floating structure to the seabed in the shape of catenary (Fig. 1), are often employed nowadays. As the mooring system is flexible, the FPS will move back and forth under wave motion. If the FPS moves to the left in Fig. 1, the tension in the SCR increases, the riser is picked up and the touchdown point (TDP) moves to the right. If the FPS moves to the right, the tension in the SCR reduces, and the TDP moves to the left. Close to the TDP, the riser is alternately picked up and set down. Fig. 2 shows schematically the variation of curvature in the boundary layer close to the TDP for (a) a rigid seabed and (b) a deformable seabed. As the TDP moves back and forth, the change in the riser bending moment is much larger than that in the suspended span. As fatigue damage primarily depends on the induced stresses, the region close to the TDP is a fatigue hotspot.

The response of the seabed to repetitive picking up and setting down has a major influence on the bending moments at the fatigue hotspot, and therefore on the fatigue life of the riser. In current practice, a ‘touchdown factor’ that multiplies the embedment depth by 2 or 3 times is employed to consider such fatigue effect. This empirical approach is crudely oversimplified as there is little justification for the magnitude of the factor adopted. Various researchers had carried out studies on this issue. Aubeny et al. (2008), Clukey et al. (2005), Langford and Aubeny (2008) and Hodder et al. (2008) performed cyclic pipe tests under plane strain condition. Bridge et al. (2004) and Hodder and Byrne (2010) conducted three-dimensional model studies to investigate SCR responses at touchdown zone on clay and sand, respectively. However, relatively little study has been made to examine the effects of different parameters on the soil strength degradation over the lifetime of SCR.

To better understand the effects of riser fatigue, a series of centrifuge model tests were conducted in the present study to investigate the interaction between the soil and the riser subjected to repetitive upward and downward movements, with particular attention on the degradation of riser penetration and uplift resistances with increasing penetration/uplift cycle. Parametric studies were also carried out to evaluate the effects of soil strength, riser displacement rate and loading mode on riser–soil interaction. The test results are presented in detail in this paper.

Section snippets

Centrifuge model study and procedure

Centrifuge modelling technique is employed in the present study as the prototype stress levels can be appropriately simulated in a centrifuge model under forced gravitational field. Zhang et al. (2009) provided a brief description of the centrifuge modelling technique and its principles for a marine geotechnical problem involving breaking wave impact on caisson breakwaters. Details of centrifuge modelling techniques and the relevant scaling laws are given in Taylor (1995).

Base case

Among the 3 soil strength profiles shown in Fig. 6, the test with OCR=5 is expected to yield the most consistent outputs. As such, this test (Test 1 in Table 1) is chosen as the base test to facilitate the comparison of results with other tests (Tests 2–6). In this test, the model pipe diameter used was 50 mm. This larger pipe diameter was adopted to facilitate the installation of pore water pressure transducers while maintaining the one-piece feature of the model pipe. The test was conducted at

Parametric studies

In this section, the effect of soil strength, riser pipe displacement rate and loading mode on the interaction between the riser pipe and soil will be investigated. Details of the tests are shown in Table 2.

Conclusion

A centrifuge model study is conducted to evaluate the pipe penetration and extraction responses under repetitive vertical pipe penetration/uplift cycles. The results obtained from the base case with extensive instrumentation reveal that a trench would be formed upon first pipe penetration causing reduction in pipe penetration resistance in subsequent penetration cycles. During pipe extraction, the observed negative pore pressure readings at the pipe base suggest the development of suction

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