Fatigue Life Improvement of Steel Catenary Risers due to Self-Trenching at the Touchdown Point

Langner, Carl G.

doi:10.4043/15104-ms

Cited by 12 publications

(9 citation statements)

References 2 publications

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“…A fatigue concentration occurs around the TDZ, which is influenced by the bathymetry of the seabed through the TDZ as well as the stiffness (or the non-linear load-displacement response) as the riser moves vertically (Langner, 2003;Clukey et al, 2007;Randolph et al, 2013;Shiri, 2014). This paper is focussed on the changing strength of the seabed, and the evolution of a seabed trench, around an element of riser pipe undergoing vertical and combined vertical-horizontal motions on a soft clay.…”

Section: Introductionmentioning

confidence: 99%

The evolution of seabed stiffness during cyclic movement in a riser touchdown zone on soft clay

2017

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Steel catenary risers are pipelines that convey fluids from the seabed to floating structures. The stiffness of the pipe-seabed response, which is the ratio between soil resistance and pipe embedment, in the touchdown zone strongly affects the fatigue accumulation rate, so is an important design parameter. This paper reports a centrifuge modelling study into the longterm pipe-seabed interaction forces on soft clay seabeds, with tests representing many months of behaviour at prototype scale. The results show that the penetration and extraction resistance during large amplitude cycles degrades during the initial few tens of cycles, in the same way that cyclic penetrometer tests capture the fall in soil strength from the intact to the remoulded state. Calculations using bearing capacity factors for a cylinder provide good predictions of this response, although if the cycles of movement involve the pipe breaking away from the soil then the resistance reduces by more than the ratio of intact to remoulded strength, and this is attributed to entrainment of water in the soil around the pipe. However, with further cycles, as pore pressure dissipation occurs, the seabed stiffness recovers due to the gain in soil strength from consolidation. Eventually, the remoulding and water entrainment effects are wholly erased, and the stiffness exceeds the initial state. These observations suggest that current design practice-which factors down the soil stiffness to represent the influence of the cyclic degradation and remoulding process-may overlook a significant effect that raises the seabed stiffness, and potentially also reduces the fatigue life. The evolution of seabed stiffness during Yuan, White & O'Loughlin cyclic movement in a riser touchdown zone

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Section: Introductionmentioning

confidence: 99%

The evolution of seabed stiffness during cyclic movement in a riser touchdown zone on soft clay

2017

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“…Langner Langner [2003] investigated this self-trenching phenomenon of an SCR at the TDP of soft seafloor. By comparing the fatigue damages with rigid seafloor, the curved trench was found to reduce the fatigue risk at the TDP.…”

Section: Problem For An Offshore Structure: Fatigue Damage At the Tou...mentioning

confidence: 99%

Review of Low Cycle Fatigue: Issues in Naval and Offshore Engineering

Delgado-Morales¹,

Palma²,

Gu³

et al. 2022

Fundamental Concepts and Models for the Direct Problem

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Fatigue can be understood as a process of progressive localized plastic strain that occurs in a material subject to cyclic stresses and strains at high stress concentration locations, whose concentration can cause cracks and culminate in the material's fracture. The fatigue process begins with the appearance of the crack, later on with growth, and finally propagation. The study of this phenomenon can be divided into the function of the material life in low and high cycle fatigue, but some studies address problems in the ultra-low and high number of cycles. This chapter addresses aspects of low cycle fatigue, as it is a relatively unexplored area and for which there are few references. Therefore, the main objective of this text is to describe the main aspects of low cycle fatigue, presenting diverse applications and potential problems in the context of fatigue behavior of materials. For this contribution, a review of the technical literature will be presented, considering relevant publications in the last decade. The text overview considers experimental and numerical aspects, fatigue damage, fracture behavior, thermomechanical fatigue, as well as real cases describing how a low-cycle fatigue problem can be defined and the way to approach it through the use of engineering tools found in the literature. Even though the low-cycle fatigue approach and analysis technique covers all engineering, the purpose of this chapter will be on marine and ocean engineering. Several cases described in the reviewed works will be addressed, where problems related to welded structures, turbines, pipelines, risers, mooring chains, floating systems, and wind platforms are presented, situations related to marine and offshore equipment will also be described, showing how low cycle fatigue can present itself in the various situations above, considering that low-cycle fatigue problems can cause structures to fracture, which could consequently cause serious injuries to life and marine environment. At the end of the chapter, potential applications and problems where this phenomenon can appear will be presented.

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“…• Fatigue performance assessment of the SCR in the TDZ remains a serious design challenge for SCR behaviour. Despite some research papers presenting a reduction in fatigue damage [34,40,50] due to riser embedment in the TDZ, other studies have proposed an increase in fatigue damage [7,49]. These confounding results due to different geotechnical parameters have been imposed with trenches.…”

Section: Further Researchmentioning

confidence: 99%

Geotechnical Response Models for Steel Compliant Riser in Deepwater Clays

Elosta¹

2020

Geotechnical Engineering - Advances in Soil Mechanics and Foundation Engineering

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The touchdown zone (TDZ) often proves to be a spot where cyclic bending stresses are the largest and is therefore a critical location for fatigue. Catenary steel compliant pipelines or risers (SCRs) are subject of much ongoing research, particularly with respect to their fatigue life, which is strongly influenced by seabed soil conditions in the TDZ. This chapter reviews the recent publications that might have an impact on the SCR-seabed interaction. The review starts by looking at the SCR general arrangement. Thereafter, the focus moves to the review of the recent research that studied the interactions between deepwater SCRs and the seabed. In addition, the review went over the analysis techniques of the SCR, including the modelling philosophy and models for geotechnical response. The research gap and the need for future research are identified.

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Fatigue Life Improvement of Steel Catenary Risers due to Self-Trenching at the Touchdown Point

Cited by 12 publications

References 2 publications

The evolution of seabed stiffness during cyclic movement in a riser touchdown zone on soft clay

The evolution of seabed stiffness during cyclic movement in a riser touchdown zone on soft clay

Review of Low Cycle Fatigue: Issues in Naval and Offshore Engineering

Geotechnical Response Models for Steel Compliant Riser in Deepwater Clays

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