Formation and Development of Seabed Trenching from Subsea Inspection Data of Deepwater Gulf of Guinea Moorings

Colliat, Jean-Louis; Safinus, Stefanus; Boylan, Noel; Schroeder, K.

doi:10.4043/29034-ms

Cited by 13 publications

(5 citation statements)

References 4 publications

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“…Some recent reports have presented examples of seabed trenching of various sizes that have been observed through ROV around mooring chains in the Gulf of Guinea and North Sea (Bhattacharjee et al, 2014;Sassi et al, 2018;Colliat et al, 2018;O'Neill et al, 2018). The sites consisted of soft clay deposits with strength increasing with depth.…”

Section: Contribution Of Chain-soil Interaction and Potential Trenchi...mentioning

confidence: 99%

Insight into the Behavior of a Caisson Anchor under Cyclic Loading in Calcareous Silt

Mohiuddin

Hossain

Kim

et al. 2022

J. Geotech. Geoenviron. Eng.

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This paper provides insight into the behaviour of a stiffened caisson anchor under inclined cyclic loading in calcareous silt. A series of tests was conducted in a beam centrifuge. A monotonic test was first performed, quantifying the pure monotonic capacity, and then four cyclic loading tests varying mean load, amplitude and number of cycles. Cyclic soil characterisation T-bar tests and caisson tests were linked. Undrained cyclic T-bar tests led to generate excess pore pressure, resulting in degradation of soil strength and stiffness. For partially drained cyclic caisson tests, the excess pore pressure generated during initial undrained monotonic loading experienced partial dissipation. Healing due to consolidation outweighed the damage due to initial pore pressure generation. Post-cyclic monotonic capacity was found to be up to 35% higher compared to the pure monotonic capacity unless the anchor failed during cyclic loading. Measured rotation indicated the evolution of anchor failure mechanism. Caisson capacity under inclined loading was presented as a failure envelope, with the effect of cyclic loading accentuated. The contribution of the soil-chain interaction on the caisson capacity was minimal. No trenching was apparent on the soil surface, and no gap was formed around the anchor, in the considered centrifuge testing conditions.

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Section: Contribution Of Chain-soil Interaction and Potential Trenchi...mentioning

confidence: 99%

Insight into the Behavior of a Caisson Anchor under Cyclic Loading in Calcareous Silt

Mohiuddin

Hossain

Kim

et al. 2022

J. Geotech. Geoenviron. Eng.

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show abstract

“…The findings from the Serpentina FPSO inspections (Bhattacharjee et al 2014) After the Serpentina trenching observations came into the public domain, accurate bathymetry inspections of seabed trenches were conducted for the moorings of five FPSOs and off-loading terminal buoys in the Gulf of Guinea. Colliat et al (2018) presented longitudinal and transversal trench profiles and statistically related trench development with the mooring-line loads.…”

Section: Observations Of Trenching and Trench Morphologymentioning

confidence: 99%

“…(a) and (b)) at the centroids of segments(#18 and #23) confirm the trench opening near the mudline during the initial chain lifting (stage L1) and then the development of the trench (in stages L1 to L8). The trench develops as the chain moves up and down between loading and unloading stages whilst progressively penetrating the trench bottom downwards during unloading.The limitations of the CEL analyses, without consideration of hydrodynamic effects, water entrainment and out-of-plane motions, do not allow full trench development, or the observed breadth of the open trenches (reaching 50% of the trench depth) reported byColliat et al (2018).…”

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confidence: 99%

Numerical Investigations into Development of Seabed Trenching in Semitaut Moorings

Sun

Bransby

Neubecker

et al. 2020

J. Geotech. Geoenviron. Eng.

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Field observations of semi-taut mooring systems have shown severe trench development in front of suction caissons. Trenches extend back from where the chain emerges from the seabed under ambient mooring loads to relatively close to the caisson, and with depths approaching the padeye depth. To help understand the process, a Coupled Eulerian-Lagrangian (CEL) approach has been used to apply cyclic loading to a mooring chain system, treating the chain as a series of linked cylindrical elements and the soil as a strain-softening Tresca material. The chain extended through the soil into the water column, and cyclically changing boundary conditions were applied to the far-end of the chain to reflect a typical mooring layout and metocean conditions. In spite of rather idealized conditions, with loading restricted to the vertical plane of the chain and no consideration of hydrodynamic effects, a stable trench profile developed after only a few cycles of loading. The chain profile straightened during the cycles, with the section nearest to the padeye gradually cutting deeper into the soil and with the amplitude of cyclic motions of the chain, and hence tendency for a trench to form, increasing at shallower depths. The magnitude of soil resistance acting on the chain reduced by increasing cycles, with particularly low resistance in the zone where the trench was most developed. The long-term longitudinal profile of trenching was predicted based on the current modelling, and was found comparable to that inferred from field observations, even without consideration of hydrodynamic and out-of-plane effects.

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“…Then, some trenches were found in the North Sea [21]. The latest report presented three-dimensional trench profiles [22]. front of the anchor in soft clay [19].…”

Section: Introductionmentioning

confidence: 99%

A Framework for Mooring and Anchor Design in Sand Considering Seabed Trenches Based on Floater Hydrodynamics

Rui

Long

et al. 2023

Sustainability

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Seabed trenches reduce anchor capacity. However, the adverse influence is not considered in the current design, and no guideline is found in engineering practice. This paper presents a framework for mooring and anchor design in sand considering seabed trenches based on floater hydrodynamics. First, a hydrodynamic calculation of the studied floater coupled with the mooring system was conducted. Then, the potential trench profile was assessed using a mooring line–seabed dynamic model. Third, after assessing the suction anchor performance based on its installation and capacity, a refined anchor, caisson–plate gravity anchor (CPGA), was proposed, and the capacity mechanisms were analyzed. It was found that the tensions of mooring lines l1 and l2 resist maximum loads under a 45° load condition. Due to the sand’s high strength, a trench is difficult to form in this mooring type. The suction anchor is not suitable as an anchor for the carbonate sand, considering its installation and capacity. The analyses indicate that the increase in soil stress and soil mobilized range in front of the caisson promotes the capacity of CPGA. This paper provides a framework/example to design moorings and anchors considering potential trenches and provides a solution to the anchor design in carbonate sand.

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Formation and Development of Seabed Trenching from Subsea Inspection Data of Deepwater Gulf of Guinea Moorings

Cited by 13 publications

References 4 publications

Insight into the Behavior of a Caisson Anchor under Cyclic Loading in Calcareous Silt

Insight into the Behavior of a Caisson Anchor under Cyclic Loading in Calcareous Silt

Numerical Investigations into Development of Seabed Trenching in Semitaut Moorings

A Framework for Mooring and Anchor Design in Sand Considering Seabed Trenches Based on Floater Hydrodynamics

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