Susan Gourvenec outlines toolbox approach for optimizing geotechnical design of subsea foundations
Fig. 1: Evolution of offshore architecture. Image created by M Cocjin, COFS, UWA.
Subsea foundations are becoming increasingly widespread as offshore development moves away from the conventional template of a fixed platform over a set of wells to subsea development of multiple wells and fields tied back to a single facility.
Subsea developments comprise a network of infield flowlines and assorted pipeline and wellhead infrastructure, which is typically supported on shallow, mat foundations – or “mudmats.”
The geotechnical design challenge of subsea mudmats is to withstand greater dead and operational loads on softer seabeds without increasing the footprint size or weight. The motivation is to reduce costs associated with installation – for example eliminating the need for a heavy-lift vessel to place the mudmats alone if handling limits of pipe-laying vessels are exceeded – whilst providing acceptable in-service reliability.
There is a toolbox of solutions for optimizing the geotechnical design of subsea foundations, to reduce the size of mudmat foundations for the same operational conditions, compared with designs based on conventional practice. Such methods may improve the viability of projects, contributing to the unlocking of valuable but currently ‘stranded’ hydrocarbon reserves.
Optimization of capacity assessment methodology
Classical bearing capacity theory is recommended by most industry guidelines and is typically used to design subsea mudmats, but has been shown to poorly represent the actual response for a range of offshore foundation and loading conditions.
The alternative “failure envelope approach” allows ultimate limit states to be defined in terms of individual load components and explicit definition of the boundary conditions, such as foundation geometry and soil strength characteristics. The result is a failure envelope or surface defining ultimate limit states in combined load space, such that the effect on factor of safety of a variation in any independent component of load can be assessed.
A new failure envelope framework has been developed to predict undrained ultimate limit states of subsea mudmats under loading in six degrees of freedom. The industry partner involved in the project reported that the new design methodology has led to the possibility of reducing the size of shallow foundations such as pipeline end termination mudmats by 20%, or alternatively are able to withstand larger jumper loads.
Fig. 2: Load carrying benefits of a pin-piled hybrid subsea foundation.Image from COFS.
Optimization of foundation configuration
Hybrid subsea foundations A hybrid subsea foundation involves a mudmat and a deeper foundation system acting in consort to increase load carrying capacity above the mat alone ultimately leading to smaller required footprint sizes. Two concepts for hybrid subsea foundations have been considered at the Centre for Offshore Foundation Studies (COFS), one involving corner pin-piles as the deeper foundation solution and another using mid-line suction caissons.
Considering the pin-pile hybrid foundation, in practice the mat would be laid on the seabed and the piles then jacked through a tapered slot in the mudmat, with a locking cap to restrain the pile head from vertical displacement while allowing pile rotation. Centrifuge modeling has been carried out at COFS to assess the viability and potential gains in capacity of the pin-pile subsea foundation. A simplified lower-bound approach has been developed for predicting capacity of pin-pile subsea foundations whereby the mat carries the entire vertical design load and the pile group carries the entire sliding and torsional loading. Even considering the two foundation systems independently, i.e. not relying on interaction between the mat and the piles, indicates considerable increase in capacity can be achieved over the mat alone (Figure 2).
Research on pin-piled hybrid subsea foundations has continued at COFS looking at fully combined load response in six degrees of freedom and at the load-sharing of the mat and pile group when acting in consort. The design framework developed at COFS shows that provision of pin-piles can reduce footprint areas by up to 60% for typical deepwater field conditions. The industry partner on this project has adopted this innovative pin-pile hybrid foundation system on Esso’s Erha North project offshore Nigeria as a cost-effective mitigation solution against pipeline walking.
Internal shear keys Provision of sufficient internal shear keys or “skirts” to prevent shearing within the confined soil plug of a skirted foundation enhances mudmat capacity. Results from a program of finite element analyses have been distilled into simple design charts defining the optimal number of internal shear keys as a function of equivalent embedment ratio for intervals of strength heterogeneity index and vertical load mobilization (Figure 3). It is seen that the commonly adopted shear key interval of s/d = 5 in engineering practice over-estimates the critical number of internal skirts for cases of low vertical load mobilization, low soil heterogeneity index and low embedment ratio but becomes unconservative with increasing vertical load mobilization, soil heterogeneity index and foundation embedment ratio.
Fig. 3: Optimal number of internal shear keys for subsea foundation under 6 dof loading. Image from COFS.
Optimization of geotechnical input
Best available site investigation data
A sound understanding of near-surface soil strength is essential for the accurate prediction of the response of structures laid on or shallowly embedded in the seabed, such as subsea mudmats (and pipelines). A number of novel tools suited to near-surface strength characterization are being developed at COFS, including the hemi-ball and toroid and the pile penetrometer.
Consolidated undrained strength In an offshore scenario, a mudmat and supported structure may be set down on the seabed several months in advance of operation of the attached pipelines, when the horizontal loads (and therefore moments and torsion) come in to play due to thermal expansion of the attached pipelines. Consolidation of the soil in the vicinity of the foundation will take place under the self-weight of the foundation and structure it is supporting in the period following set down and before operation.
Further efficiencies in subsea foundation design can therefore be realized if the consolidation-induced strength gains can be banked, i.e. it may be possible to rely on a higher value of undrained shear strength than measured in situ. The time lapse between installation and operation may range from a few months to a year depending on the project, over which time considerable gains in shear strength may be achieved, depending on the consolidation properties of the sediment.
A theoretical method for predicting consolidated strength gains in capacity of shallow foundations under vertical preloading, based on a critical state framework has been developed at COFS. The theoretical method has been applied to a range of foundation and pipeline problems under multi-directional loading, providing a quick and easy method for predicting consolidated undrained resistances.
Effect of cyclic loading All offshore structures and hence the foundations that support those structures are subject to cyclic loading. Cyclic loading of subsea foundations may arise from environmental, installation or operational loading. Unnecessary conservatism in parameter selection is required to account for uncertainty in the effect of cyclic loading on engineering parameters, which will lead to conservatism in design output. Current work at COFS is investigating a framework to predict cyclic degradation of soil properties, with particular attention to subsea mudmats. Foundation capacity could then be assessed by accounting for consolidated gains in capacity, modified by a reduction factor to account for cyclic loading degradation.
Optimization of operational mode
Challenging the traditional but conservative paradigm that a foundation should remain stationary, optimization can be achieved through tolerable foundation mobility.
The concept of mobile foundations is that they are designed to move tolerably across the seabed to absorb some of the load imposed by thermal expansion of the pipeline rather than being designed large enough to resist all the operational loads and remain stationary. The concept of mobile foundations is radical, but a logical progression from the now widely-accepted practice of permitting subsea pipelines to buckle laterally either across the seabed or on engineered structures in response to thermally-induced expansion during operation (OE: April 2014).
A tool box of solutions to optimize the geotechnical aspects of subsea foundation design has been highlighted. The various options can be described by an ‘optimization class’ in terms of (1) optimizing the design methodology, (2) optimizing the configuration of the foundation, (3) optimizing the geotechnical input parameters and (4) optimizing the mode of operation. The techniques described are examples of some tools in each class, but the implication is not that the tool box is complete. Much scope exists for adding new tools in each class. Current research at the Centre for Offshore Foundation Systems at the University of Western Australia is investigating new tools for predicting cyclic load response and settlements – tools that are simple enough to use but sophisticated enough to capture the necessary aspects of soil behavior. Many of the technologies described in this paper have been applied in practice supporting projects offshore Australia and globally.
This article is based on a longer paper Frontiers in Deepwater Geotechnics: Optimizing Geotechnical Design of Subsea Foundations by Susan Gourvenec and Xiaowei Feng, published in a special edition of Australian Geomechanics, December issue, 49(4).
Susan Gourvenec is a Professor at the Centre for Offshore Foundation Systems at the University of Western Australia. Gourvenec has more than 15 years of geotechnical engineering experience, with particular interest in offshore geotechnics. She is a consultant offshore geotechnical engineer to industry and member of the ISO and API Committees for Offshore Geotechnics.