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Installation Constraints of Suction Assisted Foundations and Anchors for Offshore Energy Development

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Installation Constraints of Suction Assisted Foundations and Anchors for Offshore Energy Development International Journal of GEOMATE, July, 2018 Vol.15, Issue 47, pp.14-21 Geotec., Const. Mat. & Env., DOI: https://doi.org/10.21660/2018.47.3695 ISSN: 2186-2982 (Print), 2186-2990 (Online), Japan INSTALLATION CONSTRAINTS OF SUCTION ASSISTED FOUNDATIONS AND ANCHORS FOR OFFSHORE ENERGY DEVELOPMENT *Kingsley Osezua Akeme1, Alireza Rezagholilou2 and Meysam Banimahd3 1,2Faculty of Engineering, Curtin University, Australia; 3 University of Western Australia, Australia *Kingsley Osezua Akeme, Received: 30 May. 2017, Revised: 27 Jul. 2017, Accepted: 14 Aug. 2017 ABSTRACT: The application of suction caissons has increased over the last two decades for offshore energy developments. Their installation challenges in different seabed types affect their execution process and load bearing capacity. Hence, the identification of these challenges and understanding their root causes are highly important. As such, this paper aims to review the recorded installation constraints due to different seabed conditions, discuss the various factors related to each of these constraints, and finally provide some suggestions to rectify each constraint and/or its relevant factors. To do so, the approach is to evaluate the geological (geophysical and geotechnical) conditions in multiple case studies and analyze the stability of suction caisson installation in different soil types. Results show that some factors such as: plug heave contributes about 29% of the installation issues in both homogeneous clay soils and layered clay soils, soil piping and bottom resistance failure contribute about 16% and 10% respectively of the installation issues in both sand and layered sand soils, while high penetration resistance contributes about 23% of installation issues in layered soils. Also, the uncertainties of soil parameters or behavior is a complimentary factor which add more complexities into the above-mentioned factors. Therefore, a good understanding of the seabed conditions and soil parameters before and during installation, as well as constant monitoring of the induced suction created during penetration with respect to the penetration depth is essential to mitigate the likely issues. Keywords: Suction, Plug Heave, Soil Piping, Penetration Resistance, Bottom Resistance Failure. 1. INTRODUCTION structure e.g the Gullfak C platform, or as (2) a standalone structure when used for mooring and 1.1 Definition anchorage support [6]. Some examples of other suction assisted foundation and anchors include: Suction assisted caissons have been defined as suction anchor, suction pile, bucket foundation, large cylindrical steel structures with large skirted foundation and skirt piles etc. diameter opened at the base and closed by a lid at the top. The top contains a “valve controlled vent” which allows the escape of water from the caisson during installation [1], [2], [3]. However, in more detail, they are defined as foundation structures that are installed because of a differential pressure created within the caisson compartment through the application of a pump attached to the caisson top which reduces the internal pressure within the caisson compartment as shown in Fig 1. Suction caissons are classified into two categories based on their application and function. By application, they are classified as: (1) Fig 1 A Cluster of suction anchors adapted from Compression caisson foundations when they are [6] used in shallow water applications as foundation supports, and as (2) Suction anchors when they are 1.2 Installation Procedure used in deep water applications as mooring and anchoring supports [4], [5]. In general, suction assisted foundations and In terms of their function, they are classified anchors have the same installation principle in either as (1) an integrated foundation structure which a suction induced force is required for their when they are attached to the base of a foundation penetration into the seabed. Also, their installation process occurs in two-stages. In the first stage, 14 International Journal of GEOMATE, July, 2018 Vol.15, Issue 47, pp.14-21 they penetrate the seabed by their self-weight. As value (allowable suction) which corresponds to the the process occurs, the water displaced inside the point where the water within the caisson begins to caisson is evacuated through an open vent. The vaporise. This maximum value is equal to sum of velocity of water that can be discharged through the atmospheric pressure and hydrostatic pressure the open vent with respect to the over pressure outside the caisson. Once the design depth has within the suction caisson is given by the been reached, the valve at the top is closed and a following equation: passive suction is generated within the caisson. This passive suction increases the holding capacity V=C when an Orifice is used (pull - out resistance) of the caisson by engaging 2 negative pore pressure from the soil plug inside (1) √ and at the bottom of the caisson [4], [8], [10], [11]. Generally, the allowable suction for caisson V= when a piping system (valve) is used penetration before plug failure (Reverse end 2 bearing failure) occurs can be calculated using the (2) � ∑ following equation: Where V= steady velocity of water flow, C= = / + / ( FS) Coefficient of discharge (C=0.61 for sharp edged (3) orifice surface and C= for sharp edged orifice surface with short tube), P= over pressure in the Where = allowable suction, = internal suction caisson, = sea water density, and = caisson wall frictional resistance, = Caisson Resistance coefficient for the piping system ( = internal crossectional area, = reverse end 1.7 - 1.8 for a 2 ft or 3 ft diameter butterfly valve bearing over the all soil plug area at the caisson tip, ∑ system). FS = factor of safety [7]. However, suction caisson would stop Once the suction assisted foundation has been penetrating under its own weight when the soil installed, a set up time is allowed to enable the resistance, including external and internal surrounding soil to regain some of its strength lost frictional resistances and tip resistance is equal to during installation before any load attachment is the weight of the suction caisson [7]. done. During this period, the caisson remains An important phenomenon in this stage is the unloaded. Set up time is mostly observed during formation of a seal between the seabed surface and mooring application [5]. the caisson. The importance of the seal is to prevent the formation of localized piping and loss of suction during penetration. It is thus important that the seal formed must be sufficient to guarantee suction assisted penetration. However, if an insufficient seal is formed this would result to failure of the entire installation process [4], [8]. The second stage consist of the later part of the penetration process of arriving at the design depth. This is achieved by the application of an additional installation force which is provided in the form of an under pressure or suction created by pumping water out of the caisson through a “submersible pump” attached to the top of the caisson as shown in Fig 2 [4], [8], [9]. The suction generated, is the total pressure difference between the caisson internal vacuum pressure and the external hydrostatic pressure. If the pressure difference created is relatively higher than the external hydrostatic pressure, the suction caissons may buckle during installation. Also, an excessive Fig 2 Suction installation process adapted from [6] under pressure or suction created within the caisson compartment could result in the upward METHODOLOGY movement of the internal soil within the caisson which could lead to plug failure. Therefore, the The approach adopted in achieving the amount of suction available for penetration ranges objectives of this paper is carried out in two parts. from a minimum value when the caisson internal The first part is a review of the geological pressure is at zero absolute pressure to a maximum (geotechnical and geophysical) survey conducted before and during installation as well as the 15 International Journal of GEOMATE, July, 2018 Vol.15, Issue 47, pp.14-21 stability assessment of suction caisson during 2.1.3 Soil strength profile installation. While the second part is a This comprises of the drained and undrained comparative analysis of the installation issues of shear strength profile of the soil from the mud line suction assisted foundations and anchors obtained down to the seabed. This is very important to from literature reviews and over 50 case study know especially in homogeneous clay soils and in field reports. layered structured soils to avoid plug heave and punch through during penetration [7]. 2.1 Geological Properties of Soil That Affects the Installation of Suction Caissons 2.1.4 Seabed topographical irregularities The uneven nature of the seabed could cause Prior to the installation of suction caissons, a problem during installation especially for suction proper site investigation is conducted to select the caissons in cluster form where cracks are formed right location for installation, and to establish the in the caisson walls during penetration. To geotechnical installation parameters needed for a overcome this challenge, an optimal levelling successful installation campaign. To this end, a procedure of the suction caisson should be done geotechnical survey involving in-situ and immediately after self-weight penetration process laboratory tests is conducted to obtain information but before the suction assisted penetration process about the
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