It is essential in the design of gravity foundations under cyclic loading that inadmissibly large reductions of the subsoil bearing capacity due to the accumulated pore water pressure (loss of stability) and tilting of the foundation caused by the accumulation of settlements (loss of serviceability) are ruled out. This paper describes a procedure to predict these phenomena in non-cohesive subsoils. Real irregular loads during the service life of the structure can be idealized into parcels of uniform cyclic loads. Using the “High Cyclic Accumulation Model” proposed by Niemunis [1] the strain accumulation and thus the settlement of the subsoil due to the cyclic loads occurring within the service life of the wind turbine can be estimated. The pore pressure development is analyzed by dividing the mechanism in a pore pressure build-up phase in undrained condition (analytical calculation) and a dissipation phase (3D-FEM-Calculation).
Difficulties are encountered, in soft clay and silt sediments, during extraction of 25 the spudcan foundations of 'mobile' jack-up rigs. This paper reports results from centrifuge 26 model tests undertaken to provide a simple, effective and practical means for easing spudcan 27 extraction in soft normally consolidated and lightly overconsolidated clay. The tests were 28 carried out on model spudcans of two different geometries typically used in the field. The 29 spudcans were extracted from penetrations of 2~3 diameters and after an operation period of 30 ~19 months. The measured pore pressure at the bottom face of the spudcan during extraction 31 process provided insight into the development of suction at the interface between the spudcan 32 base and the underlying soil. 33The tests were carried out with and without a sand layer placed locally in-between the 34 spudcan and the mudline at the onset of penetration. In the field, a gravel layer can be placed 35 by means of a flexible fall pipe system. The presence of a sand layer (with a thickness less 36 than the spudcan tip length) beneath the penetrated spudcan assisted in easing leg extraction 37 difficulties allowing the base suction to be reduced by 73~83% and hence the breakout force 38 by 31~32%. For a given penetration depth, the installation resistance was also increased by 39 24~26%. This yielded a 15~17% reduction of spudcan penetration depth under a given 40 preload and hence the breakout force as much as 50%. Guidelines and cautions for using the 41 proposed measure are provided. Additional advantages of the new recommended method in 42 enhancing the efficiency of the conventional measures of lessening spudcan extraction 43 difficulties are discussed. 44
It is now recognized that seabed trenching in taut deep water moorings can reach the suction anchor and the chain padeye depth, with the safety of the mooring system becoming a concern. However, the actual formation of such trenches and their development with time are still largely unknown, as well as the relation with the characteristics and dynamics of the mooring systems. The analysis of a database of subsea inspection data from deep water Gulf of Guinea mooring systems is presented. The trench data, from in-service subsea inspections of mooring systems for five FPSO vessels and off-loading terminal buoys, comprise the measurement of trenches width and depth and of anchor-to-trench distances. Because similar metocean and soil conditions apply to the ten mooring systems, the study offers a unique database for better understanding the formation and evolution of this seabed trenching. The subsea inspections were carried out between 2014 and 2017 but the study covers a much larger time span of moorings performance as the FPSOs and off-loading buoys were installed between 17 years and 5 years ago. The trench development is studied in relation with the moorings characteristics and dynamics and is illustrated with typical features and trench geometries for the two types of floaters. The results obtained represent a key improvement for understanding this seabed trenching phenomenon and are most probably applicable to all deep water moorings of the Gulf of Guinea area. Preliminary recommendations about design aspects and possible mitigations that can be considered for limiting the seabed trenching are also addressed in the paper.
Jack-up geotechnical hazards such as unpredicted leg penetration, rapid legpenetration and punch-through continue to occur at an increasing rate despiteefforts by the jack-up industry to minimize these risks. Improvements ininstallation guidelines and site specific assessment are essential to achievesafe jack-up rig installation. Assessment of spudcan penetration is one of thekey aspects required in a jack-up site specific assessment. An accurate spudcanpenetration prediction underpins reliable site specific assessment. In thispaper, current practice for spudcan penetration prediction is reviewed. Designapproaches to obtain a spudcan penetration curve from field penetrometer dataare proposed. The design approaches to predict spudcan penetration are thenincorporated into an integrated jack-up installation system. The systemcalculates the spudcan penetration curve based on penetrometer data and thenclosely monitors progression of spudcan installation. The aim is to assistjack-up operators in making decision on what measures should be taken during anoffshore installation to prevent or mitigate a potential geotechnicalhazard. Jack-up Installation and Associated Potential Geohazards Most of the world's offshore drilling in water depths up to 150 m is performedfrom mobile jack-up rigs (see Figure 1). A unique feature of jack-up rigs istheir self-installing capability. This differentiates the design of its spudcanfoundations from most conventional offshore and onshore foundations, as theirperformance has to be reevaluated each time the jack-up installs at a new site. Prior to commencing jack-up operations, spudcans are routinely proof tested bystatic vertical preloading, with each of the (usually) three spudcans installedsequentially or simultaneously. Geotechnically, this increases the size of thetheoretical yield envelope in combined vertical, moment and horizontal loadspace, and thus ensures each spudcan has sufficient reserve capacity in anextreme storm event (SNAME, 2008). For three-legged jackups, preloading isaccomplished by pumping seawater into holding tanks within the hull. Thiscauses the spudcan foundations to penetrate into the seabed until the load onthe spudcan is equilibrated by the resistance of the underlying soil. Thepreload is then dumped and the hull is elevated to provide an adequate air-gapduring subsequent operation. Several potential geohazards arise due to this method of installation. Hazarddescriptions and statistics collated from reported case histories are providedin McClelland et al. (1981), Sharples et al. (1989), CLAROM (1993), Jack et al.(2001, 2007), Kvitrud et al. (2001), Hunt & Marsh (2004) and MSL (2004). Rapid leg penetration and spudcan ‘punch-through’ were identified as the mostcommon geohazards that lead to serious consequences. These incidents mostlyoccur during installing and preloading jack-up rigs in stratified deposits, where a surface or an interbedded strong layer overlays a weaker layer (Figure 2). Punch-through may lead to structural damage or failure of the rigs and evenrisk safety of the personnel on board (Aust, 1997; Maung & Ahmad, 2000;Brennan et al., 2006; Kostelnik et al., 2007; Chan et al., 2008). Due to theseinstabilities, the jack-up may also collide with the adjacent fixed platform itis drilling for. Punch-through is defined as a sudden leg penetration due to adrop in bearing capacity (see Figure 2), and where the legs cannot be jackedfast enough to maintain the hull at a horizontal level. The extent to whichrapid leg penetration may be controlled by the rig, avoiding punch-through, hasimproved with advances in jack-up technology and geotechnical knowledge.
Spudcan foundations for mobile drilling rigs continue to exhibit a high failure rate in the offshore oil and gas industry. The more frequent use of larger jack-ups in highly stratified regions, such as the Sunda Shelf in Southeast Asia, contributes to this concerning increase in ‘punch-through’ incidents, which can lead to buckling of the leg or even toppling of the rig. This paper reports measures for mitigating spudcan punch-through in multilayered soils. Centrifuge model tests were carried out on spudcan foundations penetrating through multilayered soils with interbedded stronger layers.The model tests included half- spudcan tests against a transparent window, allowing visualization of the soil flow, and full-spudcan tests to measure penetration resistance. The soil conditions tested simulatedoffshore strength profilesthat have reported punch-through failures, such as on the Sunda Shelf in Southeast Asia, Gulf of Thailand, South China Sea, offshore Australia and the Arabian Gulf. An experimental method for ‘drilling’ sites in the enhanced gravity environment was developed. An innovative spudcan was developed to integrate directly to a jack-up rig to be used where there is a potential for punch-through hazard. The experimental results show that for layered clay deposits, potential of severe punch-through failure can be eliminated by means of perforation drilling or using an innovative spudcan. For sand-over-clay sediments, using skirted foundations or the innovative spudcan allowed for eliminating the likelihood of failure. The results also show that spudcan-footprint interaction issues for reinstallation of a rig can also be mitigated through perforation around the spudcan periphery.
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