Determining Appropriate Stiffness Levels for Spudcan Foundations Using Jack-Up Case Records

Abstract

The level of soil stiffness under spudcan footings is an area of intense interest and debate, with many practitioners believing that current jack-up assessment guidelines are overly conservative. In order to evaluate appropriate stiffness parameters, back-analysis of case records of jack-up platforms in the North Sea has been performed. The records relate to three different rigs at a total of eight locations, which include a variety of soil conditions, water depths and sea-state severity. For each site the horizontal deck displacements and the sea-state conditions under storm loading are available. Numerical simulation of the platforms under storm loading was undertaken with varying levels of foundation stiffness. For each set of stiffness one-hour of numerical simulation was performed, with the most severe recorded environmental loading conditions for that site used. The horizontal deck displacements of the measured data and the numerical simulation results have been compared in both the frequency domain and by the magnitude of response. On the basis of the analyses, recommendations can be made for higher stiffness factors then are currently suggested in the SNAME, 1997 Technical & Research Bulletin 5-5A, Site Specific Assessment of Mobile Jack-up Units (SNAME, 1997).

Jack-up push-over analyses featuring a new force resultant model for spudcans in soft clay

“…For clay, the value of shear modulus G may be taken as proportional to the undrained shear strength by G ¼ I r s u0 , where I r is known as "rigidity index". The industry guidelines (SNAME, 2008; ISO, 2012) and Cassidy et al (2002b) provide recommendations on selecting the appropriate I r values. The proposed model adopts the elastic stiffness coefficients reported by Zhang et al (2012b) which apply to a spudcan shaped footing and consider the influence of increasing shear modulus with depth as well as the effect of backflow.…”

Identification of jack-up spudcan fixity by an output-only substructural strategy

“…A field exercise of monitoring the dynamic behaviours and environmental conditions of three jack-up units (Monitor, Galaxy I and Magellan) was commissioned by GobalSantaFe (GSF) in the early 1990s [1,3,7]. This programme was continued with the Magellan at the Franklin and Elgin sites in the harsh North Sea [8,9].…”

“…As jack-ups move into deeper waters and harsher environments, there has been an increasing demand to understand jack-up behaviour better so as to avoid overly simplistic analysis and excessive conservatism. In particular, the appropriate value of soil stiffness under jack-up spudcan footings is a hotly debated topic and an area of intense research interest within the offshore engineering community [1,2]. This debate is fuelled in large part by uncertainty in our ability to predict accurately a jack-up's physical response to its environment as well as uncertainty as to what effect that response has on the jack-up structure [3].…”

“…Three jack-up drilling units in harsh environment, namely, Monitor, Galaxy I and Magellan operating in the North Sea, were monitored by permanently installed instruments [3,7,8]. Using the case records from the three jack-up units at a total of eight locations, Cassidy et al [1] determined the appropriate stiffness levels for spudcan foundations by using a trial and error method to adjust the numerical values until there was a good correlation with the measured results in both the frequency domain and the magnitude of response. Nataraja et al [9] calibrated Magellan's foundation fixity with adjusted soil spring stiffness resulting in the same measured and computed natural frequencies.…”

Offshore foundation systems for resource recovery: Assessing the three-dimensional response of jack-up platforms

“…Eqns (3) and (4) Suggestions for calculating a representative shear modulus are based on calibration with monitored offshore jackups, as outlined in Cassidy et al (2002b).…”