3DENT Entries for the 2017 SNAME Offshore Symposium
Feb 6, 2017 News Archive
As mentioned in the Fall 2016 Newsletter, 3DENT personnel collaborated with various colleagues and submitted three abstracts for 2017 SNAME OFFSHORE SYMPOSIUM proceedings. All three papers were accepted and are included in the proceedings. Two of the papers were selected for presentation and the third was an “alternate.”
Before delving into the specifics of the three papers, we hereby thank our co-authors and their respective companies, for the privilege of working together and making the publication of these technical papers a reality. Additional thanks go to Wei Wang, for his work on the Case Study for Optimizing Rig Move Performance in the North Sea paper.
The first of our papers was presented by the lead author, Roy Cottrell. This paper is a follow up on a patent we applied for last year for a more efficient semisubmersible brace with improved fatigue performance and weight savings. I presented the other paper. The sections below present the specifics for the three papers.
GSF GALAXY I JACKUP CASE STUDY FOR OPTIMIZING RIG MOVE PERFORMANCE IN NORTH SEA USING AN ADVANCED SIMULATION MODEL
Damien Carre, Luis McArthur – TOTAL E&P UK
Andy Simpson, Philip Zhang, Ph.D. – TRANSOCEAN
Jose H. Vazquez, Ph.D. – 3Dent Technology
Through collaboration among TOTAL, TRANSOCEAN and 3DENT TECHNOLOGY, an initiative was launched to investigate the capabilities of the GSF Galaxy I jackup to perform a rig move in waves with periods greater than 6 seconds. The objective was to improve the rig move performance by reducing waiting on weather time.
Consulting for TRANSOCEAN, 3DENT started the study by conducting so-called “Going on Location” studies in regular waves and random waves to establish permissible wave heights as a function of period. 3DENT later developed a methodology for analyzing the coming off location and preloading at large draft conditions.
The response and therefore the limiting wave heights under which the various operations can safely be undertaken, requires a clear definition of the waves. Utilization of a wave radar, providing much-improved information on the waves when compared to either estimates by human observers or weather forecasts, was key to providing a verified and reliable input.
The response assessment program was successfully used while moving the GSF Galaxy I from the north to the south side of the TOTAL Elgin platform, saving 10 days of waiting on weather over a 15-day rig move. This application demonstrated it is possible to safely increase the environmental envelope for rig moving operations.
USING CEL TO ACCOUNT FOR SEABED DEFORMATION EFFECTS FOR JACK-UPS GOING ON LOCATION
Jose H. Vazquez, Ph.D., Barton D. Grasso, P.E., Marcus A. Gamino – 3Dent Technology
JACK S. TEMPLETON, III, D. ENG., P.E. – SAGE USA
State-of-the-art analysis methods to establish adequate limits for going on location include nonlinear time-domain response analysis of the units, accounting for jacking speed and spud can shape as well as wave directionality, water depth and soil conditions. The majority of analyses represent the seabed as a nonlinear spring, attempting to match the penetration curves which account for spud can shape and soil properties.
There are two significant effects that have not been addressed by these methodologies: the fact the seabed profile changes after each leg impact, and the energy dissipation properties of the soil. In this paper, the Combined Eulerian-Lagrangian (CEL) capabilities of Abaqus are used to determine both vertical and lateral soil resistance to leg motions of a representative jack-up while going on location. The CEL results are compared with the forces from idealized representations of the soil-structure interaction in the nonlinear response simulation.
The results of this study show that the impact loads for jack-ups going on location based on elastic representations of the seabed are over-predicted, and as such they can be considered to be on the conservative side when used in the determination of permissible wave heights.
SEMI-SUBMERSIBLE BRACE TO COLUMN CONNECTION – A MEANS FOR IMPROVED STRENGTH AND REDUCED STRUCTURAL WEIGHT
ROY H. COTTRELL, P.E. – COTTRELL & COTTRELL, ENGINEERING AND BUSINESS SERVICES
BARTON D. GRASSO, P.E., JOSE H. VAZQUEZ, PH.D., JUAN SANTOS – 3DENT TECHNOLOGY
Structural braces are provided between port and starboard columns or pontoons of semi-submersibles to reduce the bending moment at the column deck box connection created by squeeze/pry loads from hull-wave interaction. Traditionally, the braces have been designed as fixed end beam/columns for withstanding axial loads. The racking displacement of the brace ends results in brace end bending moment and induced stresses at the brace to hull connection. To handle the high bending moment, the brace wall thickness must be increased, which results in a stiffer connection. Because the racking deflection is controlled by the deck box, and not the stiffness of the braces, the magnitude of this deflection remains essentially constant regardless of brace stiffness. Thus the wall thickness increase simply attracts more bending moment, more stress and further increases in wall thickness. The additional reinforcement to resist the bending stresses typically results in substantially heavier structure than required by the compressive and tensile squeeze/pry loads alone, that they are intended to resist and control.
The authors have developed and analyzed an alternate structural arrangement that reduces the bending stiffness of the braces at their connection to the hull, allowing the braces to act more like pin ended column elements.