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This Part 2 of our writeup on self elevating units, focuses on a transition between modes of operation seldom addressed in design. This mode goes by different names such as “installation,” “pinning,” “leg lowering,” or “going on location.” We use the going on location term (GoL), as it most clearly describes what the unit is attempting to do when its legs are lowered to transition from the afloat condition to an elevated condition at a specified location.
As of the time of this writing, there is no clear accepted methodology for going on location analyses, but over the past few years, 3DENT personnel have worked with ABS Americas, jackup designers and drilling contractors to develop a methodology that closely represents the behavior of a jackup during this transition when the legs make contact with the seabed. To date, 3DENT has performed GoL analyses on over a dozen jackup designs. From these analyses, it is clear that some jackups have higher GoL capacities than others, due to their overall size, jacking system capacity and spudcan shape.
It can easily be argued that a number of parameters need to be calibrated before fully trusting the results of an analytical study, but we believe that the methodology we have developed is already beneficial for understanding trends, and certainly for comparing capabilities of different designs.
A good GoL response simulation will account for jacking speed and spudcan shape. 3DENT has developed two approaches for generating permissible wave height and rotation curves for a given water depth and representative soil stiffness: A simplified approach and a more robust approach. Both approaches are based on three analyses (diffraction, structural and non-linear response). The main difference between the two approaches is in the “pass/fail” criteria.
The simulation can be divided into four segments, as illustrated and described below.
Ramping segment – Time prior to the actual start of the simulation during which all wave heights are gradually increased from 0 to the target value.
Free-Floating segment – Time starting at t=0 sec, to the first time there is contact between the seabed and any of the members representing the spudcans.
Transition segment – Time from the first time there is contact between the seabed and any of the members representing the spudcans, until all legs have made contact and the hull starts to come out of the water. In other words, the transition segment is the time between the free-floating segment and the elevating segment.
Elevating segment – time after all legs have made and remain in contact with the seabed, resulting in minimal motions except for the elevation of the hull.
3DENT has conducted simplified GoL analyses for a number of different jackups of various designs. The result from a simplified GoL analysis is a curve of permissible wave heights for a single water depth and representative soil stiffness. In addition to the permissible wave height curve, a second curve is generated to show the corresponding associated rotation angles (referred to as the permissible angle curve). These curves are intended to be used together when making a decision as to go on location or not. For simplicity (as well as for cost considerations and conservatism), a single curve is usually produced for waves from the most onerous direction and used for all wave directions.
3DENT has also conducted more robust GoL analyses for a few different jackup designs, producing curves for different directions, water depths and soil stiffness values for both regular and random waves (i.e., nomograms).
For sure, each unit exhibits a unique response, producing a unique permissible wave height curve. However; most permissible wave height curves tend to follow a generalized concave-up shape and then they flatten out. At smaller periods, both heave and pitch are small, thus permissible wave height values are high. Then, as both pitch and heave increase, the permissible wave height values reach their lowest points (at periods in the vicinity of the natural periods). Then, as pitch decreases for the larger periods, the curve starts to increase again, and eventually the results become heave dominated.
To be sure, the results vary with water depth, direction and whether the analyses are based on regular waves or random waves. When directionality is included, using the results is best done with the aid of a simple Excel Program. This program allows for the combination of swells (thought of as represented by regular waves) and wind-driven waves. Instead of showing a curve with permissible wave heights or hull rotation angles, the program calculates the utilization ratio (often referred to as the unity check or UC) for the conditions at hand. The program can also accommodate cases when the user has access to multi-peak spectral data. The figure below shows the simple GUI for this program.
Finally, it is noted that terms such as “stiff soil” or “soft soil” are not clearly defined. The following distinctions are offered for consideration:
Very Stiff Soils - Soils that produce penetrations at full preload of 1-2 feet past the spudcan tip.
Stiff Soils - Soils that produce penetrations at full preload beyond 2ft past the spudcan tip but do not engage the full bearing area of the spudcan.
Intermediate Soils - Soils that produce penetrations at full preload that engage the full bearing area, but do not exceed 10ft.
Soft Soils - Soils that produce penetrations at full preload in the 10-30ft range.
Very Soft Soils - Soils that produce penetrations at full preload greater than 30ft.
Alternatively, a sense for stiff or soft soils can be assessed for sands based on their internal friction angle, f, and for clays based on their shear strength value, Cu.
As of now, GoL analyses have been carried a number of simplifications in the modeling of the soil. In 2017, and working with Jack Templeton (of SAGE USA), we published a paper entitled SEABED MODELING EFFECTS ON JACK-UP RESPONSE WHILE GOING ON LOCATION. The paper “presents the results of a study which uses the spudcan motions from a time domain GoL analysis in OrcaFlex as input to a Combined Eulerian-Lagrangian (CEL) analysis completed in Abaqus.”
It is our expectation that soon we’ll be doing these analyses fully coupled. With calibration, the coefficients needed to make valid predictions will soon arrive and GoL operations will become safer.
Reference Material
Vazquez J.H., Michel R.P., Alford J.H., Quah M. and Foo K.S. “Jackup Units: A Technical Primer for the Offshore Industry Professional.” Published by Bennett & Associates and Offshore Technology Development, 2005.
Vazquez J., Grasso B., Gamino M. and Wang, W. “Jackups Going on Location - Understanding Energy Principles on Leg Impact Loads.” Proceedings of the 21st Offshore Symposium, February 2016, Houston, Texas.
Carre D., McArthur L., Simpson A. Zhang P. and Vazquez J.H. “GSF GALAXY I Jackup Case Study for Optimizing Rig Move Performance I North Sea Using an Advanced Simulation Model.” Proceedings of the 22nd Offshore Symposium, February 2017, Houston, Texas.
Vazquez J.H., Grasso B.D., Gamino M.A., Templeton J.S., “Using CEL to Account for Seabed Deformation Effects for Jack-ups Going on Location.” In: Proceedings of the 22nd SNAME Offshore Symposium. February 2017. Houston, Texas.
Vazquez J.H., Grasso B.D., Gamino M.A., Templeton J.S., “Seabed Modeling Effects on Jack-ups Going on Location.” In: 2017 International Conference: The Jack-Up Platform at City University in London. September 2017.