Self Elevating Units - Part 2
Oct 1, 2014 News Archive
The second edition of our write up on self elevating units focuses on one of the modes of operation often not 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, as it most clearly describes what the unit is attempting as it lowers its legs 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 three 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 as the legs are lowered and make impact with the seabed. While it can easily be argued that a number of parameters need to be calibrated before fully trusting the results of an analytical study, we believe that the methodology we have is already beneficial for understanding trends, and certainly for comparing capabilities of different designs.
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 of 1-2 feet past the spudcan tip.
- Stiff Soils – Soils that produce penetrations that do not engage the full bearing area of the spudcan.
- Intermediate Soils – Soils that produce penetrations that engage the full bearing area, but do not exceed 10ft.
- Soft Soils – Soils that produce penetrations in the 10-30ft range.
- Very Soft Soils – Soils that produce penetrations greater than 30ft.
Alternatively, a sense for stiff or soft soils can be assessed for sands based on their internal friction angle, and for clays based on their shear strength value, Cu.
A good 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 is in the “pass/fail” criteria.
The simulation can be divided into four segments, as described and illustrated 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.
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. As mentioned at the start of this write up, the results vary with the choice of damping value used. For larger damping values, permissible heights increase, but associated angles decrease. Therefore, since the curves are intended to be used together, there is a self-correcting aspect to this approach.