Looking for Damping Values – In-House Testing
Aug 8, 2017 News Archive
This is the first installment in what we hope is an informative foray into (informal and inexpensive) model testing. We are not professional model makers and have no experience in building a model from scratch. While you will not see the results of these tests presented at a conference, they will lay the ground work for future testing.
We at 3DENT are interested in investigating the total effective damping of a Jackup, as it pertains to Going on Location and Coming off Location operations. In a near perfect world, we (the industry) would pool some money and pay for some extensive model testing at a top-notch facility to validate each and every little thing our hearts desired. Unfortunately, this isn’t a perfect world. With limited resources for testing we must, in the words of Gunnery Sergeant Highway, “Improvise, Adapt, and Overcome.”
Our goal is to create a model that is large enough to produce meaningful results and small enough to be portable, tested in a small tank, and involve minimal handling equipment. Size-wise, we could easily transport a 1:70 scale model at roughly 3 ft long x 3 ft wide, with the legs being 6 to 7ft. However, the scale weight would be around 75 lbs. The hull model would need to be designed to handle that weight without excessive deflections. This is all very possible, however the time commitment and cost rise with the complexity of the structure. The “Test Facility” is yet to be determined and it will most likely be a small tank, somewhere in the range of bathtub size. We are considering putting something together ourselves, a 4ft x 4ft x 3ft deep tank (~ 300 gallons) with one side made of clear acrylic. A swimming pool is also a possibility.
With the size restrictions in mind we settled on a 1:160 scale for our model. The particulars are below for the generic Jackup:
At this size, we can use our 3-D printer to print the hull and spudcans. Our 3-D printer has a 12” x 12” footprint capacity, breaking the hull into 3 sections it was easy enough to create the hull with 3 prints. The 3-D printer lays down layers of ABS melted plastic, the print is created by adding additional layers until the full object is completed. The print is rigid enough to support the 7 lbs model weight without further reinforcement.
The hull and spudcans are straightforward, the legs presented us with a couple of issues to overcome. A truss leg at this scale isn’t easily done. While we could obtain small diameter tube, building the leg would be tough and the scale weight would be much larger than our target. We borrowed a page from SNAME 5.5A and will use an equivalent diameter leg. The equivalent leg calculation gave us a number close to 1”. We were able to source 1” OD aluminum tubing that is light enough to get us to our target weight per length of the leg. The leg particulars are below:
The equivalent leg will be flooded during our test and the impact of the water in the “leg” will be examined. Our plan is to compare a series of different legs to gauge the impact of different configurations of holes drilled in the legs. We are considering the following options:
- A single hole (1/4”) at the bottom of the leg (plus a venting hole, of course).
- A series of equally-spaced vertical holes (1/4”) in one side of the leg.
- A series of equally-spaced vertical holes (1/4”) through the leg alternating 90 degrees.
Currently, the hull, spudcan, and single hole legs are finished (see picture below). The next installment in this series will cover adding weights to approximate the mass distribution of our target Jackup and a discussion of our test equipment. If things progress smoothly we may have some preliminary results to discuss as well. Please feel free to chime in with any thoughts comments or feedback.