Oct 1, 2015 News Archive
The picture above, courtesy of Dr. Choo, was taken during the conference’s dinner. One thing to note is that the conference is truly international, as everyone at my table was (at least) bilingual.
3DENT attended the 16th biennial International Jackup Conference at City University in London this past month. It had been a number of years since I last attended and I found it to be a good experience. It was good to catch up with some old acquaintances and see some of the advances in soil-structure interaction as well as on the going on location analyses, which is one of 3DENT’s expertise areas. One paper, in particular, caught my attention. With the presenting author’s permission, I am including a brief summary of that paper below.
The paper I am reviewing is entitled “Overcome Jack-up Rig Installation Problem in Difficult Soil Condition in East Sea – A Case History” and it was written by X.Li, P. Handidjaja, T.C. Teh, and D.Q. Nhut, of Braemar Technical Services (Offshore) Pte. Ltd. This overview and the pictures from the paper are included herein with permission from the presenting author, Paulus Handidjaja, to whom I hereby express my sincere thanks for allowing me to do so. Paul can be reached by email at firstname.lastname@example.org.
Figure 1, below, is a composite picture show from various figures on the paper, showing the general site location, seabed bathymetry and the top 4 meters of borehole data. As can be seen, the soil consists mostly of loose brown silty sand with an internal friction angle of 20 degrees, and the bathymetry shows the presence of sand ripples (the result of bottom currents 1.3m/s).
In 2009, a Mitsui MODEC 400 C-35 jackup was installed at this site, but it was operated only for a short period of time during the monsoon. In 2011, a wellhead platform was installed over the existing well. A Keppel Fels Mod V B jackup was installed after the platform with the predicted shallow penetrations. As described in the background section of the paper, “with the unit operating at a high airgap… over the Northeast Monsoon period… a Tropical Depression approached the unit in December 2011; [and] the unit “walked” and later on found structural failure on the legs… The unit was removed shortly thereafter.”
In August 2012, another B-Class jackup (this time a “Big Foot”) was installed but its operations were limited to the end of the southwest monsoon. During this installation, regular ROV surveys of the spudcans were conducted and it was found that the movement of the sand around the spudcan was erratic and high current was imminently present even in the Southwest monsoon (See Figure 2).
With the operator wanting to have a solution that would allow operations during any typhoon season, a novel approach was taken. As mentioned in the paper, “in 2013, a Friede & Goldman JU 2000E design was selected to drill over the platform. But this time, ground penetration was studied and considered for the installation to eliminate or minimize the scour potential problem.”
The proposed solution was to use Mass Flow Excavation (MFE) to “prepare the ground such that potential scour undermining the footing foundation can be minimized or eliminated… The MFE is a tool of non-contact dredging operations, providing an efficient method of clearing sediment from subsea structures without damaging the structures themselves.” Figure 3 shows a compilation of pictures from the paper showing the MFE device and a schematic of the way it is used from a floater. The figure also shows seabed scans before and after the excavation. Figure 4 shows soil cross-section profiles for all legs. The idealized design excavation profile had 40m diameter at the seabed, 18m diameter at the bottom and 5-6m depth.
Results and Conclusions
As indicated in the paper, the excavation was achieved within the allotted time frame of 5 days, and although the achieved profiles did not match the design profiles perfectly, they were of sufficient depth for spudcan embedment, with a minimum “nominal” depth of 3.5m achieved at all spudcan locations. Despite the rig being delayed, the cross-sections held and upon installation it was confirmed that the spudcans reached the minimum target penetration such that the section of maximum bearing area was below the nominal seabed elevation. Lightship and full-preload penetrations were the same, and recorded as 4.75 +/- 0.10 meters for all three legs (with the starboard leg having the lowest penetration at 4.65m). ROV survey inspections showed the starboard leg penetration was marginally penetrated to the widest section.
To conclude the paper, the authors highlight the following points about the use of the MFE and the excavation as a way to minimize or eliminate the [scour] risk:
- Quick to clear the sediment
- Excavating the area can be seen in real-time…
- Improves the foundation and structural integrity
The authors also mention some disadvantages of MFE, as follows:
- Once the holes are made, [that] set of footprints is only applicable for that typical size of rig with a specific rig position.
- A thorough soil information assessment must be carried out in order to know if the MFE is able to be carried out. There is a limitation to types of material and strength that can be cut.
Not mentioned in the paper, but discussed during the presentation, is the fact that continued survey during the year-plus drilling campaign showed that the spudcan bottoms remained below the mudline (i.e., there was essentially no scouring). The jackup has since gone off location and a new campaign is under way. MFE will be used again, as the new jackup has different leg spacing.