Jack-ups are designed to withstand specific environmental conditions at a defined water depth in combination with certain types of soil conditions. A balanced design results in an almost full utilization of the design elements under such conditions. Typical design elements used are the holding system (either the leg fixation system if present, or the jacking system), leg chord elements or the preload capacity. This means that if an owner wants to use the jack-up beyond the original design conditions, for example to operate in deeper water or in harsher conditions or with bearing pressure limitations, some of these elements would have to be upgraded. However, these elements are fully integrated into the overall design of the jack-up and thus an upgrade is always a very costly and often also a time-consuming affair.
A simpler way of upgrading the overall capacity of a jack-up is through the use of spudcan extensions. Spudcan extensions can be designed to replace the existing spudcan in the form of a new spudcan below the existing one at the bottom of the leg, a so-called add-on spudcan, or as a temporary or permanent extension of the original spudcan. In these cases, the impact of the spudcan extensions on the total jack-up are very limited, where in the best case the jack-up remains as-is and in the worst case only the lower ends of the legs are affected. The purpose of spudcan extensions is generally twofold:
Limit the leg penetration into the seabed, effectively reducing the free leg length below the hull;
Move part of the overall leg bending moment from the hull towards the spudcan, effectively reducing the loads acting at hull level;
The first purpose is relatively simple and usually results in just an enlargement of the total horizontal projected area of the spudcan. In this case, the spudcan extension can be a welded permanent structure, or if the owner wants to be able to go back to the original design it will be designed to connect temporarily to the chords of the lower end of the legs or the top plate of the spudcan, fitting around the existing spudcan.
The second purpose is a bit more complicated, as the extension must now be able to attract overall leg bending moments. This usually means that the extension must either be significantly bigger than the original spudcan or equipped with skirts, or a combination of both. The size and type of spudcan required depends heavily on the soil conditions of the intended area of operation. The end result is mostly in the shape of an add-on spudcan, but can also be executed as a permanent extension.
GustoMSC shall present the design considerations of typical spudcan extensions, apply spudcan extension to three well known types of GustoMSC jack-ups and show the influence of spudcan extensions to the capacity of the jack-ups.