The authors address economic impact of quality and availability of site soil information on safe jackup installation. Preload operations based on sufficient and on limited (or insufficient) soil data are analyzed. Reliability and applicability of a variety of soil information sources are discussed.
In order to prevent jackup settlement during drilling or during storm conditions, the jackup unit is preloaded during the initial stages of the installation procedure. The preloading of a three leg unit is normally performed by elevating up to a 3-5 feet air gap. Ballast water is then gradually added to preload tanks until the weight of the unit loads the soil under the spud cans to a level equal to or in excess of the anticipated spud can loads for the design storm conditions. During the preload process, increasing rig weight usually causes the legs to penetrate deeper until the bearing capcity of the soil becomes equal to or greater than the spud cans loads. One of the main potential problems during preloading is a sudden uncontrolled leg penetration caused by (a) a spud can punching through the stronger soil layer into the underlying weaker soil, or (b) by unit leaning instability in uniform soft clays.
The results of uncontrolled penetrations can be disastrous. There are some preloading strategies that can lessen or even eliminate the magnitude of uncontrolled penetrations and reduce the possibility of rig leg damage. One strategy is to preload at zero air gap (also termed "preloading in the water"). As preload is applied and the spud cans penetrate into the soil, the rig draft increases. However, to achieve full preload, the draft of the rig hull typically must be limited to about 2 feet. At drafts greater than this, the buoyancy of the hull will not allow full spud can loading. As shown on the Fig. 1, this procedure can significantly reduce the potential peak value of the soil loading during punch through, and resulting differential leg penetrations.
Another strategy to lessen the magnitude of uncontrolled penetrations is to sequentially preload each leg of the rig. Because the total weight of the rig is reduced, the load during the punch through is also reduced. Also, the other legs, due to lower load can be jacked during a punch through to lessen the out-of-level condition of the rig, and lessen the load increase on the punch through leg. When this procedure is used together with zero air gap, dramatic reductions in spud can loading during punch through can be achieved, as illustrated in Fig. 2.
To plan preloading operations, expected rig foundation performance should be evaluated. This information requires subsurface soil data including soil stratigraphy and engineering properties. The following information is typically gathered to assess MODU foundation stability:
Geological information for the area, to understand physical environment.
Site bathymetric data, to provide accurate water depth and seabed topography.