FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
THE PATENTS RULES, 2003
Provisional/ Complete specification
[See section 10 and rule 13]
1- Title of invention :
"PUNCH THROUGH PREVENTION DURING PRELOADING OF OFFSHORE JACK UP RIG IN 80M WATER DEPTH BY BREAKING HARD SEA-BED OVERLYING SOFT CLAY"
2. Applicant(s):
Name Nationality Address
Oil and Natural Gas India Institute of Engineering and
Corporation Ltd. Ocean Technology, ONGC
Complex, Phase - II, Panvel -410221, Navi Mumbai, Maharashtra, India. AI-410221
3. Preamble to the description:
The following specification particularly describes the invention and the manner in which it is to be performed.

PUNCH THROUGH PREVENTION DURING PRELOADING OF OFFSHORE JACK UP RIG IN 80M WATER DEPTH BY BREAKING HARD SEA-BED OVERLYING SOFT CLAY
BACKGROUND OF THE INVENTION
1. Field of the Invention:
Present invention relates to a method of breaking the hard crust of sea bed formation overlying soft clay in any offshore location in water depth up to 80 meters to avoid punch through situation during preloading operation of three-independent-legged jack-up rigs.
2. Description of the Prior Art:
Exploration of hydrocarbon in offshore involves installation of independent leg type offshore jack up rigs with spudcan foundations. The three-independent-legged jack-up rigs are huge steel structures with light ship weight of about 8,000 metric tons fitted with equipments for drilling into seabed in search of hydrocarbon. When all the three legs are raised, the rig becomes floating barge and can be towed to desired location for drilling or other intended purposes. The leg or the barge is raised or lowered by rack & pinion type jacking system or by hydraulic jacking system. For installation of the rig at location, the legs are lowered at the planned location and pinned to seabed. The barge

is supported on its three legs at seabed. Before the barge is raised to desired air gap of about 14 to 16 meter for drilling operations, the sea bed soil formation is tested to prove the bearing capacity of soil to be strong enough to support the dead weight of the rig and also additional working load and environmental/storm load. This process of testing the bearing capacity of seabed is called preloading or pre-driving. During preloading; sudden penetration normally of one leg because of failure of soil below the rig footing is called punch through of the rig. Punch through causes the tilting of the barge, damage to its structure and other nearby structures.
One such incident had occurred in Indian offshore oil field, west of Mumbai, at a location 'ID' (Latitude 19°18'58.31" N, Longitude 71°18'36.19" E, water depth 76.4 m below chart datum). The soil strata description of the location 'ID' is shown at Figure 1 and the phenomenon of punch through is shown at Figure 2.
The Possible options/available techniques for installation of Jack up
rig for location with geotechnical data showing potential punch
through or rapid penetration probabilities are given below:
(i) Breaking the hard crust by drilling maximum number of holes of
maximum possible size in desired locations. Drilling numbers
of holes in hard crust in the expected area of footing of the rig
has not been successful to solve the punch through risk for

safe deployment of jack up rig. (ii) Dredging and deepening of the location of the pug marks.
Dredging and deepening of the desired location /foot-print in 80
meter water depth is practically not viable. (iii) Loosening of the top crust by controlled explosion is ruled out
as it can have adverse effects on pile foundation of the vyell
platform. (iv) Jacking and drilling of the location by suitable type of rig or mat
type jack-up rig having hydraulic jacking system which can put
preloading pressure to break the hard crust safely. However,
rig operators do not come forward to offer service at location
with potential punch through risk. Hence, in normal situations the punch through locations is avoided for rig deployment. With the depletion in worldwide hydrocarbon reserves, one can not afford to skip the exploitation of oil and gas reservoirs from locations having potential punch through probability. Hence, there is a need for a technique to facilitate deployment of jack up rig in offshore locations which are prone to punch through.
Principal object of the present invention is to break the hard crust of sea bed formation overlying soft clay in any offshore location in water depth up to 80 meter to avoid punch through situation during

preloading operation and installation of offshore jack up rig safely to facilitate exploitation of hydrocarbon from such offshore locations.
DETAILED DESCRIPTION OF THE INVENTION:
The present invention relates to a method to break the hard crust of sea bed formation overlying soft clay in any offshore location in water depth up to 80 meter by using Offshore, Houston 4 leg design class jack up rig. The Offshore, Houston 4 leg design class jack up rig has hydraulic jacking system. Its footing has two components, viz. Integral Tank Footing (ITF) and an Auxiliary Tank Footing (ATF). The ITF is the bottom part of the leg heavily reinforced and the ATF is a removable type of footing. Normally ATF remains connected to bottom part of leg to increase the projected footing area and in this case the footing pressure on sea bed is distributed and is much less to break hard crust of sea bed. In order to break hard crust, the footing pressure on sea bed has to be much more. An idea was conceptualized and a method was developed to increase this footing pressure on sea bed. It was conceptualized to disconnect the ATF from the bottom part of leg and attached to the hull. When ATF is detached from the leg and attached to hull, the footing area being only of ITF gets reduced significantly and on pre-driving (preloading) it will exert greater pressure on the sea bed. The said concept was

applied on one of the above said design class jack up rig. From the operation manual of the rig it was revealed that with a hydraulic pre-driving pressure of 1000 psi; loading of 573 kPa can be put on the seabed to break the hard crust which will be sufficient to break the hard crust at sea bed, although the rig's hydraulic system can develop maximum jacking/pre-driving/pre-loading hydraulic pressure of 3800 psi. This simple fact was used to design an innovative technique for breaking the hard crust at the seabed. Calculations details are as below:
• Footing size of four legged Jack up rig (Sagar Samrat) with ATF: 15.5 mx9m
• Therefore area of the footing with ATF=139.5 square meter
• Footing size without ATF i.e. ITF only: Triangular base of 6.1m and height 5.28 m
• Therefore area of footing without ATF= 16.1 square meter
As per operational manual of four legged Jack up rig (Sagar Samrat), hydraulic pressure of 2700 psi in the jacking system is equal to 2800 short tons axial load per leg. Extrapolating the above relation, it can be worked out that predriving pressure of 1000 psi in the jacking system will put pressure of 573 kPa on seabed with ATF disconnected from ITF. This will break the hard crust overlying the soft clay.

The above novel idea was put into test at 'ID' platform location where earlier a jack up rig had 8.5 meter punch through of its port leg. The positioning of the Offshore, Houston 4 leg design class jack up rig for the test is shown in figure 3. By predriving leg no.1 and leg no.4 of four legged Jack up rig with 400 to 700 psi jacking pressure, six holes were made as shown in the figure 4. The layout and dimensions of the foundation of the jack-up rig used to break the crust is shown in figure 5. The operation took 3 days to break hard crust at six positions each in two areas. After breaking the hard crust at positions within the expected area of footing of suitable rig, the jack-up rig was positioned and preloading was done on one leg at a time, keeping the air gap to minimum. Forward and starboard leg penetrated beyond the crust and thereafter full preload was held for 2 hours and rig jacked up successfully. Thus any punch through could be avoided as the resistance of hard crust at seabed is destroyed prior to installing a three legged jack up rig.
SUMMARY OF THE INVENTION:
The invention relates to a method for breaking hard crust of soil formation at sea bed overlaying soft clay in any offshore location in water depth up to 80 meters. The method is to use smaller footing to pre-drive the legs of Offshore, Houston 4 leg design class jack up

rig equipped with hydraulic jacking system. The objective of breaking the hard crust is to eliminate the risk of punch through of the rig during preloading operation and to prevent the listing of rig as well as structural damage to the rig itself or/and to the platform. The invention facilitate the safe deployment of jack up rig for exploration of oil and gas from offshore locations in water depth of up to 80 m having potential punch through probability.
BRIEF DESCRIPTION OF DRAWINGS
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
1) Figure 1 is the soil strata description of the location where the punch through phenomenon had occurred.
2) Figure 2 is the illustration of the phenomenon of punch through.
3) Figure 3 is the illustration of the position of the four legged Jack up
rig
4) Figure 4 is the illustration of the method of making holes.
5) Figure 5 is the layout and dimensions of the foundation of the jack-up rig used to break the crust.

WE CLAIM:
1. The hard crust at the seabed in any offshore location can be broken /punched through using a four legged Offshore, Houston 4 leg design class jack up rig with hydraulic jacking by significantly increasing the applicable foundation pressure on the soil by using reduced foundation size.
2. The solution to facilitate the deployment of three-independent-legged jack up rigs in a safe way at any offshore location of punch through prone areas with hard crust at seabed overlying soft soil, the procedure at claim no. 1.
3. The process at claim 1 and the solution at the claim 2 in any offshore location in water depth up to 80 metres below chart datum and that are prone to punch through, for hydrocarbon exploitation activities by jack- ups.