Drilling platform

An off-shore drilling platform or the like supported by support legs standing on the bottom of the sea is converted to withstand ice pressure occurring in ice filled waters by providing those portions of said drilling platform, which are to be protected against ice pressure, with protective structures below the platform. The drilling platform and/or the protective structures are then, when ice occurs, lowered along the support legs to a level where the protective structures partly extend below the surface of the ambient water.

The invention relates to a method for use in ice conditions of a drilling 
platform or the like supported by means of support legs on the bottom of 
the sea. 
Drilling platforms with a sea bottom support, so called jack-up platforms, 
are usually constructed only for openwater conditions. Oil prospecting is, 
however, nowadays carried out also in areas which are at least during a 
part of the year, fully or partly covered by ice. Special drilling 
platforms for ice conditions have been constructed and these are usually 
heavy floating or bottom support structures with no height adjustment 
possibilities. Conventional jackup platforms cannot be used in areas, 
where ice problems may occur, and hence, it has been necessary to use 
expensive, fully ice-protected structures in such areas, where the ice 
conditions are rather mild and/or there is ice only occasionally. 
It has been suggested to use heat transfer and/or ice cutter means for ice 
protection as shown in U.S. Pat. No. 3,759,046 and Swedish Patent 
Specification No. 391 549. A vertically movable and rotatable ice shield 
is shown in U.S. Pat. No. 4,102,144. None of these constructions would be 
particularly useful in realistic ice conditions, because they fail to 
recognize the true nature of moving natural ice and the ice pressure 
caused thereby. 
The object of the invention is to present a method for modifying the 
conventional support leg carried drilling platform, so that it can be used 
also in ice conditions. According to the present invention there is 
provided a method for using an off-shore drilling platform or the like in 
ice-filled waters, which platform is supported by support legs standing on 
the bottom of the sea, said method including the steps of providing those 
portions of said drilling platform, which are to be protected against ice 
pressure, with protective structures below said platform and, when ice 
conditions occur, lowering said platform and/or said protective structures 
along said support legs to a level where said protective structures partly 
extend below the surface of the ambient water. By these means the object 
of the invention will be obtained. In a platform of this type, the most 
important thing is to protect the drill shaft and the support legs of the 
platform against the horizontal load caused by a moving ice field. 
In a preferred embodiment of the invention, the protective structures are 
made vertically adjustable. Thus, it will not be necessary to adjust the 
vertical position of the platform itself, which is a slow and complicated 
operation. 
By forming the protective structures so that, in dependence on their 
vertical position, they bend the ice either upwards or downwards, it is 
possible, by choosing the bending direction of the ice, to regulate the 
vertical forces acting on the platform so that they counterbalance the 
overturning forces acting on the platform due to the movement of ambient 
ice. 
The ice bending portion of the protective structures can be cone-shaped, 
converging in the ice bending direction and having a vertical extension of 
at least twice the thickness of the thickest level ice to be broken. 
The inclination of the ice bending surface relative to a horizontal plane 
is chosen within the range 25.degree. . . . 65.degree., preferably within 
the range 35.degree. . . . 55.degree.. 
A tube like portion can be attached to the narrower end of the cone-shaped 
portion of the protective structure. The axial extension of this tube 
portion is preferably at least one quarter of the axial extension of the 
cone-shaped portion. 
If the protective structures of the drill shaft and the support legs are so 
constructed, that they have a different cross-section area in different 
directions, the protective structures are made turnable about a vertical 
axis, so that the most favourable cross-section can always be turned 
against the ice pressure. 
Since the support legs of a jack-up platform usually have such a 
construction that they poorly withstand ice pressure, it is important that 
the support legs also are provided with protective structures reducing the 
horizontal ice pressure acting on the support legs. For vertical 
adjustment of these protective structures it is advantageous to use the 
same rack or other force transmission arrangement by means of which the 
platform itself is adjusted vertically. The protective structures of the 
support legs or some of the support legs and of the drill shaft can also 
be integrated into one unit vertically movable independently of the 
platform. 
The vertical position of the protective structures of the support legs can 
be so chosen, that the overturning moment generated by the ice load is 
balanced by so selecting the ice bending either upwards or downwards at 
different support legs, that thereby proper balancing vertical reaction 
forces are generated.

In the drawing, numeral 1 indicates the actual drilling platform and 2 its 
support legs by means of which the platform 1 is supported on the bottom 3 
of the sea. The platform is a jack-up platform, and can be raised or 
lowered on the support legs by means of jacking mechanisms 10. The 
platform 1 has a drilling tower 4, dwelling and service facilities 5 as 
well as other necessary buildings and equipment not shown in the drawing. 
In the drilling tower 4, there is a drill shaft 6 having a drill at its 
lower end for drilling a hole into the ground forming the bottom of the 
sea. 
When the drilling platform works in open sea, the distance h between the 
bottom of the platform 1 and the water surface is so great that not even 
very high waves can hit the platform 1. This is a common principle in 
jack-up platforms. 
One way of using the invention is shown in FIG. 2. In ice conditions the 
platform 1 is lowered on its support legs 2 so, that protective structures 
7 attached to the platform 1 and arranged around the drill shaft 6 and 
preferably also around the support legs 2 are positioned at the level of 
the water surface, so that moving ice 8 floating at the water surface hits 
the tapered portions of the protective structures 7 and is broken by means 
of bending against the tapered surfaces of the protective structures. It 
is necessary to make sure that the protective structures 7 have a 
sufficient extension below the water surface. It is usually sufficient 
that the protective structures go down to a level at least 5 m below the 
water surface. 
FIG. 3 shows a more sophisticated method of using a platform according to 
FIG. 1 in ice conditions. According to this alternative, only the 
protective structures 7 are moved vertically, as shown by arrows 11. They 
may be provided with a portion 7a tapered downwardly for bending the ice 
downwards as well as a portion 7b tapered upwardly for bending the ice 
upwards. The vertical reaction force caused by the bending of the ice can 
then be used for improving the stability of the entire platform 
construction as shown in FIG. 3. 
By applying the invention as shown, a normal jack-up platform can be used 
even in rather severe ice conditions. In a very severe emergency case, 
when there is a risk of the whole platform being destroyed due to strong 
ice pressure, the platform 1 can be lowered so that it by its weight 
crushes the ice mass gathering around the support legs, whereafter the 
platform is again lifted up out of direct contact with the ice. 
FIG. 4 illustrates a protective structure 7' that has different 
cross-section areas in different directions and is turnable about the 
vertical axis of the leg 2 as indicated by the arrow 12 so that the most 
favorable cross-section can always be turned towards the ice pressure, 
indicated by the arrow 13. 
The invention is not limited to the described examples, but several 
modifications thereof are feasible within the scope of the attached claims 
.