KULIKOV S.N., PROKINA.N. JSC “AMIGE” Russia

From 2000 to 2003 Arctic Marine Engineering Geological Expeditions (JSC AMIGE) have been carrying out a great scope of engineering survey on the detailed sites intended for exploratory oil and gas drilling. The major part of investigations has been focused on the Ob Bay and shallow Pechora shelf. 13 sites have been surveyed in the Ob Bay and 5 - in the Pechora Sea during the last four years.

Prospecting and exploratory deep drilling on the sites has been earned out from jack-ups. At present two platforms are in operation on the Arctic shelf: jack-up "Mumianskaya" (ARCTICMORNEFTEGAZRAZVEDKA) and jack-up "Amazon" (GAZFLOT Ltd.)

When engineering surveying for such a specific problem as jack-up installation on a drilling location estimation of bearing capacity of seabed soil is essential for it is one of the most important engineering features.

In Russia so far there is no comprehensive standard procedure which would allow calculating penetration of jack-up legs with proper accuracy. There are three methods used in AMIGE to estimate bearing capacity of seabed soils. The main is home-developed procedure based on the Russian standard SNIP 2.02.01-83 "Foundations of buildings and constructions". Another method is also based on the domestic standard SNIP 1.02.03-85 "Pile foundations". The third procedure used for estimation of leg penetrations is so called Skempton's method.

Analysis of different calculation procedures and comparison of estimated penetration data with those actually acquired at sea allow to make the following conclusions:

1. Estimation of bearing capacity of seabed soils made on the base of SNIP 2.02.01-83 is most suitable for accurate calculation of leg penetrations of jack-up "Murmanskaya". Method of calculating leg penetrations from strains gives more accurate data which differ from actual penetrations no more than by 10%, whereas the data calculated from bearing capacity may differ significantly (up to 50% and more).

2. For jack-up "Amazon" the most reliable leg penetrations have been estimated on the base of SNIP 1.02.03-85 "Pile foundations". It is our opinion that the reason lies in installation technology. "Amazon" has been every time installed without pads, so the legs behaved in the soil as hanging piles. For such behaviour of legs this method of penetrations estimation seems to give the most accurate results.

3. Skempton's method of leg penetrations estimation gives the results significantly different from the actual penetrations. The greatest discrepancies have been found between estimated and actual bearing capacity of very soft soils and soils with high strength properties. For very soft sediments bearing capacity calculated by this procedure has been every time underestimated, and for soils with Irish strength - overestimated.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004