MOROZOV A.N., POTAPOV V.M., BLAGOVIDOV L.B.
OJSC “CDB “Corall”, Ukraine

In 1999, OJSC “CDB “Corall” (Sevastopol) developed conceptual design of ice-resistant drilling platform (IRDP) for the Tazov Bay, and specifically for the Ader-Payutinskoye gas field, as ordered by the OJSC “Gazprom”. Previously worked-out technical solutions were modified and refined in this Project.

Severe arctic conditions, with prolonged winter and shortened cool summer periods, are peculiarities of the local climate.

In winter period, all the surface of the Bay is covered with ice field 140-170 cm thick to increase up to as high as 250 cm in coldest years. Ice loads are considerable in winter and ice floating periods, while available data on upper seabed layers at the site suggest lower strength of those layers. That provided reason enough to select pile-supported platform as early as at basic design stage.

Considering peculiarities of local climate (specifically, the shorter ice-free summer period), the ice-resistant platform is designed as a combined structure consisted of two parts as follows:

• floating submerged platform itself,
• substructure.

Such a configuration makes it possible to preinstall substructure and secure it with piles during a shorter summer period. The submerged platform is mounted on during the consecutive summer period.

The ice-resistant platform is intended for installation in the area of the Tazov Bay or other areas characterized with similar water depths and hydrometocean conditions, to ensure the following:
• drilling of up to 12-production-well bay,
• simultaneous drilling and producing of gas wells,
• producing of gas wells.
Design hydrometocean conditions are listed below:
• 40 m/s 100-year wind speed,
• 2.55 m height of wave with 1% probability of exceedance,
• 0.6 m/s current speed in ice-free period,
• 2.5 m maximal ice thickness,
• 0.11 m/s ice drift speed,
• 1.86 MPa ice compression strength,
• 0.93 MPa ice bending strength.

Type of the platform is ice-resistant, floating, submerged, with pre-installed piled substructure.
There are a total of 16 piles, of 2.0 m diameter and 25 m length each.
Number of berths in the accommodation module is 42.

Endurance (days) is as follows:
• 15 days with regard to provision and fresh water supplies,
• 20 days with regard to process supplies.
Principal ship equipment parameters are as follows:
• electric power plant is rated for about 2,000 kW,
• cargo hoisting gear includes two x 20 t BOS boom cranes,
• life rescue gear includes two auger-type amphibious survival craft with capacity of 42 men each,
• Ka-115 helicopter landing area is available.
Water depth at site is about 8 meters.

Weight/overall dimensions of the platform and substructure are listed below:

platform substructure

length, m 48.0 56.4

width, m 24.0 32.4
depth, v 14.5 3.0
weight lightship, t 3,400 500
draft lightship, m 3.0 1.0

Principal parameters of process equipment are:

maximal length of inclined well up to 5,000

quantity of production wells, pcs 10

quantity of injection wells, pcs 2

type of production flowing
gas production rate of process facilities, m³/day 5,000,000
maximal production rate of a well, m³/day 500,000

reservoir pressure, MPa 8.7

distance to integrated gas treatment unit, km 200 (approximately)

Production facilities provide the following:

• gathering of products derived from gas wells,
• measuring of liquid and gas production rate for each well,
• gas cleaning from mechanical admixtures,
• primary separation of gas,
• separation of liquid-phase-products into condensate and produced water,
• gathering of condensate with further pumping it into transmission pipeline.

To save costs for the platform as much as possible, accepted production equipment includes just the equipment for primary treatment and transmission of gas field products. Where integrated gas treatment is a must, the corresponding equipment is more feasible to install onshore.
T-38 drilling unit incorporates DRECO derrick having the following characteristics:

• 317.5 t cargo hoisting capacity (maximal),
• height of 41.45 m (for triples), and 34.13 m (for doubles).

Both the substructure and the production platform are floating. Their transportation to the site is intended by means of standard towing. First, the substructure is delivered, submerged and piled to the seabed. Then the production platform is inserted inside the piled substructure.

This design approach has the following advantages:
1. it enables various options of constructing the IRFP, i.e. either with both parts of the platform constructed at Tyumen Region shipyards or at Severodvinsk/Murmansk shipyards, or production platform constructed at Severodvinsk and the substructure constructed at Tyumen shipyard,
2. utter simplicity of substructure arrangement makes it possible to construct it in the shortest possible time and install it at the site at as early stage as the production platform still being constructed,
3. upon completion of field operations the production platform can be easily (by de-ballasting) emerged off the substructure for repair, subsequent application or salvage. The substructure may be dismantled later.

Conclusion

In the process of development of the Project, a wide range of issues was considered and examined to deal with ranking and optimizing results of calculation of ice load affecting the IRFP, soil engineering analysis, design approaches related to the IRFP providing its reliable all-the-year-round operation in the Tazov Bay.

The conclusion which can be drawn based on results of calculation performed is that the IRFP structural components strength and buckling stability are ensured for the case of acting combined design loads for the entire period of IRFP operation.

Study of the structure and equipment to be applied has shown that the IRFP can be constructed in shorter terms, i.e. for 44 months if constructed at Tyumen Region shipyards, or for 21 months if constructed at Tyumen Region shipyards.

Developed as part of the conceptual design, the process facilities incorporating only the minimal required equipment package provide Ader-Payutinskoye field development at minimal possible production expenditures, as well as reliable operation and distinguished by not high price.

The basic design stage shall include a more detailed study of material and heat balance of the process to be performed onboard the IRFP and parameter of two-phase well product transmission.

Based on the scope of work performed and results gained it can be said with confidence that the IRFP can actually be constructed and reliably operated at the Ader-Payutinskoye gas field of the Tazov Bay.

Oil and gas of Arctic shelf 2008