ABSTRACT

Conoco And Norwegian Contractors have recently completed a study on the feasibility of using floating structures for production of hydrocarbons in the Norwegian Barents Sea. This location is in relatively deeper water depths of about 500M, and occasional Icebergs and other ice features may be encountered. A survey of feasible floating system configurations resulted in the selection of a deep draft concrete TLP with four corner columns and one central riser protection column. Comer columns are protected by cable type fendering units. Other fender types were also studied. This study demonstrated the feasibility of using TLP's in deep Sub-Arctic waters for oil and gas production. The paper presents the studied hull and fender configurations and associated geometric, cost, and schedule data.

INTRODUCTION

The area of interest lies in the Northern Barents Sea in the vicinity of 74 degrees North Latitude and 16 Degrees East Longitude. Sea ice can extend into this region as often as three years in every ten year period, and as much as 40% of the sea can be covered with ice. Additionally, icebergs may drift into the area. The structures used for production must therefore be able to withstand the ice floes and pressure ridges that may occur.

Water depths in parts of the area can be as deep as 500M (16500). For these water depths, and expected magnitudes of iceberg impact loadings, fixed platforms, such as a concrete Gravity Based Structure (GBS), will be costly. Therefore, only compliant floating structures have been considered.

DESIGN CONDITIONS

Since the objective of the study was to develop a platform concept, only the hull, mooring, and riser systems were evaluated. Deck was simulated as a box with 100M square plan area and 45M height. Payload, including the deck structure was 35,000 metric tons (tonne) dry and 52,000 tonne wet weight (including riser pretension, snow, and ice on deck), with 200, OOOBOPO plus associated gas production capacity, 46 each 9-5(8in 00 production and two 18in. OD pipeline risers. Design life was 50 years.

100 Year ULS wave, wind and current conditions were represented by a maximum wave Height of 24.3M with 14.5 S period, an associated 1-minute sustained wind speed of 40M/S, and a surface current of 0.85M/S.

Maximum 100 year ULS Iceberg size was represented by a 500,000 tonne ice mass with 20M sail height, 60M keel draft drifting at a velocity of 0.6M/S. Iceberg crushing strength was taken as 2.0MPa for global force calculations and 4.0MPa for local design. No other environmental effects were combined with loads from the 500,000 tonne iceberg.

Maximum 100 year ULS iceberg plus wave loading combination was represented by a 50,000 tonne iceberg (also defined as the 10 Year Iceberg), set to motion by a 7.7M significant height and 13.0S spectral peak period wave (also defined as the 10 year wave). A wave/iceberg Interaction study indicated that the significant iceberg impact velocities generated by this combination will be in the order of 2M/S.

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