KR Webzine Vol.108

In 2012, the global market for marine structures in polar and extreme climaticenvironments was estimated to be about 15 trillion won, and by 2030 it is expected to command a market of more than 100 trillion won (1/5 of the global offshore structure market). Offshore structures for marine resource development are either fixed or floating type according to the depth of the water. In areas with a depth of 100m or more and 500m or less, it is advisable to install a floating offshore structure. In these harsh environmental conditions, a number of technical requirements must be complied with such as optimization of function, ensuring safety from collision with drift ice, securing survival under extreme temperature conditions below -50oC, and eco-friendlydesign.

Floating offshore structures can be divided into semi-submerged structures; TLP, SPAR and FPSO. Semi-submerged, TLP and SPAR structures are disadvantaged in that their buoyancy changes abruptly when a load is applied. This is because most of the buoyancy is dependent  on the underwater structure. Therefore, these structures are generally used for drilling. If they are used in production, an additional ship or structure is generally needed for storage. In ice-covered water, the FPSO type structure with its own storage is considered to be most suitable as the carrier’s access to a remote storage facility is often restricted due to the ice conditions.   

(a) Kulluk drilling-rig, (b) DrillMax Drillship, (c) semi-rig

FPSOs operating in ice-covered waters tend to fall into two concepts: ship-types - 

including turret moorings to minimize extreme ice loads -  and cylindrical-types. The ship-type offshore structures (including turret moorings) minimize the ice load through an ice-vaning concept. This utilizes a propeller which is installed on the fore and aft sides to adjust the direction of the ship to the drift of ice. Cylindrical-type offshore structures, however are designed to receive a uniform ice load in any direction.  

In terms of wave load in the general sea area, it is advantageous to use a cylindrical offshore structure. However, it is necessary to develop a new hull form to avoid infringement of the patent – exclusive rights are owned by Sevan. Additionally, it is difficult to optimize the arrangement of the upper structure because of the limited space. To overcome these challenges a new hullform for the ship-type offshore structure including turret moorings should be developed.  

(a) Arctic FPU (Moss maritime), (b) Arctic FPSO (Sevan) 

Arctic floating offshore structures must optimize operational efficiency and facilitate minimum ice resistance whilst – at the same time – ensure that the station system is kept safe for stable operation. Typically dynamic positioning and mooring systems are used as station keeping systems. In ice-covered waters particularly, station keeping systems which include both dynamic positioning and mooring systems are required to secure the position while minimizing the ice load through directional rotation.

KR has undertaken a relevant project titled “Development of hullform of year-round floating-type offshore structures based on the Arctic Ocean in ARC7 condition with dynamic positioning and mooring system”. Through the project, KR has developed

procedures to evaluate the performance of a station keeping system and has analyzed

the domestic and international regulations as well as related technology. In order to secure the algorithm and mooring analysis technique considering ice load, KR also participates in a consortium that develops the technology to evaluate the performance of DP propellers and mooring systems according to various ice scenarios.

[Figure 3] Examples of platform installed both of DP and mooring system

In order to penetrate the polar markets, operators must be prepared to compete with the world’s major players such as Aker and Technip. Currently, oil prices are low and the offshore industry is suffering, but the market will re-emerge. When it does, measures will be needed to ensure a lesser reliance on foreign designs and the delivery of high quality verification and evaluation techniques.