SS: Spar Technology: A Comparison of the Spar and Single Column Floater in an Arctic Environment
Authors: Murray John , Yang Chan K. 
Published: July 2009

The Arctic is the next frontier for oil and gas production, and it will offer new challenges for engineers who must design safe and efficient systems to operate under new environments. Some designs will be based on existing ones modified for these new conditions. In this paper, two floater designs, a Spar and Single Column Floater (SCF), are evaluated for deployment in an arctic environment. The floaters are assumed to support identical topsides and riser payloads and to be subjected to the same environments composed of level ice sheets and ice ridges, wind, waves and currents. Hull steel weights and mooring systems are similar for both designs. The comparison is based on general arrangements and global performance comparing motions responses and moorings loads. Results show that the motion global performance of the SCF is marginally better than the Spar. The ice breaking cone, used on both designs, shows unacceptable heave response on the hulls, requiring them to change drafts depending on whether the environments present ice cover or open-water conditions.

Introduction

Increased interest in exploiting oil and gas reserves in more harsh environments, such as deep water and the Arctic, present new challenges to designers of offshore structures to provide safe and economic systems. There are several industry proven solutions available for deepwater developments such as the semisubmersible, the tension leg platform (TLP), the Spar and ship-shaped FPSOs. All of these designs are capable of supporting flexible or steel catenary risers, but top tensioned risers are limited to the TLP and Spar because of their requirement for low heave motions.

The arctic environment presents challenges of long distances from infrastructure and frigid temperatures that have effects on the strength of common steel materials. In addition, wind driven freezing spray from waves and green water ingress cause ice accretion on topsides and other exposed structures. As a result, additional winterization measures have to be taken to protect facilities. A number of bottom founded designs have been proven in the arctic regions with heavy ice cover and icebergs. These are found mainly in shallow water of depths less than 100 m and contend mainly with small icebergs because shallow water causes large bergs to become grounded before they encroach on the structure. In deeper water, however, the threat of large ice masses such deep draft ice ridges and large bergs makes disconnecting a safe and practical option.

Structures can be strengthened locally to withstand impact, but in the case of very large icebergs, the floater will have to be disconnected and moved out of the berg's path. When the hull is disconnected from its moorings, riser systems must also be detached and reconnected after the threat has passed.

Various types of ice management are employed to protect the floater from contact with ice that could damage its hull. These include towing bergs to alter their trajectory, using icebreakers to open the ice cover and move ice floes to reduce pressure around the hull, and moving the hull by means of its moorings to avoid contact with passing ice masses.