Introduction

For offshore oil and gas production, storage, and offloading, particularly in marginal fields, existing tankers are often converted to ship-shaped offshore units instead of using a new-build option. In fact, more than two thirds of all such ship-shaped offshore installations worldwide are currently thought to be built from converted tankers. More recent practice indicates that the application of converted offshore structures is more common in relatively benign environments, although a new-build installation may be more appropriate for special purposes in harsh conditions and/or for longer term use (e.g., more than 10–15 years). These are general statements, and operations using conversions have also been considered for fields with harsh environmental conditions as well. In marginal fields, it is often the relative cost advantage and better ability to fast track that makes a conversion more compelling than a new build. The number of vessels potentially available for conversions, however, have continued to dwindle over time.

In Section 1.6 of Chapter 1, the many advantages and disadvantages of the conversion option for ship-shaped offshore installations were discussed. The possible advantages include reduced capital costs, a less expensive and fast-track design and construction schedule, increased choices regarding construction facilities, and perhaps reduced overall project supervision requirements (Parker 1999). The disadvantages may include shorter design and remaining lives; greater site-specific environment limitations; perhaps increased operating costs because of difficulty in building in high safety factors; reduced or minimal resale and residual values; reduced reusability opportunities; and the relatively greater need for increased risk-mitigation measures related to regulatory compliance, which, as expected, have increased over time.