Conversion of Existing Tanker to North Sea FPSO Use

ABSTRACT The use of existing tankers as a permanently moored floating storage offloading (FSO) or floating production storage and offloading (FPSO) has been wide spread in theoil industry ever since tankers were used for storage. However, the application of an existing tanker for North Sea FPSO use meeting the stringent demands of the regulatory bodies and the harsh environment of the North Sea has withheld operators from using existing tankers in the North Sea. The acceptance of FPSO technology in the North Sea and the proven ability to moor a tanker with apassive mooring system in a harsh environment has opened the avenue to use existing tankers for use as an FPSO in the North Sea. This is also driven by the search for cost effective solutions for field development plans in order to make marginal fields economically exploitable. ThePOtential reuse of an FPSO on a different location also makes the depreciation period of the FPSO vessel life related instead of field life related. Many issues related to the use of an FPSO in the North Sea are currently subject of papers at various forums. This paper, however, only addresses three crucial areas:the criteria for selection of an existing tanker;the use of a passive mooring systemthe fatigue assessment and required modifications. The paper is based on experiences gained during the engineering, design and construction phase of the ULSGE GORM, the FPSO, which Blue water will initially use for Amerada Hess Limited's Fife Field on the UK continentalshelf and confirms that the conversion of a suitable tanker to North Sea FPSO Use is a technically flexible and aneconomically attractive solution. Figures at end of paper SELECTION CRITERIA OF AN EXISTING TANKER 1.1Introduction This section addresses three issues namely:criteria for selection of a suitable shipusability of existing machinery and systemsupgrading of existing systems. The advantages of using an existing tanker versus a new built tanker lie mainly in the area of availability of a suitable ship and the associated reduction in project execution time. Furthermore, less capital expenditure is required to acquire the base vessel and also the full data of the ship are available from very early in the project. The disadvantages mainly are found in a less long remaining life time when compared to a new built vessel, acceptance of older technology and some equipment becoming redundant or superfluous. 1.2Tanker Selection Criteria Initially the principle characteristics to be determined are based on expected fields of application for the FPSO. The required characteristics basically regard only the hull with the prime criteria being the age of the vessel and the anticipated flow rate of the field(s). The estimated or desired shuttle tanker capacity and frequency determines the required storage capacity while the minimum freeboard remaining at that capacity will define overall capacity as shown in figure 1. The basic tank lay out required is also of importance as this is governed by the percentage produced water during field life.