Suitability of Tankers for Floating Production Systems in Hostile Environments

Abstract

ABSTRACT This paper discusses the advantages and disadvantages of semisubmersibles and large tankers (150,000 to 250,000 DWT) when used as floating production and storage systems. Primary technical considerations which are discussed include:Mooring LoadsProduction RisersVessel MotionsCrude Storage and OffloadingSystem ReliabilityField Shut-In ParametersCapital ExpendituresLead Times The data included is based on computer simulation, model tests, and industry successes in meeting similar challenges. INTRODUCTION In the past decade several fields have been produced in the North Sea with semi submersible drilling units which were converted to production units. As a result of the proven suitability of semi submersible production units in hostile environments (Argyle & Buchan Fields), many people believe that only semi submersibles are suitable for those locations. With the increasing use of converted tankers as floating production, storage and offloading units, it is appropriate to evaluate their suitability for more hostile environments. SYSTEMS EVALUATED Five basic configurations which are considered in this paper are:Converted Semi; Shuttle Tanker. This includes a semisubmersible drilling unit (modified with production equipment), with continual offloading to dedicated shuttle tankers.Converted Semi; Storage Tanker. This includes a semi submersible drilling unit (modified with production equipment), a permanently moored storage tanker, and periodic offloading with shuttle tankers.Custom Semi Production and Storage. This includes special design and construction of a large semisubmersible, with periodic offloading with shuttle tankers.Production and Storage Tanker; SPM. This includes a very large crude carrier (VLCC), or a converted tanker (150,000 to 250,000 DWT), permanently moored with an articulated tower or single anchor 1eg mooring (SALM) with a rigid yoke, and periodic offloading with shuttle tankers.Production and Storage Tanker; Turret Mooring. This includes a converter a tanker or VLCC permanently moored with a turret mooring system, and periodic offloading with shuttle tankers. For each case, it was assumed that major pipe1ines are not in place, necessitating the use of shuttle tankers. MOORING LOADS Typical mooring loads (without shuttle tanker attached) are shown on Figure 1. The mooring forces are significantly lower for the tanker based unit which weathervanes than for a semi submersible exposed to beam seas. Therefore, a much greater factor of safety can be maintained with similar mooring systems, or the size of the tanker-based mooring can be reduced in order to minimize capital expenditures. The mooring loads for the tanker may be slightly larger than shown when the wind, waves and currents are not collinear.