ABSTRACT

This paper will deal with positioning strategies for a floating production unit and an offloading shuttle tanker. The positioning of the two vessels in a tandem configuration while weather vaning around theterminal point is a complicated task. Experience gained from ten years of operation, 12shuttle tankers and 19anchored units, will shed light on some of the problems encountered along the way and their solutions.

Introduction

SIMRADALBATROSSA/S has long experience with positioning and station keeping of vessels with large ?traditional" hull shapes. Already in 1978, a DP system was installed on the "Seaway Sandpiper", a 20,000 dwt. bulkship converted to a dynamically positioned stonedumper.

In 1981, M/T Wilnora was the first tanker to be converted for dynamically positioned loading at the Articulated Loading Platform (ALP) on the Statfjord field. The many advantages of this concept soon became apparent to the operator of the shuttle tankers at Statfjord, K/S Statfjord Transport A/S, and soon all the shuttle tankers in operation on the Statfjord and Gullfaks fields were converted to DP operation. The concept has since been developed further, both with respect to positioning strategies and number of reference systems. In the British sector of the North Sea, both ESSO and Shell are using dynamically positioned tankers.

The floating production ship may be held in position usingjust a mooring spread or its position maybe maintained by a wholly redundant Dynamic Positioning system. However, the most common method is to combine the use of anchors with some form of thruster control. The Petrojarl I, a turret moored production ship owned and operated by GolarNor Offshore A/S, is already in operation in the North Sea. She is currently producing at a rate of 25,000 b/d from two subsea wells via two flexible risers, on the Amerada Hess operated Angus field in the British sector of the North Sea. A new floating production ship (a Tentech 850 design) is currently under construction for Ocean Production A/S and Smedvig A/S at the Astano shipyard in Spain. This vessel will be equipped with both a wholly redundant Dynamic Positioning systemand a Position Mooring system, for operation in a turret moored configuration.

The Position Mooring System

This section will give a brief introduction to the most essential functions of the Position Mooring system (PM) for anchored vessels. PM systems have been in use for 10 years on 18 moored units, used for oil/gas drilling and production in rough North Sea conditions. Accommodation rigs moored very close to freed installations and connected with a gangway also benefit from the use of these systems. The main objective of the system is to automatically monitor and control of the vessel?sposition and stability within the actual anchor pattern,

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