Design of Spread-Mooring Arrangements for Very Large Storage Tankers on Shallow Water for the Widuri Field in the Java Sea
Authors: Elzinga T. , Ikert P.L. , Dale A.J. 
Published: October 2008

The paper describes the design process for an asymmetric spread mooring arrangement for Very Large Storage Tankers in the Java Sea.

The initial design has been based on static and dynamic computer simulations and has been certified by a classification society.

At a later stage model tests have been carried out to verify some design aspects. The test results showed good agreement with computer simulations.

INTRODUCTION

The WIDURI field is located in 68 ft of shallow water in the Java Sea, Indonesia, at some 130 nm north of Java and some 70 nm east of Sumatra. The field comprises a number of production and process platforms. For the storage of crude oil the operator, MAXUS South East Sumatra Inc., selected a floating storage facility comprising of two VLCC's (338,000 DWT and 178,000 DWT) both moored in spread mooring arrangements. The export facility comprises two SBM's capable of receiving tankers up to 225,000 DWT (partly loaded).

A general arrangement of the field with the storage tankers and export facilities is shown in Figure 1.

The conceptual and detailed designs for the storage and export facilities were made by Frederic R. Harris B.V. in The Hague. The design process commenced in April 1989 and had to be completed in December 1989 in view of delivery and installation time to make field production in December 1990 possible.

The project showed the following special design characteristics:* The storage tankers were to be moored in a spread mooring arrangements and had to be easily disconnected to make a break-out of the tankers from the mooring arrangements possible. This resulted in the installation of Quick Release Devices for the mooring lines onboard of the storage tankers.* The initial design of the spread mooring arrangements had to be based on typical design tankers in view of the short design period. The size of tankers was based on required storage capacity as established by MAXUS and is based on field production figures, initial peak production, contingency considerations, etc. This resulted initially in two storage tankers of 150,000 DWT each, which, at a later stage, was increased to one tanker of approximately 400,000 DWT and one of 150,000 DWT, providing 3 million barrels of usable storage capacity.* Spread mooring arrangements were to be designed for a break-out period (survival storm) of 10 years with a design draft corresponding with 75% of the required storage capacity.* Design of the mooring arrangements for the typical tankers was based on computer calculations only and were certified by the classification society American Bureau of Shipping (ABS).(The tankers however have to keep propulsive power, loads in the mooring lines have to be continuously monitored and break-out procedures well documented).* The mooring arrangements were checked for the finally selected storage tankers, initially by computer simulations and later on by model tests in summer 1990. The model tests showed a very good agreement with the computer simulations.