Tanker Loading Export Systems for Harsh Environments: A Risk-Management Challenge
Authors: Williams J.S. , Brown D.A. , Shaw M.L. , Howard A.R. 
Published: October 2008

BP Shipping led the design and construction of the dedicated Schiehallion shuttle tanker "Loch Rannoch" using their marine experience to provide a cost effective and risk managed shipping solution to this West of Shetlands development. At the forefront of shuttle tanker technology, Loch Rannoch has been built with a number of features specific to the requirement to trade West of Shetlands and also has the capability of returning VOCs to the Schiehallion FPSO during off loading operations. To operate continuously between Schiehallion and Sullom Voe close involvement with the Sullom Voe Terminal and Sullom Voe Harbour Authority ensured that they had an input to the design and outfitting of the vessel. BP Shipping worked closely with Sullom Voe Harbour Authority in the lead up to operations to ensure that local concerns and vessel operating requirements were fully addressed to preserve Sullom Voe's safety and environmental track record. This paper discusses the involvement of BP Shipping in Schiehallion, the design and construction of the shuttle tanker, the management of marine risk, the approach to officer and crew training and the liaison with the relevant authorities to ensure reliability and maximum availability of Schiehallion shuttle tanker operations.

Introduction

The Loch Rannoch is a purpose built, high specification, shuttle tanker designed to operate in conjunction with the FPSO as an integral part of the Schiehallion development. West of Shetland is recognised as an environmentally sensitive area. The prevailing wind could carry oil spills ashore within 48 hours. Therefore ensuring a safe operation that minimises the risk of pollution is a fundamental requirement. Measures to minimise VOC emissions have been incorporated in the design. Operating relatively close to shore with frequent port operations the ship has redundancy built into its propulsion and manoeuvring systems.

Design and Construction

A number of shuttle tanker configurations could be used to off load the Schiehallion FPSO. Factors affecting the choice include export destination, environmental considerations, field life production profile and environment/operating thresholds. Simulation work carried out by BP Amoco showed that the greatest field efficiency would be achieved by using shuttle tankers capable of operating at the upper end of achievable thresholds for connection and remaining connected to the FPSO as weather conditions deteriorate. The parameters modelled are illustrated in figure 1. This modelling utilised 40 years of weather data for the West of Shetland area.

The modelling work showed that the efficiency of the shuttle tanker operation was sensitive to four parameters:-Ship size/capacityOperating thresholdsTransit speedCargo threshold

The first two are the major sensitivities.

Ship Size.

High efficiencies are obtained with ships of 800,000 to 850,000 bbls capacity (Figure 2) having good station keeping capabilities. This corresponded with the larger superior specification shuttle tankers then trading e.g. the Heidrun vessels. Whilst shipyards would undoubtedly be willing to develop designs for a larger vessel this would have introduced unnecessary risks to the project e.g. delivery, performance, availability. Therefore Loch Rannoch was sized at 850,000 bbls. Smaller ships would result in under utilisation of the FPSO and be less operable in the higher sea states.