Experience with offloading in the North Sea,Development of new cost efficient technology for marine storage and production
Authors: Breivik K.G. 
Published: October 2008

Statoil has 15 years of experience with offshore loadingor "offloading" in the North Sea. Initially, this operationwas based on a rather complex articulated loading platform (ALP) and a few modified conventional tankers. The technique has developed into today's simple but efficient submerged turret loading (STL) system and a significant fleet of highly specialised vessels. These include the multipurpose shuttle tanker (MST), the first of which was recently ordered.

The efficiency and regularity of offloading operations match that of pipeline transport. Experience gainedduring 15 years of offloading has yielded a constant series of improvements in operational safety and developments in standardised operating procedures.

Statoil ranks today as one of the world's largest exporters of crude oil, and offshore loading plays a key role in the group's operations. The recent development of integrated offloading and vessel technologies opens the way to an interesting and promising future for offloading as well as marine storage production.

Introduction

Offshore loading was originally adopted by Statoil in 1979 for the Statfjord A platform in the North Sea, where it was regarded as a temporary solution that would be replaced by a permanent pipeline.

However, initial experience was very promising. The availability of the offloading system was higher than expected and costs compared favorably with a permanent pipeline installation.

Close to 5000 cargoes of crude oil have been lifted by Statoil-operated shuttle tankers from the North Sea up to May 1995, amounting to about four billion barrels. During 1994, the fleet of shuttle tankers operated by Statoil completed 672 offloading operations in the North Sea, comprising 543 million barrels. Of this otal, 592 loadings were performed in the Norwegiansector and 80 on the UK continental shelf (UKCS). Statoil will operate dedicated offshore tankers on seven British and five Norwegian fields in 1995 (Fig. 1), making it the biggest operator of such vessels in both these sectors.

Roughly 90 per cent of all cargoes have been lifted by vessels in the dynamic positioning (DP) mode. Statoil was the first company in the world to adapt DP to improve the safety and efficiency of offshore bow- oading operations. The group will by 1995 operate afleet of 16 shuttle tankers and two floating storage and offloading (FSO) units (Fig. 2).

Technical development

Offloading in the North Sea has advanced through an nterplay between the development of loading facilities nd improvements in vessel design.

A constant focus on safety and operational efficiency as contributed to technological innovation and standardisationof key elements that represent major contributions to progress in these areas.

The recent development of the submerged turret loading (STL) system, which allows crude to be loaded virtually regardless of weather conditions, means that a new offloading era is about to begin. Current operational limits permit a regularity close to 100 per cent - on a par with a pipeline even without field storage, Such direct shuttle loading (DSL) will be applied on the Heidrun field off mid-Norway. Development of STL was initiated by Statoil in 1991,