Metering systems aid accurate offloading in harsh weather

The CTM meter is a hostile environment system being used on BP's Schiehallion FPSO.

BP's Schiehallion floating production, storage, and offloading vessel deployed west of the Shetland Islands is using a hostile environment metering system for offloading and allocation duties. The CTM meter is claimed by Able Instruments and Controls to be gaining wider acceptance for FSPO duty.

The production rate at Schiehallion was 30,000 b/d of oil initially, being extracted from the first of a potential 29 subsea wells. According to Reading, UK-based Able, the system should lower both capex and opex compared to traditional metering methods. Schiehallion is subject to hostile weather conditions, but the non-invasive metering system should improve offload times, allowing windows in weather conditions to be fully utilized.

Shell's Curlew Field, 220 km east of Aberdeen, is also located in a hostile environment. Various shuttle tankers are being deployed to transport oil from the Curlew floater - once the field becomes uneconomic, the FPSO and the non-invasive offload metering system can be moved on to another location.

This particular CTM system package includes RF admittance level systems, radar level measurement, and magnetic level indicators. The system also features a comparatively small footprint and low weight in comparison to a conventional metering skid. Furthermore, its low maintenance requirements allow pigging to be enacted without the complication of bypassing the metering system.

A third recent reference is the Berge Hugin FPSO for Enterprise Oil's Pierce development in the UK North Sea. With a long-term view being taken on the floater's operational life, the process train is being designed with a modular approach. The intention is to be able to upgrade or downsize process according to the vessel's future duties, and the offload metering system's flexible nature gained it selection on this account.

The system is being fitted directly onto the 32-in. offload line. Its externally mounted flow transducers mean that there are no intrusions or mechanical parts to wear or cause a pressure drop. The flowstation's practically limitless turn-down ration meets all possible flow rates through the full bore pipeline for the Pierce development or any future application. The metering system is also suited for any type of crude - light or viscous, clean or waxy.

Evolving needs

According to Able, the system design is evolving in line with emerging FPSO needs to operate in increasingly adverse conditions, coupled with a demand for system verification. Improved stainless steel transducer shrouding, combined with the Rigilock transducer location system, insures protection of the transducer assembly. Quick release submersible connectors are employed as standard on all junction boxes.

Software is programmed to constantly monitor the differing input parameters relating to the flow signal integrity. To aid automated system verification, the VirtualProver can be implemented if an appropriately specified cargo tank gauging system is available.

The VirtualProver interface in the CTMsystem receives both the calculated product volume information and a sequence of further data variables. The additional data is used by the software to validate that the volumes qualify for use in the flow system verification routine. Once appropriate conditions are established, the software then makes a batch comparison of offload measurements and reports any deviation within the system.

Routine calibration is not required. The software allows confirmation of calibration stability and provides operator confidence in the CTMsystem's long-term reliability.