Telemetry and Control System for Interplatform Crude Loading at the Statfjord Field
Authors: Malmin P.C. , Lassa P. 
Published: December 2007

A control system for crude loading to tankers at the Statfjord field has been designed to allow tanker loading to take place at all times to prevent production shutdowns caused by loading-buoy problems. This paper discusses how the control system was designed to maximize the flexibility of loading operations and to meet all safety and regulatory requirements. The experience gained from more than 4 years of operation of the system is reviewed. The system has allowed maximum use of total field crude storage capacity while loading to 125,000-DWT [127 000-Mg] tankers nearly every day throughout the year. It has been possible to maintain a high production rate even through the periods of difficult weather conditions experienced in the northern North Sea.

Introduction

The Statfjord Platforms A, B, and C are located in the Norwegian Sector of the northern North Sea. The platforms are of the gravity-base type with total crude storage capacity of 5 million bbl [795 000 m3]. The current total crude production rate is on the order of 720,000 BOPD [114 000 m3/d oil]. The crude production and storage capacities are different on each platform. Statfjord A has the highest production level at approximately 300,000 BOPD [47 700 m3/d oil], but the storage capacity is only 1.2 million bbl [191 000 m3]. Statfjord B and C have production levels greater than 200,000 BOPD [31 800 m3/d oil] and storage capacities of 1.9 million bbl [302 000 m3]. The crude is exported by 125,000-DWT [127 000-Mg] shuttle tankers, each tanker capacity being about 800,000 bbl [127 000 m3]. This means that one tanker loading is required nearly every day to avoid production shutdown. During loading operations, the tankers are moored to a single-point mooring (SPM). Each loading operation takes 16 to 20 hours. Currently, two SPM's are installed: one at Platform B and the other at Platform C. A third loading buoy, at Platform A, was removed from service in 1984 and replaced by a new subsea loading system, the offshore loading system (OLS). To maintain production and to optimize storage capacity, 36-in. [91-cm] crude tie lines were installed between Statfjord B and C SPM's and the Statfjord Platform A, as shown in Fig. 1 (see the Appendix for explanation of terms). This has enabled the field to be operated with only the two SPM'S. Initially, an operational procedure was considered that used physical sectioning of the tie lines with blind flanges to achieve different loading modes. It was realized, however, that such operations would be time-consuming and would present safety hazards resulting in delays to crude export with a subsequent reduction in production. As an alternative to this physical sectioning, a telemetry and control system was designed. This system was commissioned on Statfjord A and B in Nov. 1982, just before production startup on Statfjord B. At that time, three tankers were in operation for crude export from Statfjord A. The changeover to the new control system from the previous concept used when Statfjord A was the only platform in the field took place between two crude loadings without causing any delay in the loading schedule. This paper describes the design and operation of the telemetry and control system and summarizes the operational experience gained over the last 5 years.

Objectives of the Control System

Two major objectives were identified for the interplatform crude-loading control system. The first was to assist control-room operators in completing all preparations required for loading operations. This implies that upon completion of loading-mode selection, valve operations, radio initialization, and mooring operations, a "ready-to-start" signal was to be sent to the crude-pump-protection logic to enable manual start of the pumps. The second objective was to ensure automatic crude-pump shutdown in case of an emergency outside the platform itself. Specifically, the tanker must be able to stop the pumps under all circumstances, regardless of loading mode selected, and no failure in the control system must prevent this emergency action from taking place when required. Additionally, the following design requirements were established.1. To obtain maximum flexibility of loading operations. This is a requirement that any platform shall be able to use any of the loading buoys for crude export. Additionally, all platforms must, if required, be able to export crude simultaneously. Finally, crude transfer between platforms must be possible to maximize crude storage capacity whenever tanker loading is impossible as a result of bad weather. This is particularly important for Statfjord A because of the high production rate and low storage capacity. As a consequence of this requirement, an emergency stop from a tanker should involve only that particular loading operation, not simultaneous loading operations to other tankers. On the other hand, the actual crude pumps to be stopped by a tanker, e.g., at the Statfjord C loading buoy, would vary from time to time, depending on the loading mode selected. Finally, predefined emergencies on the loading buoys, as well as on all platforms involved in the loading operation, should lead to automatic shutdown of crude pumps, again depending on the loading mode selected.2. To obtain maximum availability of the system. The important requirement here was that no failure in the control system itself should ever prevent or abort any loading operation. To meet this requirement, all shutdown circuitry was duplicated, high-quality components were used throughout the system, and manual over-rides with key switches were provided on signals where duplication of circuitry was not viable. The crude tie lines and the control system ensure that crude can be exported from any platform, regardless of maintenance activities on a particular loading buoy.3. To comply with regulations. The fiscal metering of the exported crude should take place at the source platform, and all crude that is metered must go to the assigned receiver. Accordingly, pumping from one platform to more than one tanker simultaneously is not permitted.

JPT

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