Ormen Lange Long Step-out Power Supply
Authors: Gedde Hans , Slåtten Brede , Virtanen Esa , Olsen Espen 
Published: July 2009

This paper gives an overview of the Ormen Lange Long Step-out Power Supply (LSPS) system which shall provide power and communication over the 120 km step out distance from Nyhamna west coast of Norway to the subsea gas compression station. The document will present status including technical description and qualification programmes.

Transmitting 65 MW over a distance of 120 km is challenging not only related to voltage and power loss at full load, but also to the voltage stability at number of different load scenarios, such as reactive power production in the cable which is also present in no load situation. The subsea mounted Variable Speed Drive (VSD) dictates stringent requirements to voltage variation and stability of the power supply system.

Limited track record and maturity of components and equipment meant that development and qualification testing would be necessary to ensure sufficient reliability of the subsea power supply system.

On this basis StatoilHydro decided to include the LSPS system in the pilot testing of the subsea gas compression at Nyhamna west coast of Norway. The LSPS pilot project was kicked off in July 2006. Since then development and testing of LSPS equipment and components has been ongoing. The status of the Long Step-out Power Supply project is that all the qualification programs have been successfully completed together with most of the manufacturing for the pilot LSPS. System test for the LSPS pilot system is scheduled to take place during October 2009. Hook up and mechanical completion at the Nyhamna test site is scheduled for first quarter of 2010.

The purpose of the LSPS pilot system is to test and qualify components and equipment to be used in the LSPS permanent system as far as practical.

Introduction

The Ormen Lange Long Step-out Power Supply (LSPS) system shall provide power and communication over the 120 km step out distance from Nyhamna west coast of Norway to the offshore subsea gas compression station.

The permanent Long Step-out Power Supply system consists of a power cable/umbilical, a subsea transformer and High Voltage dry mate penetrators. The onshore power supply system includes step down transformer, Static VAR Compensation (SVC) and reactor for reactive power compensation.

This paper describes the approach for design, testing and verification of the permanent LSPS installation. This involves system design, electrical analysis technical descriptions and the strategy for the pilot LSPS. The pilot LSPS has been carefully defined in order to represent the permanent system as far as possible. The pilot system includes a 20 MVA subsea transformer, a short length of the power cable/umbilical with field splice and high voltage penetrators which provide the dry mate connection between the power cable and the subsea transformer. With this system set up all the main components and equipment from the permanent system are represented.

The LSPS system design and engineering is performed by VetcoGray, the pilot transformer is delivered by ABB, the power umbilical is delivered by Nexans and the High Voltager penetrators are delivered by VetcoGray. A number of different qualification programs have been conducted.

What is not represented in the pilot test set up is the 125 km power cable. Building a cable simulator which consists of pi equivalents mounted in series has previously been ruled out due to size and complexity and a number of uncertainties. The approach to verify the long step-out power supply system has hence been based on electrical system calculations. The calculations have been done with two sets of independent software simulation programs run by different design teams. Comparing the results shows that there are good correlations between the calculations.