A Novel Flexible Offshore LNG-Pipe System With Continuous Operating Control
Authors: Schimmelpfennig K.K.H. 
Published: October 2008

A new flexible pipe system is being introduced which has been especially developed for the Offshore Transfer of cryogenic fluids. The pipe system which consists of several individual pipes is equipped with devices for leakage control.

The manufacturing process and praxis oriented model tests are described and the results of the latter are stated.

Possible applications for an offshore installation are outlined.

INTRODUCTION

At the present time small and distant offshore gas deposits cannot be exploited commercially by conventional technologies such as fixed production platforms and gas transfer by pipelines. However, by means of mobile natural gas liquefaction systems even those so called non-commercial fields could be exploited commercially. This can be achieved by moving the liquefaction to a second and even third location, thus allowing for a better distribution of the capital investment costs among more than one project. A group of six German companies called ARGE 76 (Arbeitsgemeinschaft Offshore Erdgasverflussigung 76) has developed such a system which consists of–a steel semisubmersible carrying the liquefaction plant–a crude gas riser system–a concrete LNG storage tank anchored in a distance of several hundred meters–a flexible LNG transfer system connecting both floating structures.

The purpose of this paper is to describe the LNG transfer system which is one of the most important components of the ARGE 76 concept. It is a completely new design which had been developed especially for the ARGE 76 system, but which can be installed for similar applications, too. The most important requirements which have to be fulfilled by such a system are–suitability of the inner pipe to transport cryogenic fluids–good thermal insulation characteristics and flexibility of the insulation material–outer pipe design which is resistant to seawater and outside pressure–high degree of flexibility of the total pipe system–self-compensation of elongations which are caused by temperature changes–suitability to install a control system for safety reasons which under operating conditions allows a continuous leakage control of the pipes over their total length–continuously running pipe without additional connecting flanges–suitability for large transport capacities–relatively simple installation process at the location–easy to repair in case of pipe-leakages–variable ballasting characteristics in order to adjust the pipe system to different water depths.

The basis for a pipe system with above said characteristics is a corrugated pipe design which since many years has proved a success in praxis for district heating distribution.

THEORETICAL DETERMINATION OF TRANSPORTATION LOSSES

Flow-and resistance-conditions in pipes are characterized by the pipe-friction or resistance coefficient. Since the flow conditions in corrugated pipes show a number of special features, the flow resistance in irregular rough pipes cannot be applied to the flow in regular corrugated pipes. Kauder has investigated the flow characteristics of water and air in corrugated pipes for a range of Reynolds numbers from 5.104 to 3.105. It was shown that for one-phase flow the pressure losses could be calculated and that they in general depend on the geometry of the corrugations and in particular on the depth and the pitch of the corrugations. It was the subject of the conducted experimental tests with fluid nitrogen to find out whether the results of Kauder?s investigations could also be used for a characterization of the flow behaviour-of cryogenic liquids in corrugated pipe