CNG Solution for Offshore Gas Transportation
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17231, "Pressurized Natural Gas - An Efficient and Reliable CNG Solution for Offshore Gas Transportation," by P. Lothe, Knutsen OAS Shipping, prepared for the 2005 Offshore Technology Conference, Houston, 2–5 May.

A fleet of ships can deliver natural gas by use of compressed-natural-gas (CNG) technology. These ships can serve as both storage and transport vehicles, discharging directly into a land-based gas-utility grid through an onshore or offshore discharge terminal, an offshore platform, or offshore buoy. A new type of ship uses a large number of vertical pipes, designed according to enhanced-pipeline-design principles, to transport the CNG. The weight of the containment system is 50% of that required for conventional pressure-vessel ship-design codes. The result is a possible large storage volume.

Introduction

The idea of transporting gas at ambient temperature and high pressure is as old as the liquefied-natural-gas (LNG) solution. The CNG option seemed especially tempting because this would allow for cost savings in liquefaction, storage, and regasification installations. Offshore loading of gas could take place directly from the gas source using established offshore loading principles.

A few decades ago, design codes implied large thickness and weight of the containment tanks, leaving little capacity for carrying gas, which prevented economic solutions for CNG transport. Technological developments with high-strength materials, welding, nondestructive testing, and production have advanced, mainly because of requirements in deeper water and over longer distances. Reliability codes and standards, based on first principles, have been accepted, and explicit risk-acceptance criteria have been set implicitly.

One of the most used worldwide pipeline standards is the Det Norske Veritas (DNV) standard for submarine pipelines, DNV: Offshore Standard—Submarine Pipeline Systems, DNV-OS-F101, January 2000. For design against burst, the steel weight of the cargo tanks may be reduced by 50% compared with the pressure-vessel code. By defining the correct failure modes for a CNG-containment system on the basis of submarine-pipeline technology, CNG could be an economically viable option.

Marine Transportation of Natural Gas

Demand for natural gas is increasing worldwide as a result of the increase in world energy consumption and increasing environmental awareness. Among fossil fuels available today, natural gas is the cleanest and most environmentally friendly energy source, and it will be increasingly important for the next 20 to 30 years. Natural-gas consumption in 2025 is projected to a total of 151 Tcf, nearly 70% higher than the 2001 total of 90 Tcf. The natural-gas share of the total energy consumption is projected to increase from 23% in 2001 to 25% in 2025.

Often, new natural-gas resources are far from markets. Approximately 30% of the discovered gas is considered stranded. It is stranded because the reservoir gas fractions are high, preventing the development or optimal production from an oil or gas field because of the distance from the gas market or existing gas-gathering infrastructure.

An additional 20 to 30% of the world's proven natural gas reserves are found in oil reservoirs (associated gas). This gas is often uneconomical to commercialize and is flared or reinjected into the reservoir.