Offshore Methanol Production: Selection of Carriers for Different Plant Capacities
Authors: Widbom G.C. , Thorsson C.O. 
Published: October 2008

Methanol production is considered a feasible alternative to flaring associated gas or development of marginal gas fields. In order to fully appreciate the problems for a floating process plant, a series of scale model tests and theoretical calculations were carried out for different type of barges for petrochemical process plants designed for operation offshore. Motions and forces caused by waves, wind and current are compared with those for semi submersibles and tankers as other potential carriers of process plants. Further, extensive tests were performed to evaluate the technical feasibility of operating a methanol-water distillation process subject to wave-induced irregular motions.

Conceptual designs and cost estimates are made for methanol plants ranging from 100 MTPD to 3,000 MTPD with natural gas or associated gas as feedstock. As carriers of the plants, purpose-built barges and semi submersible platforms as well as converted tankers are considered. A comparison is made of the methanol production costs using different plant carriers and related to different plant capacities.

INTRODUCTION

Floating Production System (FPS) are today attracting more and more attention and there are several good reasons for this - both financial and operational.

Over the last five years the offshore industry has made the first tentative steps and the 80's will see a major breakthrough in the use of floating production platforms.

The relative cost of production in deep water will be high, and it will be how this cost can be minimized that will lead to success or failure of a particular scheme.

The above has been said concerning oil production offshore. This paper will deal with offshore methanol production but the two products may in the future often be mentioned together. Maybe one way of producing oil economically is - by also producing methanol.

For marginal gas field development or instead of flaring associated gas in connection with oil production many alternative uses of the gas have been studied, such as generation of electric power, refrigeration to LPG and LNG, conversion to methanol or ammonia. Swedyards Development Corp. has concentrated on conversion of the gas. Especially concerning floating offshore production, methanol has been of special interest with regard to its predicted growing market.

METHANOL PROCESS

The advantages of the methanol process over other possible ways of converting natural gas to a commercial product are the relative simplicity of the process and the ease in handling the methanol. The conversion of natural gas to methanol usually requires a pre-treatment stage to remove any hydrogen sulphide which would poison the catalysts used in the conversion process. The desulphurized gas is then passed with high pressure steam over a nickel catalyst in the reformer to produce a gas consisting of hydrogen, carbon monoxide and dioxide. This gas is then compressed and passed over a catalyst in radial flow converters to produce methanol and water. The reaction is shown schematically in figure 1.