The pathfinder prospectus for the flotation, which is expected to value the company at £50m, was published yesterday.
The venture capitalists who invested in the 1992 management buy-out from BP will see their £600,000 investment in the ordinary shares of the company rise in value to £21m.
The venture backers are 3i, Prudential Venture Managers and Charterhouse, which will keep the majority of their ordinary shares.
The money raised will finance the repayment of £9.2m of preference shares, also owned by the venture capitalists, and net bank borrowings of £4m, giving the company an ungeared balance sheet.
The company has seen turnover grow by 30 percent since it was sold by BP to the managers for £20m.