Tanker VOC recovery plants to cut emissions caused by offshore loading

Oct. 1, 2001
Kværner Process Systems (KPS) has received orders from both Statoil and Navion for non-methane volatile organic compounds (nmVOC) recovery plants.
The pilot VOC absorption plant at time of installation on the Anna Knutsen shuttle tanker.
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Kværner Process Systems (KPS) has received orders from both Statoil and Navion for non-methane volatile organic compounds (nmVOC) recovery plants. This suggests technology has finally come of age after almost a decade of development. KPS is to supply three plants worth in excess of US$17 million, with options to supply a further five plants to Statoil. Two of the plants will be installed on tankers serving Statoil's Statfjord and Gullfaks fields, while the package for Navion is destined for a tanker serving Noske Shell's Draugen development.

Offshore loading is a popular means of exporting crude on the Norwegian continental shelf and is currently used on 10 fields - Statoil's Statfjord, Gullfaks, Heidrun, Norne, and

The problem of VOC emissions offshore is correspondingly large. During the loading of a typical 130,000 dwt tanker, some 150-200 tons of VOCs are emitted. The offshore sector as a whole is responsible for 55-60% of Norway's total VOC emissions. In 1999, offshore loading was responsible for 206,000 tons of VOC emissions out of the country's total of 343,000 tons. By 2010, Norway is committed to making a substantial reduction in these emissions, cutting them by 30% from the 1989 level. Both the overall total and the offshore contribution have risen steadily since 1989, as oil production has increased and more fields have employed offshore loading.

The technology development was carried out through the VOC/OLF project, supported by 18 operating companies belonging to the Norwegian Oil Industry Association (OLF). The project was managed by Statoil. The goal for the absorption project was met, according to Magne Holla, Statoil's project manager, with the 70% recovery target for the pilot plant being achieved.

"Experience leads us to believe we can achieve 78% recovery through absorption," he said. Although the rate of VOC emission varies according to the composition of the crude in each field, the plants developed in the project are suitable for all the fields that currently use offshore loading.

Regulations are being introduced concerning required use of the absorption technology and permitted VOC emission levels. Based on the available technology, the Norwegian Pollution Control Authority (SFT) proposes a 78% rate of recovery. When the regulations were first issued last year, they were, however, contested by the four affected oil companies on the grounds that they were too strict.

The SFT has now recommended to the environment ministry that the timetable for achieving full use of the technology should be eased. A number of stages are proposed - that the absorption technology should be used for 40% of loadings by March 2003, 70% by end 2004 and 95% by end 2005. Regularity for the equipment is set at 95%. A final ruling from the ministry is expected in the autumn. With a delivery period of some 15 months for the recovery plants, operators will have to place further orders in coming months if they are to meet the SFT targets.

At one point a voluntary agreement was being negotiated with the environment ministry by the oil companies in the VOC/OLF project - in return for favorable tax treatment, the companies would have undertaken to limit emissions to an agreed level using the recovery technology. However, the agreement, which should have been finalized two years ago, in the end failed. Several of the companies were unwilling to accept the terms proposed by the government.

Absorption process

In the first stage of the recovery and absorption process developed by KPS, VOC emissions in the ship's cargo tanks are collected by a manifold system, passed through a de-mister, and condensed. The lighter compounds - methane, ethane, inert gases - are not susceptible to condensation and are vented. Once condensed, the heavier non-methane elements are passed into an absorption column where they are absorbed into a stream of crude and returned to the cargo tank.

A pilot plant was installed on the Anna Knutsen shuttle tanker in 1998. Including the absorber column, the unit weighs about 130 tons, which is not a significant additional weight for such a ship. The plants now ordered by Statoil and Navion will be a little heavier, with the same capacity. However, as a result of the trials, some design modifications have been made, with the operational pressure somewhat reduced and the dimensions of the knock-out drum somewhat altered, according to Holla.

Under an exclusive licensing agreement signed with KPS, Hitec-Vision has responsibility for selling the recovery plants. It has a fabrication facility suitable for constructing the equipment, and will also provide an engineering and installation service. KPS will supply the process and main equipment design, enabling it to continue developing the technology.

HitecVision Marine has developed a system for the return of VOC gases from a shuttle tanker to a floating production, storage, and offloading (FPSO) vessel. The system is installed on the Loch Rannoch shuttle tanker, which is exporting oil from BP's Schiehallion FPSO in the UK sector.

In parallel with the absorption project, Statoil has also been working on technology to capture VOCs and use them as ship's fuel. This technology requires a VOC fuel plant, the main contractor for which is Hamworthy KSE, and modification of the ship's engine for dual fuel use, for which MAN B&W Diesel is responsible.

Successful trials have been performed by Statoil and MAN on a prototype engine in Copenhagen, and the development has won several prizes. Demonstration of the technology is still ongoing through operation of a pilot plant, which was installed on the Navion Viking shuttle tanker in summer 1999.

Tests so far have indicated that further engine modifications are required, Holla says. One of the attractions of this technology is that it offers savings on fuel costs. Sufficient VOCs could potentially be recovered to fuel a tanker's journey to the Mongstad refinery on Norway's west coast or to Rotterdam.