The decommissioning of Frigg field bases is being planned for 2002.
Elf Petroleum Norge, now part of the merged TotalFinaElf group, is preparing a strategy for the decommissioning of the Frigg gas field, which straddles the border between the Norwegian and UK offshore sectors. The task presents a series of challenges, not least of which is how the field's three concrete gravity-base platforms are to be dealt with.
The cessation plan, which is due to be submitted to the two governments next October, will consist of two parts: a disposal plan evaluating and comparing the various disposal options, and an environmental impact assessment (EIA) identifying and evaluating the environmental effects of the options. A further complication is that approval has to be sought from both governments, and public consultations on the EIA carried out on both sides of the North Sea.
The final shutdown on Frigg, which is expected to take place in late 2002, will mark the end of an era. Since it came on stream in 1977, the field has been an important source of gas for the UK market, at times accounting for more than one third of its supplies. Just over 60% of the field lies in Norwegian waters. Almost 70% of the license interests are held by companies in the TotalFinaElf group, in addition to which two Norwegian licensees, Norsk Hydro and Statoil, together hold just over 30%.
In addition to the concrete platforms, the facilities include two steel platforms and the wreck of a jacket which was too badly damaged on installation to be used; a subsea template which supported a now removed flare tower; and a complicated network of in-field pipelines and cables. Drilling and production platform No. 1 (CDP1) has already been mothballed, while treatment platform No. 1 (TP1) is only used as a riser platform. There is a pile of drill cuttings under drilling and production platform No. 2 (DP2), the result of operations using water-based drilling mud, and further cuttings within the base of CDP1.
Clearance needs
Under regulations agreed by the Oslo-Paris (Ospar) Commission, the topsides of all the platforms and the steel jackets have to be recovered to shore. The heaviest topsides is that of treatment and compression platform No. 2 (TCP2), which weighs 23,210 tons and includes processing facilities for several satellite fields, Elf's North-East Frigg, East Frigg, Lille-Frigg, Frøy, and ExxonMobil's Odin. Of the satellites, only Frøy remains in production, and it is expected to be shut down later this year.
Using current lifting technology, Elf says, the TCP2 topsides will have to be removed in the reverse sequence from installation, requiring more than 20 lifts. In the case of TP1, the modules will have to be lifted off one by one, with the module support frame on which they sit coming last. For DP2, removal can be done in two lifts, but first the load carrying members will have to be strengthened, and the topsides cut vertically in two. A number of small lifts will be needed in the case of CDP1, while the topsides of the quarters platform (QP) can be removed in a single lift.
The operations requiring multiple lifts and extensive preparation are clearly likely to be costly. Along with other companies with large North Sea platforms to be decommissioned - Phillips Petroleum in Norway, and BP Amoco, Kerr-McGee and Shell in the UK - Elf is investigating the potential of reduced operations and costs offered by new single-lift vessel concepts. In mid-year, the group was preparing to support the further development and demonstration of some of these concepts through joint industry projects.
Topsides removal by single lift offers another potential benefit. Some of the equipment to be recovered to shore is relatively new and in good condition for re-use, such as the five-year old M35 module on TCP2, which is used for processing Frøy oil and gas. Many single items - compressors, generators, cranes, and life-saving appliances - also have second-hand value. When removing topside modules with existing crane-barges, a critical moment comes when the module is placed on the transport barge and has to be fastened in place as quickly as possible in what is a weather sensitive operation. The risk of damage is potentially much less if the whole topsides is lifted from beneath.
Barge lifts
The capability of jacket removal is also claimed for the new single-lift concepts, which could also be interesting for Elf. Based on the use of existing crane barges, the company says the jackets would be removed by cutting them in three or more sections before lifting. The main concern is cutting the legs beneath the seabed - as the seabed conditions around the legs are not fully known yet, and the actual cutting process could require localized excavation.
Uncertainty also surrounds the DP1 jacket, which was badly damaged during installation in 1974 when the ballast tanks failed, causing the jacket to sink to the seabed out of control and in the wrong place. The structure has neither been piled, nor maintained over the years, and its structural condition is therefore unknown. It may have to be cut into several sections before being removed, Elf says.
The question of the fate of the three concrete gravity bases, two of which are located in UK waters and one in Norwegian, is another key issue. This will be the first time the UK government has had to decide what to do with this type of platform, while its Norwegian counterpart is already considering the fate of Phillips' Ekofisk Tank concrete base and protective wall, which Phillips has recommended be left in place.
According to the Ospar regulations, application can be made to leave concrete gravity bases in place, either with the shafts intact or toppled, or to refloat and dump them. Its apparent assumption that attempting to remove such structures to shore would be impractical and unsafe has been contested by concrete contractors. A JIP carried out in Norway concluded that in principle such structures could be refloated in a controlled and safe manner and recovered to shore.
Elf has not revealed what it is likely to recommend with respect to the concrete bases, though it warned in the EIA proposal that the feasibility and safety of removing them had not been established. The company was not involved in the JIP and officials have expressed skepticism about its conclusion. The JIP examined the case of a Condeep structure, but only one of the Frigg concrete bases, TCP2, is a Condeep - CDP1 is a Doris design, like the Ekofisk Tank, while TP1 is a Sea Tank. It is also the case that the Frigg bases were all designed and installed before the requirement for removability of concrete bases was introduced in Norway.
Wells and pipelines
Well abandonment will only involve operations on one platform, DP2. Here 24 wells have been drilled, though some of them are no longer in use. A modular rig will need to be brought in to plug and abandon the wells as the platform rig was removed several years ago. Meanwhile, the 24 wells on CDP1 have already been plugged and abandoned.
The probable recommendation for the pipelines and cables is that they be left in place. The larger pipelines - two 26-in lines between CDP1 and TP1 and a 24-in line between TP1 and the flare tower template - are covered with rock, while two 26-in lines between DP2 and TCP2 have self-buried. A recent comprehensive study carried out for the Norwegian government concluded that broadly speaking the preferable option was to leave pipelines and cables in place, if necessary, burying or trenching them to avoid causing any problems for fishing activities. A white paper on pipeline decommissioning is due to be published this summer, on the basis of which the Storting will decide its policy. A similar approach is believed to be taken in the decommissioning guidelines which the UK government is expected to issue in final form later this year.
In parallel with the technical work of evaluating disposal options, Elf has carried out an extensive exercise in public consultations in both countries on how the EIA should be conducted. In Norway, the 11 non-governmental organizations which must be consulted are officially identified, while in the UK, the government only recommends which NGOs it thinks should be consulted. Keen to approach the matter in a proactive manner, Elf took out adverts announcing the upcoming consultations and invited interested parties to get in touch. A number not identified by the government did, with the result that the company has consulted with a total of 35 NGOs in the UK.
The exercise has been a positive one, Elf says, and many of the comments made have been taken on board. An appendix will be included in the EIA summarizing the comments received and indicating how they have been addressed in the various studies. Although the consultation phase is over, Elf plans to maintain an ongoing dialogue with the various stakeholders. Preparation of the EIA has been in the hands of DNV.
Once the cessation plan has been submitted, the oil and energy ministry and other ministries involved will take some time to analyze and verify it. It will then go before the Storting for final approval. This process is likely to take up to three years, and it is unlikely that removal operations will get under way before the middle of the decade.
