Solid anchoring at the Skarv field
Date: 4/6/2011
The development of the oil and gas field Skarv on the continental shelf in Northern Norway is well underway. In October 2010 the installation of suction anchors for the only visible part of the field development started - The production and storage vessel Skarv FPSO, which will receive and process the oil and gas and act as an intermediate storage prior to pumping towards onshore.
The first suction anchors in the North Sea were installed at the Gorm field already in 1981, but most people have never seen pictures of how this is done. The anchor installations at the Skarv field were made by use of a custom made vessel and two ROV - Remote Operated Vehicles. One of the ROV's was equipped with cameras which documented every detail of the sea floor operations, while the other operated a pump which is a vital part of the installation process. The events on the clayey sea floor at 360 metres water depth were also followed by a shoal of curious fish, which were attracted by the headlamps.
A very solid and sturdy anchoring system is required in order to keep the almost 300 metres long floating production and storage vessel with a storage capacity of 875 000 barrels of oil and gas in place and under control in the harsh weather conditions in this area. The assignment to design the anchoring system was won by NGI, which resulted in that the Skarv FPSO required a total of 15 suction anchors of up to 14,5 metres height and a diameter of 5 metres, and a wall thickness of about 4-5 cm.
The vessel will receive oil and gas through pipelines from five production facilities on the sea bottom and can rotate around the anchoring system. But apart from this rotation the vessel must be as much as possible in one position, regardless of wind, waves and ocean currents.
The installation of suction anchors at the Skarv-field started by loading the first five anchors onto the Aker Marine Contractors custom made vessel "Boa Deep C" and transported out to location.
Enormous tin can
A suction anchor can be described as an enormous tin can open ended in the bottom. During installation the tin can is carefully placed on the sea bottom and allowed to penetrate into the seabed soil by its own weight. Then one starts to pump out the trapped water inside the tin can. This leads to a substantial under pressure which will make the anchor penetrate into the seabed. Petroleum exploration in deep waters could in many cases not have been made without the use of suction anchors, and NGI has been a central part of the development of this technology from the very start.
Subsequent to the first installation at the Gorm field in 1981, suction anchors have been used all over the world. The last statistics from 2002 showed that a total of 485 suction anchors have been installed in more than 50 locations, and in water depths down towards 2000 metres. Suction anchors have been used in most of the oil producing regions in deep waters: The North Sea, Gulf of Mexico, outside the coast of West Africa and Brazil, West of Shetland, South China Sea, the Adriatic Sea and Timor Sea. NGI has participated in the design and/or field installation of suction anchors in all of these areas.
- Comprehensive analysis is required in order to be able to successfully install a suction anchor, explains geotechnical engineer Pasquale Carotenuto at NGI. The suction anchors must able to withstand great loads, which means that the soil condition where the anchors shall be placed must be thoroughly investigated before one can start the design of the anchors. It is also important to analyse and predict the amount of suction required to pump out the water from within the anchor after placement on the sea bottom, to make sure that the anchor will penetrate to the required design depth.
Kranen om bord på "Boa Deep C" løfter sugeankeret og senker det mer enn 300 meter ned til sjøbunnen.
Installation
The installation of suction anchors at the field started with deploying the first five anchors on board Aker Solutions' custom made vessel "Boa Deep C" and subsequently transport them out to the field. The chain to fasten the anchor to the common point on the sea floor was hooked up when the anchor still was on the vessel deck. This chain is roughly 1050 m long and each chain ring weighs more than 450 kg.
Thereafter each anchor was separately lifted from the vessel deck and carefully lowered down the 360 m to the sea floor. An operator on board the "Boa Deep C" used one of the remotely controlled robot arms on one of the ROV-vessels to install a measurement device to monitor and control the angle and rotation of the suction anchor, before the anchor was carefully placed on the sea bottom. - Asuction anchor will roughly penetrate one to five metres into the sea bed due to its own weight, NGI's senior engineer Egil Solhjell explains.
It takes 10 to 15 hours to install one anchor, and the operation requires relatively calm weather conditions with wave heights less than two metres. During the installation at the Skarv field NGI had one experiences person on board the vessel, primarily to control and verify that the operation goes according to plan and to be able to do analysis if required. Technically it is possible to retract a suction anchor if the initial installation does not go according to plan, but fortunately this is seldom necessary.
Heavy chains are required to keep the production and storage vessel Skarv FPSO in place and on location utside the coast of Northern Norway in the variety of weathher conditions. Each single chain ring is almost one metre long.
Facts on the Skarv-field
The Skarv field offshore Sandnessjøen on the coast of Nordland in Northern Norway was discovered in 1998. Plans for development and production was approved by the Norwegian Parliament in December 2007, and the production start is scheduled for August 2011. BP Norway AS is the appointed operator for the field and responsible for the development.
The Skarv field includes both an oil and gas field and a clean gas field. Skarv contains hydrocarbons in three reservoir layers and the total recoverable resources are estimated to 16,8 million standard cubic metres oil and condensate in addition to 48,3 billion standard cubic metres wet gas. Oil and gas from the Skarv field will be transported onshore through an 80 km long pipeline to the Åsgard Transport system, which terminates at Kårstø.
NGI has designed the suction anchors for the Skarv field development based on data from soil investigations, including laboratory testing of sampled soil, done by Fugro Norway. Semar AS was responsible for structural engineering design of the anchors. The floating production and storage vessel, Skarv FPSO (Floating Production and Offloading) is built at the Samsung yard in South-Korea and arrived at STord Verft in Norway in the beginning of March.