Oil and gas recovery at great depth could have been a very difficult task without suction anchor technology that were used for the first time in the North Sea for about 25 years ago. NGI has played a major role in the development and application of the suction anchor concept. NGI is still among the world leaders concerning the further development and geotechnical design of suction anchors.

There is a great difference between the first suction anchors that were installed for Shell at the Gorm field in the North Sea and the suction anchors that were installed for the Diana platform in the Gulf of Mexico in 1999. The twelve suction anchors on the Gorm field were relatively small in size and were intended to secure a simple loading buoy device at a depth of 40 m, while the installation of suction anchors for the Diana platform was a world record in itself at that time, concerning water depth and size of anchors.

The height of these suction anchors is 30 m, their diameter 6.5 m and they were installed at a depth of about 1500 m on soft clay deposits. Since then, suction anchors at depths more than 1500 meters were installed, but the Diana platform installation was a technology breakthrough for the 20th century.

NGI was involved in the design and installation of the 30 m long suction anchors at the Diana field at a water depth of 1500 m.

The use of suction anchors has now become common practice worldwide and statistics from 2002 has revealed that in total 485 suction anchors have been installed in more than 50 different localities around the world, in depths to about 2000 m.

Suction anchors have been installed in most of all deep water oil producing areas around the world. NGI has participated in the design or the installation of suction anchors in the North Sea, Gulf of Mexico, Offshore West Africa, offshore Brazil, West of Shetlands, South China Sea, Adriatic Sea and Timor Sea.

Suction anchors have a lot of similarities with foundation design principles and solutions for the big gravity oil platforms that were installed in the North Sea when the offshore oil recovery started in the beginning of 1970 years. The first gravity oil platform on the Ekofisk field had a foundation area that was as big as a football field and it was placed on a seabed with very dense sand.

The platform was designed to tolerate waves up to 24 m in height. As the installation of oil platforms continued in the North Sea, in areas with poor ground conditions such as soft clays, they were designed to survive even higher storm waves.

These platforms were founded on a system of cylindrical skirts that were penetrated into the ground under combined gravitaty load and underpressure. The oil platform at the Gullfaks C field was equipped with 22 m long skirts. The Troll platform that is founded in 330 m depth has 30 m long skirts and is the world's biggest gravity platform.

A suction anchor can be compared with a huge can that is completely open from the side that looks downwards. This can is then placed on the sea bottom. While the trapped water is pumped out it creates underpressure inside the can, which results in the penetration of the can into the seabed.

The concept of suction technology was developed for the case of insufficient gravity load that is needed to press the foundation skirts into the ground. It was also developed when the suction anchors were subject to tension forces due to waves and stormy weather.

The suction anchor technology functions very well in a seabed with soft clays or other low strength sediments; the suction anchors are in many cases easier to install than piles, that must be driven (hammered) into the ground.  

Mooring lines are usually attached to the side of the suction anchor. Once installed, the anchor acts much like a short rigid pile and is capable of resisting both lateral and axial loads. The maximum holding capacity is obtained if the chain is attached at a depth where the anchor failure mode is large translational displacements with minimal rotation ("optimum load attachment point"). Limit equilibrium methods or 3D finite element analyses are used to calculate the holding capacity.

An important development step for the suction anchor technology, emerged from NGI's cooperation with the former operator in the North Sea, Saga Petroleum AS. Their oil platform, Snorre A, started to produce oil in 1992. Snorre A was a tension leg platform of a certain type, that in other parts of the world was founded with up to 90 m long piles.

Unfortunately on the Snorre A field, it was difficult to use so long piles due to the presence of huge peebles at 60 m depth under the seabed. Saga Petroleum AS decided therefore to use suction anchors, which were analysed by NGI. These analyses were verified from extensive model tests. The calculations showed that the platform could be safely secured by suction anchors of only 12 m in length.

NGI's aim is to be among the best designers worldwide, concerning geotechnical design of suction anchors. Apart from that, NGI can also show extensive experience and knowledge on the installation procedure.  

NGI has developed special limiting equilibrium programs and 3D finite element codes to calculate the holding capacity of suction anchors. The figure shows results from a 3D finite element analysis giving the velocity distribution at failure in the soil around an anchor with an optimal load attachment point. Photo of a model anchor of the Snorre tension leg platform after loaded to failure in a field test on very soft clay.

The internationally strong theoretical position at NGI in the design of offshore platforms, is due to the fact that we directly participated in the design of the big gravity oil platforms in the North Sea in the 70's and 80's.

This forced us to develop new methods for foundation design under extreme cyclic wave loading during the harsh weather condition in the North Sea. This was a huge challenge in itself, that needed to be resolved before any foundation of gravity platform and anchored mooring in storm weather could be designed.   

NGI's suction anchor expertise is acknowledged and has been well received by both oil companies and contractors worldwide since the 90's. In 2002 NGI established the subsidiary NGI Inc in Houston. - Since we opened this office we have been awarded the detailed geotechnical design for more than 15 suction anchor projects here in the Gulf of Mexico, and among these the challenging Mad Dog SPAR project involving design of anchors located in old slide deposits below the Sigsbee Escarpment, says NGI Inc's president Stein Strandvik. For further information reference can be made to the 2006 OTC papers no 17949 and 17950.

Research and Development
NGI has been heavily involved with the concept development, design and installation of suction anchors from the start. NGI's involvement covers development of calculation procedures, geotechnical design, soil structure interaction, model testing, laboratory testing, installation and monitoring. NGI has also conducted several joint industry sponsored R&D-projects. 

The project "Application of offshore bucket foundations and anchors in lieu of conventional designs" (1994-1998) was sponsored by 15 international petroleum and industry companies and was one of the most important studies. The project "Skirted foundations and anchors in clay" (1997-1999) was sponsored by 19 international companies organized through the Offshore Technology Research Center (OTRC), and the project "Skirted offshore foundations and anchors in sand" (1997-2000) was sponsored by 8 international companies. The main conclusions from the projects were presented in the 1999 OTC paper no 10824.

An industry sponsored study on the design and analysis of deepwater anchors in soft clay was completed in 2003, where NGI participated together with OTRC in the USA and COFS in Australia. The overall objective was to provide the API Geotechnical Workgroup (RG7) and the Deepstar Joint Industry Project VI with background, data and other information needed to develop a widely applicable recommended practice for the design and installation of deepwater anchors.

As "ground truth" data, installation data for 6 well defined prototype suction anchor cases in normally consolidated and lightly overconsolidated clays were used. For capacity, the hypothetical cases were also analyzed by 3D finite elements analyses. Independent 3D finite element analyses were performed by NGI, COFS and OTRC to ensure the quality of the 3D finite element results.

The overall conclusion is that suction anchoring is a mature and reliable technology that can be used safely in deep water and difficult conditions, provided that they are designed, engineered and installed with the best available competence. For further information reference can be made to the Keynote Lecture at the ISFOG conference in Perth, Western Australia in 2005.

The Norwegian company, DNV, (Det Norske Veritas) that is active worldwide in risk analysis and safety evaluation of special constructions, has produced a recommended practice report on the design procedures for suction anchors which is based on close cooperation with NGI. The main information from the project was presented in the 2006 OTC papers no 18038.