Østby, Erling (SINTEF Materials and Chemistry, Trondheim, Norway) | Nyhus, Bård (SINTEF Materials and Chemistry, Trondheim, Norway) | Hauge, Mons (StatoilHydro ASA, Trondheim, Norway) | Levold, Erik (StatoilHydro ASA, Trondheim, Norway) | Sandvik, Andreas (StatoilHydro ASA, Trondheim, Norway) | Thaulow, Christian (Norwegian University of Science and Technology, Trondheim, Norway)
McClimans, Thomas A. (SINTEF Fisheries and Aquaculture, Trondheim, Norway) | Eidnes, Grim (SINTEF Materials and Chemistry, Trondheim, Norway) | Moshagen, Hermann (BHM Engineering Services, Trondheim, Norway)
Horizontal density changes in the sea have the potential to drive extreme bottom currents. The focus in this paper is on sill overflows of Atlantic water from the Norwegian Trench into Fensfjorden in Western Norway. Fensfjorden is used for routing pipelines to the oil terminal at Mongstad. The paper describes the total study program that was carried out over the past years and outlines the challenges met in establishing a bottom current data basis for pipeline design and integrity management applications. The work included a study of large scale oceanographic and meteorological conditions believed to influence the actual area, field measurements and a thorough evaluation of the current and density data describing the flow system in the fjord and finally an advanced laboratory testing program in a rotating (Coriolis) basin. A theoretical model was set up connecting large scale ocean dynamics and extreme events of bottom current inflows. The model was tested and confirmed as a valid extreme event forecast model. The model is believed to be of considerable value for a variety of future purposes along the actual part of Fensfjorden. The total work is believed to be relevant for similar coastal locations around the globe.
Sudden increases of near-bottom currents are known to occur in Norwegian fjords, as well as other places around the globe, and must be taken into account in the design of submarine power or communication cables, pipelines and other seabed installations. Causes of these events include suspension and turbidity currents on slopes and horizontal density differences creating sill overflows to closed basins. The focus here will be on sill overflows of Atlantic water from the Norwegian Trench into Fensfjorden in western Norway. The outer part of Fensfjorden has water depths varying between about 250 m at the sill and down to 540 m at a location about 3 km further into the fjord.
For submarine pipelines installed in an area with high fishing activity, special attention must be drawn to the structural integrity of the pipelines related to interference with fishing gear such as trawl equipment. In a worst case scenario the impact/pull-over by trawl equipment will result in damage of the pipeline with possible fracture or leakage as a result. This paper deals with the design methodology developed for verifying the integrity of flowlines installed by the reeling method. The design methodology includes determination of design loads and resulting strains as a function of material stress-strain properties and Engineering Criticality Assessment (ECA) of pipeline girth welds including defects equal to maximum allowable size according to the welding fabrication acceptance criteria. For the ECA analyses the FEM program LINKpipe was applied. This program is well suited for analyses of pipelines including defects subjected to axial load, bending moment and internal/external pressure, which is typical for submarine pipelines during operation.
Ormen Lange Southern Field Development (OLSFD) is a part of the phase 2 development of the Ormen Lange gas field located about 120 km offshore the coast of Norway. The OLSFD includes an 8 slot template, two 16 inch infield flowlines, one 6 5/8 inch MEG line and one umbilical located at about 850 m water depth. The design philosophy with respect to possible interference with fishing equipment was initially adopted from the Phase I development; The MEG and the umbilical line shall be fully protected by means of trenching and subsea rock installation, while the production line shall be left exposed on the seabed designed to withstand loads from intervention with fishing gear such as trawl impact/pull-over and hooking. However as the OLSFD infield flowlines are reduced to 16” compared to the 30” production lines from phase 1, the utilization in the pipe was significantly increased by adopting this philosophy.
Woodward, Neil (Isotek Electronics Ltd, Leeds, England) | Fostervoll, Hans (SINTEF Materials and Chemistry, Trondheim, Norway) | Akselsen, Odd M. (SINTEF Materials and Chemistry, Trondheim, Norway) | Ahlen, Carl Henrik (StatoilHydro ASA, Trondheim, Norway) | Berge, Jan Olav (StatoilHydro ASA, Haugesund, Norway) | Armstrong, Mike (Isotek Electronics Ltd, Leeds, England)
Akselsen, Odd M. (SINTEF Materials and Chemistry, Trondheim, Norway) | Aune, Ragnhild (SINTEF Materials and Chemistry, Trondheim, Norway) | Olden, Vigdis (SINTEF Materials and Chemistry, Trondheim, Norway) | RØrvik, Gisle (Statoil, Trondheim, Norway)
In this investigation, we investigate the effects of solid state phase transformation on residual stresses in welding supermartensitic stainless steels, using the Satoh experimental test approach. The results obtained clearly demonstrate that the austenite-to-martensite phase transformation has remarkable effects on the final tensile residual stress level, and stresses as low as 70 to 170 MPa were found, depending on the weld thermal program. These results may imply that unnecessary conservatism is often used when assuming residual stresses to be of the same order as the base metal yield strength.
During the last decade, the use of supermartensitic 13% Cr stainless steels has grown in offshore installations (e.g. Enerhaug, 1997, 1999; Olsen, 1999). This allows cost reduction due to much lower material costs than those of alternative duplex grades. Supermartensitic steels also possess a certain corrosion resistance. In some cases, problems have been reported related to the pickup of hydrogen, either from welding (e.g. RØrvik, 1999; Aune, 2003; Akselsen, 2004a) or from the environment (Rogne, 2001, 2002a and b, 2003, 2004). Since their microstructure is mainly martensite, they are in principle susceptible to hydrogen-assisted cracking. Besides the microstructure and hydrogen supply, residual stresses are also important to the risk of hydrogen cracking. The buildup of residual stresses in welding of supermartensitic steels is not very well known. The present investigation was then undertaken to provide information on thermal stresses developed during weld simulation of supermartensitic 13% Cr steel, using the Satoh experimental test. Also included are superduplex 25% Cr and welds made of 13% Cr and between 13 and 25% Cr, both made with superduplex wire. The results obtained will clearly demonstrate that the tensile residual stresses developed in super 13% Cr steels are surprisingly lo w, and much lower than those of 25% Cr superduplex steel.