Narve Oma

Fatigue life improvement of welded doubling plates by grinding and ultrasonic peening

Fatigue life assessment is important for all floating offshore structures, both not only related to the new building stage but also related to lifetime extensions. Fatigue cracking occurs normally due to uncertainties in estimated fatigue life and this is a well-known problem for floating structures. In this paper, the aim has been to look deeper into the effects of grinding and ultrasonic peening (UP) in order to improve the service life of structural connections. Fatigue testing of full-size fillet-welded doubling plates has been performed. The paper presents results from these tests including as-welded condition and ground and UP improved specimens. The paper also presents alternative S-N curves for improved details. The primary goal has been to assess the possibility for fatigue life improvement of fillet-welded doubling plates where fatigue cracking may initiate from the weld root. The information gained in this project is considered to be important for the offshore industry working with floating structures as well as for development of new fatigue assessment procedures and requirements in standardisation.

The Effect of Measured Whipping and Springing on LNG Vessels

Strain measurements of structural members onboard two LNG vessels have been collected for a period of 5 years. The vessels have been sailing in the world wide trade except for the North Pacific area. The time spent in the North Atlantic is about 40%, which is higher than for typical LNG vessels. The vessel speed has been relatively low in average and well below the service speed, still the effect of springing and whipping has been significant on the fatigue and extreme loading. Previously, results from the strain sensors have been presented versus the fatigue and extreme loading on a general level, independent on the environmental conditions. In this paper the focus is more towards how the vessel behaves in wind/waves with respect to springing and whipping in order to understand more of the relationship between accumulated fatigue damage, heading and loading condition.The vessels have been equipped with wave radars and wind sensors. The effect of whipping and springing on accumulated or part fatigue damage versus relative heading is demonstrated. The effect as a function of the wave height/wind speed is shown for selected headings. One of the objectives is to check if the wind sensor can be a useful alternative to the wave sensor to capture the physics. Some of the data is studied for ballast and cargo condition separately in order to see if these loading conditions can be merged, which is desirable from an assessment point of view.Copyright

Assessment of Ageing Structures: Case Studies

In all phases of the service life of a structure, uncertainties will occur, hence probability based methodologies may be an important and valuable tool in order to verify structural integrity and the corresponding expected safety level. This paper gives a brief description of an approach for how to demonstrate the safety of facilities in the life extension phase by the use of risk based inspection (RBI) plans. This is illustrated with four separate cases used for assessment and life extension of FPSOs operating in harsh environments on the Norwegian continental shelf (NCS). The assessments are partly based on the methodology introduced in NORSOK (2009).Copyright

Fatigue Life Improvement of Welded Doubling Plates

Fatigue life assessment is important for all floating offshore structures related to the new building stage but also related to lifetime extensions. Fatigue cracking occurs normally due to uncertainties in estimated fatigue life and this is a well-known problem for floaters.In this paper the aim has been to look deeper into the effects of improvement methods for improvement of the fatigue life of structural connections.Fatigue testing of full size fillet welded doubling plates has been performed of a specimen in as welded condition for comparison with two specimens improved by grinding. The paper also presents an alternative S-N curve for grinded details.The primary goal is to assess the possibility for fatigue life improvement of fillet welded doubling plates where fatigue cracking may initiate from the weld root. The information gained in this project is considered to be important for the offshore floater industry, as well as for development of new fatigue assessment procedures and requirements in standardization.Copyright

Offshore unloading of semi-pressurized CO2 to an oilfield

This chapter presents a new concept for offshore unloading of cryogenic pressurized CO 2 . The offshore unloading system transports the liquid CO 2 from the dedicated CO 2 ship to the wellhead on the platform at the required temperature and pressure. During the unloading phase, the ship is connected to a submerged turret loading (STL) system. The CO 2 is pumped to a pressure high enough to avoid phase transition in the transfer lines. A flexible riser, a sub-sea pipeline, and an insulated pipeline in the platform shaft bring the CO 2 from the unloading location to the topside of the platform. The CO 2 is pumped to the injection pressure and heated to avoid operational problems before it is injected into the reservoir for IOR (Increased Oil Recovery), using the existing water injection wells. Several challenges have also been discussed including icing problems when unloading CO 2 at -50°C and the ways to avoid dry ice formation during the depressurization of the CO 2 pipeline. Due to the large investment costs of the ship, a high unloading rate of the CO 2 is crucial to lower the total transportation costs to a competitive level. Despite the technical and economical challenges with offshore unloading of CO 2 , the ships flexibility to deliver CO 2 directly from the source to the oil fields might be a key element in providing CO 2 for IOR to the North Sea.

Fatigue Capacity of Stiffener to Web Frame Connections

Daewoo Shipbuilding & Marine Engineering Co., Ltd. (DSME), StatoilHydro and DNV established a common project to investigate the reason for the difference between calculated fatigue lives and the in-service experience and to assess the fatigue capacity of stiffener web connections subjected mainly to web frame shear stresses. The main objective of the work was to establish fatigue test data and perform numerical analysis of collar plate connections in order to provide improved confidence in analysis methodology for fatigue life assessment. Large scale fatigue tests of different types of connections were carried out to obtain fatigue test data of collar plate connections. Finite element analyses were carried out for comparison with fatigue test data and with measured stresses on the test model. Based on this work recommendations on fatigue design analysis of connections between stiffeners and web frames have been derived. The background for this is presented in this paper.Copyright