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    Technical Papers

    Comparison of Simulated and Full-scale Disconnection of Turret Buoy

    2013 | A. Bech and G.O. Hovde, NOV APL Norway; J. Daniel, Petrobras America Inc; T. Vinje

    Abstract

    This paper addresses mooring design methods, reveals records of site tests, and also compares the conventional theoretical numerical modeling approach with a real full-scale disconnection of this challenging scenario. The FPSO BW Pioneer set a new world record for deepest mooring system on a production unit at 2,500m (8,200ft) of water depth in the US Gulf of Mexico. The unit also pioneered this type of facility (FPSO) in the US Gulf of Mexico.

    Fatigue Design of Roller Bearing for Large FPSO Turrets

    2012 | Tom Lassen and Svein Erik Hansen, Sigmund Askestad

    Abstract

    The present report presents the fatigue design and a fatigue life prediction method for large roller bearings applied in the turret turn table for large Floating Production Storage and Offloading (FPSO) units. The contact point between wheel and rail in these bearings is subjected to a multi-axial stress situation and both surface wear and fatigue cracking may occur. Stress analyses with contact elements are carried out and a methodology based on the Dang Van fatigue criterion is adopted. The criterion is based on an equivalent stress defined as a combination of the fluctuation of the shear stress from its mean value and the associated hydrostatic stress at a critical plane at any time. The criterion has been used in the railway industry and is regarded as an appropriate approach for the crack initiation phase in the high cycle fatigue regime. A realistic case study is included where material combinations with high strength stainless steel S165M and high strength carbon steel 42CrMo4 are applied. It is demonstrated that a favorable wheel-rail design is characterized by an extreme load condition with a stress situation within the linear elastic behavior of the steel. Furthermore, the Dang Van equivalent stress function shall be below the shear stress fatigue limit. These two design criteria will ensure robust design and long durability of the turret bearings.

    Deep Water SAL - an Offloading Station for Large Water Depths

    2011 | Vidar Aanesland, Arild Bech, NOV APL

    Abstract

    The use of FPSOs in the development of oil and gas fields in deep waters requires accessibility to safe and reliable off-take solutions. The Deep Water SAL (DW SAL) design moves the well proven Single Anchor Loading (SAL) concept to deep water applications. The SAL system has been used for the last ten years in the North Sea at eight different locations.

    Mooring Line Components with Semi-Brittle Behaviour: Verification of Fitness for Purpose

    2011 | Tom Lassen

    Abstract

    Low impact energy for Charpy V Notch (CVN) specimens and associated low Crack Tip Opening Displacement (CTOD) values have occurred on several occasions in high strength steel offshore mooring components. In the present work an Engineering Critical Assessment (ECA) for shackles has been carried out to demonstrate fitness for purpose. Typical values for CVN and CTOD are 17 Joules and 0.01 mm respectively at design temperature. The purpose of the present work is to demonstrate that even in the case where normal quality requirements are not met, the shackles may still have enough structural integrity and fatigue durability to withstand the load spectrum in the field during the planned target service life of typically 20 years. The ECA is based on applied fracture mechanics as outlined in the BS7910 document. Shackles made of QR4 high strength steel with different geometries and loading modes are analyzed. Extreme load cases and fatigue load spectra are treated and fracture mechanics modelling is discussed. A CTOD value as low as 0.01 mm may give critical surface crack depth close to 1% of the diameter in an ultimate limit state condition. For normal ductile steel behaviour the critical crack depth is usually close to 15% of the diameter. However, under the assumption that the pre-existing crack depth is 0.25 mm, the predicted fatigue life based on fracture mechanics analysis is still over 100 years. For a target service life of 20 years this corresponds to a Design Factor Fatigue (DFF) of 5. This is close to the requirement given by DNV for mooring chains. Based on the present analysis it can be concluded that the shackles are fit for purpose as manufactured even under unfavourable and unlikely assumptions.

    Ultimate Strength and Fatigue Durability of Steel Reinforced Rubber Loading Hoses

    2010 | Tom Lassen, Trond Stokka Meling, Anders L. Eide

    Abstract

    Loading hoses in an offshore loading buoy system in the North Sea were investigated with respect to extreme load resistance and fatigue durability. Both experimental work and fatigue life analyses were carried out. The FLS test is based on the principle of a service simulation test according to the American Petroleum Institute (API) 17B guidelines. The test results given in number of endured cycles from the laboratory test are scaled to the in-service conditions. Although the life estimate is based on one full scale test only, an attempt has been made to account for the inherent scatter in fatigue life. Furthermore, the results are validated by large test series with small scale test specimens for the critical reinforcement components in the composite structure of the hose wall. Test series with steel wires and samples of the steel helix were carried out. Statistically based S-N curves with characteristic scatter are established. Finally, all experimental facts were assembled and fatigue life predictions made. Redesign is considered and a scheduled inspection and replacement program is presented. The rubber-steel composite structure has sufficient strength for both the ULS and FLS case. For a planned replacement interval of 10 years the thickness of the standard steel end fittings has to be increased and the shape of the fitting should be optimized with respect to fatigue.

    Fatigue Life Prediction of Mooring Chains Subjected to Tension and Out of Plane Bending

    2009 | Tom Lassen, Arild Bech, Erik Storvoll 

    Abstract

    The present paper describes the behaviour of chain segments that are subjected to pretension and a rotation angle at the segment end. The behaviour such segments has been investigated both experimentally and by finite element modelling. The purpose is to carry out fatigue life predictions. A full scale test with a studless chain segment with a diameter of 125 mm has been carried out to shed more light on the behaviour. 

    Disconnectable FPSO - Technology to reduce risk in Gulf of Mexico

    2007 | Vidar Aanesland, Jens P. Kaalstad, Arild Bech, NOV APL; Anders Holm, Nexus Floating Production

    Abstract

    The paper describes the Submerged Turret Production System (STP) and a state of the art disconnectable FPSO. The STP technology is well proven in the North Sea, South China Sea and other offshore arenas worldwide for both disconnectable FPSO's and for permanent mooring configuration. Lately it is being used for turret mooring of one of the worlds largest FPSOs located in Mexican waters in Gulf of Mexico on the PEMEX KuMaZa field. The FPSO is scheduled for first oil April 2007 and a short update of this project is presented.

    Crack Growth in High Strength Steel Subjected to Fatigue Loading in Corrosive Environment

    2005 | Tom Lassen, Jose Luis Arana, Luis Canada

    Abstract

    The presentation of the fatigue crack growth behavior of new high strength steels designated R4S grade. Eight compact tension specimens with 25mm thickness were subjected to constant amplitude loading while exposed to seawater without and with cathodic protection.

    An Efficient System for Floating Production, Storage and Loading in Ice-Covered Sea

    1997 | A. Beth, C.M. Beth, K. Syvertsen, T. Vinje, NOV APL

    Abstract

    The feasibility of using a submerged turret loading (STL) system for offshore loading of oil in icy waters is discussed. It is shown that the system is easily adaptable to icy conditions. Since the STL system is easily disconnectable there will be an economical tradeoff between investments in mooring system, investments in oil-storage capacity, and investment in icebreaking assistance, specially in waters with heavy ridge formation.

    Basic Features of the Submerged Turret Production (STP) Concept

    1997 | V. Aanesland, O. Skjåstad, T. Vinje, F. Major

    Abstract

    The Submerged Turret Production (STP) system is a standardized and compact mooring and production system for FPSO applications. The system is applied in the development of the Lufeng oil field in the South China Sea. The system consists of a complete spread mooring system integrated with a STP Buoy, two risers and two umbilicals. When the Buoy is pulled into and locked to the mating cone the vessel is free to weather-vane. The STP Buoy can be disconnected from the FPSO and will then sink to an idle position of about 40 m below the water surface and the FPSO is free to leave the site.

    Fatigue Reliability and Life Cycle Cost Analysis of Mooring Chains

    1996 | Tom Lassen, K. Syvertsen

    Abstract

    Mooring line failure is a major concern in the design of floating production and loading systems. Chain breakage may occur during a dynamically extreme loading condition or due to fatigue damage introduced by the repetitive dynamic load spectrum during long term operation. Reliability methods can be applied by the designer to prevent a high probability of line failure and to achieve a cost effective design. In the present paper, a lognormal model is used to derive the probability of fatigue failure as a function of service time. The uncertainties in the fatigue capacity and the modeling errors of both the dynamic loading and the damage calculation are included. A parametric study is carried out, and expected life cycle costs are estimated. The results are discussed in the light of the fatigue design factor specified in current rules and recommendations.

    Fatigue and Overload Reliability of Mooring Systems

    1997  |  Geir Olav Horde, NOV APL; Torgier Moan, Norwegian University of Science and Technology

    Abstract

    The methods of structural reliability, taking uncertainties related to both loading and strength into account, have been applied to mooring system design. Both the ultimate line tension and the fatigue failure mode are considered. One of the major concerns of the methodology is the combination of different response components, such as those due to low-frequency wind and wave motions, and wave-frequency wave motions. Different combination strategies have been compared. Efforts have also been devoted to obtaining a systems formulation for a single mooring line which accounts for correlation between individual components properly. The example application is concerned with a turret-moored vessel with 12 mooring lines.

     

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