This paper gives a summary of the structural design of the Snorre Tension Leg Platform (TLP) and presents the final platform configuration. The most important parameters and requirements governing TLP configuration are presented. Key aspects of the sizing process and final geometry are summarized and finally an outline of the analysis methodology is provided. The selection of a TLP configuration is an interative process rather than the result of one particular exercise.
Summary This case summarizes tension-leg platform (TLP) rigid riser designconsiderations and presents results of TLP riser analysis for aproduction/injection riser in the North production/injection riser in the North Sea. Analysis methods, design criteria, and design optimization are addressed. The riser is designed for 300-m water depth in compliance with Norwegianregulations. Emphasis is placed on application of the regulations andquantitative comparison of alternative methods for analysis of fatigue andextremes. Introduction This paper summarizes TLP rigid riser design considerations and presentsresults of the TLP riser analysis. TLP functions include drilling, production/injection, and export through rigid risers. TLP design is mostsensitive to the many production/injection risers. Such parameters as riserspacing, top tension, stroke, and pontoon clearance strongly influence globaldimensions and deck spacing/layout. Parameters used in this case study aresummarized in Table 1. Fig. 1 is a schematic of the TLP riser system. Thedesign is based on Norwegian Petroleum Directorate regulations and guidelines. Criteria for local design are summarized in Ref. 3. Analysis Methods Both frequency domain and time do analysis methods were used. Fatigueanalysis is primarily based on frequency domain and extremes on time domain. The riser is a nonlinear dynamic system, and the inaccuracy in using frequencydomain (assuming constant top tension and using stochastic linearization) isdiscussed below. The approach adopted for extreme responses is regular waveanalysis. This method was used to overcome uncertainty in statisticalextrapolation from an irregular time simulation and, for efficiency, to enablemany design iterations. Dynamic analysis was carried out without introducingload factors. Load factors are applied to responses when carrying out codechecks and when preparing design values for component specifications (e.g., stroke results). Boundary conditions include TLP motion, tensionercharacteristics (see Stroke section) and template/subsea wellhead interface(see Components and Interfaces). The riser analysis included TLP offsets in therange corresponding to TLP extreme motions. First-order response is includeddirectly as a transfer function in the riser analysis, and all other TLP motioncomponents are taken into account by defining a range of riser mean positionsupon which the first-order response is superimposed. TLP setdown is included ateach timestep in the time domain analysis. Sensitivity and Optimization As usual for deterministic design, an extensive sensitivity study must becarried out. The following were investigated:wave periods;currentvelocity;riser periods;current velocity;riser location andgeometric phase;hydrodynamic parameters, variation with depth, Reynoldsparameters, variation with depth, Reynolds number, Keulegan-Carpenter's number, and roughness;annulus and tubing fluids;top tension and tensionercharacteristics;TLP offset;extent of marine growth; andTLP huginfluence on wave kinematics. Quantitative sensitivity results vary widely forthe various responses along the riser, and the resulting priority is influencedby those responses that govern the design. The rigid risers tend to be inconflict with the overall TLP optimization on such parameters as top tensionand riser spacing. parameters as top tension and riser spacing. Optimization, therefore, is an iterative process carried out on the basis of the sensitivityresults for both the risers and the TLP. Handling and operational weatherlimitations are most sensitive to riser spacing and wellbay layout. Theserequire early detailed consideration because they are important to TLPefficiency in service. An alternative analytical approach to sensitivityanalysis and optimization with probabilistic methods could be used whereprobabilistic methods could be used where quantitative sensitivity results aregenerated directly in one analysis. This method has been applied successfullyto TLP tethers. Extreme Stresses Allowable tensile stresses in the riser wall were checked against values in Ref. 2. The Von Mises (combined) stresses were solved with a minimum wallthickness, taking into account corrosion/wear allowance and wall-thicknesstolerances. Ref. 9 is used for sum calculations. Compressive stresses also werechecked for load where axial stresses were low. Local buckling checks did notlead to additional design requirements. Extreme results vary considerably withanalysis method.
This case study presents a Production/Injection rigid riser. The riser is designed for 300 meter water depth in compliance with Norwegian regulations. Emphasis is placed on application of the regulations and quantitive comparison of altemative methods for analysis of Fatigue and Extremes. Introduction TLP functions include Drilling, Production/Injection and Export through rigid risers. TLP design is most sensitive to the many Production/Injection risers and parameters such as riser spacing, top tension, stroke and pontoon clearance strongly influence global dimensions and deck spacing layout. Parameters used in this case study are summarised in table 1. Figure 1 shows a schematic of the TLP Riser System. The design is based on NPD Regulations (ref. 1) and Guidelines (ref. 2). API RP 2T (ref.3) was used for local design. Analysis Methods Both Frequency domain and Time domain analysis methods (refs 4,5) were used. Fatigue analysis is primarily based on Frequency domain and Extremes on Time domain. The riser is a non-linear dynamic system and the inaccuracy in using Frequency domain (assuming constant top tension and using stochastic Iinearisation) is discussed below. The approach adopted for extreme responses is regular wave analysis. This method was used to overcome uncertainty in statistical extrapolation from an irregular time simulation and for efficiency in order to enable many design iterations. Dynamic analysis was carried out without introducing load factors. Load factors are applied to responses when carrying out code checks and in the process of preparing design values for component specifications (eg. Stroke results below). Boundary conditions include TLP motion, Tensioner characteristics (see Stroke) and Template/Subsea Wellhead interface (see Components). The riser analysis included TLP offsets in the range corresponding to TLP extreme motions (ref. 6). First orderresponse is included directly as a transfer function inthe riser analysis and all other TLP motion components are taken into account by defining a range of riser mean positions upon which the first order response is superimposed. TLP setdown is included at each time step in the Time domain analysis. Sensitivity and Optimisation As usual for Deterministic design, an extensive sensitivity study must be carried out. The following were investigated:wave periodscurrent velocityriser location, geometric phasehydrodynamic parameters, variation with depth, Re, KC and roughnessannulus and tubing fluidstop tension and tensioned stiffness characteristicsTLP offsetextent of marine growthTLP Hull influence on wave kinematics Quantities sensitivity results vary widely for the various responses along the riser and the resulting priority is influenced by those responses which govern the design. The rigid risers tend to be in conflict with the overall TLP optimization on parameters such as top tension and riser spacing. Optimization is therefore an iterative process carried out on the basis of the sensitivity results both for the risers and the TLP. Handling and operational weather limitations are most sensitive to riser spacing and Waubay layout. These require early detail consideration as they are an important input to TLP efficiency in service.
This paper gives a summary of the design basis and requirements of the Norwegian Field Development, Snorre B hull. It describes the fabrication of the hull at the Dragados Offshore yard in Spain, and highlights the critical and important features in the design. It describes the method used, the main elements in the hull and the shape of the final product. The paper includes a description of the fabrication sequence, load out, launching and towing to Norway. Experiences gained during fabrication will be discussed and summarised. A full description of the fabricated sections will be performed and this will be visualised with photos and drawings from the yard. Description of the Floater The Snorre B platform is a floating drilling and production unit (SSPV) consisting of a steel hull and deck structure. The hull consists of four "square rounded" columns supporting the deck. The columns are connected in the corners at the ring pontoon, which is rectangular in shape. The deck structure has two main truss-lines in both the transverse and longitudinal directions. In addition to these trusses, there are also two major transverse bulkheads. Each corner of the deck consists of a "rounded square" box. There is a main deck and a cellar deck including a double bottom structure. Fig. 1 shows the floater, hull and deck. Fig. 1. Snorre B deck and hull Design Most of the design work for Snorre B was initiated in 1998. Key factors accounted for in the structural design basis are:Regulatory requirementsEnvironmental CriteriaSafety ConsiderationsFunctional and Operational Requirements Design criteria Snorre B is designed for a 100 years storm condition for the location in the North Sea at a water depth of 350 metres. The extreme significant wave height for the area is 15.5 metres ULS, respectively 18 metres PLS. This is combined with a storm wind of 41 m/s. The design has an allowance as shown below:Margins: Mai 1999 August 2001Company: 3,000 tonnes 1,570 tonnesContractor: 1,850 tonnes 1,170 tonnesThe design life is 20 years. Factors are used for the fatigue life calculations.Outside hull below Splash zone: 2Splash zone: 3Above Splash zone: 1Inside hull: 1 and 2Deck general: 1Critical areas not possible to inspect: 10 The deck shall accommodate both drilling (RAM RIG), production equipment and aluminium living quarters. Snorre B has 16 anchor lines which combine wire and 145 / 137 mm chain. These are connected to suction anchors. Regulatory Requirements Based upon experience gained from more recent platform development in the North Sea, design criteria and rules were established for the structural design. Det Norske Veritas Rules for Mobile Offshore Units (DNV MOU) including supplementary requirements of Norwegian Petroleum Directorate / Norwegian Maritime Directorate and NORSOK, among other, were used as basis for design.
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