TLP Rigid Riser: A Case Study
Authors: Rooney P.P. , Engebretsen K.B. , Pettersen D.J. 
Published: December 2007

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.