TLP Rigid Riser: A Case Study
Authors: Rooney P.P. , Engebretsen K.B. , Pettersen D.J. 
Published: October 2008

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.