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Dynamic Amplification Factors for Response Analysis

of Steel Catenary Risers at Touch Down Areas

Lucile M. Quau, Mehrdad Kimiaei & Mark F. RandolphCentre for Offshore Foundation Systems, University of Western Australia

Perth, WA, Australia

ABSTRACT

Steel catenary risers (SCRs) are one of the most popular and cost

effective types of risers for deep water field developments. There aredifferent complex issues for engineering of riser systems, with fatigue

design, particularly in the touch down area, one of the most challengingfor SCRs. Traditional fatigue designs, which are mainly based on the

static response of risers lying on linear seabed springs, usually lead tooverly conservative results. Recent availability of advanced numerical

software packages using nonlinear riser-soil interaction models hasprovided an opportunity to improve fatigue design approaches. In

general, SCRs are known as dynamic and fatigue sensitive structures.

In this paper, dynamic and static responses of a particular SCR system

are investigated. The main aim of this pilot study is to explore theusefulness of the DAF (dynamic amplification factor) approach for

dynamic response of SCR systems, which are basically nonlinear

structural systems. The sensitivity of the DAFs to some of the keyinput parameters is investigated with the main parameters considered

being: amplitude of the vessel motions, period of the vessel motionsand soil stiffness. It is shown that vessel motions (amplitudes and

periods) can significantly influence the DAF, while the soil stiffness

has no major effect on it.

KEY WORDS: Steel catenary risers; fatigue; touchdown zone;dynamic amplification factor.

INTRODUCTION

In the last two decades, oil and gas exploration and production in

deepwater offshore fields have increased and have moved into deeper

waters (more than 2000 m in the Gulf of Mexico). The deepwater

developments are pushing riser technology to the limit and steel pipes,

rather than flexible pipes, are now the most common approach. Steelcatenary risers (SCRs) are regarded as one of the most cost effectivetypes of risers among the different riser concepts, allowing large

diameter or production from remote wells. Different engineering issuesexist for SCRs when used in deepwater and fatigue design is the most

complicated challenges (Campbell, 1999; Aggarwal et al., 2007).

Unlike risers connected to fixed platforms, SCRs connected to floatingstructures are subjected to much greater dynamic forces due to motionsof the floating structures and hydrodynamic loads on the body of the

riser (Mekha, 2001). These dynamic stresses result in two criticalfatigue areas: At the vessel hang-off point and in the touchdown zone

(TDZ) (Kimiaei et al., 2010). The structural response of SCRs in theTDZ is complex and highly sensitive to nonlinear riser-soil interaction.

Therefore in structural analyses of SCRs, time domain simulations aremore appropriate than frequency domain analyses to capture possible

nonlinearities (Mekha, 2001). In advanced engineering approaches for

fatigue design of SCRs, dynamic nonlinear time history analyses maybe carried out. Dynamic simulations of structural systems such as SCRs

with geometrical and material sources of nonlinearities need high

computational effort and are very time consuming (Xia et al., 2008).

However, predicting the shape and general forces on an SCR in a static

analysis is a relatively simple process and can be achieved

approximately by solving the standard catenary equations and allowingfor the boundary layer near the TDZ (Bridge et al., 2003; Lenci and

Callegari, 2005).

Quantification of the dynamic response, relative to the static response,can be represented in a dimensionless form by defining dynamic

amplification factors (DAF) expressed as (Barltrop and Adams, 1991):

(1)

Use of DAFs is widespread for linear structural systems as a

simplification of structural dynamic analyses. By knowing the DAF

value and the static response amplitude of a system, the dynamic

response amplitude of the system can easily be evaluated. In such an

approach, there is obviously no need for complicated and time

consuming dynamic analyses.

In general, SCRs are structural systems with both geometrical and

material nonlinearities in the TDZ. This paper presents a pilot studyperformed to explore the usefulness and the possible applications of the

DAF approach for SCRs. Results of dynamic time history analyses ofan example SCR system are presented and compared with the static

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Proceedings of the Twenty-first (2011) International Offshore and Polar Engineering Conference

Maui, Hawaii, USA, June 19-24, 2011

Copyright 2011 by the International Society of Offshore and Polar Engineers (ISOPE)

ISBN 978-1-880653-96-8 (Set); ISSN 1098-6189 (Set); www.isope.org