Drilling and production in deeper water set new requirements to the design of marine risers. In this paper the most common methods of dynamic analysis of such risers are reviewed and discussed with reference to a set of full scale data.

The measurements were taken by CONOCO in 1975 on a drilling riser operated from a semisubmersible. A spectral approach is applied in evaluating these data and the results include standard deviations, response, spectra and transfer functions. The quality of the data is found to be generally good. Some doubt is express as to the exact conditions at the top support, but it is still considered that important trends are revealed.

In the theoretical analysis four different computer are investigated, covering six different methods. The programs employ either linearized frequency domain solution or time integration techniques with varying degrees of non-linearities included and for either regular waves or irregular sea. Most of comparisons with measurements are carried out in terms of linearized transfer functions. The validity of this approach is discussed as part of the evaluation of the results.

Introduction

Riser analysis methods have receive considerable attention in the technical literature but little has been published on the verification of such methods against full scale measurements. The mason for this is that few instrumented programs have been conducted, and that the resulting data usually have been kept confidential.

In 1974 CONOCO conducted an instrumented investigation on a drilling riser from the submersible Sedco 702 while operating an the Hutton field in the North Sea. The resulting raw data were offered for sale and Det norske Vertias, Kongsberg Vapenfabrikk and Norsk Hydro joined Aker Engineering in an effort to analyse this data and to compare the results against several available riser analysis programs.

The instrumentation adopted by CONOCO was quite extensive, but for the present paper the following instrumentation has been made use of :

  • wave rider

  • platform mounted wave staff with heave compensating accelerometers

  • horizontal platform offset by acoustic position indicator

  • bending strain gauges at four locations along the riser

The reduction of the raw dam was done by the SAMPAN data analysis package (Ref. 1). Generally the sea states analysed were found to contain relatively moderate wave conditions. For this presentation three sea states were selected (A, I and R) Table 1 that presentation three sea states were selected (A, I and R) Table 1 that were found representative for the range of sea states. Sea state 1 with significant waveheight of 8.4 m was the highest sea state measured.

Four different analysis programs containing six different methods were used in the present comparison:

  • SEARISER (Ref. 2) Non-linear time integration according to the Wilson -method. Both regular and irregular seas may be synthesized.

  • RISANA (Ref. 3) Non-linear time integration according to the Newmark beta - method. Regular wave only.

  • CONOCO STATIC/DYNAMIC (Ref. 4) regular waves.

  • NV457 (Ref. 5) Several methods are included but the ones used here am the stochastic linearized and directly solved frequency method and nonlinear irregular sea simulation according to Wilson - method.

A brief review of the theoretical basis for the methods adopted are given in the following.

RISER DYNAMICS
Equations of Motion

The equation of motion of a segment of a marine riser may be written as :

Mx + B/Vrel/Vrel + T'x" + Elx" + Wx' = F

where:

M is the riser mass including added mass

x is the riser acceleration

B is the unit drag force

Vrel - is the relative velocity between the riser and the surrounding water, ie. vrel = × + Cu - w where Cu is the current velocity and w is the wave particle velocity

T - is the so called "effective" tension which is the true tension plus buoyancy effects (Ref. 6).

x" - is the riser curvature or the second derivative of the deflected shape

EI - is the riser bending stiffness

x"" - is the forth derivative of the distorted riser shape

W' - is the inclination of the riser to the vertical

F - is excitation force which consists only of wave inertia since wave drag has already been included on the other side of the other side of the equal sign

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