Partially Displacement Controlled conditions in the overbend

Abstract

The S-lay pipeline installation method is characterised by its fast installation process and its application in all water depths. Offshore standards and design codes provide local buckling checks for combined loading criteria for load controlled and displacement controlled conditions, where a load controlled condition is defined as a case in which the structural response is primarily governed by the imposed loads and a displacement controlled condition is defined as a case in which the structural response is primarily governed by imposed geometric displacements. It is recognised by the offshore pipelaying industry that the condition with the pipeline on a rigid stinger in S-lay is not fully displacement controlled. Using the methods by DNV only allows for situations that are either 100% load controlled or 100% displacement controlled to be checked. In order to account for partially displacement controlled conditions, a method is needed to calculate the level of displacement control in a specific situation and to translate this to a partially displacement controlled buckling check.

This graduation thesis proposes a method to calculate the level of partial displacement control of an S-lay pipeline installation situation using pipe stiffness variations in beam elements. First, a static pipelaying configuration is modelled using beam elements. Next, the same situation is modelled with a different pipe stiffness over the stinger section. The changes in moment and curvature at the rollerbox locations between the base case and the modified model give an indication of the level of displacement control on the stinger. An approach to translate the calculated level of displacement control to modification of the displacement controlled buckling check by DNV is presented.

Detailed finite element models composed of shell type elements are created to validate the proposed method. A first step towards validation of the proposed method has been taken with satisfactory results. The calculated levels of displacement control with the proposed method are within 2% of the calculated level of displacement control in the detailed partially displacement controlled model using shell elements. The use of the proposed method allows the calculation of the level of displacement control using Euler-Bernoulli beam element models and with reduced computational time.

A parametric study is performed using the proposed method to study the effect of most relevant variables in the S-lay configuration on the degree of partial displacement control. The effects of the stinger radius; rollerbox spacing; rollerbox load; lay tension; pipe diameter; and pipe diameter to wall thickness ratio have been studied, giving greater understanding of the partially displacement controlled behaviour.

The research done in this thesis provides an engineering approach to determine the partially displacement controlled conditions in the S-lay configuration using stiffness variations in Euler-Bernoulli beam element models. First steps have been taken to validate this engineering approach with satisfactory results. A method for modifying the displacement controlled local buckling check to account for partially displacement controlled conditions is presented, as well as an understanding of the influence of most relevant pipelaying variables on the partially displacement controlled condition.