Offshore Pipelaying: Significance of Motions and Dynamic Stresses During Laying Operations
Authors: Clauss G.F. , Weede H. , Saroukh A. 
Published: October 2008

When installing an offshore pipeline, a long pipe span between the vessel and the seabed is exposed to severe static and dynamic loads. A numerical simulation of its dynamic response is developed and experimentally validated. The calculation methods are based on large deflection circular beam theory. The solution is composed of a linear approach in frequency domain and a modally decomposed nonlinear correction in time domain. If the pipe follows the axial motion component of the stinger, a nonlinear interaction between dynamic tension and bending is included. Alternatively, the tensioner may hold the tension force on a constant value, and the resulting relative axial motions between pipe and vessel are determined. Linearized soil reactions are included to simulate the partial absorption of the bending oscillations. Model tests are performed and accompanied by theoretical simulation; the good agreement proves the reliability of the calculation methods. This validation simulates existing full scale prototypes following the special similarity laws of offshore pipelaying. A highly efficient and flexible model testing technique and equipment is used. The significance of dynamic stresses is illustrated by a full scale parameter study.

Introduction

The technique of offshore pipelaying is shown in Fig.1. Segments of steel pipe, weighted and protected by a concrete coating, are connected on board and lowered to the seabed.

In the S-method (Fig. 1), the angle between the horizontal manufacturing position and the steep elastic line is bridged by a circular stinger equipped with rollers. Its curvature radius corresponds at least to the maximum bending stress. To avoid a bending moment peak at the last roller, the pipe must lift off smoothly from the stinger well ahead of its lower end.

In extremely deep water the elastic line becomes so steep that the required stinger length may not be feasible. In this case, the so called J-method is recommended: All welding equipment and the tensioner can be variably inclined to minimize the bending moment at the clamped end.

The effective weight per unit length, as required to fix the pipeline on the seabed, and a possible current yield large static bending stresses, which are limited by pulling the pipe horizontally with the moored vessel. In the seaway, the vessel moves in all degrees of freedom and imposes oscillations to the pipe. Secondarily, the pipe is also loaded by direct wave action near the surface.

A tensioner supports and retains the pipeline on the vessel and supplies the required tension to the unsupported span. The tensioner drive can be operated for partially compensating the dynamic axial loads. This paper discusses both full compensation (no dynamic tension, maximum axial pipe motion relative to the ship) and blocked tensioner (maximum dynamic tension, no axial pipe motion relative to the ship). These two extrema are the boundaries of more sophisticated tensioner driving methods [13] which can be simulated arbitrarily.

Only a few papers on the dynamics of pipelaying have been published up to now.