Finite-Element Response Analysis of a Lay-Away Pipeline xpansion Curve on Soft Clay
Authors: SvanØ Geir , Jostad H.P. , Bjaerum R.O. , Stokkeland Alfred 
Published: October 2008

The response analysis of a lay-away expansion curve on a seabed of soft clay requires careful consideration of soil reaction (consolidation effects) and geometrical nonhnearities. The general purpose ftite element program FENRIS was specifically customized for this application and utilized for the anrdysis of a typical lay-away configuration for the Troll Phase I wet gas pipelines.

It was found that the response was governed by the relatively high soil resistance caused by consolidation effects and partial embedment in the soft clay. After breakout the system became unstable due to large compressive pipe forces, and an arc-length solution algorithm was employed to overcome this unstable situation.

The configuration analyzed was found not to be acceptable, and concept modifications are currently being engineered.

INTRODUCTION

Thermal expansion of an offshore pipeline towards a platform riser base is traditionally controlled by expansion spools installed on the seabed to fit the riser/pipeliie geometry and tied-in using hyperbaric welding. With increasing water depth this method becomes progressively less attractive due to the inherent need for manned subsea intervention. For the tie-in to the Troll Phase I platform (water depth 300m) the so called curve lay-away was identified as a promising alternative in combination with pull-in into a dry riser shaft via dedicated sealtubes. A general background for the Troll Phase I pipelines is available elsewhere [1].

On the Troll seabed of very soft clay the pipe will be partially embedded, and the lateral breakout resistance will substantially exceed that of a pipe on top of the seabed. Model tests by Brennodden et al [2] further demonstrated that the resistance increases with time after pipe installation. This effect, which will be referred to as a "consolidation effect", introduces significant soil reaction capacity, and may reduce the functionatily of the lay-away expansion curve. After laterrd breakout in the curve the system becomes unstable due to the large compressive pipe forces. This situation requires special attention regardiig the solution procedure, and an arc-length algorithm is utilized to follow the pipe through the breakout process to a new stable position. The fidl history of pipe forces and displacements is predicted, allowing a detailed evaluation of the studied expansion curve configuration.

PROBLEM DEFINITION

The detailed layout is depicted shown in Figure 1, and representa an S-shaped curve. The modelled pipe section is 6882 m long. The pipe coordinate X is a line coordinate along the pipe. Between the platform end at pipe coordinate X=O and the "jar end" of the model at X=6882 m, there are two expansion curve sections with 3000 m radius. The enclosed curve angles are 37.5 and 39.0 degrees. The pipe coordmtes of the curve points (CP1 through CP4) are listed in Figure 1. Beyond the "far end" of the model, the pipe extends in a straight lioe over several kilometres. Pipe data The steel pipe inner diameter and wall thickness assumed are 853.8 × 32.6 mm. The overall outer diameter is 1010.8 mm.