Critique of Offshore Pipelay Criteria and Its Effect on Pipeline Design

ABSTRACT The pipelay criteria currently used for pipeline design is based mainly on submarine static pipelay analysis results. The criteria recommended in the codes and by operators are sometimes more stringent than necessary and affect the layability of the pipeline, and the capability of pipelay barges. This problem is illustrated herein using both static and dynamic analyses of a typical pipelay operation. The analysis show that for large diameter pipelines, the design is restricted to the maximum layable submerged weight to satisfy the pipelay criteria. In order to achieve this, the pipeline may need trenching to provide additional stability, or require a higher awlied tension, over and above the installed tension capacity of the pipelay barge. It is shown here that this can lead to an increase in the project cost and schedules. The results presented herein show that by Modifying the arrrent pipelay criteria, it is possible to lay heavier pipelines, thus i..rrq;>roving the cost effectiveness of a project. An example is presented illustrating the effect of pipelay criteria on the design of a large diameter pipeline and show that further cost savings could be made on pipeline trenching and pipelay barge modification required. INTRODUCTION The design of an offshore large diameter pipeline is dependant to a large extent on the availability of pipelay barges to install the pipeline. The pipe layability, using the chosen barge, will Impact on the maximum design concrete coating at a particular water depth and thus provide an upper bound to the amount of negative buoyancy available to satisfy on-bottam stability requirements. The negative buoyancy requirement for a pipeline is dependant on the pipe size, hydrodynamic forces and the pipe/soil interaction characteristics. This is an area whidl has been the subject of considerable research activity in the recent past. As a result, there are a number of computer programs developed to help the designer to achieve a submerged weight requirement for the pipeline to satisfy the various stability criteria. In most cases, the resultant submerged weight of the pipeline can be laid by a pipelay barge and the pipeline will be stable on the seabed during its lifetime. However, in sane cases, the resultant submerged weight of the pipeline is too heavy an:i hence cannot be installed by the pipelay barge to satisfy the pipelay criteria. To overcome this problem, it is necessary to either trench the pipeline and reduce the negative bouyancy requirement, thus Improving the layability of the pipeline, or to increase the capability of the pipelay barge tension/overbend support system so that a heavier pipeline can be installed. The first option depends on the suitability of the seabed for trenching, e.g. sandy or clay. If the seabed consists of rock or boulders, this may not be a viable option. In this case, the pipeline has to be designed to be stable on the seabed, with the pipelay barges being modified to facilitate installation.