Buckling Considerations In The Design Of The Gravel Cover For A High-Temperature Oil Line

ABSTRACT Total Oil Marine have recently installed a 15.3 km long 12" pipeline in the UK sector of the North Sea as part of the development of the Alwyn Field. The pipeline is completely covered by gravel for physical protection. This paper discusses design engineering problems related to anticipated buckling phenomena associated with high oil temperatures during operation of the pipeline. Special studies were performed by LLOYD'S REGISTER OF SHIPPING and DELFT HYDRAULICS LABORATORY concerned, respectively, with buckling and heat transfer aspects of the pipeline design. In addition, full scale pull-out tests were carried out. INTRODUCTION 12" Alwyn-Ninian Oil pipeline The Alwyn North field is located in the northern part of the UK sector of the North Sea, some 160 kms east of the Shetlands and 110 kms north of the Frigg field and has been developed by Elf UK (66.7%) and Total Oil Marine (33.3%), TOM being the Operator. The development plan consists of the following facilities:–one drilling and accommodation steel platform (NAA)–one production steel platform (NAB)–two export subsea pipelines:a 12" pipeline for the transport of the oil production from Alwyn NAB to Ninian Central.a 24" pipeline for the transport of the gas production from Alwyn NAB to Frigg TP-The Alwyn-Ninian oil pipeline is 15.3 kms long and made up of 12" X65 (API 5LX) -0.406" W.T. seamless, epoxy and 2" concrete coated pipes. For physical protection purposes the Alwyn Ninian pipeline is covered by a continuous gravel embankment over its entire length. As such it is amongst the longest pipelines completely covered with gravel in the North Sea. The gravel embankment has a triangular shape of which the height relative to the seabed varies from 1.4 m to 1.0 m and a maximum embankment slope of 20- to meet the requirements of the fishing industry. Design Engineering Problem Due to high oil temperatures to be sustained in operation (design temperature 84°C at riser sea bottom extremity), a potential bar buckling problem induced by large thermal stresses was investigated as part of the normal pipeline engineering design. Lloyd's Register of Shipping - Ocean Engineering Department (LRS) were commissioned to perform a specific study to analyse all the aspects of this potential bar buckling. LRS first findings were that, assuming the pipeline to be an extremely long beam "snaking" on the sea bottom, there was no risk from bar buckling as such. However an increase of the initial lateral deflection would occur which did not induce dangerous additional bending stresses. Such a model was particularly suitable for an uncovered pipeline simply laid on the seabed. This concept, however, did not allow the proper analysis of the local phenomenon, i.e. a straight section of 100-200 m long completely laterally restrained by gravel. LRS therefore developed an alternative model which indicated that the pipeline might buckle locally and emerge vertically from the gravel embankment if the depth of cover was not sufficient.