Seabed-pipe interaction for deepwater applications

Dr Cheuk, Chi Yin Johnny   (Principal Investigator (PI))

Professor Bolton Malcolm David   (Co-Investigator)

Dr White David John   (Co-Investigator)

30

2006-10-01

106733

Seabed-pipe interaction for deepwater applications

deepwater applications, Seabed-pipe

Geotechnical,Others - Civil Engineering, Surveying, Building and Construction

HKU 1115/06E

General Research Fund (GRF)

2006

Completed

1. Offshore hydrocarbon developments are extending into deeper waters, with oil or gas fields in water depths of >2000m becoming viable. The development of these resources requires new installation techniques, which present new geotechnical challenges. In deep-water, a floating facility is held in place by flexible mooring lines anchored at the seabed, and the risers often take the form of catenaries draped down to the seabed, which continue as on-bottom pipelines. The facility is permitted to deviate laterally by typically 10-30% of the water depth. Since pipelines undergo cycles of thermal loading during operation and shutdown periods, the resultant thermal expansion must be relieved. An emerging approach is to allow the pipeline to buckle in the horizontal plane, which is a more cost-effective option than restraining the pipeline. Also, the facility movement means that the pipeline is vulnerable to structural damage in the riser touchdown zone, during large cyclic movements. 2.The soil conditions in the new hydrocarbon development regions often comprise soft sensitive clay, meaning that it shows a significant drop in strength when sheared and remoulded. Significant environmental loads with a typical period of 10seconds arise from wind and wave action on floating facilities and near-surface sections of the risers and mooring lines. Also, in certain regions, seasonal or medium-term sea currents can create significant additional loading with periods of weeks or months. Pipeline systems suffer further loading during shutdown cycles (which can last for hours or days) due to thermal contraction when not transmitting hot oil. These rates of loading span from undrained to drained conditions. These arduous conditions, soft sensitive clay, drained and undrained cyclic loads, and large amplitude motion (of the buckling pipeline and in the riser touchdown zone), present new challenges for the geotechnical design of pipelines and risers where they interact with the seabed. 3.The proposed research aims to study the response of an on-bottom pipeline, which is also relevant for steel catenary risers, as they both involve axial and lateral loading of a partially embedded cylinder due to external cyclically applied loads. The results will be used to improve existing models by taking into account the behaviour of large amplitude cycles, self-burial, and berm-building. In order to reach the ultimate goal, the following intermediate objectives have to be achieved: (a) To fabricate a model chamber with a 2-dimensional actuator to examine the failure mechanisms of a pipe section under combined horizontal and vertical loading (b) To conduct numerical modelling of a partially embedded pipe under combined horizontal and vertical loading at large deformation (c) To deduce from these results, refinements to current models for predicting pipe response 4.Improved models for pipe-soil interaction based on realistic mechanisms are the most important outcome of the proposed research, which will benefit practicing engineers. The outcomes will influence the feasibility and economic viability of deep-water hydrocarbon developments by providing practical guidance for offshore engineers designing on-bottom pipelines and SCRs; these new technologies are currently under-exploited due to the high conservatism required when using current design methods. The derived models for pipe-soil interaction will allow more realistic analyses to be conducted during design than is currently used, reducing the uncertainty in the structural loads experienced by the pipelines and risers, allowing more economic and safe design. 5.China, being the second largest consumer of petroleum products in 2004, has a high demand for energy products to maintain the pace of the economy growth. This has put offshore hydrocarbon development a very high priority. Recent offshore exploration interest has centered on the Bohai Sea area, believed to hold more than 1.5 billion barrels in reserves, and the deepwater block 43-11 (1500-3000m deepwater), located about 350 km southeast of Hong Kong. In addition, a large network of pipelines are being planned or constructed to link up with nearby oil suppliers, such as Venezuela, Russia and Kazakhstan, for oil import. All these large-scale developments present challenges to engineers looking into the geotechnical design of the offshore structures. Robust design of facilities in offshore exploration and transportation is therefore a dependent variable for maintaining the economic growth. The technological development will be boosted by the outcome of the proposed investigation. 6.The proposed research tackles a fundamental problem in foundation engineering, and will generate new theoretical developments supported by high quality numerical and experimental results, that will be of wide interest to academics. The research strategy builds on physical and numerical modelling, which are perfect counterparts in tackling this geotechnical problem. The use of a powerful image analysis technique enables the mechanisms to be identified more precisely than has been previously possible, whilst analysis using the finite element technique, once verified against the physical models, will allow broader parametric studies to be conducted. These two packages of work will aid the development of upper and lower bound plasticity solutions for limiting load combinations that are more amenable to routine design than FE analysis. Plasticity solutions can also be combined and cast into unified ‘force resultant plasticity models’ to capture the full load-displacement response of a pipe element. 7.The proposed research also promotes advancement in research technology. Image analysis has been proven an invaluable tool for investigating failure mechanisms in soil-structure interaction problems. However, most applications have been using granular materials. The present research will further develop the technique, applying it in fine-grained materials, such as clay, by artificially imposing soil texture. In terms of numerical modelling, the entire physical process will be studied with the help of remeshing technique, replacing the conventional wished-in place approach. This paradigm of modelling the process instead of simply the instant is particularly vital for this large-displacement problem.