Submarine Pipeline Design in the South China Sea

Transcription

1 Submarine Pipeline Design in the South China Sea ZHAO Dang 1, HE Ning 1, LIU Run 2, WANG Bo 1, LIU Meng-meng 2, WANG Meng 1, HUANG Hui-di 1 1. Offshore Oil Engineering Co., Ltd. Engineering Company, Tianjin, , P. R. China 2. Tianjin University, School of Civil Engineering, Tianjin, , P. R. China zhaodang@mail.cooec.com.cn Abstract: Submarine pipelines in South China Sea have been under consideration for more than a few years, but only few have been built. One reason for this is that petroleum prices have not increased as fast as had been expected, but another is that there have turned out to be several technical difficulties, among them high pressure and high temperature petroleum resource, deepwater and harsh environments, uneven seabed feature, and possible fishing activities. The harsh environment and geological conditions resistant to South China Sea petroleum development, and has a substantial influence on regulatory approvals. Forward environmental data and geotechnical data must be predicated on a complete and convincing resolution of technical questions. This paper examines each problem area, and sets out to reach a judgment on the question that have been resolved and how far the industry is easy to construct submarine pipelines in the South China Sea. Keywords: HPHT; Internal wave; Routes selection; CRA Lined Pipeline. 1. Introduction A few years ago, the hydrocarbon resources of the South China Sea seemed on the point of rapid and energetic development. Many schemes were promoted, some of them very ambitious. Nevertheless, much research was carried out, and a very few projects were actually carried through, partly because costs and technical uncertainty made the economics seem questionable, among them the LIWAN Deepwater EPCI, Shallow Water Facilities of the South China Sea Deepwater Gas Development and PANYU 35-1/2 Subsea EPIC Services project are underway. With research continued, and some ideas changed markedly. The oil price has recovered, at least for the moment, and some offshore projects appear economically and technically feasible in the South China Sea. Most of these projects involve marine pipelines. They are receiving renewed interest, though there are still technical questions and environmental unfavorable factors. The objective of this paper is to review the state of knowledge, and to attempt to assess how far the pipeline industry is now ready for the new challenge. It is clear that some projects are very much more demanding than others. Rightly and unsurprisingly, all the indications are that the industry will begin with the less ambitious and less difficult projects, and expects to apply the lessons learned to the larger projects that may be reached later, probably not for five or ten years. This paper should not be interpreted as a comment on any specific project. As in the case of deep water, some pipeline construction problems are the same as they are elsewhere, and some are easier in the South China Sea. Pipeline affected by marine growth, for example, is rarely a major constraint, because in the South China Sea, the deepwater makes the marine growth effected less. 2. HPHT In comparison to the easy and conventional sources of hydrocarbons found in other sea areas of China, the reservoirs found in the South China Sea are mostly under high pressure and/or high temperature (HPHT) condition (e.g. pipeline from PANYU 35-2 A2H well to PANYU 35-2 CM, the parameters are 38.49MPa for hydrotest pressure, 31.0MPa for operation pressure and the max design temperature up to 84.0 C; and pipeline form PANYU 35-1 A2 PLET to PANYU 35-1 A1H ILM, the parameters are 33.09MPa for hydrotest pressure, 27.0MPa for operation pressure and the max design temperature up to C). Transporting the oil and gas by flowlines and pipelines from these HPHT reservoirs is a major challenge. A pipeline laid on or buried in the seabed responds to high pressure and/or high temperature by expanding against the frictional resistance from the soil and other restrictions, resulting in axial forces, axial displacement (also known as end expansion), lateral buckling, upheaval buckling, or a combination of these, depending on whether the pipeline is partly/fully restrained or unrestrained. In some cases, pipeline walking may occur after the pipeline in operation is cooled down, e.g. in a shutdown, heated up for operation, and then the thermal cycles repeated. These pipeline movements/forces can cause 125

2 failures in the midline or at the tie-ins connected to the pipeline end, and are critical to the integrity of a pipeline. When a pipeline is subject to high pressure and high temperature, its ends expand longitudinally and exert forces and bending moments onto adjacent tied-in structures or pipes connected to it. The tied-in structures/pipes must be designed to withstand these expansions and loads. Dumping rocks along the pipeline has conventionally been adopted to reduce the end expansion, but is challenging for deepwater pipeline. Trenching and burying pipelines are costly. A giant spool installed at the pipeline end is another alternative which is commonly used in the design of subsea pipelines for the South China Sea. They are often used in combination to eliminate end expansion when it is very large. However, such post-lay intervention work is costly, requires long offshore time to accomplish and its installation is restricted by the lay barge and environmental data. Thus, a major challenge is to improve on the ways the pipeline end movements can be controlled ways which are simple, safe and cost effective. A global team of experienced engineers has developed a new concept SliPIPE[1], shown in Figure 1, to deal with the end expansion of a rigid pipeline subject to HPHT. Key advantages of SliPIPE are that it avoids the fabrication and complicated installation associated with giant spools; minimizes costly post-installation subsea intervention work; and SliPIPE is space-efficient, ideal in areas congested with many subsea facilities, as are often encountered in brownfield modification work, where safeguarding their integrity during intervention work can be formidable. Although SliPIPE is conceptual and will require refinement and engineering through basic and detailed design before it can be adopted in an actual project, it is an ideal way put forward to cope with HPHT pipeline design. Figure 1 Sketch of SliPIPE(Chia Chor Yew and Asle Venas, 2013) When pipelines operate at higher temperatures, the likelihood of buckling becomes more pertinent. Global buckling analysis will be performed to identify whether the global buckling is likely to occur. If it is, then further analysis is performed to either prevent buckling or accommodate it. Likelihood of above-mentioned dumping rocks may induces higher loads in the line pipe when prevents it from buckling, and which will not provide enough restraint, then localized buckling may occur (i.e., upheaval buckling), which can cause failure of the pipeline. Another method is to accommodate the buckling problem by permitting the pipeline to deflect on the seabed, by using a snake lay or buckle mitigation methods such as sleepers[2-4] or distributed buoyancies[5]. This method is obviously cheaper than rock dumping, and results in the pipeline experiencing lower loads. However, the analysis will probably have to be based on the limit-state design, because the pipe will have plastically deformed. This method is becoming more popular. This method can also be used with intermittent rock dumping; by permitting the pipeline to snake and then to rock dump, the likelihood of upheaval buckling is reduced. The methods employed in calculating upheaval and lateral buckling, as well as the pullover response, are detailed in Ref. DNV-RP-F110[6]. 3 Internal wave Internal wave is a nonlinear wave, and often happens in the South China Sea, which known as the natural laboratory for the internal wave study. It is only in research step that the effects of nonlinear ocean internal wave make on submarine structure. Studies have shown that two main reasons arise internal wave, one is the stable layer structure of sea water, which should be determined by the seasonal temperature, atmospheric circulation, and geographical position of 126

3 the sea water. For a given sea zone, the density of sea water is stable for different seasons, and the deeper the heavier density of the sea water. The other reason is the disturbance activity, which can be divided into external and internal disturbances. The external contents the rise and fall of barometric pressure, wind inducing at ocean surface and seismic effect from the ocean bottom, while the internal contents the interaction of tide and submarine topography, resonance of surface wave and induced wave caused by the effect of current on the roughness seabed terrain and so on. In the South China Sea, the sea water has acquired the requirements for internal wave. Distribution of the internal solitary waves has shown in Figure 2 and Figure 3. Compared the two-dimensional wave at the ocean surface, the internal wave current (soliton) is three-dimensional. The data used for the PANYU 34-1 marine pipeline design refer to Table 1. There is a proposed riser at the PANYU 34-1 jacket platform, the primary design parameters are: water depth 190m, OD 355.6mm and WT 15.9mm of Grade API 5L X-65, PSL2 (API 5L, 2007). Figure 2 - Distribution of the internal solitary waves in the South China Sea [7] (The iso lines denote water depth in meter) Figure 3 - Monthly distribution of the internal solitons in the northeast of South China Sea from SAR [8] 127

4 Table 1 Main Environmental Extreme Value Wave Current Soliton current H s (m) 13.8 Water depth(m) Data (cm/s) Water depth(m) Data (cm/s) H max (m) T s (s) T p (s) For the soliton current is a three-dimensional, assent the effect on the riser analysis by considering eight directions and two conditions (with soliton current and without). Refer to Figure 4 for the riser model (AutoPIPE) and Figure 5 for the soliton current directions. Riser Figure 4 - Schematic of riser model Figure 5 - Schematic of soliton crrent direction for riser analysis According to DNV-OS -F101[9], analyze the installation case of riser model by LRFD, and the results (load control combined a and b ) list in Figure 6(with soliton current effect) and Figure 7(without soliton current effect). From above analysis and other documents, it is known that the internal wave current may be hazardous for marine riser. For a proper assessment, site specific data are required on the internal wave velocity, direction, duration and frequency of occurrence. In the future, should pay more attention to the internal wave characteristic, and proposal relevant methods as a safeguard against the effects of the internal wave on the submarine pipeline. 128

5 Figure 6 - Results of riser analysis with soliton current effect Figure 7 - Results of riser analysis without soliton current effect 4. Pipeline Routes selection The proposed pipeline route should be performed form the results of seabed soil feature evaluation, pipeline upheaval buckling analysis for HPHT pipeline, exact location and burial status of existing pipeline and cables shall be identified prior to installation activity, pipeline on-bottom roughness and scour induced free span assessment, and pipeline and anchor area QRAs. The water depth in the South China Sea is shown as Figure 8. The water depth and route survey corridors within PANYU and LIWAN zones refer to Figure 9, and the routes for proposed pipelines are generally flat with local variations of seabed level for a few meters due to some irregular seabed areas. 129

6 Figure 8 - Schematic of water depth in the South China Sea Figure 9 - Schematic of survey corridor and pipelines general layout within PANYU and LIWAN zones The main discernible prominent seabed features within this zone are areas of hard irregular seabed and sand waves, and undesirable seabed features and obstructions i.e. isolated pock marks, hard undulated seabed features are found within some proposed pipeline route. Hard irregular seabed features are seems to be the exposed locally strong clay due to erodible soils around. However the effect of these seabed features to pipeline integrity during installation and operating shall not be negligible. 130

7 4.1 Soil characteristic Geotechnical investigation has been carried out along the pipeline route to understand the presence of soil type and derive the geotechnical data required for the proposed pipeline. Liquefaction potentials have been evaluated from seismic response analysis results and cyclic triaxial test results. Analysis results indicate that the liquefaction will not occur along the pipeline route. The survey has not identified any marine activity such as access channel, anchors scars and debris within pipeline route survey corridor. However, there are trawl gear scars existing. If the trawling board hits on 6 (PANYU 35-1 zone) or 10 (PANYU 35-2 zone) pipeline within this zone, according to the results, it will cause damage. According to national regulation, trawling is forbidden on pipeline route, and need enhance management to ensure no trawling on pipeline route. Likelihood of trawling on pipeline is very low, so the risk level is acceptable within national regulation. Sand waves are identified along the pipeline route from LIWAN 3-1 CEP to PANYU 34-1 CEP and PANYU 35-1 zone. But the impact of sand waves to integrity of the pipeline is considered to be insignificant due to smaller height and the wave lengths of the sand waves. These sand waves and potentially scour may cause spans along the pipeline. The evaluation of sand waves and ridges and pipeline scour assessments are performed. The recommendations from analyses include trenching and burial for the selected pipeline (e.g. pipeline form LIWAN 3-1 CEP to PANYU 34-1 CEP). 4.2 Sand waves and sand ridges evaluation Sand waves and sand ridges evaluation are to assess the dynamics of the seabed and to evaluate the impact of sand waves and ridges on the design, maintenance and operation of the pipeline. Possible causes of seabed mobility and dynamics are as follows: a) Seabed changes due to general sediment transport if the seabed is erodible; b) Seabed changes due to development and migration of bed forms. The mobility of the seabed depends on its erodibility. If the seabed is erodible, then sediment transport occurs under the combined action of waves and current, causing potential seabed scouring. In addition, bed forms may occur on an erodible seabed. The bed forms may develop, migrate and disappear depending on variations in the environmental conditions Seabed Erodibility Erodibility and potential seabed scour depend on local soil conditions as well as wave and current conditions. In terms of erodibility and potential of seabed scour, the soil condition in the PANYU and LIWAN areas can be roughly classified as follows: 1) Sandy seabed: Non-cohesive or almost non-cohesive seabed (sandy seabed). The seabed consists of sand with or without silt. There is no clay or the fraction of clay is small (order of 5%). The seabed is generally (easily) erodible. For non-cohesive or almost non-cohesive seabed, the erodibility is determined by the Shields number (van Rijn, L.C., 1993) [10]. The critical Shields number for initiation of motion is about 0.05 for a sandy seabed. The seabed will be mobile if the Shields number exceeds the critical Shields number. 2) Clayey seabed: a) Partly cohesive seabed (clayey seabed): consisting of a small fraction of clay (about 10-20%). The seabed is less erodible than sandy seabed; b) Cohesive seabed: consisting of a significant fraction of clay (>20%). The seabed is more resistant to erosion than partly cohesive seabed. The clayey seabed is cohesive or partly cohesive. It is more resistant to erosion due to waves and current. The flocs on the surface of a cohesive sediment bed are bound together by inter-particle attractive forces. To remove a floc by flow requires a bed shear stress sufficient to overcome the attractive forces. Erosion of bottom material occurs if the bed shear stress exceeds a threshold shear stress Bed forms Bed forms are generated by the combined effect of currents and waves. The type of the bed form in the above classification is related to the peak wave and current velocities, water depth, sediment size and the availability of sediment [10]. According to the amplitude and cause of formation, bed form can be defined as: ripples, mega-ripples, sand waves and tidal sand banks. Based on the evaluation of bed forms in the PANYU and LIWAN zones, these bed forms are mega-ripples and sand waves. They are mobile but their impact on the pipeline is not important. However, in view of a design life of 50 years for the pipeline, the long-term variations should not be neglected. Their impact on the pipeline should be taken into account. The small sand waves may develop and migrate depending on the wave and current conditions. On the basis of the sediment transport calculations, it can be considered that they are relatively stable under the normal conditions and they are dynamic under the storm conditions. Based on the observed seabed features and soil conditions as described above, note that in identifying potential areas of concern, emphasis was placed on the seabed mobility and seabed irregularity, and which will 131

8 lead to free spans. There are two types of free spans: free spans due to seabed roughness and free spans due to local scour. 4.3 Pipeline on-bottom roughness study The on-bottom roughness assessment shall be performed for the surveyed bathymetric data along the pipeline route, and will adopt a realistic seabed profile as well as temperature/ pressure profiles along the pipeline, which can be used to determine the locations, along the pipeline, which are prone to upheaval buckling and will give a guidance to indicate the span location and type of seabed rectification may be required. Rectify the spans exceeding the allowable limit or exceeding the stress criteria due to seabed undulations for the whole length of the pipeline route. It was found that there were no spans exceeding the maximum allowable span along the pipeline route at PANYU 35-1 and LIWAN 3-1 zones. While one proposed pipeline with a greater elevation differences refer to Figure 10 shown that three places (the max at about KP7350) of free span have exceeded the allowable span length calculated from the free span analysis reports. The soil along the pipeline route consists of loose to medium sand, very soft clay and hard undulated seabed feature. The hard seabed feature at the above locations are seems to be the locally hard clay surrounded by more erodible and mobile soil. Historic seabed movement have resulted in rapid seabed elevation changes along the design route and laying the pipeline over these features results in excess spans. Also the pipeline bending moments and stresses are exceeding the allowable limit as per DNV - OS - F101 (2012) (90% of SMYS limit). Therefore the pipeline around this location has been re-routed to avoid the seabed feature to mitigate the free spans and reduce the forces and moment within acceptable limit. The trimming of elevated seabed will result in the pipeline stresses and bending moment within allowable (e.g. refer to Figure 11 for the proposed pipeline trimming) or re-routed with a S-curve. Based on the seabed intervention assessment and cost comparison the conclusions can be drawn: the cost of seabed intervention is significantly more than the increase in cost if the pipeline is re-routed at the seabed feature locations. Figure 10 - On-bottom roughness analysis results for one proposed pipeline (Installation case) 132

9 Figure 11 - Trimming at KP7350 for the proposed pipeline (Installation case, green line is for trimming, red line for pipe elevation and black line for seabed elevation) 4.4 Self-lowering and free spans due to scour In the pipeline design located in the South China Sea, an assessment was made of self-lowering and free span development due to local scour for the proposed pipeline. The assessment of free span development due to local scour was made by combining empirical approach and numerical modeling. The initial self-lowering of the pipeline due to its weight after the pipe laying will be 5-10% of the pipe outer diameter. After laying, the free spans due to scour may generate and develop and further self-lowering may occur. 4.5 Upheaval buckling analysis Upheaval buckling is of particular concern to high temperature and/or high pressure buried pipelines when the pipelines are restrained both axially and laterally, with inadequate resistance to pipeline upward movement. Upheaval buckles could occur when a pipeline is resting on an imperfection, and/or an uneven trench bottom. The pipeline will tend to release the axial force by buckling in the direction with the lowest resistance. When imperfections occur in the vertical plane, this is usually upwards. The pipeline will be analyzed for the minimum backfill materials to resist upheaval buckling. The preliminary upheaval buckling analysis has been carried out to identify whether upheaval buckling is likely to be a problem. The analysis will be performed to calculate the required download to resist the pipeline tendency to buckle upwards due to installation on an uneven trench bottom or seabed. The design methodology of the upheaval buckling is based on the semi-empirical approach given by Palmer et al. [11]. The methodology will calculate the required minimum downward load to counteract upheaval buckling. The required downward load is intended to be provided by the use of concrete weight coating. Where this downward load is beyond the practical limits of concrete weight coating alone, then additional downward load shall be provided by soil burial or rock dumping. Where burial is provided, the uplift resistance may be calculated as follows: For sand: H q= sub H OD 1 f OD For clay: H q= c OD min 3, OD The soil will be in disturbed state after trenching and backfilling, which reduces the shear strength of the clay. The cohesionless backfilled soil is assumed to be fluidised soil. Therefore remoulded shear strength of clayey soil shall be considered for calculating the resistance against upheaval buckling of pipeline at the time of commissioning of the pipeline conservatively. In the deep-water area (e.g. pipeline from LIWAN 3-1 CEP to PANYU 34-1 CEP), trenched to a depth of 1.5m to top of pipe, natural backfill due to waves and currents is expected to occur compared to mechanical backfill. In the shallow water area (e.g. pipeline from LIWAN 3-1 CEP to onshore plant) where the seabed consists of mainly sand, backfill during to waves and currents are expected to occur and the backfill rate is expected to be considerable. In the nearshore area (water depth less than 30 m) where the seabed consists of mainly clay, backfill due to waves and currents could be limited because a) the clay fraction of the backfilled local sediment is likely to be 133

10 washed away; b) natural backfill of the backfilled local silt and sand fraction may be limited due to lack of upstream supply and the fine sizes of the silt and sand. 4.6 About Installation Minimum allowable lateral curve radius For the soil feature characteristic, the proposed pipeline route is not only straight, there also S curve routes (e.g. pipeline form PANYU 35-2 CM to PANYU 35-2 ILM), and the minimum allowable lateral curve radius is calculated by SF T R L, The bottom tension used in the minimum radius calculation is based on pipeline W installation feasibility analysis Spool choose code For there are two proposed platforms (PANYU 34-1 CEP and LIWAN 3-1 CEP) will be built for connection the subsea manifold to transport fluids/gas through long distance pipelines to the onshore plant for further processing. It is unavoidable to use spools to connect the subsea pipeline and riser on the platform at recent design phase. The following conclusions can be drawn from the oil and gas development of PANYU and LIWAN filed: 1) 90 degree bends are more effective in absorbing pipeline expansion and convenient for spool fabrication; 2) L-shape spools with minimized spool length easy for lifting and installation compared to Z-shape spools; 3) Installation contractor shall consider minimum safety distance between the laybarge and structure/jacket during laying down pipeline at platform and install (lifting/lowering) spool and PLET into the specified location in project area; 4) Tolerances will be considered during the spool design, which include: structure installation tolerances, metrology tolerances, fabrication tolerances and connector stroking tolerances (if applicable). The installation tolerances for subsea structures are: 1) Horizontal tolerance: ±2. 2) Plan tolerance: ±2.5 m in any direction for single piece subsea structures installed on the seabed, and ±0.5 m in any direction relative to the pre-installed piping for protection structures. 3) Plan orientation: ±3 from defined orientation. The installation tolerances for pipeline initiation/ termination usually are: five (5) metres long by five (5) metres wide. A suitable acoustic system to position the pipe accurately with respect to the platform and other critical areas in accordance with relevant specifications shall be provided. The fabrication tolerances for spool bend usually are: ± 0.75º. The installation tolerances for spools are: maximum setdown velocity 0.5 m/s. Movement toward deeper waters and harsher environmental conditions has required the continuous development of new technologies, and will continue to do so. Compared with the horizontal spools described above, vertical jumpers had used in South China Sea recent years. The comparison considered from both technical and economic standpoints may refer to Ref. Corbetta, G. and D. Cox [12]. Based on the above summary of results and conclusions the following recommendations can be made: 1) If seabed is rough, a local re-routing may be considered. If the re-routing is outside the available pipeline survey corridor, then a local re-survey is required; 2) If seabed is expected to be dynamic at a time scale of months to one year, then a resurvey of the dynamic areas may be considered in order to better quantify the short term seabed dynamics; 3) For local span length calculated by on-bottom roughness analysis exceeds the allowable span length calculated from the free span analysis, the method to cope with this problem usually contents local trimming of the seabed and/or re-routing the pipeline route; 4) Be reasonable to choose the spool shape. 5 CRA Lined Pipeline For the high corrosion of petroleum resources in the China South Sea, lined pipe has chosen for the subsea pipeline design. 28.6km of 6 lined pipeline and 18.3km of 10 lined pipeline, with 3mm CRA, have designed in PANYU 35-1 and 35-2 zones for oil and gas transition. The cost of lined pipeline is significantly less than the increase in cost if the pipeline to be used traditional steel pipeline design in accordance with 20 years design life. 134

11 Compared to carbon steel pipeline design, there are some key points for the lined pipeline design: 1) Manufacture Pressure: There are hydraulic expansion method, thermal-hydraulic shrink-fit method and hydro-exploding method to manufacture lined pipe. The pressure is a control point for manufacture, if the pressure is higher, which will make the liner pipe be devastatingly plastic deformation. However, if lower, which will make the gripping force of liner pipe and carbon steel pipe less than the demanded. 2) Welding Quality: the welding will use two materials for two pipe connection welding at least, one for liner pipe and the other for carbon steel pipe. It is difficult to meet the welding quality duo to the out-ofroughness of CRA pipe end and carbon steel. 3) Welding Efficiency: Because welding quality is demand high, the welding efficiency may be influenced. This determines the cost of laying vessel. 4) In the analysis (e.g. in-place, on-bottom stability, installation, bottom roughness and so on) for proposed CRA lined pipeline, it is not necessary to consider the utilisation of the strength of the liner, but only consider the mass of the liner with the utilisation of the strength of the backing material. Each length of finished lined pipe shall be measured for conformance to wall thickness and CRA layer requirements. The wall thickness shall conform to the tolerances specified in API 5LC [13] and DNV-OS-F101 [9]. Wall thickness measurements shall be made with a properly calibrated non-destructive inspection device of appropriate accuracy. Four points bend testing shall be carried out for each type of pipe in PANYU 35-1/35-2 project to mimic the deformation which will take place during the laying processes. Before four points testing, test joint shall be liner collapse tested. The test pipes will be subjected to displacement controlled loading in order to achieve the specified level of strain. Monitoring of the test pipe during the bend test will be carried out using internal video cameras and external strain gauges. This data will be captured and recorded for posttest analysis and review. Bending test can be treated as a quasi-static process, use the backing material quasi-static allowable stress and strain based on DNV-OS-F101 [9] limit states to check if the lined pipe meet the MPQT after cycles. The test shall continue bending to liner failure and the maximum strain in pipe shall be recorded. Except the testing of pipe in absolute bending stress condition, the tensile bending test could be additional. The axial tensions applied to the pipe specimen should consider laybarge tension capacity or pipeline submerged weight from end of laybarge stinger to touch down point on the seabed multiplied with dynamic safety factor. Normally tensile testing of both the carbon steel base material and the CRA liner material shall be carried out at the maximum design temperature in addition to the ambient temperature testing. Tensile test will be done separately for the steel and CRA liner material. Out of roundness for pipe body and pipe ends shall according to DNV-OS-F101 [9] and MPQT. 6 Conclusions A strictly personal assessment is that pipeline route selection and CRA lined pipeline construction remain the difficult questions, and that the other issues are either unlikely to be significant or can be solved in a routine way. It is plainly possible to eliminate the effect of the uncertainties by adopting a very conservative design strategy and by incorporating large margins of safety. In the case of pipeline route selection, this can readily be done in areas where the hard undulated seabed and sand wave seabed features are not severe, and the cost of doing so is quite acceptable. In severe undulated and sand wave seabed features, the cost of re-finding a generally flat pipeline route is likely to be very high, and then a greater understanding of trimming processes is likely to be beneficial. The same arguments apply to CRA lined pipeline construction, but there the environmental consequences of getting it wrong are less serious and effective quality control procedures and welding inspection will be enormously valuable. 7 Nomenclature API American Petroleum Institute CEP Central Processing Platform CM Central manifold CNOO China National Offshore Oil Corporation C COOEC Offshore Oil Engineering Co., Ltd. CRA Corrosion Resistant Alloy DNV Det Norske Veritas EPCI Engineering design, Procurement, Construction and Installation 135

12 EPIC H max H s ILM LRFD MPQT OD PLET QRA SF SMYS T T p T s WT W q H c sub f Engineering design, Procurement, Installation and commissioning Maximum wave height Significant wave height In-line Manifold Load and Resistance Factor Design Manufacturing Procedure Qualification Test Outside Diameter Pipeline End Termination Quantitative Risk Analysis Safety Factor Specified Minimum Yield Stress Tension at touch down/bottom Spectral peak wave period Significant wave period Wall Thickness Soil lateral friction factor Pipeline submerged weight Uplift resistance Required minimum backfill height to top of pipe Soil undrained shear strength Submerged density of soil uplift coefficient 8 Acknowledgments The authors would like to thank Chia Chor Yew, DNV GL Singapore, and Asle Venas, DNV GL Hovik, for allowing the use of the results [1] and helpful suggestion. They also would like to thank the South China Sea oil resources development Project Group of CNOOC and Subsea Pipeline Engineering Department of COOEC for encouraging this study and allowing this paper to be published. The paper was supported by: a. Ministry of Industry and Information Technology of P.R.C Offshore Engineering Equipment Scientific Research Project( )- Research & Development for Subsea Production System Design & Key Subsea Equipment (Phase I); b. National Key Technologies R&D Program of China (Grant No. 2011ZX05056). 9 References [1] Chia Chor Yew and Asle Venas, SliPIPE: a new concept to deal with pipeline expansion. Journal of Pipeline Engineering, 12(2). [2] Randolph, M.F., et al., Recent advances in offshore geotechnics for deep water oil and gas developments. Ocean Engineering, 38(7): [3] Esaklul, K., et al Active Heating for Flow Assurance Control in Deepwater Flowlines. Offshore Technology Conference held in Houston, Texas, U.S.A. (OTC 15188). [4] Harrison, G., M. Harrison, and D. S. Bruton, King flowlines-thermal expansion design and implementation. Offshore Technology Conference held in Houston, Texas, U.S.A. (OTC 15310). [5] Y Bai and Q Bai, Subsea Engineering Handbooks. Gulf Professional Publishing is an imprint of Elsevier 225 Wyman Street, Waltham, MA 02451, U.S.A., [6] DNV-RP-F110, Global buckling of submarine pipelines Structural design due to high temperature/high pressure. Det Norske Veritas, Norway. [7] Fang Xin-hua, Du Tao, Fundamentals of OceanicInternal Waves and Internal Waves in the China Seas, QingDao, China Ocean University Press, [8] Zheng, Q., et al., Statistical and dynamical analyses of generation mechanisms of solitary internal waves in the northern South China Sea. Journal of Geophysical Research-Oceans, 112(C3). [9] DNV-OS-F101, Submarine Pipeline Systems. Det Norske Veritas, Norway. [10] Van Rijn L C Principles of sediment transport in rivers, estuaries and coastal seas. Amsterdam, Aqua Publications. [11] Palmer, A., et al Design of submarine pipelines against upheaval buckling. Offshore Technology Conference held in Houston, Texas, U.S.A. (OTC 6335). [12] Corbetta, G. and D. Cox, Deepwater tie-ins of rigid lines: Horizontal spools or vertical jumpers? Oil Production & Facilities, 16(3): API 5L, Specification for Line Pipe. Edition March, Det Norske Veritas, Norway. [13] API 5LC, Specification for CRA Line Pipe. Det Norske Veritas, Norway. 136