Challenges of HP/HT Pipelines In Deep Water
High-pressure, high-temperature (HP/HT) pipelines are increasing in deepwater areas such as West Africa and the Gulf of Mexico. In deep water, severe conditions lead to challenges in installation, flow assurance, thermal buckling, and management, all of which are critical issues for HP/HT pipeline design.
Flow assurance is one crucial factor for HP/HT pipeline design, which includes:
(1) Steady-state performance of the flowline as governed by the overall heat transfer coefficient or U-value, and
(2) Transient performance of heatup and cooldown requirements.
During the cooldown period after shutdown of the fluid flow, the product temperature must remain above a specified value to avoid hydrate formation. In deep water, pipeline design must seek a balance between the steady-state and transient cooldown requirements to meet the specific field conditions, along with issues of weight, installation needs, and other commercial and economic factors. For the thermal insulation system, conventional single pipe and pipe-inpipe (PIP) systems are two options and the final selection depends on a range of factors, such as cost, installation and thermal management.
PIP systems may offer the best balance of thermal efficiency and steady-state performance for HP/HT pipelines; however, the increased weight is a factor of considerably greater significance for installation in deep water than in shallow. The wet insulation materials used with HP/HT pipelines will result in an extremely thick coating for good steady-state performance, which introduces increased cost and seabed stability issues due to the low submerged weight. For the transient cooldown procedure for a flowline, low thermal conductivity and high thermal inertia are very important to determine the “no-touch” time during a shutdown and also the thickness of the insulation layers. The higher density also results in an increased submerged weight, which, in turn, eases seabed stability issues.
HP/HT pipelines laid on the seabed are susceptible to global lateral buckling, resulting in deflections that can lead to the pipe cross section yielding. This is caused by compressive axial force building up as the pipeline tries to expand thermally but is restrained due to soil resistance between the pipe and the seabed. The key to an effective solution is the pipe’s ability to extend axially. Control of the axial and lateral movements of a pipeline can be obtained by using either a buckle initiation device or expansion spools and it will result in economic relief of the axial force. Various approaches have been tried and applied to manage pipeline global buckling. Finite element (FE) modeling of the pipeline installation and operation has been widely used to identify the issues related to the management of thermal buckling, which includes pipeline walking under cyclic thermal loading, global buckling interactions, etc. The detailed analysis and modeling of thermal buckling will enable cost-effective mitigation options.
Installation in Deep Water
The installation of HP/HT pipelines is more challenging and costly in deep water than in shallow water. As the reservoir and field go deeper, the weights involved with these pipeline systems may become too heavy for conventional installation methods. PIP systems are more installation-vessel dependent than conventional pipe, and this dependency is even more pronounced with increasing water depth. Lay rates and reliability are key factors for the pipeline installation in deep water. The high tensions associated with the J-lay method and the high bending in the steep S-lay method may cause high stress in the pipeline. The strain-based design approach has been used in many installation analyses. Pipeline installation is detailed in Chapter 5.
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