Subsea Riser End Fitting With Offset Hub for Vertical Installation
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Solution Overview
Problem
The subsea oil and gas industry faces challenges in connecting and installing subsea risers with a substantially vertical bottom orientation to subsea equipment, particularly in dynamic environments with high sea states and strong currents, where conventional rigid steel risers experience fatigue and flexible risers are expensive and have limited flow capacity.
Innovation Solution
An end fitting for subsea risers is introduced, featuring an engagement head for longitudinal engagement with a subsea foundation, a connector hub offset laterally from the flow axis, a connector pipe in fluid communication, and a counterweight opposed to the connector hub and pipe, allowing for vertical installation and pivotability, which simplifies the connection process and reduces the need for jumper pipes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional rigid steel risers are used in dynamic environments with high sea states and strong currents, then the riser structure provides mechanical strength and flow capacity, but the riser experiences fatigue and rupture over time due to repetitive motion
Solution Approach 1:
The riser system is divided into multiple segments: a rigid upper riser section, a flexible intermediate section, and a rigid lower riser section. This segmentation allows each part to perform its optimal function - the rigid sections provide structural strength where needed, while the flexible section absorbs motion-induced stresses through bending, preventing fatigue propagation throughout the entire system.
Solution Approach 2:
The riser employs a composite structure combining rigid steel pipe sections with flexible pipe sections. The rigid steel sections (API 5L) provide mechanical strength and structural integrity, while the flexible pipe sections (API 17J) provide fatigue resistance through their ability to bend and accommodate motion. This composite approach resolves the contradiction between needing strength and needing fatigue resistance.
2Reliability
If flexible pipes are used to reduce fatigue, then the riser can accommodate motion and currents, but the cost increases significantly and flow capacity is limited
Solution Approach 1:
Instead of using flexible pipe for the entire riser length, the system segments the riser so that only the intermediate section requiring fatigue resistance is made flexible. The upper and lower sections near the platform and seabed equipment are made of rigid steel pipe, which is more cost-effective and provides higher flow capacity. This segmentation reduces the overall cost while maintaining fatigue resistance where most needed.
Solution Approach 2:
The flexible pipe properties are applied locally only in the intermediate section of the riser where motion accommodation is most critical, rather than throughout the entire riser length. This localized application of flexibility reduces material costs and manufacturing complexity while still providing the necessary fatigue resistance in the high-motion zone.
3Ease of manufacture
If a simple free-hanging rigid riser is used, then the installation is low cost and easy, but the riser is not technically viable in challenging projects with high sea states and strong currents
Solution Approach 1:
The riser is segmented into rigid and flexible sections, with the flexible intermediate section specifically designed to handle dynamic environmental conditions. This segmentation maintains relative installation simplicity while adding the necessary adaptability to withstand high sea states and strong currents, making the system technically viable for challenging projects.
Solution Approach 2:
The riser system incorporates dynamic characteristics through the flexible pipe section, which can bend and move with waves and currents. This dynamic capability allows the riser to adapt to changing environmental conditions while maintaining overall structural integrity, enabling deployment in challenging marine environments that would be impossible for a completely rigid system.
4Ease of operation
If the riser bottom is substantially vertical, then the installation is simplified, but connecting to horizontal subsea equipment becomes challenging
Solution Approach 1:
The riser system is designed with a predetermined configuration where the flexible intermediate section is pre-formed with appropriate curvature and orientation. This preliminary shaping allows the riser to naturally transition from the vertical upper section to the horizontal lower section, simplifying the connection process to subsea equipment while maintaining installation simplicity.
Solution Approach 2:
The flexible pipe section is designed with specific curvature characteristics that allow it to bridge the angular difference between the vertical upper riser and the horizontal lower riser or flowline connection. This curvature enables a smooth transition in orientation, making the connection to horizontal subsea equipment more straightforward while keeping the overall installation simple.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution facilitates cost-effective vertical installation of subsea risers without the necessity of jumper pipes, enabling reliable connection to subsea equipment and accommodating various riser configurations, including Steep Wave and CVAR, while allowing for pivotability to withstand ocean currents.
Implementation Method 1
a counterweight opposed to the connector hub and the connector pipe about the flow axis... in a longitudinal engagement direction
Data Source
AI summary
An end fitting for a subsea riser comprises an engagement head to be engaged with a subsea foundation in a vertically downward longitudinal direction. A structure of the end fitting surrounds a longitudinally extending flow axis and is arranged to bear a tensile load on a load path extending longitudinally from the engagement head parallel to the flow axis. The structure includes a pivot joint assembly having a first part attached to or integral with the engagement head and a second part pivotable relative to the first part about a centre of rotation substantially aligned with the flow axis. The structure can be fixed to the engagement head or removably attached to the engagement head. The engagement head supports an upwardly facing connector hub that is offset laterally from the flow axis. A connector pipe in fluid communication with the connector hub extends from the flow axis to the connector hub. A counterweight is opposed to the connector hub and the connector pipe about the flow axis.