Subsea Load Lowering with Rigid Rod Transfer Below the Splash Zone
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Solution Overview
Problem
Current methods for lowering and recovering subsea loads, such as wellheads or BOPs, face challenges with heavy loads on vessels lacking specialist heavy-lift equipment, requiring complex operations and expensive equipment like separate crane vessels or flexible pipes prone to fatigue and damage, and struggle with safe passage through the splash zone.
Innovation Solution
A method using a rigid rod supported by a hold-back system on a pipelay vessel to lower loads, transferring weight from the rod to a wire at a predetermined depth, and a reverse process for recovery, employing a tensioner system and winch to manage the load safely through the splash zone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If a separate crane vessel is used to lower heavy subsea loads, then the load capacity is sufficient, but the cost increases greatly and availability depends on weather windows
Solution Approach 1:
The patent combines the functions of a crane and a pipelay vessel by using the existing hold-back system and tensioners on a pipelay vessel to support and lower heavy subsea loads. This merging eliminates the need for a separate crane vessel while maintaining adequate load capacity through the integrated use of available equipment.
Solution Approach 2:
The hold-back system and tensioners originally designed for pipelay operations are made multi-functional by enabling them to support and lower heavy subsea loads in addition to their primary function of laying pipelines. This universality allows a single vessel type to perform multiple heavy-lift operations.
2Ease of operation
If pipelay equipment or a small crane is used to start lifting the load, then the operation can begin from the deck, but complex operations are required to transfer the load underwater to a winch
Solution Approach 1:
The load is preliminarily positioned and secured to the rod on the deck before the lowering operation begins. The rod is then directly lowered through the water using the hold-back system, eliminating the need for complex mid-operation transfers to a winch that would otherwise be required.
Solution Approach 2:
A rigid rod serves as an intermediary element between the hold-back system and the subsea load. The rod enables direct transmission of force from the hold-back system to the load, simplifying the operation by eliminating the need for complex underwater transfer mechanisms to a winch.
3Ease of operation
If a flexible pipe is used as a wire to lower loads, then the load can be suspended, but the flexible pipe is expensive and mechanically weak, being prone to fatigue and damage
Solution Approach 1:
The patent uses a rigid rod instead of a flexible pipe as the load-bearing element. The rod is a more reliable, mechanically stronger alternative that is not prone to fatigue and damage. While the rod may need to be recovered and reused, it provides superior mechanical properties compared to flexible pipes.
Solution Approach 2:
The solution employs a rigid rod made of strong materials (such as steel or composite materials) that provide high mechanical strength and fatigue resistance. This replaces the flexible pipe's composite structure with a rigid composite or metal structure that offers superior reliability for heavy load suspension.
4Speed
If a rigid rod is used with a hold-back system, then the load can be lowered controlledly, but complex operations are required to detach the load from the rod underwater
Solution Approach 1:
The detachment operation is segmented into distinct phases: the rod is first detached from the load at or near the surface where access is easier, then the load is separately lowered on the wire. This segmentation avoids the complexity of underwater detachment operations by performing the detachment at a more accessible location.
Solution Approach 2:
Instead of attaching the load to the rod and then detaching it underwater, the process is inverted: the load is attached to the rod, the rod is detached at the surface, and then the load is transferred to the wire. This inversion of the sequence simplifies the operation by performing detachment at the surface rather than underwater.
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
Enables safe and controlled deployment and recovery of heavy subsea structures, reducing operational complexity and costs, and allowing subsea operations with fewer weather restrictions, using existing pipelay vessel equipment without the need for complex or expensive heavy-lift vessels.
Implementation Method 1
a friction clamp that is able to travel along the pipelay tower and engages the rod by friction
Implementation Method 2
operating the hold-back system to advance the rod downwardly relative to the pipelay tower
Implementation Method 3
when the load reaches a predetermined depth underwater, transferring the weight of the load from the hold-back system to the wire
Implementation Method 4
a winch to retract the rod upwardly relative to the pipelay tower, lifting the load from the subsea location to the surface
Data Source
Figure 1
Figure 2
Figure 3a~3b
AI summary
A method of lowering a discrete load from a pipelay vessel comprises: holding a rigid rod on an upright launch axis where the rod extends through a hold-back system; coupling a lower end of the rod to the load; connecting a wire to the load directly; operating the hold-back system to advance the rod downwardly, hence submerging the load, while the weight of the load is suspended from the hold-back system via the rod; when the load is underwater and beneath the splash zone, transferring the weight of the load from the hold-back system to the wire by detaching the load from the rod while the rod is held by the hold-back system; and continuing to lower the load in the water, suspended directly from the wire. The method may be reversed to recover a load from a subsea location, such as when lifting a wellhead or blowout preventer from the seabed.