Zap-Lok™ mechanical pipe connector performance

Scope

A client came to Jee looking to develop a new gas field using a pipeline constructed using the Zap-Lok connection system, instead if conventionally welded pipe. The Zap-Lok connection, by Tuboscope™, is a push-fit type of connector that relies on axial friction to resist pipeline tensile loading and retain pressure. Reservoir conditions meant the pipeline inlet could experience fluid temperatures as low as minus forty degrees Celsius which is significantly lower than the temperature range previously qualified for this connector system.

The client required assurance that the connection would maintain axial capacity and pressure seal, when exposed to such extreme cooling during start up and early operation. 

Solution

The Jee had carried out similar work on the Zap-Lok connection system to verify its performance at high temperature. Jee’s approach was based on use of these models, updated for project specific parameters including wall thickness, thermal behaviour and a full range of expected loading scenarios at the low temperature service conditions.

Analysis was completed using an axisymmetric Abaqus model representing the pin and bell geometry with insertion, internal pressure application and temperature changes applied sequentially. Sixteen cases were assessed to capture pressurisation scenarios, ambient conditions and a range of low temperature combinations.

The base case was to cool both the pin and the bell to the same temperature. This is appropriate for steady state conditions where the pipe wall is in thermal equilibrium. Additional cases represented a theoretical worst situation where the pin cools first while the bell stays at ambient temperature.  This represents the initial start-up condition which would temporarily increase the temperature gradient through the connector wall but only exists as a rapid transient condition.

Technical highlights

●    Connector behaviour under equal cooling showed negligible change in axial capacity across all temperatures tested.

●    Pin and bell diameters contracted by almost identical amounts under cooling which maintained contact force and preserved predicted performance.

●    Internal pressure was shown to enhance axial capacity by increasing reaction forces between components.

●    When only the pin was cooled a reduction in axial capacity of up to fifteen percent was predicted.  Further study work will assess the rate of thermal transfer between the pin and bell, to define the maximum predicted thermal gradient versus time.

●    Results confirmed that performance assumptions remain valid provided the line pipe meets the Charpy requirements at minus fifty degrees Celsius as defined in the project line pipe specification.

Benefits and outcomes

Jee demonstrated that the Zap-Lok connector maintains axial capacity in the low temperature conditions expected when both connector components cool together.  The outcome of the study identified the need for a more detailed study to assess the dynamic response of heat transfer through the connector so that behaviour under initial start-up conditions, with rapid temperature change, can be verified. 

The work gives the client clear evidence, based on previously validated analysis methods, that the selected connector will operate safely within the predicted temperature and pressure envelope for normal operation. 

This study provided the client with confidence in the suitability of the Zap-Lok connector for low temperature service in the proposed development. By quantifying the effect of cooling, confirming the negligible impact on capacity and identifying where further optimisation could be targeted, Jee delivered a robust and practical assessment aligned with real subsea behaviour. This gave our client the validation needed, to be able to progress to the next stages of the field development.