Novel Techniques to Improve Sea-States During 1st End PLET Initiation

Abstract 1st End PLET initiation is a common method considered for offshore pipeline installation. The 1st End PLET helps in supporting a direct permanent hold back system without any other operations by secondary, vessels. However, there are some associated factors that limit the use of 1st End PLETs. These limiting factors include the complicated design and fabrication of a hinged system, the relatively large anchor forging loads and seabed features that limit the length of chain/wire length for initiation. These factors have a direct impact on the limiting sea-states for offshore installation which may result in long weather downtime. Subsea7 has considered novel ideas to improve the sea-state for a scenario where the wire length has been restricted for a 1st end initiation PLET with a permanent hold back system. Subsea7 has investigated the use of a stretcher wire that has the capability of straining up to 25% at very high loads without compromising the structural integrity of the initiation wire system. The stretcher wire was connected between two chain/wire segments for a hinged 1st end PLET installation. It has also been considered for a Stab and Hinge PLET initiation method. In both scenarios, there is an improvement in the limiting sea-state compared to the case using an initiation wire/chain without the ability to stretch. This innovative method using a stretcher wire provides a damping effect in the initiation wire system which is highly beneficial. This damping effect reduces the peak stresses and strains that typically occur at the anchor forging section of a 1st end PLET/pipe interface during initiation. Furthermore, the stretcher wire does not have any limitation to use under any external hydrodynamic pressure. Hence, this selection could be beneficial for subsea operations at any water depth. This paper will discuss and analyze the installation methods used and the screening for a suitable stretcher wire for given 1st end PLET initiation. The limitations and drawbacks that should be accounted before considering the use of stretcher wire for initiation will also be discussed. Subsea7 has successfully installed a number of 1st end PLETs with such a stretcher wire. Some of these projects will be listed here. Finally, this paper will also introduce potential future developments that are being considered to further improve the PLET initiation sea-state by further dampening the 1st end PLET system.