Design Features for Wye Sled Assemblies and Pipeline End Termination Structures to Facilitate Deepwater Installation by the J-lay Method
Authors: Dyson K.C. , McDonald W.J. , Olden P. , Domingues F. 
Published: October 2008

This paper presents an evaluation of some novel installationrelated design features required for the installation of fourteen in-line wye sled assemblies (WSAs) and five pipeline end termination structures (PLETs). The WSAs and PLETs were designed for installation by the DCV Balder utilizing J-lay techniques. WSA features included structural frames designed to flex, hinged yokes for anchoring buoyancy modules, and yoke stoppers to provide stability for the WSA pipe-spool during upending. Due to tower width limitations, some of the larger WSAs had sliding, or separately installed mudmats.

PLETs were designed with hinged mudmats for 'first-end' installation lay-away, but also had to be capable of second-end recovery to surface for repair and re-installation, if necessary.

Introduction

BP and its partners are developing several deepwater prospects in the Gulf of Mexico. These include the Holstein, Mad Dog, Thunder Horse and Atlantis floating production facilities in water depths ranging from 4500 feet to 7300 feet. Oil and gas will be exported via the Mardi Gras Transportation System. The deepwater development consists of a combination of 16-inch to 28-inch oil and gas trunk and lateral pipelines, tied-in using piggable wyes.

At the time of the release of this paper, the Balder has successfully installed eight WSAs and two PLETs in the Southern Green Canyon Area. Typical WSA and PLET arrangements are shown in Figures 1 and 2 respectively.

The initial challenge for tie-in structure design was the non-existence of suitable vessel installation equipment, because Heerema, the selected installation contractor, was still designing, fabricating, and modifying the Balder's J-lay equipment. Anticipating installation restrictions on WSA and PLET design consequently required intense cooperation between the design team and the installation contractor.

The team's primary objective was to develop structures that would allow the tie-in of a lateral pipeline. The tie-in was to have the following requirements, as a minimum:Must be piggable.Must consist of a WSA and PLET.The WSA to be installable as a single, inline structure, as part of the J-lay operation. It was not to exceed the initially specified limits of the J-lay system.The trunkline and lateral pipelines to have isolation capability using full bore deepwater subsea valves.The connection between the WSA and PLET must be made using a jumper, and diverless connectors.WSAs and PLETs must have adequate mudmat capacity. Due to the J-lay tower's capability, and the sheer size of the pipeline components, the WSA designs were split into three major groups. The split related to mudmat design.Integral: All pipeline components, plus the support structure and mudmats are built as an integral part of the WSA. No ROV intervention is required during the installation process.Sliding Mudmats: Due to mudmat size requirements, the WSA mudmat needs to be extended after the WSA has been installed in the pipeline and before landing the WSA/pipeline in J-mode on the seabed. Hence, ROV intervention required.Separate mudmats: The mudmat requirements exceed the limits of the sliding option. Hence, the mudmat has been separated from the WSA structure and is installed before the piping and support structure part of the WSA. These are separate installation activities.