Diverless Maintained Cluster Subsea Architecture: 14 Years of Experience
Authors: Roberts Peter M. , White John , Wood Richard J. , Tucker Ron 
Published: October 2008

Diverless hook-up and tie-back of subsea oil & gas production systems is an increasingly significant activity. Once exclusively associated with " very deep water??, diverless tie-ins are increasingly considered for use within saturation diving depth. The successful experience with diverless tie-ins has made them less risky, less time consuming and less costly, whereas increasingly onerous HSE requirements and wider appreciation of their provisions is heightening the perception of diver assisted tie-ins as a potentially high-risk and high-cost activity.

Experience of over 14 years' diverless tie-in activity in the West of Shetlands area supports the points this paper makes regarding more general recommendations and expectations for future developments in the industry worldwide. Many of the lessons learned could be classified as conventional wisdom but that does not reduce their importance. The deepwater environment is very unforgiving of poorly thought out procedures and inadequately tested hardware. A case study of deck-to-deck tie-in time of the same connector design over several successive seasons is presented and the reasons for the classic exponential " Learning Curve?? analysed. The significant impact of Connector Design and Contracting Strategy on collateral hardware costs, and perhaps more importantly on installation vessel times, is discussed and recommendations presented.

Introduction

‘Diverless Maintained Cluster’ subsea architecture was conceived in 1989 as a solution for field developments West of Shetlands. In order to cope with the harsh environs, the design philosophy called for compact, modular and inter-changeable hardware with the following development targets:Hardware had to be deployable via the moonpool of a semi-submersible drilling vessel as it was the most stable working platform in the field.The installation process had to be fully interruptible to accommodate short working windows.The installation process had to minimise both vessel installation time and hard specific duration targets for installation and connection operations.Connections were required to be reversible and re-connectable for either single bore or multi-bore systems to maximize flexibility of the subsea architecture.The field architecture had to accommodate a ‘best-in-class’ philosophy, which allowed different subsea hardware brands to be interconnected and avoid risks due to obsolescence of components over the field life.

The operators recognised early that significant installation and through life cost savings were driven by the connection system. Because the connection system is the physical embodiment of the installation interface between all of the subsea hardware components, its dimensions and capabilities were critical to achieving the above development targets. A diverless connector with a compact pitch porch spacing of 1.0 m and appropriate functionality was unavailable, so the technology development of a dedicated diverless connection system for flowlines, umbilicals and flying leads was launched.

Exxon led an extensive design and build programme from 1989 to 1991 to design a bespoke diverless connection system, which culminated in ROV trials of DMaC tooling in Lock Linnie, Scotland. A 1993 JIP composed of Exxon, BP, BHP and Statoil led to the building of a pre-production set of tools to be used from a drill rig West of Shetland. In recognition of its significance to Diverless Maintained Cluster field architetcture, the connector was named the acronym DMaC.