Experiences in Classification of Deepwater Mooring Systems for Floating Installations
Authors: Huang Ken , Lee Ming Yao 
Published: October 2008

This paper discusses key technical issues for classing deepwater mooring systems, Special applications of new mooring components such as synthetic fiber ropes, suction pile anchors and Vertically Loaded Anchors (VLA) have been presented. This paper establishes the design criteria for synthetic fiber ropes, suction pile anchors, and VLAs. The rationale as presented is based on recent projects and research and development results from joint industry studies. Testing and installation considerations for synthetic fiber ropes and field test requirements for suction pile anchors and VLAs are reviewed and discussed. Additional design and analysis requirements for the use of synthetic fiber ropes are also proposed for a taut leg mooring system. This paper represents a summary of experiences gained through the application of these innovative mooring technologies in mooring components and anchoring devices. The guidelines set by ABS as presented in this paper are intended to enhance safety standards in marine operations while meeting the new challenge to moor floating units in deep water.

Introduction

With the recent explosive growth in deepwater drilling and production operations worldwide. American Bureau of Shipping (ABS) is actively involved in classification of several conversions to deepwater and new-built floating platforms including semi-submersibles, ship-shaped drilling units and tanker-based FPSOs (Floating Production. Storage and Oftloading), Most of these floating units will be moored in the ever increasing water depths. In deep water, the conventional catenary mooring systems become less stiff and have a larger horizontal spread, with much higher mooring component costs and payload penalties. To cope with these challenges, new mooring systems for stationkeeping of floating units have been developed for ongoing projects. such as those offshore of Brazil installed or to be installed. by Petrobras in 1997 and 1998. These include polyester synthetic fiber ropes. VLA and suction pile anchors applied in the taut leg mooring systems.

Classification of Deepwater Mooring Systems

General.

ABS has established the position that the scope of classification of an FPSO system shall include three major items, Le., the mooring system, the vessel, and the production facilities [Ref. 1]. Other equipment and systems such as import and/or export risers may also be classed, if requested by the owner. Since the early 1970s, ABS has provided numerous classification services for floating production systems. Table I lists some recent projects in water depths greater than 500 meters. There are special considerations to be addressed for the design and analysis of mooring systems in deepwater applications. The key element, when it comes to novel systems involving new components such as synthetic fiber ropes, suction anchors, vertically loaded anchors, etc., is to provide a comparable and compatible safety standt1rd to that of the traditional catenary mooring with chain wire and drag embedment anchors. It is well recognized that the mooring line loads tend to favorably adjust themselves when movements of drag embedded anchors occur in the traditional catenary mooring system, This is the rationale for the safety factors of the anchor holding capacity being much lower than those of the mooring line tension,