Development Of A Novel Tieback Connector With High Mechanical Strength Capacity For Spar Floating Production Facilities And Their Production Risen
Authors: Skeels H.B. , Johnson J.B. , Koleilat B.M. , Frishrnuth R.E. 
Published: October 2008

The advent of Spar Floating Production Systems to the oil industry presents unique operating conditions with respect to environmental loads, offsets and motions, on the one hand and restricted equipment sizes on the other. This pushes the technical limits of oil field equipment. This paper addresses one such technical area, the development of a slender profile high strength tieback connector for the Spar's well production risers. The tieback connector must be slender in shape to navigate the internal guide structure of the Spar's hull, land and lock remotely inside a subsea wellhead. After installation, the tieback connector must withstand both external loads generated by the Spar's movements, along with internal wellbore pressures.

The Spar's slender profile requirement necessitates the need for an internal connection system which generates enough extra lock force (preload) to prevent seal separation. An extensive 2D finite element analysis program was undertaken to model and optimize connector parts and estimate the amount of preload needed to prevent separation under design loads dictated by various extreme spar and production riser motions. The 2D analysis was refined to give strategic components the ability to flex and compress during the locking preload operation so that the preload can be stored by these components. A novel 3D analysis model was created to predict complicated relationships between external bending moments and tensile forces acting on the tieback connector and wellhead through the motions of the riser. These relationships were then fed back into the 2D model to finalize the Connector design.

The predicted stresses and the empirical relationship of bending moment to an equivalent tension were verified by prototype testing. Testing also established design and operating parameters for the manufacture and factory acceptance of seven production units.

Introduction

Several multi-discipline design teams were formed to work concurrently to develop subsea, surface and structural facilities to build a Spar production facility on a "fast track" schedule within 2 years. One such design team was charged with developing a remotely operated connector which can pass through narrow conductor guide passages inside the spar's structure and subsequently lock and seal inside previously drilled subsea wellheads. Other design teams examined the Spar's hull and up to sixteen 9-5/8 inch diameter production risers and how they behave under several operating, workover, and severe storm conditions. From the riser team's analysis came ten severe loading cases which the tieback connector design team had to deal with (see Table 1). Another problem that the design team had to face is one common to "fast track" projects. As the hull and riser motion teams reviewed and refined more operating conditions, the load and moment requirements changed several times until they were finalized nearly one year into the project. The third problem was physical size. Well spacing inside the Spar's moonpool lead to conductor guides that could only drift equipment no larger than 26 inches in diameter. This limitation automatically means that the tieback connector must be of an internal latching variety since 18-3/4" subsea wellheads have outside diameters in excess of this limit.