SS: Deepwater Riser Technology: Design of Retrofit Hang-Off Receptacle Clamp for Tie-Back of Steel Catenary Risers to the Pompano Platform
Authors: Chen Jack , Sriraman Sharan , Clark Robert , Lange Steven , Conner Monte , Cooley Bruce 
Published: July 2010

It is not unusual that the existing riser hang-off structures such as receptacles and I/J-tubes on offshore facilities are all occupied and are unavailable for future riser tie-backs. In such cases, it may be necessary to design and install retrofit hangoff structures to accommodate the tie-back risers. A number of hang-off structures, essentially in the form of jacket clamps at various elevations of the jacket leg were designed and successfully installed for tie-back of one 6-inch and two future 10-inch steel risers to the Pompano platform, a fixed jacket platform located in 1,294 ft water depth in Gulf of Mexico (GoM).

This paper focuses mainly on a novel SCR hang-off Clamp design. The clamp was installed 323 ft below the Mean Water Line (MWL). It weighs 51 kips in air with an internal diameter of 102 inches and a height of 14 ft. It was made up of two semi-cylindrical shells connected through a hinged joint for ease of diver installation and was clamped to the jacket leg using stud bolts. The SCR Clamp was provided with three hang-off receptacles in the form of receiving baskets for the SCR tapered stress joints. The baskets were supported on the front half shell of the clamp through a series of tubular brace members. A deadweight transfer clamp (Stop Clamp) was provided below the SCR clamp and supported by jacket bracing members through two saddle supports in order to provide a direct load transfer to the jacket. The SCR Stop Clamp design offered additional redundancy to the system in terms of longitudinal and rotational slippage modes of failure. The Stop Clamp also provided significant assistance during the installation of the main SCR hang-off Clamp.

The SCR Clamp was designed as a standalone friction clamp to withstand all operational and extreme loading from the SCR without the need for the Stop Clamp. In case of an unlikely partial or full loss of friction, the SCR Clamp will be supported by the Stop Clamp, which will then transfer the loading to the jacket's horizontal and diagonal bracing members.

The Stop Clamp's saddle footings sit on the jacket bracing members to guarantee direct load transfer, and assisted in vertical and rotational positioning of the SCR Stop Clamp during installation. Thus, the SCR Clamp offers a dual redundant design philosophy using both friction and direct load transfer to the jacket structure.

Detailed 3D finite element analysis was performed on the SCR Clamp using ABAQUS. The finite element analysis was complementary to industry standard manual design calculations and provided confirmation of the design.

Introduction

MC72 is a Gas Well drilled and completed by LLOG. The subsea tree is 6 miles South-East of the Pompano platform, a fixed steel structure standing in 1,294 ft of water in block Viosca Knoll 989 in the Gulf of Mexico. LLOG MC72 was tiedback to the platform. The well was expected to produce a maximum of 40 Million Standard Cubic Feet per Day (MMSCFD) of gas with an initial completion shut-in tubing pressure of approximately 8,260 psi and an arrival temperature at the platform of 50 °F. Minimal amounts of condensates and water were expected initially. Flowline size is 6-inch nominal.

A 6-inch riser and 3-inch umbilical were designed to complete the tie-back to the Pompano platform. The 3-inch umbilical was to be accommodated through the only available J-tube on the platform. Due to the unavailability of additional I/J-tubes and/or hang-off porches, a retrofit hang-off structure had to be designed to support the 6-inch riser. In addition, it was decided that provisions were required on the hang-off structure for two future 10-inch risers.