SS WHO DAT Project - Unique SURF Interface on the OPTI-EX Generic
Authors: Nagel M. Edward , Murphy Matthew , Wang Aimin , Schilling Brianna , Fudge Dion , Gosset Benita , Cruz Pete , Book Shawn 
Published: April 2012

This paper describes the design, manufacture, and installation of subseaumbilicals, risers and flowlines (SURF) interface on the OPTI-EX floatingproduction system (FPS). For the purposes of this paper, the SURF interfacewill be limited to the riserto- hull structural connection. This structuralconnection consists of the riser end fitting, riser porch, and riser porchadapter. The riser porches are very efficiently designed. For the OPTI-EX, 7.5MT of steel weight has a top tension capacity of 300 MT. The design is highlyflexible at accommodating field architecture, required no internal hullmodifications or foundations, and the OPTI-EX, located on the WHO DAT field, accommodates flexible riser sizes from 5 inches (for production risers) to 11.5inches (for oil export lines) in 1000 m water depths.

The riser porches and mating riser adapters/I-tubes are simple structures;precision manufactured to ensure proper load distribution in-service and toensure ease of in-yard and future offshore installation. The design is robust, fatigue resistant, and fabrication-friendly.

Mounted on the outboard facing surfaces of the FPS hull pontoon, these mountsfacilitate riser installation and pipeline laying in directions away from theunit. At WHO DAT, the riser adapters accommodate pipeline approach anglesrelative to the FPS from perpendicular to a given side of the unit to as muchas 26 degrees from perpendicular thus providing great pipeline layoutflexibility relative to the unit.

The rigging of the flexible pull-ins, though " involved", was not overlycomplex. Maximum pull-in loads were low as the catenary span of all risers atinitial hang off was short. This short length was due to utilizing the " layaway" method of deploying the flexibles once hung off. This allowed pull-inequipment on the FPS to be relatively small and relatively easy to handleoffshore.

Introduction

OPTI-EX was speculatively built as a generic FPS able to operate in a broadrange of water depths and MetOcean conditions. Since it had no operator andrelated specific field location, field architecture and riser design specificswere unknown prior to unit fabrication. Hence, no riser porches weremanufactured and installed during construction even though a riser porch designhad been developed for it that would accommodate a wide array of riser sizes, weights, and configurations utilizing either flexible risers or SCRs (steelcatenary risers). The porch design was such that the unit could accommodate upto 20 risers. The maximum single riser design top tension capacity was 500 MT. This design was presented to LLOG and its pipeline engineer for use on the WHODAT project. It was selected as the solution to accommodate up to 12 productionflowlines and 2 export pipelines. Only three production risers and two exportrisers were initially installed, the rest of the porch slots were reserved forfuture production risers. A fourth production riser will be installed duringthe first quarter of 2012. When installed, the four production risers willcomprise two each piggable flowline loops, one loop serving one well cluster. Refer to Figures 1 and 2 for field layout.