Three Semisubmersible Floating Production System Options With Steel Catenary Risers
Authors: Wilson Adam 
Published: February 2015

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 25265, ’Field Development Using Semisubmersible Floating Production System With Steel Catenary Risers in Western Australia Harsh Environment,’ by Alaa M. Mansour, Shankar Bhat, Dharma Pasala, and Dhiraj Kumar, Intecsea, prepared for the 2014 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed.

Semisubmersible floating production systems (SFPSs) with steel catenary risers (SCRs) are attractive field-development solutions, especially in deeper water of Western Australia. However, SCR design is challenging in the offshore Western Australia environment because of severe cyclonic environments and persistent swells. This paper presents a comparison of three different field-development alternatives that use SFPSs with SCRs in deepwater Western Australia. These alternative solutions are a conventional semisubmersible hull design with steel lazy wave riser (SLWR), a deep-draft semisubmersible hull design with conventional SCRs, and a novel semisubmersible hull design with conventional SCRs.

Introduction

Building robustness in an SFPS, especially when personnel stay onboard during cyclonic events, requires the system to be designed to survive events with return periods larger than the typical 1,000 years adopted for survival events in the Gulf of Mexico. Events with 10,000-year return periods or greater need be adopted for Western Australia. Events with 10,000- year return periods have higher significant wave height and longer peak periods, both of which increase the SFPS motion response, especially in the heave direction. This, coupled with a design location in moderately deep water (&lt;5,000 ft), makes SCR strength design very challenging. In addition, the persistent swell in the Western Australia area makes SCR fatigue design more challenging.

There are potentially three alternatives to overcome the SCR strength and fatigue challenges. One option is to improve the SFPS motion by using a very deep- draft semisubmersible (VDDS) while maintaining a conventional SCR design. Another option is to use conventional semisubmersible design and adopt an SLWR. The third alternative is to use conventional SCRs with a novel semisubmersible hull design that does not require a deep draft but offers benign motion response, especially in the heave direction.

In this paper, the three different hull/riser combinations are investigated and compared for a presumed wet-tree gas field with a host facility in 4,000 ft of water.

The free-hanging-solid-ballast (FHSB) semisubmersible design fundamentally consists of a conventional semisubmersible hull with four columns connected by a ring pontoon at the keel level. The semisubmersible’s motions and overall performance are enhanced by use of a robust, low-tech feature: namely the FHSB tanks (Fig. 1). The FHSB tank is attached to the hull below keel level by means of chains, which pass through guide fairleads at the keel level and connect to deployment winches at the top of the columns. The addition of an FHSB tank to the semisubmersible results in a significant increase in the semisubmersible heave natural period, mainly through its own weight and added mass from its large surface area, while controlling the heave response in the wave-frequency range below 16 seconds.