Semisubmersible Based Dry Tree Platform with Compliant Vertical Access Risers

Abstract

It is shown in this paper that a semisubmersible-FPS can be used as a dry tree platform in ultra-deep water Gulf of Mexico when designed with Compliant Vertical Access Risers (CVARs). The Technical feasibility of the proposed dry tree semi concept is demonstrated by an example application in 8,000 ft water depth. Riser extreme response, interference and fatigue have been analyzed. Installation schemes have been considered. It is concluded that the proposed dry tree semi concept is technically feasible and that it can provide an attractive lower cost solution for dry trees in the ultra-deep parts of the Gulf of Mexico. Introduction Dry tree platforms have been used in the Gulf of Mexico in water depths up to 5,400 ft. Studies have shown that it becomes expensive to extend the application of conventional dry tree platform concepts (TLPs and Spars) to ultra-deep water depths such as 8,000 or 10,000 ft. We propose an alternate dry tree platform concept to provide operators with the benefits of direct access to wells from lower cost floating systems such as the semisubmersible-FPS (henceforth 'semi- FPS'). Dry tree wells are tied back to the semi-FPS using Compliant Vertical Access Risers (see Figure 1). The CVAR is attached rigidly to the semi-FPS (see Figure 2) via a flexjoint or a tapered stress joint avoiding the need for flexible jumpers and simplifying deck layout. Unlike a Spar or a TLP, there is no relative motion between the riser top and the vessel in operation, simplifying access to the dry trees and well access during workovers. At the sea floor the CVAR is attached to the wellhead in much the same manner as a Top Tensioned Riser (TTR). The CVAR, like Spar or TLP TTRs, has the benefit that the riser is vertical at its top and bottom - permitting vertical access. TTRs obtain their compliancy by virtue of relative motion at their tops which is accommodated by tensioner or buoyancy can stroke. CVARs obtain their compliancy from excess riser length. Since the buoyancy is distributed approximately in the middle portion of the riser CVARs have the characteristic shape shown in Figure 1. Background The idea of the CVAR dates back (at least) to 1988 when a patent was awarded to Pearce et al [1] for a flexible riser and associated equipment that could be used to obtain vertical access to a subsea well. (The patent belongs to Halliburton/KBR.) KBR carried out a number of early studies in the late 1980s and early 1990s on the feasibility of using CVARs as production and workover risers from floating production systems in 3,000 to 6,000 ft depths. Brinkmann and Whooley [2] describe the use of CVARs with a Spar (also known as Deep Draft Caisson Vessel or DDCV) in 4,800 ft of water in the Gulf of Mexico. However, a study on this subject has not been reported so far.

Riser Concepts for High Motion Vessels in Ultradeep Water

“…The CVAR (Ishida, et al, 2001), (Mungall, et al, 2004) can be thought of in some ways as a special case of the SWR because both riser configurations terminate near the seafloor at a riser base. The significant differences between the two are listed below:…”

State-Of-The-Art Of Ultra Deepwater Production Technologies

“…These have addressed CVAR studies ranging from FPSO's in Brazil and Indonesia [39], SPARs, and now recently, semi submersibles in the GOM [40]. These have addressed CVAR studies ranging from FPSO's in Brazil and Indonesia [39], SPARs, and now recently, semi submersibles in the GOM [40].…”

VIM Model Testing and VIM Induced Mooring Fatigue of a Dry Paired-Column Semisubmersible Platform

“…For the first approach, TTRs supported by buoyancy cans have been proposed by Often et al (2000 and2001); Compliant Vertical Access Riser (CVAR) was presented by Mungall et al (2004); and Ram style tensioners with a nonconventional production riser stroke range of 11 m (35 ft) or even longer were considered by William and Leverette (2009) and Wang (2012). Development approaches can be categorized into two groups, 1) use a traditional semi hull form with an innovative dry tree riser design, and 2) utilize a traditional Ram style tensioner with a proven riser stroke range and extendable or adjustable base.…”