Design Considerations Of A New Generation Swath Diving Support Vessel

Abstract

ABSTRACT This paper presents design considerations of a SWATH (Small Waterplane Area Twin Hull) Diving Support Vessel (DSV) presently under construction for delivery in 1995. It is designed to transit on twin pontoons at 12 knots and operate semi-submerged with dynamic positioning or a spread mooring. It will provide multi-purpose support for sheik w or deep water operations that include saturation diving support, pipe laying, trenching, well intervention, and offshore fire fighting services for worldwide use. This vessel greatly increases the capability of the offshore industry to perform deep water diving and other support functions in sea conditions that significantly exceed tha operating limits of mono hulls. This paper summarizes the design basis for the SWATH DSV encompassing the evolution from concept to the final design approved by the American Bureau of Shipping. Validation of analytical predictions of motion and station keeping with model tests are also presented. The vessel represents an important advance toward cost effective deep water operations. INTRODUCTION The acronym SWATH DSV stands for small waterplane Area Twin Hull, giving support vessel. Thesemisubmerged SWATH DSV hull form (Figure 1) developed by BSM is a novel design that blends the technologies of semisubmersible drilling vessels and constant draft SWATH 2. This multi-draft SWATH vessel will be the first of its kind to support a variety of functions including saturation diving support, pipeline laying and jetting, well work over, wireline servicing, ROV intervention, and fire fighting support for worldwide service including Gulf of Mexico and North Sea. The vessel will transit at 12 knots in sea state 4, and operate in sea states 5 and 6. Monohulls performing similarunctions rarely can operate beyond sea state 4. The vessel has a diesel electric dynamic position system with three 930 KW diesel engine generators in the upper hull and four 800 HP azimuthing thrusters in the pontoons. It is equipped with two heavy lift cranes, a large stern A frame, two moonpools and accommodations for 57 persons. Table 1 summarizes the principal particulars and Figure 2 shows the outboard profile and end views. The design project was performed on a fast track schedule. Conceptual design was completed in the month of September 1993. Preliminary design was completed in two months ending in March 1994. Model testing was performed in April and the final design was substantially complete by June 1994. A construction yard was selected in June and a contract awarded in September. Regulatory submittals overlapped initiation of construction, Vessel delivery is expected in the fall of 1995. FUNCTIONAL REQUIREMENTS The primary function of the vessel is diving and ROV operations in open ocean environments worldwide. The overriding requirement is for a ship which can perform diving and/or ROV operations, while under thruster assisted dynamic positioning or in a four (4) point mooring, in higher sea states than can be tolerated by equivalent monohulls. The vessel is required to support a minimum of 100 long tons of variable deck load in addition to the mission equipment. The basic necessity is to provide flexibility in the design.