Design of the Compact Semisubmersibles Rockwater Semi 1 and 2
Authors: Schepman G.J. , van Santen J.A. 
Published: December 2007

This paper describes the design of two compact semisubmersible vessels usedin the North Sea. The design stages covered range from the initial marketsurvey to the performance evaluation.

Introduction

In the early 1980's, Smit Intl. considered options for expanding itsoffshore activity beyond its traditional towing, transport, supply, and salvageoperations. Offshore inspection, maintenance, and repair (IMR) andconstruction/installation were determined to be the areas best suited to thecompany's marine background and experience. However, specialized equipment wasneeded to begin these types of operations.

Market Evaluation

To determine the requirements for vessels to be acquired, Smit Intl.surveyed IMR and medium-sized construction/installation markets worldwide. Thecompany's primary areas of interest were the North Sea, the Mediterranean Sea, the Gulf of Suez, Africa, North and South America, India, and the Middle East. Although many platforms stood in the Gulf of Mexico, this area was notconsidered. Next, the numbers of platforms of interest in each area wereidentified (Table 1). Although it was difficult at that time to figure a totalmaintenance budget, the literature indicated that costs averaged $10billion/year worldwide, including post-startup maintenance and systemsupgrading. Actual estimates for the North Sea were around $2.5 billion. It wasestimated that $500 million would be used for North Sea followup maintenance, $1 billion for underwater maintenance, and $1 billion for general topsidemaintenance and cleaning. Once the sizes and locations of the potential marketswere identified, particular potential markets were identified, particularmarket segments-primarily those of topside and subsea IMR and secondarily thoseof light to medium construction-were evaluated more closely. Subsea IMRincludes activities on completion jackets, pipeline tie-ins, etc., using diversand remotely operated vehicles (ROV's). Topside activities include generalmaintenance and refurbishment, hookup, and accommodation. Also, the markets forplatform installation and removal, construction and equipment replacement, subsea well workovers, and logistics support were evaluated. Most of thepotential work was expected to be in water depths less than 150 m, where 90% ofthe investigated platforms were located. At the time, this market segment wasbeing serviced by monohull diving support vessels (DSV's) for the subsea workand a variety of converted jackups and converted drilling semisubmersibles forthe topside work. A limited number of purpose-built units were being used, butthese were dedicated to oil companies as field support and safety vessels. After this initial market evaluation, Smit Intl. hired Marine Structure Consultants B.V. to design a vessel that would allow the company to enter themarket effectively. Our study evaluated monohulls, jackups, largesemisubmersibles, and compact semisubmersibles. All types of activities(typical day rates, workability in various sea states, flexibility ofoperations, and movements between locations) were considered (see Table 2). After extensive evaluation of all information, a compact semisubmersible designwas chosen. Oil companies were consulted to determine their future service andequipment needs. Their comments greatly affected the final investment.

Initial Design Specification

To remain cost-effective while outperforming traditional vessels, the newcompact semisubmersibles had to meet the following design criteria:a minimum payload capacity of 1,500 tons (metric), with 750 for deck load;a large net deck area of more than 1,000 m, with a crane capacity of 150 tons anda 30-m reach;utilities to accommodate 110 persons and cabins for 89;positioning achieved with ajoystick dynamic positioning (DP) system; andwire mooring systems (in water depths up to 100 m).

The vessels had to beoperable in extreme environments (from the harsh North Sea to the Africancoast, where swell conditions exist). Investment costs had to be competitivewith the monohull day rate of about $10,000/d (1985 price level).

JPT

P. 460