Topside Design Research Articles

A method to generate a pipe model to compensate for low congestion level of 3D geometry available at the early stage of detailed design

For a recent explosion risk assessment, a CFD (computational fluid dynamics)-based simulation has been widely employed using a 3D geometry model for a gas dispersion and an explosion simulation. For an accurate determination of explosion design loads, extensive scenarios covering various leak locations, leak directions, leak rate, wind directions and so on are treated in the simulation. In order to reflect the results of the risk assessment into detailed topside design, the time-consuming simulations should be begun from the early stage of the detailed design. However, the congestion level of 3D geometry modeling available at the early stage is considerably different from the as-built model due to the lack of modeling of small-sized piping, pipe supports, electrical supports, detailed structural outfitting and so on. This study proposes a method to generate 3D pipe modeling to compensate for the low congestion level. A random approach is made for the generation by randomly determining a starting point of a pipe spool, pipe diameter, pipe direction, a straight pipe length, number of pipes in a pipe spool and so on. For a pipe running through a given box range, it is cut out on the box boundary and a pipe type connecting with a neighboring module is also realized. For a longitudinally long pipe rack, the inside pipe spools have different features. The generation algorithm is adjusted to realize these features. The generated method doesn't include any algorithm to avoid existing other geometric entities such as equipment or structural members. Thus, the generated pipe length is generally reduced after combining with a module containing equipment and structural members. A study is also done about how much the pipe length of the as-built model needs to be increased considering the overlap. The proposed method is verified by combining the generated pipe model with a module containing only structure & equipment, and comparing explosion analysis results with the corresponding as-built model. A dispersion simulation is also performed for the validity of the proposed model in the dispersion simulation as well.

CLOV Project: Overview

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25789, “CLOV Project: Overview,” by Geneviève Mouillerat, Total, and Francisca Silva, Sonangol, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. This paper describes the CLOV deepwater megaproject in Block 17 offshore Angola. This major development encompasses four separate oil and gas accumulations in waters up to 1400 m and aims to recover 505 million bbl of both light and heavier oil plus associated gas in quantity sufficient to require an export solution to shore for inclusion in the Angola liquefied-natural- gas (LNG) project. Introduction The CLOV megaproject of USD 8.4 billion (to first oil) in Block 17 offshore Angola is the fourth in a series of deepwater developments. The cluster of fields that gives the megaproject its name—Cravo, Lirio, Orquidea, and Violeta—is 140 km off the coast of Angola and is west/northwest from the earlier developments at Girassol, Dalia, and Pazflor in waters ranging in depth from 1100 to 1400 m (Fig. 1). CLOV came on stream on 12 June 2014 and reached its production plateau 3 months later. The four fields have recoverable reserves expected to be produced over a 20-year period. The oil is a mixture of good-quality light Oligocene crude (75% of recoverable reserves) and lower-quality Miocene crude. The presence of the latter has had a material effect on the development scenario for the complex. Exploration Development The complex of fields was first discovered in the summer of 1998 when the Lirio-1 well was drilled, finding a significant Oligocene accumulation with a large gas cap. The oil-bearing reservoirs in the northeast compartment of the structure were confirmed by the Lirio-2 appraisal well, which also further delineated the size and extent of the gas cap. The following year, Cravo-1 confirmed further light-oil reserves before the first appearance of the heavier Miocene oil at Orquidea-1. Additional exploration wells further revealed the complicated nature of the reservoirs at the location. Violeta-1 added to the Miocene reserves, but the Orquidea-1 well, drilled into a deeper horizon and 15 to 20° downdip of the main reservoir, revealed more light oil, which would provide an additional challenge at the well-design stage as a result of difficulty in defining well objectives. Subsurface work continued, and revised reservoir models indicated that the reserves for Cravo-Lirio would exceed the 245 million bbl given by an earlier estimate. The question remained whether the reserves warranted a standalone development. The first conceptual studies for these accumulations were carried out in spring 2005 to give some definition to subsea architecture, topside design, and a central facility. After the Lirio-2 appraisal well (drilled later that year) increased reserves, preproject studies were launched on the assumption that Cravo-Lirio would constitute a standalone hub with a floating production, storage, and offloading (FPSO) vessel, with the Miocene finds as a later tieback. A year later, following additional integrated geological and geophysical studies of the Miocene finds, which included data from two more appraisal wells (Orquidea-2 and Violeta-2), it was decided to choose a development concept that incorporated the Oligocene and Miocene accumulations.

A Study on the Lifecycle of the Offshore Plant and the Simulation with DMU and Ergonomics

In this paper, the academic research on the value chain and the supply chain of offshore business are conducted for the investigation of the offshore business comprehensive configuration. Then, the assembly and ergonomic simulation was conducted for the purpose of advanced validation of offshore project.. With respect to the simulation analysis, the assembly simulation model(w.r.t. drill ship) and the ergonomic simulation model(w.r.t. FPSO) are constructed. Through the assembly simulation of drillship, the production processes could be validated in terms of availability. Also, the problem of topside design of FPSO was found by ergonomic simulation.

Navy Radar Trades at the Ship Interface

AbstractThe next generation radar suite will include a radar that has sensitivity far greater than shipboard radars in use today. Most likely the radar will be a multi‐faced, solid state, phased array system with each array consisting of thousands of individual radiating elements powered by transmit/receive (T/R) modules. It will likely need to be large, and the demands it makes on ship services will be unprecedented. Its electrical power demands could be larger than the total ship's load for present‐day ships. Array size could significantly impact topside design. This paper examines top‐level radar system design choices and illustrates the trends in the ship impact of those choices. Radar design options considered are aperture size and shape, coolant operating temperature, number of array faces, and power system architecture. These design options have an effect on the radar system's weight, footprint, power demand, and cooling load. The effects on ship design can be significant. The ship impact analyses consider a radar system that includes ship‐provided equipment such as electrical power distribution equipment and chill water plants. An analysis of radar systems, all with the same performance, shows that radar system power demand can change by as much as four megawatts for a notional surface combatant depending upon the radar designer's configuration choices. Radar system weight can vary by more than 100 metric tons. There is a cubic relationship between T/R module power and array face area for a constant sensitivity radar system. This relationship is examined to show that there is often a choice of T/R module power that minimizes radar ship impact. The exact choice of T/R module power depends upon the customer's preferences, the manufacturer's capabilities, and choice of vendors. This paper does not convey any official US government position or US Navy endorsement of any particular radar architecture or design approach.

Performance-based design of blast resistant offshore topsides, Part II: Modelling and design

A simplified analytical model for deck plates of offshore topsides structures is proposed. Based on the theory of virtual work and an assumed deformed shape function in Cartesian coordinates, a set of equilibrium equations in terms of energy are developed. The equations are formed as a static case and a dynamic case with unsolved variables defined as generalized coordinates. When these equations are re-arranged, they are equivalent to a modal analysis, which can be solved using Lagrange’s equation method. The generalized coordinates in the static case can be solved manually while in the dynamic case, the ordinary differential equations are solved numerically by developing a computer program using a symbolic software package, MAPLE. The proposed method is comparable with finite element analysis results up to a certain threshold at which development of membrane strain and plasticity modes become dominant. When comparing the results to the reported experimental data, the proposed shape functions for deformations in the lateral directions are modified in order to accommodate the observed behaviour. The results compare favourably with test data and finite element analysis as a control case. Despite inconsistent ductility ratios between the proposed method and the finite element analysis at very high overpressures, the method shows good correlation of results at practical design values. Hence, the proposed method would be a useful tool for preliminary appraisal methods especially for design engineers involved in evaluating the performance of deck plating subjected to a range of hydrocarbon explosion scenarios.

Performance based design of blast resistant offshore topsides, Part I: Philosophy

The philosophy of performance based design of blast resistant offshore systems is an approach by which structural components are allowed to behave in a controlled non-linear characteristic that satisfies the demand criteria for preventing loss of lives, asset protection and integrity of escape routes. As the approach is based on behaviour rather than a prescriptive performance, uncertainties are removed which attributes to a precise design compared with a traditional design method. In the context of offshore installations, particularly for topside structures where safety cannot be compromised, this will lead to an economical design as well as meeting the functionality of a system from pre to post explosion regimes. This paper presents a performance based design philosophy for a typical topside structure which has the merit of being informative to the client and to regulators. This method also assists engineers in the comparison of engineering solutions which can be correlated to performance levels for which the installation has been designed and protected. The paper is divided into two sections. The first section discusses the methodology of the approach accompanied by a design algorithm assessment in which the principles of performance based design are emphasized in order to achieve the specific demand level for attainment of performance objectives. The second section demonstrates its application followed by a design example of a typical offshore topside to assess deck plate resistance against a defined blast load level.

Energy absorbing passive impact barrier for profiled blastwalls

After the Piper Alpha accident, the Health and Safety Executive (UK) now require safety cases to be submitted for all new and existing installations on offshore topsides. The findings from a recent Joint Industry Project have revealed that peak overpressures developed from typical hydrocarbon explosions can be much higher than previously used in the design of offshore topsides and thus has necessitated strengthening of many existing installations, including blastwalls. This paper presents one such technique where the response of the blastwalls is modified by the inclusion of a passive impact barrier system placed at a certain offset behind the walls. An explicit non-linear finite element model has been validated against two large scale blast tests of shallow profiled blastwalls. This has included modelling the effect of contact, weld tearing, large displacement and plasticity in order to provide a realistic model. The study has been extended to investigate the response of deep profiled blastwalls with various arrangements of the passive impact barrier systems. The increase in the containment pressure and the influence of strain rate, load duration, offset of the impact barriers and its stiffness are discussed.

1.1.1 Applying Systems Engineering to Warship Topside Design

AbstractThis paper presents a high level warship topside design process. All the aspects involved such as Human Factors, Electromagnetic Engineering, the Platform, the Combat System and so on, are taken into account. To manage, plan and coordinate all the activities, a Systems Engineering approach is applied. Starting from requirements definition and their analysis, through an iterative optimization loop, the whole Systems Engineering process is described. To assist supervision, graphic tools such as risk matrices are introduced. These matrices allow a global vision of the whole process in a very simple way. So we provide an effective path towards an efficient topside configuration that minimizes the risks which would otherwise occur in a non‐interdisciplinary approach. Finally a way to “measure” the process is given.

The Case for Compact Separation

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction The mission of a production facility is to separate produced well fluids into individual phase components (oil, gas, water, and sand) and process these phases into marketable products or dispose of them in an environmentally acceptable manner.1 Gravity-based vessels (two- or three-phase, horizontal or vertical) perform most of the separation duty; however, the demands of deepwater production and remote environments command a new portfolio of technology. The term compact separation is commonly applied to separation-unit processes that do not rely on (large) gravity-settling vessels. In the evolving world of facilities technology, compact separation promises to revolutionize topside design by saving space and weight while improving process performance. Compact separation is both necessary and unavoidable because it provides benefits to separation design beyond minimal facilities. However, evaluation of any compact-separation technology should fully review all aspects of performance, not just size. Separating Forces Compact separation applies to partitioning of distinct phases - specifically oil, water, gas, and sand. All four phases are separable by gravity settling; however, "enhanced" physical forces reduce equipment size. Specie separation within a phase (e.g., hydrocarbon fractionation, water deoxygenation, or desalination) is not covered in this review, because it requires more than mechanical forces to be accomplished. Gravity. Horizontal or vertical vessels relying on gravitational force provide most of the separation duty in oil and gas production. Efficient separation can be easily achieved with minimal complexity. However, the large size and weight of these vessels, especially at high pressure, is the main driver for development of compact-separation technology. Additionally, floating production performance deteriorates because of wave motion. Normally, these vessels are the largest and heaviest pieces of production equipment in the process facility. When relying only on gravitational forces for separation, residence time and low velocity are the keys to efficient separation. Impact/Interception. One of the first developments in compact separation was the addition of impact or interceptor internal devices to reduce gravity vessel size. Including devices such as momentum breakers to aid in gas/liquid separation, perforated plates for motion suppression, matrix packing to aid oil/water separation, and vane packs and mist pads to aid in droplet removal from gas greatly increase the separation efficiency compared with bare gravity vessels and enable significant size/weight reduction in many cases. Momentum breakers provide a partial defense against motion sensitivity, but they are susceptible to plugging or channeling. Cyclonic. The current trend in compact separation is cyclonic technology. Harnessing vortex flow provides an effective separation force of 10 to 5,000 times gravitational acceleration (g force), significantly decreasing separation equipment size. Static cyclones use pressure drop to produce spinning flow patterns, which in turn applies the high g forces to separate phases of different densities. Common equipment in this category includes cyclonic inlet devices for separators, axial flow and multicyclones for gas scrubbing, and deoilers and desanders for produced-water treatment. Cyclones provide the highest throughput-to-size ratio of any separation equipment, coupled with the least complexity. They are a net consumer of pressure drop, and the high velocities produced may be a source of wear. Rotordynamic. Rotordynamic forces are developing into the next generation of compact-separation technology. Applied energy to a rotating element promises to increase separation efficiency over that of cyclones because the static wall (boundary layer) effects are eliminated. Moving to rotordynamic equipment is a major step because the complexity of equipment increases significantly; however, in many cases, the benefits are worthwhile. Equipment in this category includes produced-water centrifuges, solid-bowl centrifuges, dynamic cyclones, self- and external-powered separator turbines, and the in-line rotary scrubber.

Engineering Aegis Cruiser Topsides ‐ Enhancing Design Capabilities and Life Cycle Support

ABSTRACTThe U. S. Navy is in the process of modernizing ship structures manufactured before 3‐D computer aided design capabilities were available. Detailed 2‐D views on large engineering drawings can be confusing to personnel performing removal and installation activities. 3‐D tools and capabilities would be highly beneficial to the modernization and implementation of the desired improvements by reducing confusion.To help meet this challenge for the cruiser conversion program, it was necessary to capture cruiser topside design model data. This task involved acquiring detailed 3‐D measurements of equipment and structure on the topside of a baseline representative Aegis cruiser on a non‐interfering basis. The 3‐D measurements were achieved by using a 3‐D laser scanner to remotely capture dimensional information of a ship's topside. These measurements have been used to create an As Is 3‐D model of the topside of a baseline representative Aegis cruiser to support cruiser conversion design efforts. The 3D model has provided accurate locations for topside equipment relative to ship structure.This measurement procedure should help in every aspect of topside design by creating more accurate surface combatant topside models. Pedigreed models would aid in studies of antenna coverage, interference, and combat system performance that is sensitive to antenna placement relative to each other and to ship structure. 3‐D laser surveying provides a cost‐effective method of maintaining current topside configuration data for the entire U.S. naval Fleet.

Development of a limit state approach for design against gas explosions

The design of topsides against explosions requires the definition of a design over-pressure, however, these values are often treated as deterministic and there is a wide variation within the industry in the treatment and interpretation of the loads. This paper advocates the adoption of a number of limit state for explosion loading. Events of different magnitudes are differentiated on the basis of frequency and linked to appropriate degree of reliability thus avoiding disproportionate effects from minor events. The two principal limit states considered are a limit state for all the safety critical systems for relatively high frequency events and a survival condition for low probability events. Parallels are drawn from other branches of engineering where extreme loads have to be designed for.

Topside Design of Warships: A 100 Year Perspective

ABSTRACT The warship design process requires competition, compromise, and iteration. This is as true today as it was when David Taylor was a young officer in the Construction Corps at the turn of the century when the U. S. was struggling to develop a world class navy. The naval architects and ordnance engineers of his day wrestled over topside arrangements much as do their current counterparts. They tried to anticipate future threats, and they worried about cost because they were in an arm's race. These were total ship engineering issues, although the phrase had not yet been coined. What is different today? It is tempting to say that the short answer is technology. And, more importantly, the rate of change. Yet, an objective appraisal of David Taylor's era tells us that they too experienced an explosion in technology. Just the years 1900–1910 alone saw the introduction of dreadnought battleships, submarines, and aircraft. We are still trying to grapple with the implications of the last two warfare systems. History tells us that our predecessors got it right more often than wrong. Why? Because they recognized the need to change the way they did business in the face of new technologies and new threats.

Modeling and Simulation for Integrated Ship Topside Design

ABSTRACT A new program initiative is evolving within the Naval Sea Systems Command (NAVSEA) to define and implement an integrated, architectural approach to surface ship topside design. With any given ship suite of sensor, weapon and control systems, ship topside architecture and design has more influence over realized ship warfare mission capability than any other ship system or engineering design discipline. An integrated treatment of all the major factors that govern ship topside physical and functional attributes will produce more capable, adaptable and affordable ship designs. An integrated ship topside design engineering framework will allow efficient exploitation of emerging system technologies to improve ship capabilities and flexibility. Integrated ship topside design is critically dependent on state‐of‐the‐art computer‐aided engineering— modeling, simulation and visualization— to provide timely and affordable quantification of the numerous engineering trade‐offs and design decisions that go into the topside design.This paper outlines the need, rationale and vision for integrated topside design in the U. S. Navy. The engineering framework that forms the essential core of the ship design engineering and acquisition management disciplines required to carry out integrated ship topside design is described. The modeling and simulation tools needed to carry out the integrated ship topside design process, along with their expected sources, are defined. A practical architectural mechanization of these computer‐aided engineering tools is outlined based on successful experience with the NAVSEA Electromagnetic Engineering program. The paper concludes with an overview of the status and plan to complete implementation of this new engineering capability, and its application to the design of the next generation of U.S. Navy surface ships.

3D Visualization Applied to Electromagnetic Engineering

ABSTRACT Over the past few years, the Naval Sea Systems Command (NavSea) Topside Design Group has conducted extensive development of the electromagnetics code for naval ship topside design. Analysis of computational electromagnetic data has long been a challenging task for surface ship designers. There are a number of very good computational models for analyzing a surface ship's electromagnetic characteristics. Unfortunately, the output of these models consist of huge matrices of real numbers that are very difficult to analyze.This paper will show how NavSea, in conjunction with Naval Command, Control and Ocean Surveillance Center RDT&E Division (NCCOSC/NRaD) and Rockwell International, is using computer graphics to help surface ship topside designers optimize the electromagnetic characteristics of a ship's design. NavSea has developed 2D and 3D scientific data visualization tools that provide an intuitive “feel” for the output of the numeric computational electromagnetic models.This paper will also describe how industrial software standards and fast, inexpensive graphics engines will provide electromagnetic design engineers with much more capable, robust, flexible, portable, and economical scientific visualization tools.

EMI—The Enemy Within

ABSTRACTElectromagnetic interference (EMI) is one of the major contributors to mission degradation in our fleet today due to the increase in population and sensitivity of both topside and below deck electronic systems. Sensitive combat systems designed to counter intelligent and deceptive targets can be confused by the complex intra‐ship EM environment. This can cause identification failure or losing “track” of a hostile or incoming missile or even engaging “friendly targets.”Topside design and integration efforts have been used to reduce EMI, but this is not the total solution to the problem. A program of total ship and system EMI assessment and control must be implemented. This program must exploit electromagnetic compatibility (EMC) optimization in electronic circuit design and take advantage of and (in some cases) direct topside shapes and structures to control the propagation of desired and undesired EM energy. Positive and active control of EM design characteristics are absolutely required before optimum combat system effectiveness can be realized.This paper will describe the current topside design process, EMC improvements being made, and how the process is being integrated into, and is dependent upon, the ship design process. It will give examples of some of the major mission degrading EMI problems in the fleet today and how past problems were solved with existing EM analysis programs. It will also discuss the control of EM energy in new design through the use of techniques being developed such as ray tracing and ray casting.The paper projects where the challenges lie for future topside and EM engineering designers and describes how the equipment technology transfer process must be better integrated to meet the challenge of effective EMI control.

Ship Topside Integration and Electromagnetic Interference Control

Electromagnetic Interference (EMI) is rapidly becoming one of the major contributors to mission degradation in the U.S. Fleet today. Topside design and integration efforts have been used to reduce EMI, but this is not the total solution to the problem. A program of total ship and system EMI control must be implemented that will span battle groups and be effective during joint operations before optimum combat system effectiveness can be realized. This paper gives examples of some of the major EMI problems in the fleet today. It discusses both intraship and intership EMI problems, how these problems arose, and how they might have been prevented through the use of techniques such as frequency management, equipment design, better element placement and improved cable shielding. It describes the current topside design process, electromagnetic compatibility (EMC) improvements being made and traces the history of EMC requirements and their application to date. It also describes a developing program for managing ship topside arrangement configuration. The authors project where the challenges lie for future topside and EM engineering designers and describe how the equipment acquisition process must improve in order to meet the challenge of effective EMI control.

Control of the Topside Electromagnetic Environment

ABSTRACTThe surface combatant's topside electromagnetic environment is becoming more complex. The quantity of antennas has been increasing, radiation characteristics are becoming more varied, and there are more modes of operation and a larger library of radiating parameters. Automatic computer control of radiators is more prevalent as is increased output power. Furthermore, there has been a reduction in maximum safe exposure limits for personnel, and there is an increased awareness of the importance of radar cross section reduction. The result is that ship designers and system operators are faced with conflicting requirements. The future will require electronic equipment designers to be more innovative and replace traditional systems with those of more flexibility. Potential solutions to the situation will require increased attention to both topside design and to the tools provided to the operators. Areas warranting further attention include the physical shape of the topside structure and equipment, movable antennas, increased use of radiation absorbing material, and movable radiation barriers. Safe radiation limits for personnel need to be made more flexible. Wartime rules and battle conditions will be different than peacetime. Some of our ordnance needs to be tested and possibly redesigned, so as to be less susceptible to electromagnetic radiation. And finally, complex surface combatants will require computer‐based expert systems to assist the operators in making decisions as to what to radiate, how to permit or restrict the movement and firing of ordnance, and the limitations to be placed on personnel access topside under peacetime, wartime and battle conditions.

COMBATANT PATROL BOAT TOPSIDE DESIGN

ABSTRACTAlong with the recent development of shipboard weapon systems of substantial firepower, considerable interest has risen in the design of light‐displacement, high‐speed combatant patrol boats. However, there has been an incorrect notion about patrol boats; “little boats have little problems.” From topside designers' point of view, little boats have many and big problems. Because of extremely limited shipboard topside space of this type ship, severe constraints are imposed for topside designers to effectively arrange shipboard combat systems, such as radars, fire control systems and communication equipment, to fully optimize the overall system performance and to sufficiently reduce electromagnetic hazards and interference (EMI). Recently, computer‐aided design techniques have been extensively utilized in combatant patrol boat topside design by making effective use of NAVSEA's topside design model (TDM), which allows for interactive design in graphically illustrated topside arrangements. Combat systems effectiveness is analyzed in terms of elevation plane optical coverages, radar line‐of‐sight (LOS) range detection, and omni‐directional antenna range prediction. The TDM algorithm includes the cumulative amplitude probability distribution of the HF communications range for the broadband fan and tuned whip antennas. The program is extended to analysis of the radiation pattern of directive antennas showing gain reduction principally due to blockage by superstructures. Finally, a set of candidate topside arrangements are proposed to the appropriate ship acquisition and design manager as feasible options to be pursued in the preliminary design phase.

THE UNIMAST CONCEPT ‐ A MAJOR DEPARTURE IN SHIPBOARD RADAR ANTENNA INSTALLATION PHILOSOPHY

ABSTRACTThe best topside location for an antenna is on top of the highest mast on the ship, thus affording all‐around coverage and minimum interference. However, usually only one antenna can occupy that site. Modern naval combatants have numerous antennas, and the necessary compromises in mounting all of them means that even the best site is often electro‐magnetically compromised by adjacent structures and other antennas.A contrary approach is to mount larger antennas lower, and to design the ship to minimize blockage. This is “The UNIMAST Concept,” which argues for a single mast on which are mounted all the ship's major rotating surveillance antennas. The larger antennas rotate around the lower part of the mast, while lighter antennas, e.g., surface search, are sited above.This paper discusses the degradation of sidelobes due to present topside design and numerically relates degradation to Electronic Counter‐Countermeasures (ECCM) performance. Removal of that degradation via the UNIMAST Concept requires that a device known as an Annular Rotary Coupler (ARC) be used. The development of just such a high power, wide‐band ARC on a Navy‐sponsored R&D contract is herein described. Also described is the potential backfitting of the UNIMAST Concept to existing ships; e.g., the DDG‐993.Battle damage vulnerability of the UNIMAST Concept with respect to a conventional two mast design is addressed. It is argued that the UNIMAST Concept affords no degradation in vulnerability due to improved potential for protecting vital antenna parts.